OCR AS Chemistry F332: Factors Affecting Rates of Reaction

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Factors Affecting Reaction Rates
Different chemical reactions go at different rates.
Studying reaction kinetics helps chemists to find ways of speeding up or slowing down
chemical processes in industry. It also helps them to make predictions about important
reactions such as those that occur between gases in the atmosphere, and to understand the
mechanisms of chemical reactions.
The rate of a chemical reaction may be affected by:
The concentration of the reactants
For example, the rate of reaction of chlorine atoms with ozone in the stratosphere
increases as the concentration of chlorine atoms increase.
In the case of solutions, concentration is measured in mol dm ³
In the case of gases, the concentration is proportional to the pressure
The temperature
Nearly all reactions go faster at higher temperatures
The intensity of radiation
If the reaction involves radiation, for example, ultraviolet radiation of a certain
frequency causes O2 molecules to split into O atoms
The reaction then goes faster when the intensity of the ultraviolet radiation increases
The particle size of a solid
A solid such as magnesium reacts much faster when it is finely powdered than when it is
in a large lump because there is a much larger surface area of a solid exposed for
reaction to take place on
The presence of a catalyst

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The Collision Theory of Reactants
We can explain the effect of these factors using a simple collision theory.
The basic idea is that reactions occur when the particles of reactants collide, provided
they collide with a certain minimum kinetic energy.
For example:
Two particles, an ozone molecule and a chlorine atom, moving around in the stratosphere.
For a reaction to occur, the two particles must first collide so that they come into contact
with each other.…read more

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Enthalpy Profiles
Plots, as shown above, are a useful way of picturing the energy changes that take place as a
reaction proceeds.
The curved line is the energy pathway for a pair of colliding molecules ­ it is called the
energy (or enthalpy) profile for the reaction.
In going from reactants to products, the highest point on the pathway corresponds to an
arrangement of atoms where old bonds are stretched and new bonds are starting to
form.…read more


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