Factors that affect the rate of reaction.

Factors that affect the rate of reaction

Background

We can measure the rate of reaction by measuring the rate of change by either,

one of the reactants disappearing with time
one of the product forming with time.

A chemical reaction can only occur between particles when they collide (hit each other). Particles may be atoms, ions or molecules.

There is a minimum amount of energy which colliding particles need in order to react with each other. If the colliding particles have less then this minimum energy, then they just bounce off each other and no reaction occurs. This minimum is called the minimum energy.

Collision Theory:

particles must collide
with sufficient energy to break existing bonds
with the correct orientation

Measuring rates:

We can measure how quickly products form or we can also measure how quickly reactants are used up.

Here are some graphs which you can get whilst studying these two different methods,

Factors that affect the rate of reaction:

1.Raising the temperature has the same effect on all three reactions.

Raising the temperature makes the particles move faster. This means that more particles with each other per second.
The rate of the reaction increases. Also, the faster the particles are travelling, the greater is the proportion of them which will have the required minimum energy for reaction to occur. As a general guide,
raising the temperature of a reaction by 10 °C
will double the rate of the reaction.
The gradient of the plot will be twice as steep.

Here is a prediction graph for an experiment investigating the effect of temperature on the rate of a reaction:

2.Increasing the concentration (in solution).

Increasing the concentration of a substance in solution
means that there will be more particles per dm3 of that substance. The more particles that there are, the more will per second,
and so the rate of the reaction increases. In the reaction between sodium thiosulphate solution and dilute hydrochloric acid,

HCl+sodium thiosulphatesodium chloride+sulphur dioxide+sulphur+water.
HCl(aq) + Na2S2O3(aq) NaCl(aq) + SO2(g) + S(s) + H2O(l)

increasing the concentration of sodium thiosulphate
means that there will be less time before the can no longer be seen
(the sulphur will be produced more quickly).

This is a prediction graph, for the experiment.

When making different concentrations of sodium thiosulphate, we must
take care to use the same total volume
(of sodium thiosulphate plus hydrochloric acid)
for the comparison to be correct.

3. Increasing the pressure (in gases).

Increasing the pressure of a reaction where the reactant is a gas
is similar to increasing the concentration.
A gas at higher pressure will have more particles per dm3
of the particles (usually molecules) of the gas.

The more particles that there are,
the more will per second,
and so the rate of the reaction increases.

If the reaction is ,
then increasing the pressure will shift the equilibrium
towards the side of the reaction which has the smaller volume.

4. Increasing the surface area of a solid.

A solid in a solution can only react when particles with the surface.
The bigger the area of the solid surface,
the more particles can collide with it per second,
and the faster the reaction rate is.

You can increase the surface area of a solid
by breaking it up into smaller pieces..

A powder has the largest surface area and will have the fastest reaction rate.
This is why are often used as powders.

In the reaction between calcium carbonate and dilute hydrochloric acid,

HCl + calcium carbonate calcium chloride + carbon dioxide + water.
HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)

calcium carbonate may be used in the form of marble chips.

The reaction rates can be compared using large marble chips,
and the same mass of small marble chips.
The reaction can be followed by plotting the against time.

The reaction rate is faster (the slope is steeper)
for the reaction with small marble chips (greater surface area).

Note that the final loss of mass is the same for both reactions.
This is because the same mass of calcium carbonate (marble chips)
will give the same mass of carbon dioxide,
whether the chips are large or small.
The smaller chips will just do it more quickly.

Here is another prediction graph to show the affect of surface area on the rate of reaction:

5. Catalysts.

What is a Catalyst?

A catalyst will change the rate of a reaction.
A catalyst is often used to make a reaction go faster.

The catalyst itself does not take part in the reaction. It is not changed by the reaction, it is not used up during the reaction, it is still there when the reaction is complete.

A catalyst is usually a transition metal, a transition metal oxide, or an in living cells. An exception is aluminium oxide, ...

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Here is another prediction graph to show the affect of surface area on the rate of reaction:

5. Catalysts.

What is a Catalyst?

A catalyst will change the rate of a reaction.
A catalyst is often used to make a reaction go faster.

The catalyst itself does not take part in the reaction. It is not changed by the reaction, it is not used up during the reaction, it is still there when the reaction is complete.

A catalyst is usually a transition metal, a transition metal oxide, or an in living cells. An exception is aluminium oxide, used in the .

How does a Catalyst work?

A catalyst works by providing a convenient surface for the reaction to occur.
The reacting particles gather on the catalyst surface and
1) more frequently with each other,
2) more of the collisions result in a reaction between particles because the catalyst can lower the for the reaction.

A catalyst is often used as a powder,
so that it has a bigger per gram.

Here is an example of an experiment using a catalyst:

Hydrogen peroxide is stable at room temperature. The presence of a catalyst may cause it to decompose.

hydrogen peroxide oxygen + water.
2H2O2(aq) O2(g) + 2H2O(l)

The rate of the reaction can be followed by the volume of oxygen produced.

The catalyst used is Manganese(IV) oxide - MnO2(s). Using more catalyst will show an increase in reaction rate. This is because more catalyst will have a greater surface area for the reaction to take place.

The reaction can be performed using (the same amount) of different catalysts, to compare how well each catalyst works for the same reaction.

As you can see from the graph,
Manganese(IV) oxide - MnO2(s), is the best catalyst. The gradient of the plot is greater (steeper) than the other two. Copper(II) oxide - CuO(s), is not as good as manganese(IV) oxide but is better than zinc oxide - ZnO(s).

Without a catalyst, hydrogen peroxide will not decompose.

Here is a different prediction graph showing the affect of a catalyst on the rate of a reaction:

Pilot study

I have chosen to investigate the effect of concentration on the rate of reaction in the reaction between sodium thiosulphate and hydrochloric acid. For my final experiment, however I will carry out a pilot study so I will be able to know what molarity of hydrochloric acid to use, in order to obtain accurate results. The molarities that I will use in the pilot experiment are, 0.5 Molar, 1.0 Molar and 2.0 Molar. The reason that the change in molarity of acid would change the results is that the lower the molarity of acid, the slower the reaction will be. If the reaction is too fast that you are not able to start and stop the stop-clock in sufficient time for it to be an accurate result. Or if the reaction is too slow and therefore the experiment will take too long, resulting in me not being able to collect sufficient information to confirm my theory.

Here I have drawn a prediction graph for this experiment.

Apparatus:

flask
cross drawn on piece of paper
measuring cylinder
stop-clock
40 g/dm3 sodium thiosulphate
8 g/dm3 sodium thiosulphate
0.5 Molar hydrochloric acid
1.0 Molar hydrochloric acid
2.0 Molar hydrochloric acid

Method:

50 cm3 of 40 g/dm3 sodium thiosulphate solution was placed in a flask. This was measured using a measuring cylinder.
5cm3 of 0.5 Molar hydrochloric acid was then placed in a small cylinder. Using the burette, for accuracy.
the acid was then added to the sodium thiosulphate. And at the same time, the stop-clock was started.
The flask was swirled to mix the solutions
The flask was placed on a piece of paper marked with a cross
Looking down at the cross from above. When the cross disappeared, the stop-clock was stopped and the time was noted.
This method was repeated using the three different molarities on each of the two different concentrations. I.e the 40 g/dm3 thiosulphate was tested with 0.5,1.0,and 2.0 molar acids. And the 4 g/dm3 thiosulphate was also tested with the 05,1.0, and 2.0 molar acid.

Hypothesis

I predict that the 2 molar hydrochloric acid, reacted with the 40 g/dm3 Sodium Thiosulphate will have the fastest rate of reaction. I can prove this with the collision theory. We know, from our studies that the higher the concentration of an acid or solution is, the higher the rate of reaction will be, this is shown in the diagrams below:

The first diagram shows a highly concentrated solution. This means that it has more particles per cm3.. Therefore there are more particles colliding, resulting in the reaction occurring faster. Where as the second diagram shows a low concentrated solution. This means that it has less particles per cm3. Therefore there are less particles colliding, resulting in the reaction occurring slower.

Results for pilot study:

Conclusion:

I am going to use the 2 Molar acid in my main experiment. This is because it had the fastest results, where as the 0.5 Molar acid had results which took too long (776 secs). If I used this in my final experiment then I would not have enough time to collect sufficient evidence to support my case.

Final Experiment

Safety:

To make sure that I, and people around me are safe whilst I am carrying out my experiment, I will remember the following points,

I will remember to wear goggles to protect my eyes
I will remember to wear a lab coat to protect my clothes and skin
I will tuck my stool out of the way, so no one can trip over it.
I will make sure my bag is tucked away, so no one could trip over it
I will not rush about or run, resulting in knocking something or someone over
I will stand up whilst doing my experiment, so that just in case something spills, I will be able to get away fast
I will not leave the apparatus at the side of the table, so it cannot get knocked over
I will wear my hair up

Here is some safety information about hydrochloric acid and sodium thiosulphate which I will be handling during the experiment.

Hydrochloric acid:

Corrosive: may cause burns. The vapur is very irritating to the respiratory system. Solutions equal to and greater then 6.5 M are corrosive and those equal to and greater than 2 M but less then 6.5 are irritant. It could be deemed sensible to label 1 M solutions as irritant as well.

General Risk year 9+ Concentrated acid may be used with careful supervision. Wear eye protection and gloves. Use a fume cupboard.

Sodium thiosulphate:

Harmful if swallowed. May also be irritating to the eyes and respiratory system.

General Risk Assessments: year 7+. Wear eye protection. Reaction with hydrochloric acid. Once the colloidal sulphur had formed, the solution should be washed away immediately with plenty of water.

Fair test:

I will be varying the concentration of sodium thiosulphate in my experiment. I will be doing this by adding different amounts of water to a 40 g/dm3 concentration of sodium thiosulphate. The volume of the solution will stay the same, throughout the experiment (50 cm3). Also, the volume of hydrochloric acid will stay the same throughout (5cm3). I will make sure that my measurements are accurate by putting the burette on the bench so that my eye is level to read the measurements, also I will check to make sure that there are no bubbles in the burette, resulting in inaccurate results. I will also make sure that any other factors which would affect the rate of reaction would stay the same. I will try and keep the temperature the same, by not placing one repeat in the sunshine and another in the shade. Also, I will keep the concentration of hydrochloric acid the same, I found out the concentration that I will use in my pilot experiment. I will start the stop-clock at the same time for each repeat, as to give each experiment the same “starting point”. The cross which I place underneath the conical flask will be the same for all the repeats and different concentrations, making it a fair test.

Apparatus

Conical flask
Cross drawn on piece of paper
Measuring cylinder
Stop-clock
40 g/dm3 sodium thiosulphate
2.0 Molar hydrochloric acid
Water
Measuring cylinder

Hypothesis

I predict that as the concentration of sodium thiosulphate is increased, so will the rate of reaction. For example, the 40 g/dm3 concentration of sodium thiosulphate will have a faster rate of reaction then 8g/dm3 concentration of sodium thiosulphate.

We know that before two particles can react they must meet. In a low concentration of sodium thiosulphate, the particles will be few and highly spread. This means that the number of reactions will be limited because less particles will meet.

At higher concentrations there are more particles and so the probability of them coming into contact with other particles is increased.

This explains why it is that the rate of a reaction is dependent upon concentration of the reactent.

Here I have used the diagrams which I used to explain the pilot experiment, to further prove my case:

Method

Firstly, 50cm3 of sodium thiosulphate was measured out in a measuring cylinder, then placed in a conical flask
Using a burette, making sure that there was not any bubbles in it causing inaccurate readings, and that my eyes were level to the water level. 5cm3 of 2M hydrochloric acid was poured into a different conical flask from the sodium thiosulphate.
The sodium thiosulphate was placed on top of a piece of paper with a cross drawn on it.
The hydrochloric acid was then poured into the same flask as the sodium thiosulphate
The flask was then swirled to mix the solutions, and the stop-clock was started immediately. Taking care to start the stop-clock at the same time for all the experiments and repeats.
Looking down at the cross from above, when the cross disappeared, i.e you cannot see it because the solution has become too cloudy, the stop-clock was stopped, and the time was noted.
The experiment was the repeated with 40cm3 of thiosulphate , with 10cm3 of water added into the flask, and then, 5cm3 of hydrochloric acid. 30cm3 of thiosulphate and 20cm3 of water, 20 cm3 of thiosulphate and 30cm3 of water, 10cm3 of thiosulphate and 40cm3 of water, and finally 5cm3 of thiosulphate and 45cm3 of water
All of the different concentrations we4re repeated three times to obtain an average.

Here are two diagrams to show how to set up the apparatus:

Results for final experiment:

Analysis

My results supported my hypothesis that as the concentration was increased, so was the rate of reaction. These results were shown on two graphs (A and B). This is because, we know that before two particles can react they must meet. In a low concentration of sodium thiosulphate, the particles will be few and highly spread. This means that the number of reactions will be limited because less particles will meet. At higher concentrations there are more particles and so the probability of them coming into contact with other particles is increased. This explains why it is that the rate of a reaction is dependent upon concentration of the reactent.

Looking at the graph of concentration and time of reaction (graph A). We can see that the time taken for, the cross to disappear was shortest when the concentration of sodium thiosulphate was greatest, for example, 40 g/dm3 concentration of sodium thiosulphate took, on average 28 seconds for the cross to disappear.

As the concentration of sodium thiosulphate decreased the time taken for the cross to disappear increased. For example, 4 g/dm3 concentration of sodium thiosulphate took 328 seconds, on average until the cross disappeared.

The graph is a smooth curve.

As the concentration got higher, the gap in the time taken for the cross to disappear got smaller, for example the difference of the time taken for the cross to disappear between the 4 g/dm3 concentration of sodium thiosulphate and the 8 g/dm3 concentration was 195 seconds. This is considerably higher then the difference between the 32 g/dm3 and 40 g/dm3 concentrations, which was 7 seconds difference.

Looking at graph B, we can see the rate of reaction compared with the concentration of sodium thiosulphte. This graph shows a straight line. Looking at this graph we can conclude that the rate of reaction is directly proportional to the concentration of sodium thiosulphate. The gradient of the graph is not particularly steep. This shows that there is not a great change in the rate of reactions between concentrations.

Evaluation

I feel that my results are accurate, this is because, looking at graph B (rate of reaction compared with concentration, there are no major anomalies in the results. Also I used as accurate methods as I was able to collect the results for the experiment. As I said before, there were no major anomalies in my results, however there were two that did not fit quite on the line. These were the 4 g/dm3 concentration, which was slightly below the line with 0.003/s, and The 24 g/dm3 concentration which slightly above the line with 0.022/s as it’s rate of reaction. The reason that these anomalies could of occurred, is that I could of not measured out the acid, thiosulphate or water out correctly from the burette. Also, I could of started the stop-clock slightly after or before I actually should of.

The information that I collected could be adequate to support my conclusion fully, I could of however collected more evidence by doing more repeats and maybe doing more different molars of acid and concentrations of thiosulphate. By collecting more evidence, I would of had a wider range of information from which to make an average out of. Also I would of repeated all of the anomalies that I acquired, resulting in a more accurate average. If I had, had more time.

I do feel that the method that I used was suitable for my experiment. If I could of, I could of used a light sensor and data logging to investigate in more accurate detail. The light sensor detects the amount of light that travels through the reaction mixture in the beaker. As the mixture gets cloudier the amount of light reaching the light sensor decreases. This would of acquired much more accurate results, as it uses digital readings to get the results.

To extend my experiment I could of investigated another factor that affects the rate of reaction, for example, the effect of temperature on a reaction rate. Here is the method that I would of followed:

Put 10cm3 of sodium thiosulphate solution and 40cm3 of water into a flask.
Measure out 5cm3 of dilute hydrochloric acid in a small cylinder.
Warm the thiosulphate solution in the flask if necessary, to bring it to the required temperature. (I would do this experiment five times, with temperatures in the range 15-65 degrees Celsius.)
Add the acid from the cylinder and start the clock. Swirl the flask to mix the solutions and place it on a piece of white paper marked with a cross

- Look down at the cross from above. When the cross disappears. Stop the clock and note the time taken. Record the temperature of the mixture in the flask.

Bibliography

Chemistry in context - Graham Hill

Co-ordinated science: chemistry - L. Erdrich

Understanding chemistry - Ted Lister

Chemistry students book - Nuffield

Chemistry for you - Lawne Ryan