Dear friends,
I have been quite busy these few days. Sorry for not updating this blog. Thanks to all who have signed up for the email newsletter. I will try to get the first issue out by next week. thank you all for your support. In the meantime, please read through the notes that I have posted up. Remember repetition is the mother of all success.
Happy studying
Zhanrui
Tuesday, July 1, 2008
Sunday, June 29, 2008
E mail newsletter
Dear readers,
I will be creating weekly newsletters which contain information about how to prepare for the A Level chemistry exam including solutions to past year question. If u would like to subscribe to this newsletter, please fill up your email address in the top right hand corner of the blog.
I will be creating weekly newsletters which contain information about how to prepare for the A Level chemistry exam including solutions to past year question. If u would like to subscribe to this newsletter, please fill up your email address in the top right hand corner of the blog.
Saturday, June 28, 2008
Relation of reaction mechanism to reaction kinetics
In a nutshell, the rate equation is made up of reactants which are involved in the rate liming step or the slow step.
When chemists study reaction mechanisms, they are interested to know how the reaction actually occurs, for example which bonds are boken first, which bonds are formed first etc. We cannot determine the reaction mechanism from the balanced chemical reaction.
Let us consider the reaction of propanonne with iodine. From the balanced chemical equation, we can see that there are 2 reactants propanone and iodine. In order to determine the order of reaction w.r.t iodine, we use an excess of propanone and vary the concentration iodine, i.e. iodine will be the limiting reactant. Using the initial rates method, we can determine the order of reaction w.r.t iodine. It was determined that the reaction is zero order w.r.t iodine which means that the rate of reaction is independent of the concentration of iodine. Iodine will not appear in the rate equation
Next we determine the order of reaction w.r.t propanone and it was determined that the order of reaction w.r.t propanone is first order . Hence propanone will appear in the rate equation . Similarly we determine the order of reaction with respect to hydrogen ion concentration to be first order. Hence from the above data, we can write the following rate equation:
The rate equation also indicates that H+ and propanone are reactants in the rate determining step and a reaction mechanism can be proposed based on this data.
Note that the rate equation cannot be used to prove that a particular reaction mechanism is correct, it can only be used to prove that a particular reaction mechanism is wrong. If some one suggests that iodine is a reactant in the rate determining step, we can use the data from the kinetic analysis to say that he is wrong because the rate of reaction is independent of the concentration of iodine and hence iodine cannot be a reactant in the rate determining step.
When chemists study reaction mechanisms, they are interested to know how the reaction actually occurs, for example which bonds are boken first, which bonds are formed first etc. We cannot determine the reaction mechanism from the balanced chemical reaction.
Let us consider the reaction of propanonne with iodine. From the balanced chemical equation, we can see that there are 2 reactants propanone and iodine. In order to determine the order of reaction w.r.t iodine, we use an excess of propanone and vary the concentration iodine, i.e. iodine will be the limiting reactant. Using the initial rates method, we can determine the order of reaction w.r.t iodine. It was determined that the reaction is zero order w.r.t iodine which means that the rate of reaction is independent of the concentration of iodine. Iodine will not appear in the rate equation
Next we determine the order of reaction w.r.t propanone and it was determined that the order of reaction w.r.t propanone is first order . Hence propanone will appear in the rate equation . Similarly we determine the order of reaction with respect to hydrogen ion concentration to be first order. Hence from the above data, we can write the following rate equation:
The rate equation also indicates that H+ and propanone are reactants in the rate determining step and a reaction mechanism can be proposed based on this data.
Note that the rate equation cannot be used to prove that a particular reaction mechanism is correct, it can only be used to prove that a particular reaction mechanism is wrong. If some one suggests that iodine is a reactant in the rate determining step, we can use the data from the kinetic analysis to say that he is wrong because the rate of reaction is independent of the concentration of iodine and hence iodine cannot be a reactant in the rate determining step.
Concentration time graphs
In this section, we would look at how the concentration of reactants vary with time. Using a hypothetical A + B --> C reaction as an example, lets assume that the order of reaction with respect to A is zero order, i.e. the rate of reaction is independent of the [A]. The concentration time graph of A can be represented by a straight line. In other words, [A] decreases at a constant rate with time. This is because the rate of reaction is independent of the [A]. So although the concentration of A decreases with time, the rate of reaction remains the same. If the rate of reaction remains the same, it also means that reactant A is used up at a constant rate, hence the [A] decreases at a constant rate.
Now lets consider reactant B. Lets assume that the order of reaction w.r.t B is first order. Similarly the concentration of B will decrease with time. However the rate of reaction is dependent on the concentration of B, hence as the concentration of B decreases, the rate of reaction also decreases . As a result the concentration of B decreases at a decreasing rate. This can be illustrated by drawing tangents to the curve at different time points. You can see that the gradient of the tangent becomes gentler with time, indicating that the rate of decrease is decreasing.
Now lets consider reactant B. Lets assume that the order of reaction w.r.t B is first order. Similarly the concentration of B will decrease with time. However the rate of reaction is dependent on the concentration of B, hence as the concentration of B decreases, the rate of reaction also decreases . As a result the concentration of B decreases at a decreasing rate. This can be illustrated by drawing tangents to the curve at different time points. You can see that the gradient of the tangent becomes gentler with time, indicating that the rate of decrease is decreasing.
Thursday, June 26, 2008
Reaction kinetics I
Definition of some terms
Rate of reaction is defined as the rate of change of amount or concentration of a particular reactant or product
Rates of most reactions can be related to the concentrations of individual reactants by an equation of the form Rate = k[X]^n, where k is the rate constant, X is the reactant under consideration and n is the order of reaction with respect to X. This expression is known as a rate equation.
Students should note that the rate equation can only be determined experimentally, it is not related to the balanced equation.
The order of a reaction with respect to a given reactant is the power of that reactant's concentration of the experimentally determined rate equation.
The overall order of reaction is the sum of the powers of the concentration terms in the rate equation
The half life of a reaction is the time taken for the concentration of a reactant to fall to half its original value. Students should note that first order reactions have a constant half life. The decay of a radioactive isotope is usually a first order reaction.
Rate of reaction is defined as the rate of change of amount or concentration of a particular reactant or product
Rates of most reactions can be related to the concentrations of individual reactants by an equation of the form Rate = k[X]^n, where k is the rate constant, X is the reactant under consideration and n is the order of reaction with respect to X. This expression is known as a rate equation.
Students should note that the rate equation can only be determined experimentally, it is not related to the balanced equation.
The order of a reaction with respect to a given reactant is the power of that reactant's concentration of the experimentally determined rate equation.
The overall order of reaction is the sum of the powers of the concentration terms in the rate equation
The half life of a reaction is the time taken for the concentration of a reactant to fall to half its original value. Students should note that first order reactions have a constant half life. The decay of a radioactive isotope is usually a first order reaction.
Wednesday, June 25, 2008
Electrolysis in industrial process
Anodising of aluminum
Anodising is the process of increasing the thickness of aluminum oxide layer on the surface of aluminum in order to protect the metal underneath. The aluminum that is to be anodised is made the electrode during the electrolysis of sulphuric acid. Recall that oxygen is evolved at the anode during the electrolysis. the oxygen released combines with aluminum and thickens the oxide layer.
Electrolytic purification of copper
The impure copper rod is made the anode. At the anode, the copper ions is oxidized to Cu2+ ions. The Cu2+ ions is attracted to the cathode where is gains 2 electrons to form the copper metal. The electrolyte is copper sulphate solution. Effectively the copper is transferred from the anode to the cathode.
Anodising is the process of increasing the thickness of aluminum oxide layer on the surface of aluminum in order to protect the metal underneath. The aluminum that is to be anodised is made the electrode during the electrolysis of sulphuric acid. Recall that oxygen is evolved at the anode during the electrolysis. the oxygen released combines with aluminum and thickens the oxide layer.
Electrolytic purification of copper
The impure copper rod is made the anode. At the anode, the copper ions is oxidized to Cu2+ ions. The Cu2+ ions is attracted to the cathode where is gains 2 electrons to form the copper metal. The electrolyte is copper sulphate solution. Effectively the copper is transferred from the anode to the cathode.
Tuesday, June 24, 2008
Calculations related to involving electrolysis
Quantity of charge that passes through during electrolysis
The quantity of charge, Q that passes through during electrolysis in coloumbs is given by the product of current, I in amperes and the time, t in seconds.
Q = It
The syllabus also highlighted that students should know calculations involving the electrolysis of aqueous sulphuric acid and aqueous sodium sulphate. Realise that in both cases of electrolysis , it is actually the electrolysis of water , hence hydrogen and oxygen gas will be evolved. The half equations are presented below.
The effective reaction is the electrolysis of water. Note that the ratio of the volume of hydrogen gas to oxygen gas evolved is 2:1.
The quantity of charge, Q that passes through during electrolysis in coloumbs is given by the product of current, I in amperes and the time, t in seconds.
Q = It
The syllabus also highlighted that students should know calculations involving the electrolysis of aqueous sulphuric acid and aqueous sodium sulphate. Realise that in both cases of electrolysis , it is actually the electrolysis of water , hence hydrogen and oxygen gas will be evolved. The half equations are presented below.
The effective reaction is the electrolysis of water. Note that the ratio of the volume of hydrogen gas to oxygen gas evolved is 2:1.Monday, June 23, 2008
How to determine order of reaction?
How do we deduce the order of reaction?
The syllabus specify that students should know how to deduce the order of reaction with respect to a particular reactant using the initial rates method. Initial rate simply means the rate of reaction at the start of the reaction. For a typical chemical reaction, the initial rate of reaction is the fastest because the rate of reaction slows down as the reactants are being used up. Hence we would use the initial rate of reaction to represent the "true" rate of reaction.
Let's consider the reaction A + B --> C
The initial rates of reaction were determined and the results were presented in the table below
First we would determine the order of reaction with respect to reactant A. To do that we would look at Run(a) and Run(b) because the concentration of the other reactant B is the same. Only the concentration of A is changed. Hence whatever change in rate of reaction observed is due to A. Comparing Run(a) and run(b), we see that [A] is doubled however the reaction rate remains unchanged. Hence the order of reaction w.r.t A is zero order.
Secondly, we would dtermine the order of reaction w.r.t reactant B. Using the same reasoning we compare Run(b) and (c) where the [A] remain constant and [B] changes. We realise that the reaction rate doubles when the [B] doubles. Hence the order of reaction with respect to reactant B is first order.
The syllabus specify that students should know how to deduce the order of reaction with respect to a particular reactant using the initial rates method. Initial rate simply means the rate of reaction at the start of the reaction. For a typical chemical reaction, the initial rate of reaction is the fastest because the rate of reaction slows down as the reactants are being used up. Hence we would use the initial rate of reaction to represent the "true" rate of reaction.
Let's consider the reaction A + B --> C
The initial rates of reaction were determined and the results were presented in the table below
First we would determine the order of reaction with respect to reactant A. To do that we would look at Run(a) and Run(b) because the concentration of the other reactant B is the same. Only the concentration of A is changed. Hence whatever change in rate of reaction observed is due to A. Comparing Run(a) and run(b), we see that [A] is doubled however the reaction rate remains unchanged. Hence the order of reaction w.r.t A is zero order.
Secondly, we would dtermine the order of reaction w.r.t reactant B. Using the same reasoning we compare Run(b) and (c) where the [A] remain constant and [B] changes. We realise that the reaction rate doubles when the [B] doubles. Hence the order of reaction with respect to reactant B is first order.
Electrolysis II
Prediction of discharged substances during electrolysis
Redox series
When there is more than one anion or cation ion present in the electrolyte, how do we decide which ion will be discharged. The ion that is preferentially discharged depends on its position in the redox series. The redox series is presented below. Students should take note of the position of the hydrogen ion. In the presence of almost all other ions except copper and silver ions, hydrogen ions will be preferentially discharged.
There is a similar series for anions, albeit a shorter one. Similarly for hydroxide ions, unless the other ion present is bromide or iodide ion, hydroxide ions will be preferentially discharged.
Concentration
The concentration of ions present may also affect which ion will be discharged. Take for example if both lead ion and hydrogen ion are attracted to the cathode and lead ion is present in much higher concentration, lead ion will be preferentially discharged although it is higher in the redox series. In other words, high concentration can promote the discharge of an ion higher in the redox series.
Redox series
When there is more than one anion or cation ion present in the electrolyte, how do we decide which ion will be discharged. The ion that is preferentially discharged depends on its position in the redox series. The redox series is presented below. Students should take note of the position of the hydrogen ion. In the presence of almost all other ions except copper and silver ions, hydrogen ions will be preferentially discharged.
There is a similar series for anions, albeit a shorter one. Similarly for hydroxide ions, unless the other ion present is bromide or iodide ion, hydroxide ions will be preferentially discharged.
Concentration
The concentration of ions present may also affect which ion will be discharged. Take for example if both lead ion and hydrogen ion are attracted to the cathode and lead ion is present in much higher concentration, lead ion will be preferentially discharged although it is higher in the redox series. In other words, high concentration can promote the discharge of an ion higher in the redox series.
Electrolysis I
Relationship between Faraday's Constant and Avagadro's Constant
F: Faraday's Constant = 96500C
L: Avagadro's Constant = 6e23
e: charge of an electron = 1.6e-19
Basically, Faraday's Constant is a quantity that tells us how much charge does a mole of electrons possess. Recall that one mole is 6e23 which is also the Avagadro's number. Thus the relation F=Le
Prediction of the substance liberated during electrolysis
The ion that is discharged at the electrodes during electrolysis is affected by 3 main factors. They are the state of electrolyte (molten or aqueous), position in the redox series and concentration of the ion in the electrolyte.
State of electrolyte
If the electrolyte is aqueous, hydrogen ions (protons) and hydroxide ions are present. Due to the fact that hydrogen ions and hydroxide ions are positioned quite low in the redox series, they are usually preferentially discharged. If the electrolyte is in molten form, hydrogen ions and hydroxide ions are not present.
To illustrate this, let us consider the different products produced during the electrolysis of molten NaCl and aqueous NaCl.
During the electrolysis of molten NaCl, molten Na metal and chlorine gas is produced. Na+ is attracted to the negative electrode (cathode) and chloride ions is attracted to the positive electrode (anode). At the cathode Na+ ion gain an electron to form the molten Na metal. At the anode, Cl- loses an electron to form chlorine gas.
On the other hand, during the electrolysis of aqueous NaCl, hydrogen gas and oxygen gas
are produced. Both hydrogen ions and sodium ions are attracted to the cathode, however hydrogen ion is preferentially discharged. Hydrogen ion gain an electron to form hydrogen gas. Both hydroxide ions and chloride ions are attracted to the anode. Hydroxide ions are preferentially discharged at the anode due to its lower position in the redox series.
Sunday, June 22, 2008
Fuel Cells
What is a fuel cell?
Students should note that in the syllabus, you are not required to know the details about how a fuel cell works. You just need to know the advantages of a fuel cell as a power source. In a nutshell, the hydrogen oxygen fuel cell generates a flow of electron due to the oxidation of hydrogen gas and the reduction of oxygen gas.
Advantages of using a fuel cell
As mentioned in the syllabus, the advantages of using a fuel cell are the fuel cell is smaller in size, has a lower mass and can produce a higher voltage than conventional batteries.
Students should note that in the syllabus, you are not required to know the details about how a fuel cell works. You just need to know the advantages of a fuel cell as a power source. In a nutshell, the hydrogen oxygen fuel cell generates a flow of electron due to the oxidation of hydrogen gas and the reduction of oxygen gas.
Advantages of using a fuel cell
As mentioned in the syllabus, the advantages of using a fuel cell are the fuel cell is smaller in size, has a lower mass and can produce a higher voltage than conventional batteries.
Limitations of the standard cell potentials
We have seen how standard cell potential can be used to predict the feasibility of a redox reaction. However students should also understand the limitations of using the standard cell potentials.
Energetics vs Kinetics
In general, reactions with positive standard cell potentials are energetically feasible. However the standard cell potential does not tell us about the rate of reaction or its kinetic feasibility.
Standard conditions
Standard electrode values are determined under standard conditions such as the concentration of the aqueous metal ion is 1M. Students should not that a concentration of 1M is quite high. In practice, the redox reactions that we are interested in may not have such a high concentration of reacting ions.
Also the standard electrode values vary with the concentration of metal ions. Take for example the reduction of Cu2+. When the concentration of CU2+ is increased beyond 1M, the feasibility of Cu being reduced is increased. Thus the standard electrode potential will become more positive.
Energetics vs Kinetics
In general, reactions with positive standard cell potentials are energetically feasible. However the standard cell potential does not tell us about the rate of reaction or its kinetic feasibility.
Standard conditions
Standard electrode values are determined under standard conditions such as the concentration of the aqueous metal ion is 1M. Students should not that a concentration of 1M is quite high. In practice, the redox reactions that we are interested in may not have such a high concentration of reacting ions.
Also the standard electrode values vary with the concentration of metal ions. Take for example the reduction of Cu2+. When the concentration of CU2+ is increased beyond 1M, the feasibility of Cu being reduced is increased. Thus the standard electrode potential will become more positive.
A simple electrochemical cell
The schematic below illustrates a simple electrochemical cell.
In short an electrochemical cell functions as a battery. Recall that a current is simply a flow of electrons. In an electrochemical cell, the electron transfer that occurs during a redox reaction is made to flow through an external circuit.
How do we identify which species is donating the electron and which species is accepting the electrons? We can make use of the standard electrode values in the data booklet.
From the electrode values we can see that Cu2+ is more likely to be reduced compared to Zn2+. Hence we make Cu2+ the species that is to be reduced and Zn(s) would be the species that will be oxidized, i.e. the electron donating species. If you calculate the standard cell potential, we will see that the standard cell potential is positive, indicating that the reaction is feasible.
Students should note that for an electrochemical cell, the standard cell potential is always positive.
In short an electrochemical cell functions as a battery. Recall that a current is simply a flow of electrons. In an electrochemical cell, the electron transfer that occurs during a redox reaction is made to flow through an external circuit.
How do we identify which species is donating the electron and which species is accepting the electrons? We can make use of the standard electrode values in the data booklet.
From the electrode values we can see that Cu2+ is more likely to be reduced compared to Zn2+. Hence we make Cu2+ the species that is to be reduced and Zn(s) would be the species that will be oxidized, i.e. the electron donating species. If you calculate the standard cell potential, we will see that the standard cell potential is positive, indicating that the reaction is feasible.Students should note that for an electrochemical cell, the standard cell potential is always positive.
Electrochemistry II
How do we measure the the standard electrode potential?
Metals in contact with their ions in aqueous solutions
For example, we want to measure the standard electrode potential of Cu2+(aq)/Cu(s) half cell. The Cu2+(aq) /Cu(s) half cell consists of a copper electrode immersed in a 1M solution of Cu2+.
Ions of the same element in different oxidation state
For example, we want to measure the standard electrode potential of the Fe3+(aq)/Fe2+(aq) half cell. This half cell will consist of a platinum electrode immersed in a solution containing 1M Fe3+ and 1M Fe2+ ions
How to calculate standard cell potential by using standard electrode potential values in the data booklet?
For example if we want to calculate the standard cell potential for the following redox reaction:
We would look up the values for the standard half cell reactions from the data booklet.
Note that the standard electrode potential are always given as reduction potential. In our redox reaction Zn is oxidized, hence we have to reverse the sign of its standard electrode potential. Hence the standard cell potential would be +0.76 + (+0.34) = +1.10V
The standard cell potential is a positive value, indicating that the redox reaction is feasible.
Metals in contact with their ions in aqueous solutions
For example, we want to measure the standard electrode potential of Cu2+(aq)/Cu(s) half cell. The Cu2+(aq) /Cu(s) half cell consists of a copper electrode immersed in a 1M solution of Cu2+.
Ions of the same element in different oxidation state
For example, we want to measure the standard electrode potential of the Fe3+(aq)/Fe2+(aq) half cell. This half cell will consist of a platinum electrode immersed in a solution containing 1M Fe3+ and 1M Fe2+ ions
How to calculate standard cell potential by using standard electrode potential values in the data booklet?
For example if we want to calculate the standard cell potential for the following redox reaction:
We would look up the values for the standard half cell reactions from the data booklet.
Note that the standard electrode potential are always given as reduction potential. In our redox reaction Zn is oxidized, hence we have to reverse the sign of its standard electrode potential. Hence the standard cell potential would be +0.76 + (+0.34) = +1.10VThe standard cell potential is a positive value, indicating that the redox reaction is feasible.
Electrochemistry I
Redox processes
Redox processes can be defined in terms of electron transfer. Oxidation is defined as the loss of electrons. Reduction is defined as the gain of electrons. Redox process can also be defined in terms of changes in oxidation number. Oxidation is defined as an increase in oxidation number and reduction is defined as a decrease in oxidation number.
Standard electrode potential
The standard electrode potential of a half cell is defined as the potential of that half cell relative to a standard hydrogen electrode under standard conditions. The standard conditions are: all solutions have a concentration of 1M. Any gases involved have a pressure of 1 atm. The temperature is 298K.
Standard cell potential
Standard cell potential is the potential of the cell under standard conditions mentioned above.
Description of the standard hydrogen electrode
The standard hydrogen electrode consists of hydrogen gas at 1atm and 25 degrees Celsius bubbling around a platinum electrode. The electrode is immersed in 1M solution of H+ ions.
Redox processes can be defined in terms of electron transfer. Oxidation is defined as the loss of electrons. Reduction is defined as the gain of electrons. Redox process can also be defined in terms of changes in oxidation number. Oxidation is defined as an increase in oxidation number and reduction is defined as a decrease in oxidation number.
Standard electrode potential
The standard electrode potential of a half cell is defined as the potential of that half cell relative to a standard hydrogen electrode under standard conditions. The standard conditions are: all solutions have a concentration of 1M. Any gases involved have a pressure of 1 atm. The temperature is 298K.
Standard cell potential
Standard cell potential is the potential of the cell under standard conditions mentioned above.
Description of the standard hydrogen electrode
The standard hydrogen electrode consists of hydrogen gas at 1atm and 25 degrees Celsius bubbling around a platinum electrode. The electrode is immersed in 1M solution of H+ ions.
Saturday, June 21, 2008
Solubility product Ksp
Solubility product is used to compare the solubility of sparingly soluble ionic solids. I would use AgCl as an example to illustrate solubility product.
Ksp(AgCl) = [Ag+] [Cl-]
The solubility product of AgCl is the concentration of soluble silver ions at equilibrium multiplied by the concentration of soluble chloride ions at equilibrium. Students should note the difference between solubility product and ionic product. Ionic product is simply the product of concentration of ions at a given time, i.e the concentration of ions may not be at equilibrium.
Note that the solubility product concept applies to only sparingly soluble ionic compounds, i.e. it cannot be used for soluble compunds like NaCl. The solubility product is a modified equilibrium constant hence it is affected by temperature.
Common ion effect
The solubility of a sparing soluble ionic compound AB can be reduced by the presence of A+ or B- from a second source. For example we can compare the solubility of AgCl in water and in NaCl solution. The common ion is the chloride ion. Intuitively we know that AgCl is more soluble in water than in NaCl solution. Why is this so? Consider the solubility equilibria of AgCl:
In a solution of pure water, there are no chloride ions, so AgCl dissolves until the ionic product equals to Ksp. In a solution of NaCl, less AgCl can dissolve, as the ionic product will reach Ksp sooner as there are chloride ions already present in solution.
Its quite hard to explain in words but in the A level examinations, students will be asked to do some calculations. Based on the calculations, the effect of the common ion will be more apparent.
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Ksp(AgCl) = [Ag+] [Cl-]
The solubility product of AgCl is the concentration of soluble silver ions at equilibrium multiplied by the concentration of soluble chloride ions at equilibrium. Students should note the difference between solubility product and ionic product. Ionic product is simply the product of concentration of ions at a given time, i.e the concentration of ions may not be at equilibrium.
Note that the solubility product concept applies to only sparingly soluble ionic compounds, i.e. it cannot be used for soluble compunds like NaCl. The solubility product is a modified equilibrium constant hence it is affected by temperature.
Common ion effect
The solubility of a sparing soluble ionic compound AB can be reduced by the presence of A+ or B- from a second source. For example we can compare the solubility of AgCl in water and in NaCl solution. The common ion is the chloride ion. Intuitively we know that AgCl is more soluble in water than in NaCl solution. Why is this so? Consider the solubility equilibria of AgCl:
In a solution of pure water, there are no chloride ions, so AgCl dissolves until the ionic product equals to Ksp. In a solution of NaCl, less AgCl can dissolve, as the ionic product will reach Ksp sooner as there are chloride ions already present in solution.
Its quite hard to explain in words but in the A level examinations, students will be asked to do some calculations. Based on the calculations, the effect of the common ion will be more apparent.
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Friday, June 20, 2008
Bicarbonate buffer: Controlling blood pH
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The buffer in blood is made up of carbonic acid (weak acid) and hydrogen carbonate (conjugate base).
When protons is released into the blood, the protons combine with hydrogen carbonate (conjugate base) to form carbonic acid. As a result, the concentration of protons in the blood only increases slightly and the resulting pH change is small.
When hydroxide ions are released into the blood, the alkali that is added reacts with carbonic acid to form salt an dwater. As a result, the hydroxide ions are removed from blood, and the pH of blood only changes slightly.
The explanation that I have given above is actually a oversimplified one. For a more physiological and thorough explanation, please refer to the following website on Blood, Sweat and Buffers. For the purpose of the A Level examinations, my explanation will suffice.
Link to Contents Page
The buffer in blood is made up of carbonic acid (weak acid) and hydrogen carbonate (conjugate base).
When protons is released into the blood, the protons combine with hydrogen carbonate (conjugate base) to form carbonic acid. As a result, the concentration of protons in the blood only increases slightly and the resulting pH change is small.
When hydroxide ions are released into the blood, the alkali that is added reacts with carbonic acid to form salt an dwater. As a result, the hydroxide ions are removed from blood, and the pH of blood only changes slightly.
The explanation that I have given above is actually a oversimplified one. For a more physiological and thorough explanation, please refer to the following website on Blood, Sweat and Buffers. For the purpose of the A Level examinations, my explanation will suffice.
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Buffers and pH control
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The pH of a buffer solution can be calculated using the Handerson-Hasselbach equation.
Buffers are solutions which resist changes in pH on addition of small amount of acid or alkali. Buffers usually consist of a weak acid and its conjugate base (acid buffer) or a weak base and its conjugate acid (basic buffer).
How does an acid buffer work?
As mentioned above, an acid buffer consists of a weak acid and its conjugate base. An example is ethanoic acid and its conjugate base ethanoate ion. The conjugate base is usually supplied as a salt. The buffer is most effective at resisting pH changes when the concentration of the undissociated acid molecule (ethanoic acid) is similar to the concentration of the conjugate base (ethanoate ion).
When a small amount of acid is added, the protons combine with the conjugate base (ethanoate ions) to form the weak acid (ethanoic acid). The H+ that is added is removed, hence the concentration of H+ in solution and thus the pH changes only slightly.
When a small amount of base is added, the base reacts with the weak acid to form salt and water. The hydroxide ions are removed from the solution in the form of water and the pH of the solution only changes slightly.
Calculating the pH of buffer solutionThe pH of a buffer solution can be calculated using the Handerson-Hasselbach equation.
Students should note that this equation can only be used for calculating the pH of buffer solutions. Also the equation above can only be used to calculate the pH of acid buffers. For base buffers, use the equation below
Thursday, June 19, 2008
Acids and bases II
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3) pH, dissociation constants (Ka, Kb) and ionic product of water (Kw)
The pH of a solution is the negative logarithm to base ten of the molar concentration of hydrogen ion. Students should note that pH only tells you how acidic or basic a solution is, it does not tell you about the strength of the acid or base. A concentrated solution of a weak acid will have a low pH , but it does not mean that the acid is a strong acid
In order to measure the strength of the acid , chemists use the acid dissociation constant. The acid dissociation constant is an equilibrium constant. Basically it is the ratio of concentration of protons and conjugate base to the concentration of undissociated acid molecules. Since acid dissociation constants are equilibrium constants, they are not affected by concentration, unlike pH which is concentration dependent. The greater the Ka, the stronger the acid.
Dissociation of water
Water dissociates to a very small extent to form hydrogen ions and hydroxide ions. This is represented by the ionic product for water Kw, which is the [H+] X [OH-], 10e-14 for pure water at 25 degree celsius. Note that like acid dissociation constant, the ionic product for water is effectively an equilibrium constant, hence it is affected by temperature, i.e. Kw will vary with temperature.
4) Indicators for acid base titration
The most common indicators are litmus, methyl orange and phenolpthalein. Methyl orange is red at acidic pH, orange at pH 3 to 4 and yellow at pH 5 onwards. Litmus paper is red at acidic pH and blue at pH values of 7 and above. Phenolpthalein is colourless at at acidic pH (pH 0 to 7 ) and red at alkalike pH (pH 9 and above). Students shouldnote that not all indicators change colour at pH 7.
In a titration of strong acid against a strong alkali, at equivalence point when the acid is completely neutralized, the pH changes by a large extent from around pH 4 to 10. Thus both methyl orange and phenolpthalein can be used to indicate that equivalenc epoint has been reached.
In the titration of a strong acid with a weak base, the pH change is less drastic, from around pH 4 to 8, hence phenolpthalein is not a suitable indicator as it changes colour at around pH 8. Methy orange will be a suitable indicator.
In the titration of a weak acid with a strong base, the pH change at equivalence point is from around pH 7 to 11. Phenolpthalein will be a suitable indicator as it changes colour at around pH 8.
In the titration of a weak acid with weak base, the pH change at equivalence point is very small. hence there are no suitable indicators. You can probably detect the pH change using a pH meter.
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3) pH, dissociation constants (Ka, Kb) and ionic product of water (Kw)
The pH of a solution is the negative logarithm to base ten of the molar concentration of hydrogen ion. Students should note that pH only tells you how acidic or basic a solution is, it does not tell you about the strength of the acid or base. A concentrated solution of a weak acid will have a low pH , but it does not mean that the acid is a strong acid
In order to measure the strength of the acid , chemists use the acid dissociation constant. The acid dissociation constant is an equilibrium constant. Basically it is the ratio of concentration of protons and conjugate base to the concentration of undissociated acid molecules. Since acid dissociation constants are equilibrium constants, they are not affected by concentration, unlike pH which is concentration dependent. The greater the Ka, the stronger the acid.
Dissociation of water
Water dissociates to a very small extent to form hydrogen ions and hydroxide ions. This is represented by the ionic product for water Kw, which is the [H+] X [OH-], 10e-14 for pure water at 25 degree celsius. Note that like acid dissociation constant, the ionic product for water is effectively an equilibrium constant, hence it is affected by temperature, i.e. Kw will vary with temperature.
4) Indicators for acid base titration
The most common indicators are litmus, methyl orange and phenolpthalein. Methyl orange is red at acidic pH, orange at pH 3 to 4 and yellow at pH 5 onwards. Litmus paper is red at acidic pH and blue at pH values of 7 and above. Phenolpthalein is colourless at at acidic pH (pH 0 to 7 ) and red at alkalike pH (pH 9 and above). Students shouldnote that not all indicators change colour at pH 7.
In a titration of strong acid against a strong alkali, at equivalence point when the acid is completely neutralized, the pH changes by a large extent from around pH 4 to 10. Thus both methyl orange and phenolpthalein can be used to indicate that equivalenc epoint has been reached.
In the titration of a strong acid with a weak base, the pH change is less drastic, from around pH 4 to 8, hence phenolpthalein is not a suitable indicator as it changes colour at around pH 8. Methy orange will be a suitable indicator.
In the titration of a weak acid with a strong base, the pH change at equivalence point is from around pH 7 to 11. Phenolpthalein will be a suitable indicator as it changes colour at around pH 8.
In the titration of a weak acid with weak base, the pH change at equivalence point is very small. hence there are no suitable indicators. You can probably detect the pH change using a pH meter.
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Acids and bases I
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1) Bronsted-Lowry Theory of Acids and Bases
According to the Bronsted Lowry theory, an acid is defined as a substance that can donate proton to another substance, i.e. a proton donor. An example of an acid is hydrochloric acid. Students also need to know what a conjugate base is. An acid whhich has donated its proton becomes a conjugate base. Using hydrochloric acid as an example, hydrochloric acid is the acid and the chloride ion is the conjugate base. Using ethanol as an example, if ethanol behaves as a Bronsted acid, it loses its proton and becomes ethanoate ion. Therefore the conjugate base is the ethanoate ion.
A base is a substance which accepts a proton from another substance, i.e. a proton acceptor. An example is ammonia. Ammonia is a Bronsted base, it accepts a proton and becomes the ammonium ion. In this case ammonia is the base and the ammonium ion is the conjugate acid. Note that a bronsted base is related to its conjugate acid and a bronsted acid is related to its conjugate base.
Note that certain acids and bases do not conform to the bronsted acid and base definition. Take for example sodium hydroxide, it is well know that sodium hydroxide is a base. However NaOH is not a proton acceptor hence it is not a Bronsted base.
There is another definition of acids and bases proposed by Arrhenius. According to the Arrhenius definition, an acid dissociates in water to form hydrgen ions and a base dissociates in water to produce hydroxide ions. NaOH will fit the Arrhenius definiton of a base. There are other definitions such as the Lewis acid definition. For more information students can refer to the wikipedia entry.
2) Differences between a weak and strong acid.
A strong acid such as hydrochloric acid dissociates fully in water to form hydrogen ions and chloride ions. A weak acid dissociates partially in water. An example is ethanoic acid. When ethanoic acid is dissolved in water, some of the ethanoic acid molecules will dissociate to form the ethanoate ion and hydrogen ions. However not all of the ethanoic acid molecules dissociate so there will be ethanoic acid molecules in the wtaer in addition to ethanoate ions and protons.
Students must be careful not to mix up concentration and strength of acid. A concentrated acid might not be a strong acid. A 5 M ethanoic acid is a concentrated acid solution but ethanoic acid is not a strong acid.
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1) Bronsted-Lowry Theory of Acids and Bases
According to the Bronsted Lowry theory, an acid is defined as a substance that can donate proton to another substance, i.e. a proton donor. An example of an acid is hydrochloric acid. Students also need to know what a conjugate base is. An acid whhich has donated its proton becomes a conjugate base. Using hydrochloric acid as an example, hydrochloric acid is the acid and the chloride ion is the conjugate base. Using ethanol as an example, if ethanol behaves as a Bronsted acid, it loses its proton and becomes ethanoate ion. Therefore the conjugate base is the ethanoate ion.
A base is a substance which accepts a proton from another substance, i.e. a proton acceptor. An example is ammonia. Ammonia is a Bronsted base, it accepts a proton and becomes the ammonium ion. In this case ammonia is the base and the ammonium ion is the conjugate acid. Note that a bronsted base is related to its conjugate acid and a bronsted acid is related to its conjugate base.
Note that certain acids and bases do not conform to the bronsted acid and base definition. Take for example sodium hydroxide, it is well know that sodium hydroxide is a base. However NaOH is not a proton acceptor hence it is not a Bronsted base.
There is another definition of acids and bases proposed by Arrhenius. According to the Arrhenius definition, an acid dissociates in water to form hydrgen ions and a base dissociates in water to produce hydroxide ions. NaOH will fit the Arrhenius definiton of a base. There are other definitions such as the Lewis acid definition. For more information students can refer to the wikipedia entry.
2) Differences between a weak and strong acid.
A strong acid such as hydrochloric acid dissociates fully in water to form hydrogen ions and chloride ions. A weak acid dissociates partially in water. An example is ethanoic acid. When ethanoic acid is dissolved in water, some of the ethanoic acid molecules will dissociate to form the ethanoate ion and hydrogen ions. However not all of the ethanoic acid molecules dissociate so there will be ethanoic acid molecules in the wtaer in addition to ethanoate ions and protons.
Students must be careful not to mix up concentration and strength of acid. A concentrated acid might not be a strong acid. A 5 M ethanoic acid is a concentrated acid solution but ethanoic acid is not a strong acid.
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