RATE OF REACTION
During a chemical reaction, the reaction is caused by particles colliding together. If zinc is added
to hydrochloric acid, the zinc particles will collide with the hydrochloric acid particles, and
therefore react.
For many reactions involving liquids or gases, increasing the concentration of the reactants
increases the rate of reaction. This is because there are more particles in a given area to collide
against each other. For example:
The reaction between zinc and hydrochloric acid. This usually involves adding hydrochloric acid
to zinc granules in a test-tube (let’s say 10cm3 of acid). If the acid is diluted, that means there
will be fewer hydrochloric acid particles in the solution to collide with the zinc particles. The
more concentrated it is, the more particles there will be, thereby increasing the chance of a
collision occurring, and increasing the rate (= speed) of reaction.
Temperature is equal to energy. Therefore, if particles are heated up, they have more energy. If
they have more energy, they vibrate more vigorously, and are more likely to collide with other
particles. Therefore, temperature increases the rate of reaction. In general, the rate of reaction
doubles with every 10O C rise in temperature (i.e. it goes twice as fast).
A catalyst is a substance which increases the rate of a chemical reaction but is chemically
unchanged at the end of the reaction.
As mentioned above, during a chemical reaction, the reaction is caused by a collision of
particles. However, the particles will only react if they collide with enough energy (the amount
of energy needed to cause a reaction is called the activation energy).
In a solution of hydrochloric acid, for example, certain particles will have more energy than
others. Therefore, only certain particles will have enough energy to react. The rest of the
particles will only react because of the energy created by these first particles. Catalysts work by
reducing the level of energy required to create a reaction:
NO CATALYST
WITH A CATALYST
Therefore, if more particles can react straight away, the reaction will occur faster.
Particle size (or surface area) also affects the rate of reaction.
As mentioned above, during a chemical reaction, the reaction is caused by a collision of
particles. The more collisions that can occur per second, the faster the reaction. If the zinc is in
the form of a large lump, the hydrochloric acid particles will only be able to react with the zinc
particles on the outside of the lump; they won’t be able to react with the inside particles until
the outside ones have already reacted. This means the reaction is slow.
If the zinc is in a fine powder, the hydrochloric acid particles will be able to collide with a lot
more zinc particles, speeding up the reaction.
HOW TO MEASURE RATE OF REACTION
One way of measuring rate of reaction is through gas evolution. Gas evolution is the creation of
gas after a reaction; for example, the reaction between magnesium and hydrochloric acid
produces hydrogen gas. A way of working out how fast a reaction occurs is to time how long it
takes to collect a set amount of gas (e.g. 25cm3 of gas). The collection of gas can be done using
a gas syringe:
As the gas is
collected, the plunger
moves to the right,
pushed by the gas.
There are marks
along the syringe to
see how much gas
has been collected
Let’s say you try two different experiments, each using a different concentration of
hydrochloric acid; in the first experiment, it takes 20 seconds to collect 25cm 3 of gas, and in the
second, it takes only 10 seconds. We can see that the second experiment has the greatest rate
of reaction (i.e. it took less time).
ENERGETICS OF A REACTION
During all chemical reactions, an energy change occurs; heat is either released or absorbed.
When a reaction releases heat to the surroundings, it is called an exothermic reaction. When a
reaction absorbs (or uses up) energy from the surroundings, it is called an endothermic
reaction.
In order to start or set off most reactions, a certain amount of energy is required. This is called
the activation energy. The activation energy is used to break the bonds between the molecules
of the reactants (the substances that are reacting). The bonds then rearrange themselves and
bond again, a process which releases energy. If the energy produced is MORE than the
activation energy, the reaction is exothermic. If the energy produced is LESS than the activation
energy, the reaction is endothermic.