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Effect of Concentration on Rates of Reaction

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Effect of Concentration on Rates of Reaction


Aim
===

To find out how the concentration of Sodium Thiosulphate can alter the
reaction between hydrochloric acid and Sodium Thiosulphate.



Prediction
==========

I think that when you increase the concentration of the Sodium
Thiosulphate the rate of the reaction will also increase. I know this
because increasing the concentration means increasing the number of
reactant molecules in the solution and the collision theory states
that the rate of a chemical reaction is proportional to the number of
collisions between reactant molecules. If there are more reactant
molecules then more collisions will take place between them. The more
often reactant molecules collide, the more often they react with one
another, and the faster the reaction rate. In reality, only a small
fraction of the collisions are effective collisions. Effective
collisions are those that result in a chemical reaction.

In order to produce an effective collision, reactant particles must
possess some minimum amount of energy. This energy, used to initiate
the reaction, is called the activation energy. For every sample of
reactant particles there will be some that possess this amount of
energy. The larger the sample, the greater the number of effective
collisions, and the faster the rate of reaction. The number of
particles possessing enough energy is dependent on the temperature of
the reactants. If reactant particles do not possess the required
activation energy when they collide, they bounce off each other
without reacting and the collision has been unsuccessful.

Some chemical reactions also require that the reactant particles be in
a particular orientation to produce an effective collision. Unless the
reactant particles possess this orientation when they collide, the
collision will not be an effective one.

To summarize, the requirements for an effective collision (for a
chemical reaction to occur):

1.

The reactants must collide with each other.

2.

The molecules must have sufficient energy to initiate the reaction
(called activation energy).

3.

The molecules must have the proper orientation.

Five of the most common ways to influence the rate of reaction can be
explained using collision theory. They are:

1.

changing the nature of the reactants

2.

changing the concentration of one or more of the reactants

3.

changing the temperature at which a reaction is performed

4.

changing the surface area of a solid reactant

5.

adding a catalyst


In order to experimentally determine the effect of each of these
changes, it is necessary to perform at least two experiments in which
all but one variable is held constant and then compare the resulting
reaction rates.
The concentration alters the rate because when you have a higher
concentration it means that there are more reactant molecules. This
means that more reactant molecules will collide and therefore more
successful collisions will take place.
The rate of a reaction can be determined by measuring the rate at
which the reactants disappear or the rate at which the products form
similar comparisons can be made for other reactions in which one of
the products or reactants is coloured. In this case, the rate of
colour appearance gives us information about the reaction rate.
To find the rate of the reaction I will time how long it takes for the
cross to disappear for each result then use the formula to calculate
the rate:

[IMAGE]

[IMAGE]




The rate of the reaction = 1 1

Time s

[IMAGE]

[IMAGE]




Rate of reaction =concentration dm3

Time s

[IMAGE]

[IMAGE]




Rate of reaction= Mass g

Time s

Apparatus

Burettes x2- burettes are the most accurate measuring equipment we can
use at school.

Measuring cylinder 10 cm3- for acid.

Stop clock

Goggles

Heat proof mat

White paper with black cross

2m HCl

0.1m Na2S2O3

distilled water

stand

clamp

measuring flask- for unwanted drips from burettes.

Measuring flask- For holding acid for whole experiment.

Pipette- for acid we use it because of its high level of accuracy.

I will be measuring how long it takes for the cross to no longer be
visible when the conical flask is on it with the solution inside it.

Control variables: I will keep the concentration of the HCl the same
(2 mol dm3)and the same volume (5cm3). I will keep the temperature the
same. I will keep the equipment and the volume of liquids the same and
there will be the same person each time judging when the cross
disappears each time. To make sure that I use the same liquid each
time I am going to pour some HCL into a measuring cylinder so as it
does not get contaminated and so I do not end up using a different
acid the next time I do the experiment. I will be measuring the rate
at each concentration twice so as can take an average and be more
accurate in our results. I will be taking the following results twice-

The word equation for this experiment is;

[IMAGE]Sodium thiosulphate + hydrochloric acid Sodium chloride + water
+ sulphur dioxide + Sulphur

[IMAGE]Na2S2O3(aq) + HCL(aq) NaCl(aq) + H2O(l) + SO2(g) +S(s)

The element I will see suspended in the solution will be the sulphur
which will become separated from the sodium thiosulphate during the
experiment. It is yellow in colour.

Vol Na2S2O3(cm3)

Vol H2O (cm3)

Concentration Na2S2O3(mol/dm3)

Time1 (s)

Time2(s)

Ave (s)

Rate (1/s)

50

0

0.1

45

5

0.09

40

10

0.08

35

15

0.07

30

20

0.06

25

25

0.05

20

30

0.04

Independent variables: I will change concentration of the Na2S2O3. I
will change the ratios of liquids as shown in the table.

I do not need to go any lower in concentration than 0.04 mol. I am
measuring at each 0.01mol change in concentration so as I can get an
accurate graph with lots of readings.

For each concentration I will take two sets of results I will then
take the average of these so that I can make it more accurate. On my
graph I will plot the averages.



Safety
======

I will tie my hair back,

Wear goggles

Use a heat proof mat

Stand up when doing the experiment

Put bags under the desks

Keep the acid on the heat proof mat



Fair test
=========

I will keep the equipment the same, use the same cross, have the same
person waiting for it to disappear, same concentration of HCl, clean
conical flask with distilled water each time, use distilled water,
keep the temperature, same amount of stirring each time, and measure
accurately.



Preliminary work
================

In my preliminary work I took these results:

VolNa2S2O3(cm3)

Vol H2O(cm3)

Concentration Na2S2O3(mol/dm3)

Time1 (s)

Time2(s)

Ave (s)

Rate (1/s)

50

0

0.10

90

93

91.5

0.010928962

45

5

0.09

40

10

0.08

103

108

105.5

0.009478673

35

15

0.07

30

20

0.06

24

25

0.05

20

30

0.00

I only had time to find the rate at two different concentration but
from this I can tell that it is taking longer to react at the lower
concentration. It took longer for the reaction to happen when the
concentration was lower. This is because there were less particles
colliding and therefore less successful collisions. The results were
only marginally out, if my prediction was right then it should have
been double the rate=double the concentration. This would have made
the rate for 0.1 mol/dm3 0.014. The results I got were quite accurate
as there was only a 5% difference between them this shows that it is a
good method.

Method.

I will collect the equipment and check it all works.

Put the acid on the heat proof mat.

Put the burettes in the clamp stands and label them.(Na2S2O3 and H2O)

Using the funnel put I the Na2S2O3 into the burette with that label on
it.

Then I let a little bit out of the bottom into the spare measuring
flask to make sure that the bottom of the burette is full.

Check for air bubbles and then fill again to exactly 0 and then take
out the funnel.

Do the same with the H2O burette.

Then I poured out the HCL for the whole experiment into a measuring
flask and keep it on the heat proof mat.

Then I measured out the Na2S2O3 for the first measurement which will
be the strongest concentration into a conical flask then add the H20
which there is none of for the first reading (50cm3 Na2S2O3 )

Measure out the HCL into a measuring cylinder.

Add the acid to the conical flask stir round once and then start the
clock once it is on the cross on the heat proof mat.

Time until the person watching the solution can no longer see the
cross through it then when they say to stop the clock.

Record the reading.

Wash the conical flask out with distilled water, re-set the clock,
re-fill the two burettes and then do it again with the same
concentration of Na2S2O3.

Then record this result and find the average of the two and the rate
of the average result.

Then wash the conical flask out with distilled water, re-set the
clock, re-fill the two burettes and do this for each of the
concentrations from 0.1mol/dm3 to 0.04mol/dm3 at each 0.01mol/dm3
change in concentration. You must make sure that you wash out the
conical flask well and with distilled water between each reaction.

I have done a trial run of this which I wrote up as my preliminary
work which made sure that I knew how to conduct the experiment fairly
and alerted us of any problems I may have.

Obtaining Evidence

Vol Na2S2O3 (cm3)

Vol H2O cm3

Conc Na2S2O3

Time1 (s)

Time2(s)

Ave(s)

Rate (1/s)

50

0

0.10

29.0

31.5

30.25

0.033057851

45

5

0.09

34.0

33.0

33.50

0.029850746

40

10

0.08

37.0

37.0

37.00

0.027027027

35

15

0.07

44.5

44.0

44.25

0.02259887

30

20

0.06

56.0

58.0

57.00

0.01754386

24

25

0.05

76.0

74.0

75.00

0.013333333

20

30

0.04

93.0

97.0

95.00

0.010526316

Analysis

From my graphs I can see that there is a definite correlation between
the time and the concentration. The graph shows that they are
inversely proportional to each other as the line of best fit is a
curve. The line between rate and concentration is a straight which
shows that they are proportional. The graph shows that each time the
concentration increases by 0.01moil/dm3 the rate increased by 0.00425-1.This
means that they are proportional. This proves my prediction correct to
the extent that they are proportional but I can not prove that they
are directly proportional because the line on my graph does not pass
through the origin which means that my results were not directly
proportional. To improve on these results and to prove that it is
double the rate = double the concentration I could try it at lower
levels of concentration nearer to the origin. I stopped at 0.04 mol/dm3
because when it gets lower than that it would take a long time for the
cross to dissapear. There were a few odd results which were due to the
change in equipment because I could not use the same things in the
next lesson and the first result of the second lesson is the one which
is off of the line of best fit. I think this is also due to a change
in temperature as I did not keep it at a constant temperature.

I can back up my results with the theory of collision between
particles. This theory says that in order to produce an effective
collision, reactant particles must possess some minimum amount of
energy. This energy, used to initiate the reaction, is called the
activation energy. For every sample of reactant particles there will
be some that possess this amount of energy. The larger the sample, the
greater the number of effective collisions, and the faster the rate of
reaction. The number of particles possessing enough energy is
dependent on the temperature of the reactants. If reactant particles
do not possess the required activation energy when they collide, they
bounce off each other without reacting.

Some chemical reactions also require that the reactant particles be in
a particular orientation to produce an effective collision. Unless the
reactant particles possess this orientation when they collide, the
collision will not be an effective one.

This means that in my experiment when a sodium thiosulphate and a
hydrochloric acid particle met they did not always react when they did
not have enough activation energy. When the concentration of Sodium
thiosulphate was increased then the number of collisions did too. If
there were more collisions then there was more reactions because the
chance of a collision being successful did not change but the number
of collisions did. The rate and the concentration are directly
proportional because if you double the number of particles this
doubles the number of collisions which will double the rate of the
reaction.

Evaluation

The results I obtained were fairly accurate. On the whole they fitted
the line of best fit and the curve of best fit on the other graph.
There were two results which did not fit the line of best fit. I
thought about why these results were out. They occurred when we had a
change of equipment, when we started again the next lesson we were not
using the same equipment as we had been the lesson before which may
have effected the accuracy of our results. The equipment we used was
very accurate in measuring. We used pipettes and burettes with care
and precision. The method we used could be improved. We were relying
on someone to see when the cross disappeared which is not a very
accurate way to carry out the experiment as they can easily make a
mistake. To improve on this method we could use a data logger so that
it is measured and plotted by a computer. Another way we could improve
on the accuracy of our method would be to wait for the solution to
reach a specified colour intensity, This would make it more accurate
because it is very hard to judge exactly when the cross disappears but
if you had a chart and just said when it was that colour it would make
it much easier to judge. We could have been more accurate using our
method by doing the whole experiment in one go so as to keep the same
equipment. We could also have kept the experiment at a constant
temperature as this probably effected the level of accuracy in our
results as it is another factor which effects the rate of a reaction.

It would have been good to have done more repeats. Two tests were
managed each time but if one had been wrong this could have
dramatically changed the average time and therefore rate of reaction.

I could not prove that the rate graph was directly proportional to the
concentration as it did not pass through the origin so if I were to
carry out the experiment again I would like to see what happens with
lower concentrations nearer to the origin although these would take a
long time.

The results we obtained was enough to give us a conclusion as most of
the results fitted the line and curve of best fit and showed a clear
relationship with the concentration. We did it twice for each level of
concentration and plotted the averages on the graph which made it more
accurate. Another thing which made it may not have been accurate was
the line and curve of best fit because they were done by hand.

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"Effect of Concentration on Rates of Reaction." 123HelpMe.com. 18 Apr 2014
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