Chemical reactions
The rate of a reaction can be measured by the rate at which a reactant is used up, or the rate at which a product is formed.
The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts,
are all factors which affect the rate of a reaction.
Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.
Measuring rates of reaction
There are two ways to find the rate of a reaction:
product that has been produced.
Things to measure:
The measurement itself depends on the nature of the reactant or product:
The temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts,
are all factors which affect the rate of a reaction.
Chemical reactions can only happen if reactant particles collide with enough energy. The more frequently particles collide, and the greater the proportion of collisions with enough energy, the greater the rate of reaction.
Measuring rates of reaction
There are two ways to find the rate of a reaction:
- Measure the rate at which a reactant is used up,
- Measure the rate at which a product is formed.
product that has been produced.
Things to measure:
The measurement itself depends on the nature of the reactant or product:
- The mass of a substance - solid, liquid or gas - is measured with a balance
- The volume of a gas is usually measured with a gas syringe, or sometimes an upside-down measuring
cylinder or burette
It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical
axis, and the time goes on the horizontal axis.
axis, and the time goes on the horizontal axis.
For example, if 24cm3 of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12cm3 hydrogen / min.
Factors affecting the rate
You will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.
How to increase the rate of a reaction:
The rate of a reaction increases if:
Factors affecting the rate
You will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.
How to increase the rate of a reaction:
The rate of a reaction increases if:
- The temperature is increased,
- The concentration of a dissolved reactant is increased,
- The pressure of a reacting gas is increased,
- Solid reactants are broken into smaller pieces,
- A catalyst is used.
The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.
Collisions and reactions
You will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.
Collisions:
For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction.
The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.
Changing concentration or pressure:
If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:
You will be expected to explain, in terms of particles and their collisions, why changing the conditions of a reaction changes its rate.
Collisions:
For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction.
The collision must have enough energy for the particles to react. The minimum energy needed for particles to react is called the activation energy.
Changing concentration or pressure:
If the concentration of a dissolved reactant is increased, or the pressure of a reacting gas is increased:
- There are more reactant particles in the same volume
- There is a greater chance of the particles colliding
- The rate of reaction increases
Changing particle size:
If a solid reactant is broken into small pieces or ground into a powder:
- Its surface area is increased
- More particles are exposed to the other reactant
- There is a greater chance of the particles colliding
- The rate of reaction increases
Changing the temperature:
If the temperature is increased:
- The reactant particles move more quickly
- More particles have the activation energy or greater
- The particles collide more often, and more of the collisions result in a reaction
The rate of reaction increases
Using a catalyst:
Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. With a catalyst, more collisions result in a reaction, so the rate of reaction increases. Different reactions need different catalysts.
Catalysts are important in industry because they reduce costs.
Dangers of fine powders
The fine powders in the flour mills have a large surface area, so it can catch fire easily. If there is a lot of flour dust in the air, a small spark can cause a huge explosion
In coal mines, Methane and other flammable gases collect up in the air, a small spark can ignite them to cause an explosion.
Role of light in photochemical reactions
PHOTOSYNTHESIS is also started by ultraviolet (UV) light being absorbed by the green pigment chlorophyll in the leaves of green plants.
Carbon dioxide + water = glucose + oxygen
6CO2(g) + 6H2O (l) = C6H12O6(aq) + 6O2(g)
Reversible reactions
What is a reversible reaction?
Many chemical reactions continue until one/all reactants are used up and their products do not react together When a reaction reaches this stage it is said to have reached completion
A reversible reaction is:
A chemical reaction in which the products react together to form the products Reversible reactions never come to completion
Chemical equilibrium
This is a stage reached in a reversible reaction (in a closed system) when the forward & backward reactions take place at the same rate. Their effects cancel each other out and the concentration of reactants and products stays constant.
Changing Equilibrium
When conditions of equilibrium are changed, this will alter either the forward or backward reaction rates and destroy the chemical equilibrium.
Carbon dioxide + water = glucose + oxygen
6CO2(g) + 6H2O (l) = C6H12O6(aq) + 6O2(g)
Reversible reactions
What is a reversible reaction?
Many chemical reactions continue until one/all reactants are used up and their products do not react together When a reaction reaches this stage it is said to have reached completion
A reversible reaction is:
A chemical reaction in which the products react together to form the products Reversible reactions never come to completion
Chemical equilibrium
This is a stage reached in a reversible reaction (in a closed system) when the forward & backward reactions take place at the same rate. Their effects cancel each other out and the concentration of reactants and products stays constant.
Changing Equilibrium
When conditions of equilibrium are changed, this will alter either the forward or backward reaction rates and destroy the chemical equilibrium.