Question

What is the best way to study for a general chemistry test on determining rate and a general mechanism from experimental data?

Answer 1

• The reaction mechanism describes the sequence of elementary reactions that must occur to go from reactants to products.

• Reaction intermediates are formed in one step and then consumed in a later step of the reaction mechanism.

• The slowest step in the mechanism is called the rate determining step or rate-limiting step.

• The overall reaction rate is determined by the rates of the steps up to (and including) the rate-determining step.

The basic method of obtaining the information needed to determine rate constants and reaction orders is to bring the reactants together and then measure successive changes in concentration of one of the components as a function of time. Two important requirements are:

The time required to take a measurement must be very short compared to the time the reaction takes to run to completion; The temperature must be held constant — something than can pose a problem if the reaction is highly exothermic.

EXPERIMENAL METHOD TO FIND OUT RATE OF REACTION

The basic method of obtaining the information needed to determine rate constants and reaction orders is to bring the reactants together and then measure successive changes in concentration of one of the components as a function of time. Two important requirements are:

• The time required to take a measurement must be very short compared to the time the reaction takes to run to completion;
• The temperature must be held constant — something than can pose a problem if the reaction is highly exothermic.

Measuring the concentration of a reactant or product directly — that is by chemical analysis—is awkward and seldom necessary. When it cannot be avoided, the reaction sample must usually be quenched in some way in order to stop any further change until its composition can be analyzed. This may be accomplished in various ways, depending on the particular reaction. For reactions carried out in solution, especially enzyme-catalyzed ones, it is sometimes practical to add a known quantity of acid or base to change the pH, or to add some other inhibitory agent.More commonly, however, the preferred approach is to observe some physical property whose magnitude is proportional to the extent of the reaction.

Since reate of reaction is related via stoichiometry to the rate of consumption of a reactant and the rate of formation of a product, the rate of reaction can be determine by following the concentration of either a reactant or product as a function of time. A graph of the chemical specie's concentration versus time can be used to determine the rate reaction. This is done by drawing the tangent line to the concentration curve at the time one wishes to determine the rate of reaction. The slope of the tangent line at in any point in time is the change in specie's concentration with time at that time. The change in concentration with time is directly proportional to the rate of reaction, where the proportionality constant in the reciprocal of the stoiochiometric coefficient of the species in the balanced chemical reaction. Generally the slope of the graph changes so that the reaction rate changes with time. Usually reactions slow down as the reaction proceeds as the amount of reactants left decreases.

EXAMPLE

As an example consider the decomposition of HI at 508°C. 2HI(g) H₂(g) + I₂(g) HI and H₂ are colourless but I₂ has purple colour. So it is easiest to monitor I₂. A spectrometer can be used to monitor the change in colour intensity of the reaction chamber and relate it back to the concentration of I₂. Knowing the concentration of I₂, the concentration of HI can be calculated. Plotting the concentration of HI versus time gives the following graph. To get the rate of HI consumption at 100 s, the tangent line to the concentration curve is drawn and the slope of the tangent line is calculated to be -2.5x10-4 M/s. Therefore the reaction rate is 1.3x10-4 M/s.