Rates of reaction investigation
Rates of reaction investigation:
Aim
The aim of this investigation is to find out the effect of concentration of acid, in the
reaction between dilute hydrochloric caid and magnesium ribbon. The rate of a chemical
reaction is a measure of how fast the reaction takes place. It is important to remember
that a rapid reaction is completed in a short period of time. Some reactions are very fast,
e.g (the formation of silver chloride precipitate when silver nitrate and hydrochloric acid
solutions are mixed.In this investigation we will test different concentrations of acid
reacting with magnesium.
Prediction
I predict that as the concentration of the hydrochloric acid increases, the time taken for
the magnesium to disappear decreases. I predict that when the concentration of the
hydrochloric acid doubles, the rate of the reaction doubles.
Linking prediction to theory
Reaction rate and concentration.
The collision theory describes how the rate of reaction increase (the time taken for the
magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the
concentration of HCL increases. The theory states that if, the more concentrated the
reactants, the greater the number of collisions between particles increase. This also
explains why the greatest rate of reaction is usually as soon as the reactants have been
mixed,i.e they are both at thier highest concentrations. As the reaction continues,the
concentration of the reacting substances decrease and so does the rate of reaction. We
must consider what happens when a reaction takes place. First of all the particles of the
reacting substances must collide with each other, and secondly a fixed amount of energy
called activation energy(Ea) must be reached if the reaction is to take place. If the
particles can produce the right amount of energy(i.e if they collide fast enough and in the
right direction) a reaction will take place. The reaction is speeded up if the number of
collisions is increased. In this investigation we must consider the topic of variables.
Clearly, the time taken for the magnesium to disappear when it is placed in different
concentrations of HCL, is related in some way. The higher the concentration of HCL you
use, the less time it takes for the magnesium to disappear and so the rate reaction
increases. The concentraton of HCL you use is the independant variable because it will
vary, and the time taken for the rate of reaction to take place(i.e the magnesium to
disappear) is the dependant variable because it depends upon the concentration of
hydrochloric acid. Other variables throughout the investigation, which will vary are the
volume of water used , and the volume of hydrochloric acid. The variables which will
remain unchanged are the temperature(room temperature) will stay the same in order for
it to be a fair test, because if the temperature changes it will effect the the rate of reaction
between the reactants, either by speeding it up if the temperature rises because the
particles move faster and travel a greater ...
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disappear) is the dependant variable because it depends upon the concentration of
hydrochloric acid. Other variables throughout the investigation, which will vary are the
volume of water used , and the volume of hydrochloric acid. The variables which will
remain unchanged are the temperature(room temperature) will stay the same in order for
it to be a fair test, because if the temperature changes it will effect the the rate of reaction
between the reactants, either by speeding it up if the temperature rises because the
particles move faster and travel a greater distance in a given time and so will be involved
in more collisions. Or the temperature may slow the reaction down due to particles
moving slower. The amount of magnesium used will stay the same (2cm long), so that it
is a fair test. The time it takes for the magnesium to disappear will be measured
accurately using a stop clock as soon as the HCL is poured into beaker with the
magnesium in it. This way most results will be accurate. It is important to keep the
reactants separate while setting up the apparatus so that the starting time of the reaction
will be measured accurately. Factors which may not be easy to control are, how well the
solution is mixed when it is diluted, to get the correct concentration. A way to come
around could be(stirring the test tube twice , or shaking the testube twice). This factor is
quite important because it determines exactly what the concentration of the solution is
each time it is mixed , if the stirring factor is not carefully controlled it could lead to
inaccurate results if the concentration of the acid solution is not what we calculated it to
be. During a chemical reaction the particles have to collide with enough energy to first
break the bonds and then to form the new bonds and the rearranged electrons, so it's safe
to say that some of the particles don't have enough energy to react when they collide. The
minimum amount of energy that is required to break the bonds is called activation
energy(Ea). If the activation energy is high only a small amount of particles will have
enough energy to react so the reaction rate would be very small, however the activation
energy is very low the number of particles with that amount of energy would be so high,
so start, so the reaction rate would be higher. An example of low (Ea) would be in
explosives when they only need a small input of energy to start their exceedly exothermic
reactions. A change in concentration is a change in the number of particles in a given
volume. If we increase the volume
a)the particles are more crowded so they collide more often.
b)even though the average amount of energy possessed by a particle does not change,
there are more particles with each amount of energy - more particles with the activation
energy.
a)is a major effect which effects the rate but b) is a minor effect which effects the rate
only slightly. In this experiment we are not concerned with whether the reaction is
exothermic or endothermic because we are interested with the activation needed to start
and continue the reaction.
Apparatus/method
Hydrochloric acid(3 mole)
Water(to dilute acid)
0ml testube
magnesium(2cm long)
stop clock(sensitivity 1/10s)
beaker
Method
To get the amount of magnesium and the amount of hydrochloric acid to use in the
situation, we have to use an excess of acid so that all of the magnesium disappears.
An equation for the reaction:
Magnesium + hydrochloric acid>magnesium chloride + hydrogen
Mg(s) + 2HCL(aq)>Mgcl2(aq) + H2(g)
mole 2moles 1mole 1mole
So we can say that one mole of magnesium reacts with 2 moles of hydrochloric acid.
We know that the concentration of hydrochloric acid that we start with is 3 moles.
Throughout the experiment, we diluted the acid solution using water, as we did this the
concentration of the acid decreased. We put the acid(3 moles) into the 10ml testube and
then poured the solution into the beaker with the magnesium ribbon init(2cm long) and
then measured the time taken for the magnesium ribbon to disappear, using the stop
clock. Next, we diluted the acid by placing 9ml of acid and 1ml of water into the testube
and then poured it into the beaker with the magnesium and measured the time taken for
the magnesium ribbon to disappear using the stop clock. Then 8ml of acid and 2ml of
water solution on the magnesium, measured time taken for magnesium to disappear using
stop clock . Then 7ml of acid and 3ml of acid, repeated as above . Then 6ml of acid and
4ml of water, repeated as above. We repeated the experiment 3 times to get a fair test by
proving that all results are accurate, if they were inaccurate we repeated the experiment
again, or ignored wrong results.We tabulated the results in a clear table as soon as each
experiment was completed ,in case we lost them and to check for possible errors or gaps
in our results and it helped us to spot patterns in the results. We calculated the different,
concentrations of the acid throughout the experiment using the method below as in the
table.
When we did the experiment, we made sure that we started the stop clock, at exactly the
same time as the acid was poured onto the magnesium, to measure the start of the
reaction to the finish of the reaction where the equivalent care was taken to stop the clock
at the point the magnesium disappeared which was at the end of the reaction. While we
set up the apparatus we kept the reactants separate so that the starting time of the
reactants could be measured accurately and for it to be a fair test. We wore safety goggles
when we did the experiment, to protect our eyes from the very harmful acid. We
considered the factors in the reaction, and decided how we were going to control them.
We kept our eyes on the experiment so that we could see exactly when we sholud've
started the stop clock and stopped the clock. This way the rate of reaction(the time taken
for the magnesium to disappear) was measured accurately for a fair test. We made sure
the amount of magnesium used stayed the same in all the experiments(2cm long).
RESULTS
The results for, the time taken for magnesium to disappear when it is placed in different
concentrations of acid are summarised below.
EXPERIMENT 1
From the results in the table and the graph we can see a steady increase in the rate of
reaction as the concentration of the acid decreaes. This complies with my prediction.
The graph shows that there is an increase in the rate of reaction as the concentration
increases because the graph has it's largest gradient or it is steepest at this point.
When the graph was made into 1/time the result should have been a sraight line graph but
it did not turn out this way even though concentration~1/time. This 1/time graph could be
wrong because of inaccurate results so there seems to be no relationship between the
concentration graph and the 1/time graph. We can see from the rate of reaction graph that
when the concentration roughly doubles from (1.8 moles to 3 moles) the rate of the
reaction doubles (from 60 to 25 seconds). Also we can see that as the reaction continues
the concentration of the reactants decrease and so does the rate of the reaction as we can
see the decreasing gradient on the graph steadily falling and coming to a stop when the
reaction is complete and the magnesium has completely disappeared.
CONCLUSION
I can conclude that if you double the concentration of the acid the reaction rate would
also double, this is because the ions are closer together in a concentrated solution. The
closer together they are, the more often the ions collide. The more often they collide, the
higher the chance of a reaction between the magnesium and the hydrochloric acid. Also
because there are more particles in the solution which would increase the likelihood that
they would hit the magnesium so the reaction rate would increase. The graph gives us a
good device to prove that if you double the concentration the rate of reaction doubles. If
you increase the number of particles in the solution it is more likely that they will collide
more often. In the reaction, when the magnesium hit the acid, it fizzed and produced
many bubbles it was silver in colour(which is one magnesiums physical properties silvery
white metallic element), the activation energy of a particle gets higher with heat, the
particles which have to have the activation energy are those particles which are moving,
in the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles
which have to have the activation energy because they are the ones that are moving and
bombarding the magnesium particles to produce magnesium chloride. The graph for
/time had the form of an s curve and it did have some relationship with the other graph.
EVALUATION
There are many reasons why our results for the 1/time graph did not prove the point that
concentration~1/time,such as
)When the reaction takes place bubbles of H2 are given off ,which might stay around the
magnesium which therefore reduces the surface area of the magnesium and so the acid
cannot react properly so this effects the results.
2)We could have controlled factors in the investigation better (e.g the stirring of the
solution because if this is'nt done properly it can lead to incorrect results).
3)using larger concentrations of acid would give a bigger more accurate conclusion
instead of just using 10ml testubes use 1litre testubes, this way graphs would be more
spaced out and give an accurate form or curve
