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  • 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.

    • Concentration – The more particles there are, the bigger the chance of collisions.
    • Temperature – Particles move around more at higher temperatures, so more collisions are likely, and the collisions will have more energy.
    • Pressure – Particles in gases are very spread out. If you increase the pressure, the particles are forced together, so the chances of collision are increased.
    • Surface area – If one of the reacting chemicals is a solid, only particles at the surface can collide. The bigger the surface, the faster the reaction. Smaller particles have a larger surface area for their size than larger ones. This explains why powder normally reacts faster than lumps.
    • Catalysts – A catalyst is a substance that changes the rate of a chemical reaction, but is chemically unchanged at the end of the reaction. An inhibitor does the opposite – it slows down chemical reactions.


    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.


    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.

    Activity idea

    In this video, Teaching students how to learn, Dave Corner explains how he uses Hub resources to teach about catalysts.

    Useful link

    Watch a demonstration of how platinum acts as a catalyst with an explanation of the use of platinum in catalytic converters in this video from the BBC.

      Published 28 May 2008, Updated 10 April 2014 Referencing Hub articles
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