C2 Rates of Reactions

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  • Created on: 03-06-12 09:48
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Rates of Reactions
The rate of reaction is the speed at which a chemical reaction takes place ­ this is measured by the
rate at which either: products are formed or reactants are used up.
Collision Theory
For a reaction to take place, the reactants have to collide with sufficient energy to break their
pre-existing bonds and the right orientation for new bonds to form. The minimum amount of
energy particles must have to react and form a product is called the activation energy.
Anything which increases the chances of collisions increases the rate of reaction as it increases the
likelihood of successful collisions occurring (when the reactants collide with enough energy to
form the product).
So to increase the rate of a reaction, there should be an increase in:
1) Number of collisions
2) Amount of energy which also increases number of collisions.
Increasing the Rate of Reaction
Increasing these four main variables increases the rate of reaction:
Concentration/pressure of the reactants
The concentration of a solution is the amount of solute present in a solution. Similarly, pressure is the
amount of the reacting gas particles in air. The concentration is usually measured in moles per dm3
(1dm3 = 1 litre). A strong solution or high pressure area will have more collisions as the particles will
be closer together since there are more reacting particles in the same volume.

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Temperature °C measures heat, which is a form of energy. So when heated at high temperatures,
particles have more energy, and therefore more collisions have the necessary activation energy or
more to break bonds. Energy also makes them move faster which leads to more collisions. A 10°C
increase approximately doubles the rate of reaction.
Surface area
Smaller particles have a higher surface area to volume ratio than larger ones. Since collisions only
happen on the surface, increasing surface area leads to more collisions.…read more

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Different reactions require different catalysts.
Measuring Rates of Reactions
There are 3 ways to measure the speed of reaction:
1) Precipitation
It is the formation of a solid in a solution or inside another solid during
a chemical reaction. It can be used to measure the rate of reaction by
observing the amount of time it takes for a marker previously visible
through the solution to become invisible.…read more


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