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.