IGCSE Double Award Extended Coordinated Science
Chemistry 7.1 - Rates of Reaction
Chemical Reaction
For a successful chemical reaction to occur,
- reactant molecules must collide with enough energy.
The extra energy needed for a successful reaction is called
- activation energy
If they collide with energies below the activation energy,
- they do not react.
Typical Signs of a Chemical Reaction:
- Change in temperature
- Gas produced
- Colour change
- Precipitate (an insoluble solid) formed
Rates of Reaction
You need to know that rate of reaction is
- the speed at which a chemical reaction proceeds.
You need to know what factors can affect the rate of reaction.
- Concentration, temperature, particle size, and catalysis.
Concentration
- The higher the concentration of the reactants, the faster the rate of reaction.
If the concentration increases, simply:
- there are more molecules per volume, available to collide with each other.
- meaning there is a higher chance of collision.
- Therefore increased rate of reaction.
Temperature
- The higher the temperature of the reactants, the faster the rate of reaction.
If the temperature affects the rate of reaction in two ways.
1. By increasing the energies of the molecules
- Higher temperature means higher kinetic energy of the molecules
- This means more molecules will have enough energy to overcome the activation energy.
- Therefore increased rate of reaction.
2. By increasing the speeds of the molecules.
- Increased speed of the molecules means the molecules move around much more
- This means there is a higher chance of collision.
- Therefore increased rate of reaction.
Particle size (only solids)
- The smaller the particle size of the reactants, the faster the rate of reaction.
Here, particle size means the size of the physical particles, not the size of the molecules.
- It is easier to think of it in terms of surface area
- The larger the surface area of the reactants, the faster the reaction.
If you think of a block of chemical, only the molecules on the outside are available to react.
Larger surface area means that there are more “outside” molecules - Therefore increased rate of reaction
Catalysts
- The higher the amount of catalysts, the faster the rate of reaction.
A catalyst is:
- A chemical that increases the rate of reaction without chemically taking part in the reaction.
- Many different catalysts exist for many different reactions. Enzymes are biological catalysts (Unit B3)
A catalyst decreases the activation energy of the reaction by providing an alternate pathway for the reaction.
- Decreased activation energy means more molecules will now have enough energy to successfully collide
- Therefore increased rate of reaction
Investigating Rates of Reaction
To measure the rate of reaction of a reaction, we need to measure how fast the reaction is happening.
There are many methods to do so, by using the typical signs of a chemical reaction.
- We can measure how fast a volume of gas is produced
- We can measure how fast the colour change occurs
- We can measure how fast the temperature change occurs.
Measuring rate of reaction by gas evolution
- There are two set ups of apparatus which allows us to measure the rate of gas evolution.
Water Displacement
- In a large bowl filled with water, put a large jar (or
measuring cylinder) full with water upside down.
- Set up a reaction mixture in a sealed container with a
single tube allowing the gas to escape.
- Put the gas tube under the water-filled container
- The gas will be produced and collect in the jar,
displacing the water downwards.
- Now, the amount of gas produced can be recorded in a
constant time interval, e.g. 50cm3/min
- This will be the rate of reaction.
Gas Syringe Method
- This method is a lot simpler than the water displacement method
- Similarly, set up a reaction mixture in a sealed container with a single tube allowing the gas to escape.
- On the end of the tube, join a gas syringe.
- As the gas is produced, the gas syringe will be pushed backwards.
- The amount of gas produced can also be recorded easily, using the volume markers on the syringe.
- Amount of gas produced can be recorded in a constant time interval, e.g. 50cm3/min
For any method, is it typical to record the results in a table, and plotting a graph.
An example results table and a graph will look something like this:
Time Volume produced
(s) (cm3)
30 50
60 50
90 42
120 28
150 11
180 0
210 0
It is obvious from these results that with the duration of the reaction,
- The rate of reaction decreases
This is because as the reaction proceeds, the reactants are being used up to produce products.
This means that there are less reactants as the reaction proceeds.
- Resulting in a decrease of concentration of reactants
- And in the end, all the reactants would have been used up and there is no reaction (0 rate)
It is also obvious from these results that the fastest rate is at the very beginning of the reaction.
The syllabus says you should be able to, (SO check if you can):
- Describe the effect of concentration, particle size, catalysis and temperature on the rates of reactions.
- Define catalyst as an agent which increases rate but which remains unchanged.
- Describe and explain the effects of temperature and concentration in terms of collisions between reacting
particles (the concept of activation energy will not be examined).
- Describe a practical method for investigating the rate of a reaction involving gas evolution.
- Devise a suitable method for investigating the effect of a given variable on the rate of a reaction.
- Interpret data obtained from experiments concerned with rate of reaction.
- Describe the application of the above factors to the danger of explosive combustion with fine powders (e.g.
flour mills) and gases (e.g. mines).
