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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.