Rates of Reaction


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The rate of reaction is a measure of how fast something happens. We can
find the rate of reaction by measuring the change that happens in a set
period of time.

Introduction
------------

The rate of reaction is a measure of how fast something happens. We
can find the rate of reaction by measuring the change that happens in
a set period of time. Many factors can affect the rate of reaction;
the concentration of the reactants is one of them. In the following
experiments we will investigate how the concentration of the reactants
affect the rate of reaction. To do this we will use the reaction
between magnesium and hydrochloric acid.

GRAPH

I will measure the rate of reaction by timing how long the reaction
takes.

Aim- I am trying to find out how an increase/decrease in concentration
of the acid affects the rate of reaction in magnesium strips.

Prediction:

I predict that as the concentration of hydrochloric acid increases the
time taken for the magnesium to dissolve will decrease. I also think
that when the concentration of the acid doubles the rate of reaction
will also double.

Collision Theory:

The collision theory is how the rate of reaction increases when the
concentration of hydrochloric acid increases.

In the reaction firstly all of the particles in the reacting
substances must collide. They must collide with a certain amount of
energy called activation energy, this must be reached for the reaction
to take place. If the particles do not reach this required amount of
energy there will not be any successful collisions and therefore the
reaction will not take place. If the particles do reach the required
amount of energy a reaction will rake place and as the number of
collisions increase the reaction speeds up.

The more concentrated the reactants the larger the number of
successful collisions between the particles. This explains why the
largest rate of reaction is usually as soon as the reactants have been
mixed together, this means they are both at their highest
concentrations.

High concentration Low concentration

(18cm³ HCL + 2cm³ H²O) (6cm³ HCL + 14cm³ H²O)

The temperature in this investigation will not affect the rate of
reaction because throughout the investigation it will stay at room
temperature. If the temperature was to change during the experiment it
would effect the rate of reaction by speeding it up if the temperature
increased because the particles move a lot faster and travel a larger

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distance and so there are more successful collisions, and slowing it
down if the temperature decreased because the particles would be
moving slower.

High temperature Low temperature

The surface area will also affect the rate of reaction. It will be
affected because the larger the surface area the more space the
particles have to move and the greater the distance they have to
travel to collide with each other, this means that there will be less
successful collisions and therefore a slower reaction. On the other
hand if there is a small surface area then there is less room for the
particles to move around and so there are many successful collision
and therefore the rate of reaction is much faster.

Small Surface Area Large Surface Area

List of apparatus:

- Hydrochloric acid
- Magnesium strips (3cm long)
- Beaker
- Water
- Pipette
- Ruler
- Scissors
- Stopwatch

Apparatus set up as shown in the diagram:

Method:

- Firstly I cut the pieces of magnesium into 3cm strips

- Then I measured out 20cm³ of hydrochloric acid into one beaker.

- I placed one strip of magnesium into the beaker of acid and timed
how long it took to dissolve.

Repeat this 3 times for an average result and to cut out any anomalous
results. Also after every 3 reading decrease the volume of acid and
increase the volume of water and then repeat the steps above with the
new concentrations

E.g. 18cm ³ of hydrochloric acid and 2cm³ of water.

Why is it a fair test?

- To make it a fair test I measured the volumes of the reactants in a
measuring cylinder as accurately as possible and I repeated the same
volume of each reactant 3 times to get an average and hopefully very
accurate result.

- I measured the surface area of the magnesium strips with a ruler and
when I had cut them all to the correct length of 3cm I lined them all
up next to each other just to double check they were all the same
length.

- To get the time taken for the magnesium strip to dissolve I used a
stopwatch and timed as soon as the HCl was poured into the beaker
containing the magnesium. There has to be room for the slight human
error but my results are as accurate as possible.

- One of the factors that is hard to control is how the solution is
mixed when it has been diluted to get the right concentration. A way I
found to get around this problem was to shake the test tube twice.
This was quite important because it determined exactly what the
concentration on the solution was each time it was mixed. If this was
not controlled it could have lead to inaccurate results if the
concentration of the acid was not was we thought it was.

Priliminary results:

Conclusion:

I have come to the conclusion by looking at my results that if you
double the concentration of the acid the reaction rate also doubles,
this is because the ions are closer together in a concentrated
solution. The closer they are the more successful collisions occur and
the more collisions that occur the higher the chance of reaction
between the magnesium and hydrochloric acid.


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