Reactions which are favoured in the thermodynamic sense may not take place at a sufficient rate to be detectable. Thermodynamics has nothing to say about the rate at which reactants can be transformed into products.
Industrialists and chemical engineers are not satisfied with merely turning one substance into another. In most cases, they want to perform reactions and obtain products as rapidly, easily and as cheaply as possible.
At normal temperature and pressure, and in the absence of a catalyst, ammonia cannot be obtained from nitrogen and hydrogen. A reasonable rate can be achieved at 250 atmospheres (25,331,250 Nm-2) and 450oC in the presence of an iron catalyst.
Reaction rate can be defined as the rate of change (differential; gradient of straight line) of concentration of a particular reactant of product.
Experiments show that the rates of most reactions can be related to the concentrations of individual reactants. Consider the reaction:
Experiments may show that the rate of the reaction is proportional to the concentration of A to the power of x, i.e.:
and also the rate may be proportional to the concentration of B to the power of y.
The overall equation is:
Rate [A]x [B]y
or Rate = k [A]x [B]y
k is called the rate constant and the overall equation is called the rate equation. The order of the reaction with respect to A is x, and with respect to B is y. The overall order is x + y.
Consider the reaction between peroxodisulphate ions and iodide ions:
Experiments have shown that:
Hence the reaction is first order with respect to S2O82-, first order with respect to Isup>- and second order overall.
Consider the reaction between propanone and iodine. This reaction is catalysed by acid.
Experiments have shown that:
Hence this reaction is first order with respect to propanone, first order with respect to acid and zero order with respect to iodine. This illustrates an important point: You cannot write a rate equation from a chemical equation.
Consider the decomposition of hydrogen peroxide:
If a graph is plotted of concentration of hydrogen peroxide against time and half-life values determined, these are found to be constant. A constant half-life implies a first-order reaction, i.e.
Several experiment 'runs' are carried out at different concentrations. Graphs are plotted of concentration against time and the initial rate determined from the gradient at time = 0. A graph is then plotted of rate against concentration. If it is a straight line the reaction is first order. If a graph of rate against (concentration)2 is a straight line the reaction is second order. A reaction which is independent on concentration is zero order.
If concentration is measured over a period of time a graph may be plotted of concentration against time. By drawing tangents at a number of points, the rate of reaction at different times can be found. A graph is then plotted of these rates against concentration. If it is a straight line the reaction is first order. The rate constant is the gradient of this line.
One of the main reasons for determining the order of a reaction is to see whether the experimentally determined order sheds any light on the detailed mechanism by which the reaction occurs.
Consider the hydrolysis of 2-bromo-2-methylpropane.
It has been found that the reaction is first order with respect to 2-bromo-2-methylpropane, but zero order with respect to hydroxide ions. i.e.
The concentration of OH- ions does not influence the rate. There must be a minimum number of OH- ions present in order to obtain the product. The suggested mechanism is:
This step is a slow step.
This is a fast step.
The rate of the reaction is controlled by step 1, which is called the rate-determining step. This is why it is first order reaction.
The hydrolysis of 1-bromobutane goes by a different mechanism. It has been found that the reaction is first order with respect to 1-bromobutane and first order with respect to hydroxide ions, second order overall.
In the first slow step, the hydroxide ion approaches the carbon atom of the C-Br bond and starts to form a bond to the carbon atom. The bond between the carbon atom and the bromine atom starts to break. For an instant of time, both the incoming hydroxide ion and the outgoing bromide ion are equally associated with the carbon atom - A complex ion is formed for an instant. In the second fast step the bromide ion is lost. The rate-determining step involves both 1-bromobutane and hydroxide ions.
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