# Rates of reaction

GCSE AQA

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Rates of reaction
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.
Successful collisions
In order for a chemical reaction to occur reacting particles need to collide with enough energy
to react and form the product. The minimum amount of energy particles must have to react
and form a product is called the activation energy. When reactants collide with each other
with less than the activation energy then no reaction will take place however if the reactants
collide with the activation energy or more then a reaction will take place. When the reactants
collide with enough energy to form the product this is called a successful collision.
Collision theory
Anything which increases the chances of collisions, increases the rate of reaction as it
increases the likelihood of successful collisions occurring. The collision theory states its
not enough for particles to collide but they have to collide with enough energy to form new
substances. The minimum amount of energy particles need to collide with to form new
products is the activation energy.
Concentration in solutions
The concentration of a solution is the amount of solute present in a solution. The
concentration is usually measured in moles per dm3 (1dm3 = 1 litre) So in two solutions of
the/p same volume and concentration there will be equal number of particles present.
In concentrated solutions of the same volume the rate of reaction increases as there are
more reacting particles in the solution which are closer together which makes it more likely
for successful collisions to occur as collisions are going to be more frequent and so are are
more likely to be successful and increasing the number of successful collisions increases the
rate of reaction. So reactions happen more quickly in concentrated solutions.
In low concentration solutions of the same volume there are fewer reacting particles available
in the solution. The particles are a long way apart and will have to travel a greater distance
before they can collide and react. This is going to take time so reactions involving dilute
solutions are slow. This reduces the number of collisions which occur and in turn reduces the
number of successful collisions. Fewer successful collisions decreases the rate of reaction.
(Dilute solutions have slow reaction. Concentrated are faster produce same product)
1 mole of any solution is the Mr of the dissolved compound e.g. Co2 in 1 litre of water. If
you dissolve 1mole of a substance in two liters of water then the concentration of the
solution will half as there is more water present and so it would be a 0.5 molar solution.
Pressure in gases

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If you increase the pressure of a gas there will be more particles in a given volume of the gas
and the gas molecules will be closer together which means that the particles will collide more
frequently increasing the chance of successful collisions and increasing the rate of reaction.…read more

### Page 4

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The rate of reaction can either be measured by how quickly the reactants are
used up or how quickly the products are formed.
It can be calculated using this:
Amount of product formed or amount reactants used / time
So if I has 50cm3 of gas produced in 2 minutes the rate of reaction would be
50/2 = 25cm3 of gas produced a minute
Or 1/time
1) Precipitation
Sodium thiosulphate reacts with dilute hydrochloric acid to form a yellow

### Page 5

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Need to explain graphs
The loss in mass happened faster at first as more reacting particles
available to react with each other, more reacting particles increased the
frequency of collisions and increased chance of successful collisions occurring
­ increasing the rate of reaction.…read more