Investigation to find out if changes in concentration of acid affect the rate of a chemical reaction

Investigation to find out if changes in concentration of acid affect the rate of a chemical reaction

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Investigation to find out if changes in concentration of acid affect the rate of a chemical reaction

PLANNING –

Title – Investigation to find out if changes in concentration of acid
affect the rate of a chemical reaction.

Introduction – The rate of reaction can be defined as the speed at
which a reaction takes place. The rate of reaction tells us how
quickly a chemical reaction has happened. There are many things that
can affect the rate of reaction, (some of these are shown in figure 1
below.)

In this coursework I am going to investigate what effect the change in
acid concentration has on a chemical reaction. I will do this through
a series of experiments, where I will change the concentration of
hydrochloric acid by mixing it with water. I will measure the reaction
by recording the amount of gas given off in 60 seconds for each
experiment. I plan to keep the total amount of liquid used in the
experiments at a constant 20ml. This way I can measure the
concentration of the acid I use each time.

Aim – To find out if the rate of reaction is effected by the
concentration of an acid.

Variables – I have decided to focus on the effect concentration of
acid has on the rate of reaction. To get accurate results, I have to
make sure all my experiments are run fairly. To achieve this I will
keep all my variables constant, except the one I’m investigating, the
concentration of the acid.

The temperature, equipment and amount of substances I use will be kept
the same for all my experiments. I will also make sure the substances
I use are all prepared in the same way before each experiment. This
will avoid change in surface area. (E.g. I wont crush a substance one
time and keep It whole the next) In all my experiments I will keep the
magnesium whole. To control the temperature of the acid, I will use a
thermometer to monitor it and make sure the temperature is the same
for each experiment I do. (this will most likely be at room temp.
around 27degrees.) All my equipment will be kept identical to any
other one of my experiments, and I will use the same size measuring
cylinder (20ml) to measure out the acid and water.

The concentration of the acid is the only variable I will change
throughout my investigation. I will do this by weakening the solution
with water, starting with a very acidic solution and gradually
diluting it. This will be done by measuring the water and acid in the

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measuring cylinder to previously decided amounts, and then combine
them in the flask. However I will keep the total amount of acid and
water at 20ml every time. This will allow me to work out the
concentration of acid I am using each time.

All this will ensure I carry out a fair test and obtain accurate
results leading to a correct answer to my aim.

Variable
Reason
Effect
Value
Temperature

The higher the temperature of the acid, the faster the rate of
reaction will be.

As I increase the temperature, particles start to move faster.
Increasing the temperature increases the rate of reaction, due to the
collision theory.

A 10 degrees rise in the temperature doubles the rate of reaction.

Catalysts

Adding a catalyst increases the rate of reaction. It is not used up
itself.

Catalysts speed up reactions by providing an alternative pathway for
the reaction. This results in more collisions between molecules.

It speeds up the reaction very effectively.

Concentration

As the concentration of one substance is increased, the reaction also
increases.

The higher the concentration of a substance, the more particles
present in the same area. This increases the chance of collisions
between particles and therefore increases the rate of reaction.

If you double the concentration you will double the rate of reaction.

Surface Area.

The greater the surface area the faster the rate of reaction will be.

The more surface area there is, the more particles will spread out and
become exposed to other molecules. This creates a higher chance of
collisions and increases the rate of reaction.

If you double the surface area. You will double the rate of reaction.

GRAPH

Prediction (and some background theory) – I predict that if I
double the concentration of acid I will double the rate of reaction. I
have predicted this for the following reasons.

When I double the concentration of my acid I am doubling the number of
my particles per unit volume, (see figure 3.) The collision theory
states that when I do this, particles will collide more often in a
certain time and that it is more likely that collisions will result in
a reaction, (see figure 4.) The more collisions there are between
particles in a given time, the faster the reaction will be.

When 2 particles from different substances collide, they combine to
make one particle, (see figure 2 below.) This is the reaction. As the
reaction progresses the concentration of the reactants decreases. This
is why I also predict that most of the gas given off in my experiments
will be given off at the beginning of my timed 60 seconds. This will
mean that when I come to do my graphs, the curve to show the rate of
reactions will be steepest towards the beginning.

GRAPH

To get parts of the information above, I referred to ‘Chemistry For
You’ published by ‘Stanley Thornes Ltd.’

In the next section, I will show my preliminary work. I did 3
preliminary experiments and though these identified the parts of my
investigation I could improve and decided on the measurements I wanted
to use of each substance. I also came up with a final method.

EXP. 1 – ACID – 15ml TIME: - 30 seconds

WATER – 15ml GAS GIVEN: – 17 ml

MAGNESIUM – 0.5g

What happened? – There was some fizzing, but it took some time and the
gas was given off quite slowly.

How I plan to improve it – I will increase the concentration of acid
in the next solution and therefore hopefully increase the rate of
reaction. I also plan to increase the time from 30 seconds to a minute
to allow more time for the reaction to take place, and for gas to be
given off.

EXP. 2 – ACID – 20ml TIME: - 1 minute

WATER – 15ml GAS GIVEN: - 83ml

MAGNESIUM – 0.5g

What happened? – This time the reaction was a lot faster!! Over double
the amount of gas was given off very quickly. There was much more
fizzing taking place in the flask as well.

How I plan to improve it – I’ve decided to keep the time as 1 minute
as it gives plenty of time for the main reaction to happen. However I
think I will reduce the amount of water and acid I use, and make them
equal for my next experiment, as this time the reaction was a bit too
fast.

EXP. 3 – ACID - 10ml TIME: - 1 minute

WATER – 10 ml GAS GIVEN: - 61ml

MAGNESIUM – 0.5g

What happened? – Although the reaction was a little slower this time.
I think this is because I reduced the amount of acid used, and
increased the measurement of water. The concentration of the acid had
been reduced by 25% from EXP. 2, so the rate of reaction would be
about 25% less in my timed minute.

How I plan to improve it – I think this has been my best experiment so
far. I have decided that in my final experiments I will keep the time
as a minute as in EXP. 1 and 2. As for the measurements of acid and
water, I plan to change the concentration of my acid by using
different amounts of water to dilute it, but I will keep the total
amount of them combined at a constant 20ml. I will keep the magnesium
at 0.5g as well, as I have had no problems related to this.

PRACTICE METHOD –

* Put on goggles.

* Measure out water and hydrochloric acid.

* Then weigh out 0.5g of magnesium strips.

* Put the magnesium strips into the flask.

* Next, add the hydrochloric acid, and then the water.

* Quickly put the bung into the top of the flask and start the stop
watch.

* After a minute record the amount of gas given of in ml.

* Repeat each experiment three times to identify anomalies and
ensure your results are as accurate and fair as possible.

REVIEW – This method isn’t as good as it could be. The one thing that
really needs changing is the order in which it suggests I put the
substances into the flask.

It tells me to put the magnesium in and then add the hydrochloric acid
followed by the water. These aren’t very good instructions as once the
acid and magnesium have come in contact the reaction starts to take
place immediately. The results would then be inaccurate and unfair as
you would first of all lose the beginning of the reaction while you
added the water, and the reaction that took place wouldn’t match the
concentration of acid I said I had added, as the hydrochloric acid
would have started the reaction with no water at all to dilute it.

It would be better if it told me to first put in the water and
hydrochloric acid, and second to drop in the magnesium and quickly
push the bung into the entrance of the flask to catch the gas given
off by the reaction.

FINAL METHOD –

· Put on goggles.

· Measure out water and hydrochloric acid.

· Pour them into the flask.

· Weigh out 0.5g of magnesium strips.

· Drop this into the flask, and quickly push the bung of the syringe
into the entrance of the flask to catch all the gas given off by the
reaction.

· At the same time start the stopwatch and time a minute.

· Once a minute is up, look at the syringe and record the amount of
gas given off in ml.

· Repeat each experiment 3 times to identify anomalies and ensure your
results are as accurate and fair possible.

Apparatus –

· Goggles
· Thermometer
· Balance
· 20ml Measuring cylinder
· 250ml Conical flask
· 100ml Gas syringe
· Stop-watch

GRAPH

Substances used –

1. hydrochloric acid (2HCl)

2. water (H2O)

3. Magnesium (Mg)

Chemical Equation for the reactants -

2HC1 + Mg = MgCl2 + H2

Risk Assessment - Like any experiment that involves acid, there were
risk and hazards I had to take into consideration while doing my
experiments.

For all the experiments I carried out, I wore a pair of goggles. This
would make sure any painful eye injuries or burns could be avoided.

While pouring the hydrochloric acid, I used a funnel to help reduce
the risk of spilling the substance on me, other students or my work.

It’s important to remember that acid is a corrosive substance and
precautions must be taken while using it.

OBTAINING EVIDENCE –

Title – Results.

Control Variables – This is a list of the variables I chose to
control, with

their units and values.

The hydrochloric acid (2HC1) – this was measured in ml.

It was in the solution I used, so that it would react with the
magnesium strips when they came into contact, making a reaction I
could measure by recording the amount of gas given off.

I could control the concentration of the acid but increasing of
decreasing its amount, and adding water up to 20ml so I could work out
its exact concentration. When the concentration of the acid was
doubled, the rate of reaction would also double.

The water (H2O) – was also measured in ml.

I used the water to dilute the hydrochloric acid and reduce its
concentration. The amount of water was always the overall measurement
of 20ml, minus the amount of acid in the solution.

By controlling the water, and having a fixed amount of 20ml, I could
work out the exact concentration of acid in the solution.

Over the next page, I have displayed the results I obtained from my
experiments in tables.

I repeated each of the 5 experiments 3 times to ensure accuracy.

I have done a table for each set of experiments.

All my experiments were timed using a stop watch. I recorded the gas
given off in 60 seconds. This is because it gave plenty of time for
the reaction to take place without being too long and was an easy way
to conduct my experiments in a accurate fair way, (see preliminary
EXP. 2.)

GRAPH

0mls

Anomalies – In my results tables, I have highlighted those results I
identified as anomalies. Something I noticed was that although the
concentration of hydrochloric acid was increased by exactly 0.5m each
time, the gas given off in the reaction was often very irregular
sometimes increasing by a little and at other time doubling between
experiments. I’m not sure why this is. I will look into this later on
in the ‘Accuracy’ section.

Observations – After looking at my tables of results I can see a
pattern in the amount of gas given off, relating to the concentration
of hydrochloric acid used.

The higher the concentration of acid, the more gas is given off. This
is because the higher the concentration, the more particles per unit
volume and therefore the higher the chance of collisions happening
between them.

This suggests that what I predicted was correct. That concentration of
acid does affect the rate of reaction, and due to the collision theory
it speeds up the reaction.

I will look at my results closer when I analyze the data I collected
through my experiments later using a graph to show concentration of
acid with the amount of gas produced, and see if my predication really
was correct.

Accuracy – Like any experiment, there will have been things I did that
meant my results could not have been 100% accurate.

It was difficult to get the bung fitted securely into the entrance of
the flask to catch any gas given off, and start the stop watch at
exactly the same time. This would affect the accuracy because the
reaction starts happening as soon as the acid and magnesium come into
contact, and if the bung isn’t fitted in at the start of the reaction,
I lose some of the gas given off. If the stop watch isn’t started as
soon as the reaction starts, then it’s not 60 seconds of reaction it
would be slightly more. To fix this problem I could have asked a
friend to start the stopwatch for me, while I fitted the bung.

Although my measurements were good, they couldn’t have been perfect as
it was difficult to measure exactly 0.5 grams of magnesium on the
balance, with many other people pushing to have a go. The amount of
magnesium would affect the reaction, as the more there was of it, the
more particles would be in it to collide with the acid particles….and
therefore the higher the rate of reaction would be.

Other than these two things, I think my measurements and results were
accurate or very close to accurate. They were close enough, to allow
me to find out of it my prediction was correct or not.

Analysis – After looking back over my results and studying my graph, I
can see that my prediction was not completely correct. In my
‘Planning’ section, I made 2 predictions. These were: -

1) – I predict that if I double the concentration of acid I will
double the rate of reaction.

2) I also predict that most of the gas given off in my experiments
will be given off at the beginning of my timed 60 seconds.

Prediction 1 was incorrect. When I doubled the concentration of acid,
the rate of reaction didn’t double.
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