Rates of reaction
Calculating reaction rate
Chemical reactions occur at different speeds.
The rate of a reaction can be found by measuring
the amount of reactant used, or the amount of product
formed, over time:
Some reactions are very fast, for example, explosions or
combustion. Other reactions are very slow, for example, rusting.
rate of reaction = amount of reactant used
time
The speed of a reaction is called the rate of the reaction.
rate of reaction = amount of product formed
time
The rate of a reaction can be measured using the following units:
• g/s or g/min
• cm3/s or cm3/min
SCIENCE: Rates
of reaction
SCIENCE: Rates
of reaction
Measuring reaction rate
Activation energy
Measuring the rate of a reaction means measuring the change in
the amount of a reactant or product over time.
Chemical reactions take place when particles collide (bump into
each other). In order for a reaction to happen, particles must collide
with enough energy.
This can be done using a variety of techniques, including:
• measuring the volume of a gas product;
• measuring the change in mass of a reaction mixture;
• observing the formation or loss of a colour;
• observing the formation or loss of an insoluble solid (precipitate).
SCIENCE: Rates
of reaction
The minimum amount of energy needed for particles to react is
called the activation energy.
If particles collide with less energy than the activation energy, there
will be no reaction.
SCIENCE: Rates
of reaction
Changing reaction rate
Temperature
The rate of a reaction depends on two factors:
If temperature is increased, the rate of a reaction will increase.
• how often particles collide;
• how much energy particles have when they collide.
At a higher temperature, particles have more energy. This means
that they move faster and collide with each other more often. The
particles also collide with more energy, making it more likely that
they will react.
Temperature, concentration, pressure, surface area and catalysts
can affect the rate of a reaction.
SCIENCE: Rates
of reaction
SCIENCE: Rates
of reaction
Concentration
Pressure
If the concentration of a dissolved reactant is increased, the rate of
the reaction will increase.
If the concentration of a gaseous reactant is increased, the rate of
the reaction will increase.
At a higher concentration, there are more particles in the same
amount of space. This means that the particles collide more often
and are therefore more likely to react.
At a higher pressure, the space that the particles occupy becomes
smaller. There is less space between particles, so they collide more
often. This makes it more likely that they will react.
SCIENCE: Rates
of reaction
SCIENCE: Rates
of reaction
Surface area
Catalysts
The surface area of a solid can be increased by splitting it up into
smaller pieces.
A catalyst is a substance that is added to a reaction to increase
the reaction rate. It works by lowering the activation energy of the
reaction.
If the surface area of a solid reactant is increased, the rate of the
reaction will increase.
If there is a higher surface area, there is an increased area for
reactant particles to collide with each other. This means that the
particles collide more often and are therefore more likely to react.
SCIENCE: Rates
of reaction
Only a small amount of catalyst is needed to catalyse a reaction
because the catalyst is not used up in the reaction. It is unchanged
at the end of the reaction, so it can be re-used.
SCIENCE: Rates
of reaction
Catalysts in industry
Catalysts are often used to increase the rate of industrial processes.
This has the following important effects:
• Products can be made more quickly, saving time and money.
• Lower temperatures can be used because less energy is needed for
the reaction to happen. This saves fuel and reduces pollution.
SCIENCE: Rates
of reaction
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SCIENCE: Rates
of reaction