Factors Affecting the Rate of a Chemical Reaction. The following events must occur before a reaction can proceed:. The reactant particles must collide with each other.
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The following events must occur before a reaction can proceed:
This presentation is useful as a refresher for those of you starting Unit 2 of the A-level chemistry course.
The minimum energy required to bring about a chemical reaction.
If there were no such thing as ‘activation energy’ life would be very difficult:
Gasoline for your car would ignite as soon as it came into contact with air.
You would burst into flames.
Trees would spontaneously combust.
Activation energy is why these things do not happen, there is an energy barrier so most reactions need to be ‘started off’ by putting in some energy.
Activation energy for a reaction is shown on reaction profile diagrams (do you remember these?).
To change the rate of a reaction one or more of the following things must happen:
This is all very well but how can we follow the progress of a chemical reaction?
To find the rate of a chemical reaction we must be able to follow its progress with time.
As an example consider the reaction between calcium carbonate and hydrochloric acid.
You should already know the equation but here it is:
CaCO3 + 2HCl CaCl2 + H2O + CO2
We can follow this reaction by measuring the volume of carbon dioxide produced as the reaction proceeds.
Gas being collected
This apparatus can be used to measure the gas as it is formed. It is not the only way, look in your text book for more details.
Volume of gas/cm3
Reaction fastest at the beginning.
If you collect data for the total amount of gas produced as the reaction progresses then plot this data on a graph you should get a curve similar to that shown below.
All very well, but what does the graph tell you?
The gradient or slope of the graph shows the rate of the reaction.
Steeper slope = faster reaction.
When solids take part in chemical reactions only the surface particles are exposed so they are the only ones that can collide with particles of other reactants.
‘Inner’ particles are protected and cannot collide with other particles until they become ‘exposed’.
The surface particles are ‘exposed’ and can react.
If we break up this ‘lump’ into smaller pieces the number of particles has not changed but the there are now more ‘surface’ particles.
There is now a greater surface area with more exposed particles so more collisions can occur, hence faster reaction.
Larger surface area = faster reaction.
Consider the reaction between zinc and hydrochloric acid:
Zn + 2HCl H2 + ZnCl2
(How could you follow the progress of this reaction? Click to find out)
1M hydrochloric acid
2M hydrochloric acid
There are more particles of acid per unit volume in the 2M acid than there are in the 1M acid. So, there will more collisions between the acid and zinc particles in the stronger acid, giving a faster reaction.
Higher concentration = faster reaction
The rate of reaction between gases is increased by increased pressure.
In effect pressure is the gas equivalent of concentration.
Low pressure, particles far apart.
These two gas jars contain the same number of gas particles.
The higher pressure jar has more particles per unit volume which means a higher concentration, hence faster reaction.
Higher pressure, particles closer together.
Higher pressure = faster reaction
According to kinetic theory (do you remember this?) as the temperature increases the particles in a substance move about more quickly.
Reaction at 500C
Reaction at 300C
As the temperature increases the number of collisions increases as well as the energy of the collisions. So temperature has a big effect on the rate of reaction. For every 100C increase the rate approximately doubles.
Higher temperature = faster reaction
A catalyst is a substance that increases the speed of a reaction, without being used up. A catalyst can be ‘recovered at the end of a reaction and used again.
A catalyst reduces the activation energy of a reaction.
This is easier to understand with a diagram – see next slide.
Activation energy without catalyst.
The lower activation energy in the presence of a catalyst means the reaction will be faster. More of the collisions have enough energy to react. There is a lower ‘energy barrier’.
Activation energy with catalyst.
Catalyst = faster reaction.
Volume of gas/cm3
Reaction fastest at the beginning.
Some exothermic reactions speed up shortly after they start, this might be unexpected, but think about it!
The temperature increases and this overcomes, at least to begin with, the effect of reducing the concentration. So, in some cases the reaction will speed up then slow down and eventually stop.
Do not get caught out by this. Questions related to this effect are very common!
Rob Dickens, Doha College Science Department.