A chemical reaction involves a chemical change, which happens when two or more particles (which can be molecules, atoms or ions) interact. For example, when iron and oxygen react, they change to a new substance, iron oxide (rust). Iron oxide has different chemical properties to iron and oxygen. This is different to a physical change. For example, water can turn to ice, but ice is still water in another physical state – ice and water have the same chemical properties.
When chemicals react, particles need to collide with each with enough energy for a reaction to take place. The more often they collide, the more likely they are to react. Not all collisions result in reactions – often there is not enough energy for this to happen.
Some reactions happen faster than others. The rate depends on the likelihood of collision between particles. A number of things affect the rate of a reaction.
Catalysts play an important part in many chemical processes. They increase the rate of reaction, are not consumed by the reaction and are only needed in very small amounts. There are two main ways that catalysts work.
Adsorption. Particles stick onto the surface of the catalyst (called adsorption) and then move around, so they are more likely to collide and react. A good example is the way the platinum catalyst in a car’s catalytic converter works to change toxic carbon monoxide into less-toxic carbon dioxide.
Intermediate compounds.In this process, a catalyst first combines with a chemical to make a new compound. This new compound is unstable, so it breaks down, releasing another new compound and leaving the catalyst in its original form. Many enzymes (special biological catalysts) work in this way. Many industrial chemical processes rely on such catalysts.
One example of a catalyst that involves an intermediate compound can be found high in the Earth’s atmosphere. Up there, the chemical ozone (with molecules containing three oxygen atoms) helps protect the Earth from harmful UV radiation. But also up there is chlorine, which gets into the atmosphere from chemicals (chlorofluorocarbons, CFCs) used in some refrigerators, air conditioners and aerosol cans.
Chlorine is a catalyst, which steals an oxygen atom from ozone (O3) leaving stable oxygen (O2). At the same time, it forms an unstable intermediate chlorine-oxygen compound, which breaks down to release its oxygen. This leaves the chlorine free to repeat the process. One chlorine atom can destroy about a million ozone molecules every second. This can have a drastic effect on the atmosphere’s ability to protect us from UV radiation.
Enzymes are biological catalysts. They are proteins that fold into particular conformations such that they can help speed up very particular chemical reactions. For biochemical reactions, the reactant is typically called the substrate. The substrate is converted into the product. The mechanisms for many enzymes are very similar. The substrate(s) and the enzyme bind into a complex. The physical location on the enzyme in which the substrate binds is called the "active site". Once bound, this complex can then weaken particular bonds in the substrate such that chemistry occurs to form the product. The product is weakly bound to the substrate such that it now dissociates and the enzyme is free to bind another substrate molecule.
The active sites in enzymes can be very specific such that the enzyme will only catalyze a very specific reaction for a very specific molecule. Typically there is equilibrium between the bound complex and the free substrate and enzyme such that the binding could be reversible. In contrast, once the product is formed the backward reaction typically will never happen.
Substrate + Enzyme ↔ Complex → Product.
Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions. Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semi-metallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.
In general, catalytic action is a chemical reaction between the catalyst and a reactant, forming chemical intermediates that are able to react more readily with each other or with another reactant, to form the desired end product. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid–base reactions, oxidation–reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants; bifunctional catalysts are used extensively for reforming reactions in the petroleum industry.
