Finding catalysts for industrial reactions is a vitally important field of research. For example, without a catalyst the conversion of nitrogen and hydrogen into the valuable compound ammonia in the Haber process would be too slow to be useful. An iron catalyst speeds up the reaction and makes the process fast enough to be economically worthwhile.
Other factors change reaction rates. If reacting substances are heated, the rate of the reaction usually rises; conversely, if they are cooled, the reaction slows down. In order to react, the particles in the substances must collide with each other. Heat gives them more energy to move around and so increases the chances of a collision. Also, when particles do collide, they are more likely to react, rather than just bounce off each other, if they are moving faster. Cooling has the opposite effects. For example, when sodium thiosulphate is mixed with dilute hydrochloric acid the mixture becomes cloudy as solid sulphur comes out of the solution (precipitates). If the mixture is heated, it becomes cloudier more quickly. If it is cooled, it takes longer to become cloudy.
Increasing the concentration of reactants (the amount dissolved in a given volume of solution) can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate.
Particle size can also affect reaction rate. Marble chips will dissolve in hydrochloric acid more slowly than an equal amount of ground marble, because less surface area is exposed for the acid to attack.
The rate of a reaction can be obtained by following some measurable property that alters as the reaction occurs. Examples of such measurable properties include gas volumes, concentration of reaction mixture, and electrical conductivity.
Measuring gas volumes could be appropriate in a reaction such as the evolution of carbon dioxide gas when dilute hydrochloric acid is added to calcium carbonate,
CaCO3 + 2HCl → CaCl2 + H2O + CO2.
The rate at which carbon dioxide is evolved can be followed by attaching a gas syringe to a test tube in which the reaction is occurring and noting the volumes at fixed time intervals.
By sampling the reaction mixture and the concentration of one of the components of the reaction mixture (either reactants or products) at regular intervals, the concentration of the reaction mixture can be estimated by titration. An example is the acid-catalysed hydrolysis of an ester such as ethyl ethanoate,
CH3COOC2H5 + H2O → CH3COOH + C2H5OH.
The reaction mixture is titrated at intervals against a standard solution of sodium hydroxide; there is a constant concentration of acid present as catalyst, so as the reaction progresses, more alkali is required due to the formation of ethanoic acid.
In some reactions, electrical conductivity can be measured, as this changes as the reaction proceeds. For example, during the alkaline hydrolysis of bromoethane,
C2H5Br + OH- → C2H5OH + Br-,
the conductivity decreases because the fast-moving hydroxyl (OH¯) ions are replaced by the slower-moving bromide ions.
Other properties which can be followed in order to determine reaction rates include changes of pressure, for gaseous reactions; changes in optical rotation, where optically active materials are involved; and absorption of electromagnetic radiation, such as light, using a spectrophotometer.
CHEMICAL RATES AND MECHANISMS
Some reactions, such as explosions, occur rapidly. Other reactions, such as rusting, take place slowly. Chemical kinetics, the study of reaction rates, shows that three conditions must be met at the molecular level if a reaction is to occur. The molecules must collide; they must be positioned so that the reacting groups are together in a transition state between reactants and products; and the collision must have enough energy to form the transition state and convert it into products. Not all collisions have this energy, but more do so at higher temperatures.
Fast reactions occur when these three criteria are easy to meet. If even one is difficult, however, the reaction is typically slow, even though the change in free energy permits a spontaneous reaction.
Rates of reaction increase in the presence of catalysts, substances that provide a new, faster reaction mechanism but are themselves unchanged or regenerated so that they can continue the process. Mixtures of hydrogen and oxygen gases at room temperature do not explode. But the introduction of powdered platinum leads to an explosion as the platinum surface becomes covered with adsorbed oxygen. The bonds of the adsorbed oxygen atoms are stretched, weakening them. This lowers the activation energy for the reaction (the energy required to permit the reaction to occur). Individual oxygen atoms then react rapidly with hydrogen molecules as they collide with them, forming water and regenerating the catalyst. The steps by which a reaction occurs are called the reaction mechanism.
Rates of reaction can be changed not only by catalysts but also by changes in temperature and by changes in concentrations. Raising the temperature increases the rate by increasing the kinetic energy of the molecules of the reactants, and therefore the probability that any given molecule will have more than the activation energy. Increasing the concentration or temperature can also increase the reaction rate by increasing the rate of molecular collisions.
Rates of reaction of solid materials can also be increased by finely dividing the solid. This increases the surface area so that more molecules can collide.