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Factors the affect the rate of reaction
Increasing the temperature increases the speed of the molecules, which in turn
increases both their energy and the number of collisions.
Increasing the concentration of a solution means there are more particles in a given
volume, so collisions are more likely. However, the reaction rate falls as a reaction
proceeds because the reactants are used up.
Increasing the pressure of a gas has the same effect as increasing the
concentration, there are more particles in a given volume.
Increasing the surface area of solid reactants means that more of its particles are
available to collide with molecules in a gas or liquid.
Using a catalyst speeds up a reaction without changing the reaction itself by
offering a place for particles to meet up, and an alternative route with different
steps that have a lower activation energy.
What are catalysts?
A catalyst is a substance which speeds up a reaction, but is chemically unchanged
at the end of the reaction. When the reaction has finished, you would have exactly
the same mass of catalyst as you had at the beginning.
How do they work?
The key importance of activation energy
Collisions only result in a reaction if the particles collide with enough energy to get
the reaction started. This minimum energy required is called the activation energy for
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You can mark the position of activation energy on a Maxwell-Boltzmann distribution
to get a diagram like this
Only those particles represented by the area to the right of the activation energy will
react when they collide. The great majority don't have enough energy, and will simply
Catalysts and activation energy
To increase the rate of a reaction you need to increase the number of successful
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Adding a catalyst has exactly this effect on activation energy. A catalyst provides
an alternative route for the reaction. That alternative route has a lower activation
energy. Showing this on an energy profile:
Be very careful if you are asked about this in an exam. The correct form of words is
"A catalyst provides an alternative route for the reaction with a lower
It does not "lower the activation energy of the reaction".…read more