| What factors influence the rate of
a chemical reaction?
If you are planning an investigation, I suggest
that you investigate the effects of temperature or the effects of the
concentration of the reactants because these will allow you to choose a
suitable range of values for the controlled or independent variable. The
dependent variable will be the rate of the reaction. Keep all the other
variables fixed.
To make a prediction for your investigation you
will have to ask yourself the question: What will happen to the rate of
the reaction when I increase the temperature? or What will happen
to the rate of the reaction if I increase the concentration of one of the
reactants? The answer to that question is your prediction. The next
thing to do is to explain your prediction. You will have to answer the
question: Why will the reaction go faster if I increase the
temperature? or Why will the reaction go faster if I increase the
concentration of one of the reactions? The answer to this question is
your explanation, and to get the highest possible marks, you will have to
provide a full scientific explanation.
Once you have written your hypothesis (prediction
with explanation) you will decide how to do the experiments, i.e. write
the proposed method.
How does temperature
affect the rate of a chemical reaction?
When two chemicals react, their molecules have to
collide with each other with sufficient energy for the reaction to take
place. This is collision theory. The two molecules will only react if they
have enough energy. By heating the mixture, you will raise the energy
levels of the molecules involved in the reaction. Increasing temperature
means the molecules move faster. This is kinetic theory. If your reaction
is between atoms rather than molecules you just substitute
"atom" for "molecule" in your explanation.
How do catalysts
affect the rate of a reaction?
Catalysts speed up chemical reactions. Only very
minute quantities of the catalyst are required to produce a dramatic
change in the rate of the reaction. This is really because the reaction
proceeds by a different pathway when the catalyst is present. Adding extra
catalyst will make absolutely no difference. There is a whole page on this
site devoted to catalysts.
How does
concentration affect the rate of a reaction?
Increasing the concentration of the reactants
will increase the frequency of collisions between the two reactants. So
this is collision theory again. You also need to discuss kinetic theory in
an experiment where you vary the concentration. Although you keep the
temperature constant, kinetic theory is relevant. This is because the
molecules in the reaction mixture have a range of energy levels. When
collisions occur, they do not always result in a reaction. If the two
colliding molecules have sufficient energy they will react.
If reaction is between a substance in solution
and a solid, you just vary the concentration of the solution. The
experiment is straightforward. If the reaction is between two solutions,
you have a slight problem. Do you vary the concentration of one of the
reactants or vary the concentration of both? You might find that the rate
of reaction is limited by the concentration of the weaker solution, and
increasing the concentration of the other makes no difference. What you
need to do is fix the concentration of one of the reactants to excess. Now
you can increase the concentration of the other solution to produce an
increase in the rate of the reaction.
How does surface area
affect a chemical reaction?
If one of the reactants is a solid, the surface
area of the solid will affect how fast the reaction goes. This is because
the two types of molecule can only bump into each other at the liquid
solid interface, i.e. on the surface of the solid. So the larger the
surface area of the solid, the faster the reaction will be.
Smaller particles have a bigger surface area than
larger particle for the same mass of solid. There is a simple way to
visualize this. Take a loaf of bread and cut it into slices. Each time you
cut a new slice, you get an extra surface onto which you can spread butter
and jam. The thinner you cut the slices, the more slices you get and so
the more butter and jam you can put on them. This is "Bread and
Butter Theory". You should have come across the idea in your biology
lessons. By chewing your food you increase the surface area so that
digestion can go faster.
What affect does
pressure have on the reaction between two gasses?
You should already know that the atoms or
molecules in a gas are very spread out. For the two chemicals to react,
there must be collisions between their molecules. By increasing the
pressure, you squeeze the molecules together so you will increase the
frequency of collisions between them. This is collision theory
again.
In a diesel engine, compressing the gaseous
mixture of air and diesel also increases the temperature enough to produce
combustion. Increasing pressure also results in raising the temperature.
It is not enough in a petrol engine to produce combustion, so petrol
engines need a spark plug. When the petrol air mixture has been
compressed, a spark from the plug ignites the mixture. In both cases the
reaction (combustion) is very fast. This is because once the reaction has
started, heat is produced and this will make it go even faster. |
