| Contents for this page | Related topics |  |
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Reactants and products Definition of the reaction rate Initial rates Factors affecting reaction rates The collision theory Measurement of reaction rates Additional questions |
Catalysis Chemical equilibrium |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a)understand what is meant by the "rate of a chemical reaction", (b) and you will be familiar with the factors which influence it, and (c) have an understanding of how reaction rates are measured. | ||
A chemical reaction involves one or more reactants and products:
In the example above, zinc in a solid form, as indicated by the symbol (s) , reacts with sulphuric acid in an aqueous solution, as indicated by the symbol (aq). These are the REACTANTS. The reactants react to form PRODUCTS written to the right of the arrow. In the above example, the products are an aqueous solution of the salt zinc sulphate and hydrogen in gaseous form, which is indicated by the symbol (g). If a reactant is present in amounts that are less than will completely use up the other, it is called the LIMITING REACTANT.
The rate of a chemical reaction is defined as the change in the concentration of one of the reactants or products in unit time
For example, in the reaction above, we could express the rate of the reaction as the change in the concentration of H2SO4 (Δ[H2SO4]) in a certain time interval :
Let's consider a general reaction: A → B: If one plots the concentration of A, [A] against the time, t, one typically obtains a curve such as the one shown on the right: The fact that the graph is not a straight line tells us the rate of the reaction changes with time. |
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Now look at the graph on the left. In the time interval Δt covering t = 0 to t = t1, there is a comparatively greater change in [A] than in the same time interval Δt = t1 to t = t2. The rate of a reaction is therefore always linked to the specific time interval in which the concentration measurements were made! |
THE INITIAL RATE of a reaction is the rate right at the start of the reaction, when t = 0. It is calculated by determining the gradient of the tangent to the curve at time t = 0:
Looking at the curve again, we see that its steepest part occurs right at the start of the reaction. The gradient at t = 0 is the steepest, and that will be when the rate will have its highest value. |
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The COLLISION THEORY explain the factors that influence reaction rates. Basically, the theory postulates that the rates of reactions depend on how often and how energetically the reacting molecules collide with each other, the faster will the overall reaction proceed. We will see below how this theory is applied.
In order for two reactants to react, they must come into close contact, in other words, they must COLLIDE. However, it is not sufficient for two molecules to collide in order that they might react. The reaction will only take place if the collisions are "fruitful", and for this to happen,
There are a number of factors that increase the likelyhood of fruitful collisions will generally tend to increase the rates of reactions. These are discussed below.
For reactions that take place in solution, the greater the concentration of reactants, the greater the rate of the reaction, as the likelyhood of collisions increases the more there are reactants in a given volume.
For reactions involving gases, an increase in pressure increases the reaction rates, since, in a container having a fixed volume, the concentration of gases are directly proportional to the pressure inside the container.
The higher the temperature, the higher the reaction rate. This is because the kinetic energy of the reactant molecules increases with temperature, and thus, the collisions are more energetic.
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The graphs shown here on the left are "Maxwell-Boltzmann" DISTRIBUTION CURVES. Energy values are plotted on the horizontal axis, while the number of molecules with corresponding energies are plotted on the vertical axis. Two such curves are shown, one (blue) at a temperature T1, and one, in red, at a higher temperature T2. Ea, the so-called ENERGY OF ACTIVATION, is the minimum value that a molecule must achieve in order for the reaction to occur. At the lower temperature, T1, only a relatively small proportion of molecules (area below the blue curve to the right of Ea will have sufficient energy to react. At the higher temperature, T2, a much higher proportion of the molecules have the required energy. Thus, since the rate of the reaction depends on the number of molecules that react in a given time interval, the rate will be higher at T2 than at T1. |
Reactions between ionic compounds are usually very much faster than those involving compounds where covalent bonds have to be made or broken.
This is very important when solids are involved. The more finely divided the solid is, the faster the reaction will take place. In fact, for solids, it is the surface area, and not the concentration, that affects the rate. Of course, for a reaction involving a solid reactant and another that is in solution, the rate will be affected not only by the surface of the solid reactant, but also by the concentration of the one in solution. In terms of the collision theory, the grater the surface are of a solid, the more collisions will take place at the surface, and hence the greater the reaction rate.
These are substances which speed up reaction rates without undergoing a permanent change in the process. The process is called CATALYSIS, and it may be HOMOGENEOUS, when both the reactants and the catalyst are in the same phase (for example, when both are in solution), or HETEROGENEOUS, when the ractants and catalyst are in different phases (for example, gaseous reactants and a solid catalyst).
We have seen that the rate of a chemical reaction is defined as the change in the concentration of one of the reactants or products in unit time. For a reaction A → B, since one molecule of B is produced for every molecule of A that is consumed, it does not matter whether we measure a decrease in the concentration of A or an increase in the concentration of B - the absolute value of the rate will be the same. We can then write:
From an experimental point of view, if one wished to measure the rate of such a reaction, it would not matter whether one measured the decrease in concentration of A, or the increase in concentration of B. We would get the same results. In practice, it may well be that one of the two alternatives is much easier to perform in the laboratory, and that would then be the method of choice.
Experimentally, the rate of a reaction is found to depend (amongst other things!) on the concentration of the reactants. For a generalised reaction
we find that
where k is the RATE CONSTANT for the reaction.
There are many ways of determining reaction rates. The following will give the basic principles involved in some of them:
Changes in colour:
If a reaction produces a coloured substance, that is, one that has an absorption maximum at some wavelength in the visible spectrum, one can measure the increase in the absorbance of the solution in which the reaction takes place as a function of time. Since the absorbance of a solute is directly proportional to its molar concentration, the rate of the reaction is easy to determine. One will of course need a fairly expensive piece of equipment called a SPECTROPHOTOMETER. |
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A similar method, known as TURBIDIMETRY makes use of the scattering of light from aqueous suspensions. If a precipitate gradually forms, the solution will become more opaque with time as more finely divided particles precipitate out. The rate at which the opacity of the liquid increases can be used as a measure of the reaction rate.
Changes in volumes:
This is useful for reaction which produce gases. The reaction flask is connected to a syringe, as shown in the diagram on the right, and the volumes indicated on the syringe scale read off at regular time intervals. |
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Changes in mass:
Again, if a gas is evolved during a reaction, for example, the reaction between calcium carbonate and dilute acid, the rate at which the apparatus loses mass will be proportional to the reaction rate. Calcium carbonate powder may be suspended in water in an Erlenmeyer flask and placed on the pan of an electronic top-loading balance. The reaction is started by the addition of acid, and the mass recorded at various time intervals starting immediately after addition of the acid. |
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In fact, any measurable change that takes place during a reaction may be used to determine reaction rates. One must always bear in mind the various factors that influence the rates. (See above)