AQA GCSE CHEMISTRY PAPER 2
Mean rate of reaction (with reactant)
Rate of reaction:
Quantity of reactant used
----------------------------
Time taken
Mean rate of reaction (with product)
Rate of reaction:
Quantity of product formed
----------------------------
Time taken
Measurements for quantity of product or reactant
Either as mass in g or as volume in cm³
Measurements for the rate of reaction
Either as g/s, cm³/s, or mol/s
,Factors which affect rate of chemical reaction
- Temperature: a faster temperature means faster reaction
- Concentration / pressure: a higher concentration or pressure means a faster
reaction
- Surface area: a larger surface area: volume ratio means a faster reaction
- Catalysts: a reaction in the presence of a catalyst is faster
Collision theory
This states that a chemical reaction can only happen when reacting particles collide
with each other with enough energy.
Activation energy
This is the minimum energy required for a reaction to take place.
How surface area, concentration or pressure changes rate of reaction in terms of
collision theory
This increases the frequency of collisions, as there are more particles that are able to
collide, increasing rate of reaction.
,How temperature changes rate of reaction in terms of collision theory
This increases the frequency of collisions as well as the energy the collisions have,
increasing rate of reaction.
Catalysts
- A catalyst is a substance that changes the rate of reaction
- It is not used up during the reaction, and therefore does not show up in the products
or reactants
- Different reactions require different catalysts, for example an enzyme
- The catalyst does this by supplying a different pathway to the reaction that requires
a lower activation energy, shown in the picture
Reversible reactions
This is a reaction in which the products can react to produce the original reactants
A+B⇌C+D
Example of a reversible reaction
This reaction is reversible, meaning the reactants can be heated to get the products
as well as the products can be cooled to get the reactants:
Ammonium chloride ⇌ ammonia + hydrogen chloride
, Energy changes in reversible reactions
If a reaction is endothermic in one direction, it is exothermic in the other. The same
amount of energy is transferred each way.
Equilibrium in a reversible reaction
If the apparatus are set up to prevent the escape of reactants or products, an
equilibrium can be reached, in which the rate of forward and reverse reactions are
the same. This will always occur in consistent conditions.
The effect of changing concentration in a reversible reaction
When the concentration of one product or reactant is changed, the system is no
longer at equilibrium:
- If the concentration of the reactants is increased, more products are formed until a
balance is reached
- If the concentration of the products is increased, more reactants will form until a
balance is reached
The effect of increasing temperature in a reversible reaction
When the temperature is increased:
- More product is formed for an endothermic reaction
- Less product is formed for an exothermic reaction
Mean rate of reaction (with reactant)
Rate of reaction:
Quantity of reactant used
----------------------------
Time taken
Mean rate of reaction (with product)
Rate of reaction:
Quantity of product formed
----------------------------
Time taken
Measurements for quantity of product or reactant
Either as mass in g or as volume in cm³
Measurements for the rate of reaction
Either as g/s, cm³/s, or mol/s
,Factors which affect rate of chemical reaction
- Temperature: a faster temperature means faster reaction
- Concentration / pressure: a higher concentration or pressure means a faster
reaction
- Surface area: a larger surface area: volume ratio means a faster reaction
- Catalysts: a reaction in the presence of a catalyst is faster
Collision theory
This states that a chemical reaction can only happen when reacting particles collide
with each other with enough energy.
Activation energy
This is the minimum energy required for a reaction to take place.
How surface area, concentration or pressure changes rate of reaction in terms of
collision theory
This increases the frequency of collisions, as there are more particles that are able to
collide, increasing rate of reaction.
,How temperature changes rate of reaction in terms of collision theory
This increases the frequency of collisions as well as the energy the collisions have,
increasing rate of reaction.
Catalysts
- A catalyst is a substance that changes the rate of reaction
- It is not used up during the reaction, and therefore does not show up in the products
or reactants
- Different reactions require different catalysts, for example an enzyme
- The catalyst does this by supplying a different pathway to the reaction that requires
a lower activation energy, shown in the picture
Reversible reactions
This is a reaction in which the products can react to produce the original reactants
A+B⇌C+D
Example of a reversible reaction
This reaction is reversible, meaning the reactants can be heated to get the products
as well as the products can be cooled to get the reactants:
Ammonium chloride ⇌ ammonia + hydrogen chloride
, Energy changes in reversible reactions
If a reaction is endothermic in one direction, it is exothermic in the other. The same
amount of energy is transferred each way.
Equilibrium in a reversible reaction
If the apparatus are set up to prevent the escape of reactants or products, an
equilibrium can be reached, in which the rate of forward and reverse reactions are
the same. This will always occur in consistent conditions.
The effect of changing concentration in a reversible reaction
When the concentration of one product or reactant is changed, the system is no
longer at equilibrium:
- If the concentration of the reactants is increased, more products are formed until a
balance is reached
- If the concentration of the products is increased, more reactants will form until a
balance is reached
The effect of increasing temperature in a reversible reaction
When the temperature is increased:
- More product is formed for an endothermic reaction
- Less product is formed for an exothermic reaction
