Chemistry Paper 2 Topic 6 – The Rate and Extent of Chemical Change
The Rates of Reaction
The rate of a chemical reaction is how fast the reactants are changed into products.
To find the speed of a reaction, record the amount of product formed or the amount of
reactant used up over time.
The steeper the line on the graph, the faster the rate of reaction.
Over time, the line becomes less steep as the reactants are used
up.
Therefore, the quickest reactions have the steepest lines and
become flat in the least time.
The rate of a chemical reaction depends on:
The collision frequency of reacting particles (how often they
collide). The more collisions there are, the faster the reaction is.
COLLSION
The energy transferred during each collision. Particles have to collide with enough THEORY
energy for the collision to be successful.
Particles require activation energy (the minimum amount of energy that particles need to
react) to break the bonds in the reactants and start the reaction.
Factors Affecting Rates of Reaction
Factors affecting the rate of a reaction include: temperature, the concentration of a solution
or the pressure of a gas, the presence of a catalyst and surface area.
When temperature is increased, the particles will move faster.
As they move faster, they collide more frequently.
Also, the faster they move the more energy they have, so more of the
collisions will have enough energy to make the reaction happen.
If a solution is made more concentrated, it means there are more particles in the
same volume of water (or solvent).
Similarly, when the pressure of a gas is increased, it means that the
same number of particles occupies a smaller space.
If one of the reactants is a solid, then breaking it up into smaller pieces will increase its
surface area to volume ratio.
This means that for the same volume of the solid, the particles around it
will have more area to work on – so there will be more frequent
collisions.
A catalyst is a substance that speeds up a reaction without
being used up in the reaction itself.
They all work by decreasing the activation energy for the
reaction to occur. They do this by providing an alternate
reaction pathway with lower activation energy.
Measuring Rates of Reaction (Practical)
, Chemistry Paper 2 Topic 6 – The Rate and Extent of Chemical Change
The rate of a reaction can be observed either by how quickly the reactants are used up or
how quickly the products are formed.
When the product or reactant is in
gaseous form, measure the amount in cm3; when it’s a solid, use gram (g).
Precipitation and Colour Change:
The visual change in a reaction can be recorded if the initial solution is transparent
and the product is a precipitate clouding the solution (it becomes opaque).
You can observe a mark through the solution and measure how long it takes for it to
disappear – the faster the mark disappears, the quicker the reaction.
If the reactants are coloured and the products are colourless (or vice versa), the time
taken for the solution to lose (or gain) its colour can be measured.
Limitations for this method include it being very subjunctive and being unable to
plot a graph for the rate of the reaction.
Change in Mass:
Measuring the speed of a reaction that produces a gas can be carried out using a
mass balance.
As the gas is released, the mass disappearing is measured on the balance; the
quicker the reading on the balance drops, the faster the reaction.
Although the gas is released into the surroundings, this method of measuring the
rate of the reaction is the most accurate whilst it is possible to plot a graph.
Volume of Gas Given Off
This involves the use of a gas syringe to measure the volume of gas given off.
The more gas given off during a given time interval, the faster the reaction.
As gas syringes are quite accurate, the method gives an accurate value whilst a
graph can also be plotted for the data.
However, if the reaction is too vigorous, you can easily blow the plunger out of the
end of the syringe.
Two Rates Experiments (Practical)
Magnesium and HCl React to Produce H2 Gas:
1) Start by adding a set volume of dilute hydrochloric acid to a conical flask and
carefully place on a mass balance.
2) Now add some magnesium ribbon to the acid and quickly plug
the flask with cotton wool.
3) Start the stopwatch and record the mass on the balance. Take
readings of the mass at regular intervals.
4) Plot the results in a table and work out the mass lost for each
reading.
5) Repeat with more concentrated acid solutions; variables such
as the amount of magnesium ribbon and the volume of acid used should be kept the
same each time.
6) The three graphs show that a higher concentration of acid gives a faster rate of
reaction.
Sodium Thiosulfate and HCl Produce a Cloudy Precipitate:
The Rates of Reaction
The rate of a chemical reaction is how fast the reactants are changed into products.
To find the speed of a reaction, record the amount of product formed or the amount of
reactant used up over time.
The steeper the line on the graph, the faster the rate of reaction.
Over time, the line becomes less steep as the reactants are used
up.
Therefore, the quickest reactions have the steepest lines and
become flat in the least time.
The rate of a chemical reaction depends on:
The collision frequency of reacting particles (how often they
collide). The more collisions there are, the faster the reaction is.
COLLSION
The energy transferred during each collision. Particles have to collide with enough THEORY
energy for the collision to be successful.
Particles require activation energy (the minimum amount of energy that particles need to
react) to break the bonds in the reactants and start the reaction.
Factors Affecting Rates of Reaction
Factors affecting the rate of a reaction include: temperature, the concentration of a solution
or the pressure of a gas, the presence of a catalyst and surface area.
When temperature is increased, the particles will move faster.
As they move faster, they collide more frequently.
Also, the faster they move the more energy they have, so more of the
collisions will have enough energy to make the reaction happen.
If a solution is made more concentrated, it means there are more particles in the
same volume of water (or solvent).
Similarly, when the pressure of a gas is increased, it means that the
same number of particles occupies a smaller space.
If one of the reactants is a solid, then breaking it up into smaller pieces will increase its
surface area to volume ratio.
This means that for the same volume of the solid, the particles around it
will have more area to work on – so there will be more frequent
collisions.
A catalyst is a substance that speeds up a reaction without
being used up in the reaction itself.
They all work by decreasing the activation energy for the
reaction to occur. They do this by providing an alternate
reaction pathway with lower activation energy.
Measuring Rates of Reaction (Practical)
, Chemistry Paper 2 Topic 6 – The Rate and Extent of Chemical Change
The rate of a reaction can be observed either by how quickly the reactants are used up or
how quickly the products are formed.
When the product or reactant is in
gaseous form, measure the amount in cm3; when it’s a solid, use gram (g).
Precipitation and Colour Change:
The visual change in a reaction can be recorded if the initial solution is transparent
and the product is a precipitate clouding the solution (it becomes opaque).
You can observe a mark through the solution and measure how long it takes for it to
disappear – the faster the mark disappears, the quicker the reaction.
If the reactants are coloured and the products are colourless (or vice versa), the time
taken for the solution to lose (or gain) its colour can be measured.
Limitations for this method include it being very subjunctive and being unable to
plot a graph for the rate of the reaction.
Change in Mass:
Measuring the speed of a reaction that produces a gas can be carried out using a
mass balance.
As the gas is released, the mass disappearing is measured on the balance; the
quicker the reading on the balance drops, the faster the reaction.
Although the gas is released into the surroundings, this method of measuring the
rate of the reaction is the most accurate whilst it is possible to plot a graph.
Volume of Gas Given Off
This involves the use of a gas syringe to measure the volume of gas given off.
The more gas given off during a given time interval, the faster the reaction.
As gas syringes are quite accurate, the method gives an accurate value whilst a
graph can also be plotted for the data.
However, if the reaction is too vigorous, you can easily blow the plunger out of the
end of the syringe.
Two Rates Experiments (Practical)
Magnesium and HCl React to Produce H2 Gas:
1) Start by adding a set volume of dilute hydrochloric acid to a conical flask and
carefully place on a mass balance.
2) Now add some magnesium ribbon to the acid and quickly plug
the flask with cotton wool.
3) Start the stopwatch and record the mass on the balance. Take
readings of the mass at regular intervals.
4) Plot the results in a table and work out the mass lost for each
reading.
5) Repeat with more concentrated acid solutions; variables such
as the amount of magnesium ribbon and the volume of acid used should be kept the
same each time.
6) The three graphs show that a higher concentration of acid gives a faster rate of
reaction.
Sodium Thiosulfate and HCl Produce a Cloudy Precipitate:
