Rates of Chemical Reactions

 

Many activities and processes depend on chemical reactions. The rate or speed at which these reactions take place is important. You need to know how long to leave the cake in the oven, how long the glue is going to take to set and how long your bottle of milk will last before it goes off. This all depends on the rate of specific chemical reactions.

 

An important part of physical chemistry is kinetics which deals with reaction rates. The subject can be looked at from two directions. First we need to think about how we can measure the rates of reactions so that we can examine how those rates are affected by different conditions. Then we need to explain why those conditions affect the rates.

 

It is obvious that some reactions go very fast. Explosions are an example of fast reactions. Cookery demonstrates examples of reactions that go slower. Other important reactions are even slower, for example when concrete sets. In order to measure the rate of these reactions we must be able to measure the quantity of reactants or products of the reaction over time. So reactions that produce gases can be measured by collecting the gas produced and plotting it against time elapsed. Other quantities that can be measured are mass, temperature, or the wavelength of light which passes through in a reaction where there is a colour change.

 

Different reaction rates are affected by different conditions. Temperature is an important factor in many chemical reactions. An increase in temperature usually causes a reaction to go faster, an observation used by many famous chefs. The concentration of the reacting substances also affects the rate, as does the size of the particles involved. Explosions at custard powder factories are caused by the smallness of the powder particles, causing them to be very reactive. Light affects the rate of some reactions, for example in photosynthesis in plants and the fading of paint.

 

Catalysts are chemicals which speed up a chemical reaction without being chemically changed themselves. These include some precious metals, used in catalytic converters, and enzymes in biochemical reactions. Inhibitors are also chemicals which affect the rate of reactions, but they are negative catalysts, slowing down reactions. Anti-oxidants are used to inhibit the decay of foodstuffs so that they don't go stale quickly.

 

One of the most important factors used to explain changes in the rate of reactions is the number of collisions. When molecules of reactants collide they are likely to react. So if more collisions happen, more reactions will occur and the overall reaction will go faster. Increasing temperature makes the molecules speed up, so there will be more collisions. Increasing concentration also increases the likelihood of collisions because there are more molecules available to collide. Catalysts hold molecules in position, forcing them to collide.

The study of kinetics is a vital aspect of chemistry. If important reactions can be speeded up and inconvenient reactions slowed down, many advances can be made in many branches of science.