rate of reaction

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Speed at which a chemical reaction proceeds. It is usually expressed in terms of the concentration (usually in moles per litre) of a reactant consumed, or product formed, in unit time; so the units would be moles per litre per second (mol l^-1 s^-1). The rate of a reaction may be affected by the concentration of the reactants, the temperature of the reactants (or the amount of light in the case of a photochemical reaction), and the presence of a catalyst. If the reaction is entirely in the gas state, the rate is affected by pressure, and, where one of the reactants is a solid, it is affected by the particle size.

During a reaction at constant temperature the concentration of the reactants decreases and so the rate of reaction gradually slows down. These changes can be represented by drawing graphs.

For an endothermic reaction (one that absorbs heat) increasing the temperature may produce large increases in the rate of reaction. A 10°C rise can double the rate while a 40°C rise can produce a 50- to 100-fold increase in the rate.

Collision theory is used to explain these effects. Increasing the concentration or the pressure of a gas means there are more particles per unit volume, therefore there are more collisions and more successful collisions. Increasing the temperature makes the particles move much faster, resulting in more collisions per unit time and more successful collisions; consequently the rate increases.

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