Does Concentration affect the rate of reaction?

Sodium thiosulphate experiment

Plan:-

I must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place; this is otherwise known as the reciprocal. There are five factors which affect the rate of a reaction, according to the collision theory of reacting particles: Temperature, Concentration (of a solution), pressure (in gases), Surface area (of a solid reactant), and using a catalyst. However, I will go into more detail about these later.

I have chosen to investigate the effect concentration has on a reaction. This is because I believe that it is the most practical to investigate - it would take longer to prepare a solid in powdered and un-powdered form, and it is difficult to get accurate readings due to the inevitabilities of human errors (i.e. our reaction rates), and as gas is mostly colourless it is difficult to measure a reaction changing the pressure, also if a substance is added to give the gas colour, it may influence the outcome of the experiment. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

Aim:-

My aim is to see the effects of a change in concentration on the rate of a reaction. The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid + Water
Na S O (aq) + 2HCl (aq) + 5H O

The experiment I carry out will see if changing the concentration of the mixture (while everything else remains constant) has an effect. Both the sodium thiosulphate and the Hydrochloric acid are soluble in water, so the concentration of either can be changed. I have chosen to keep all temperatures the same, so that the experiment will be straight forward, and simplistic to record results. Temperature will be used as the temperature as it more practical and will not need to be monitored. When the temperature is being varied a water bath will be used to heat up the acid and thiosulphate to the necessary temperature.

Using my preliminary experiments I decided on using the following apparatus:

1 thermometer                1 heatproof mat                1 pair of goggles
1 stopwatch                        1 apron
1 Bunsen burner                1 beaker
X board                        2 measuring cylinders
1 conical flask                        1 gauze
1 tripod                        1 pair of tongs

Method: -

Experiment - Changing the concentration

Concentrations:

This is a preview of the whole essay

Using my preliminary experiments I decided on using the following apparatus:

Experiment - Changing the concentration

Concentrations:

As shown above, 10 cm of HCl will be used in all of the different concentrations, then an even mix of Na S O and H O are added so the total amount in each flask will add up to be 50cm . The previously measured HCL will be in a flask. The measured out Water and Sodium Thiosulphate will be in two measuring cylinders and then poured into the flask; which is placed on top of a board marked with letter X. The stopwatch will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations. The whole procedure is then repeated twice more, giving me three results for each concentration.

Repeat results will be taken to improve the credibility of the findings, and present solid grounding for an average and the final conclusion. The repeat results will help to iron out any anomalies and the average will give a good summary of the results of the experiment. However if one set of results is entirely different to the other, a fourth experiment will be performed to replace the anomalous set of results.

Safety - An apron will be worn to protect the skin and clothing.

Fair Test - In order for my findings to be valid the experiment must be a fair one. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and sodium thiosulphate will not be mixed up, and will be washed before each use so no mistakes can be made. The water, hydrochloric acid, and the sodium thiosulphate will be measured using a pipette for accuracy; this makes it easier to see if the liquids are even 1 mm off the desired amount, so a precise measurement can therefore be taken. The amount of HCl will be 10 cm each time. I will also take six different concentrations, and repeat each concentration three times; this will hopefully result in consistent results, and will clearly show any anomalies. All of these precautions will make my final results more reliable and keep anomalies at a minimum. This will make my entire investigation more successful.

Prediction -

I predict that as the concentration of the sodium thiosulphate increases the rate of reaction will decrease. This means that graphs drawn up in my analysis will have positive correlation, and will probably be curved as the increase in rate of reaction will not be exactly the same as the concentration is increased. This can be justified by relating to the collision theory. If solutions of reacting particles are made more concentrated there are more particles in the volume. Collisions between reacting particles are therefore more likely to occur. All this can be understood better with full understanding of the collision theory itself. Diagrams and descriptions are below:

Temperature:

Concentration (of solution):

Pressure (in gases):

Surface area (of solid reactants):

And using a catalyst:

For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. Only particles with enough energy can react after colliding. The minimum energy that a particle must have to be able to react is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. However the percent of successful collisions will remain the same. An increase in the frequency of collisions can be achieved by increasing the concentration, pressure, or surface area, increasing the temperature, or by using a catalyst.

My results:

I have decided to present my results in a table. I think this is the most effective way of recording them, and it is also very clear. The six recordings are in chronological order, and the volumes/concentrations used can be easily identified because they are

in corresponding sections of the table.

Analysing my evidence:

When I conducted my experiment, it was clear to see that a chemical reaction was occurring. The mixture went from being a clear, water-like liquid, to a cloudy, yellow-tinged liquid. This shows that sulphur dioxide is an end product of the reaction. Below is a graph showing the results I have recorded:

The graph clearly shows that when there is an increase in concentration of the thiosulphate solution, the reaction time decreases. This is an example of positive correlation. However, the graph also shows that the predicted smooth curve did not happen. This could be a result of human error, but all of the results for this concentration are very similar so measuring all three sets wrongly on the same concentration is very unlikely. I therefore believe that this is a result in a change of room temperature, affecting the reaction rate. This means that my prediction was theoretically correct, but it was proven wrong when conducting the experiment. If all factors had have remained constant, then the graph would show the results to be directly proportional to each other. But in this case, due to the probable change in room temperature, this cannot be shown.

Evaluating my experiment:

Overall, I think that the procedure I have used for this experiment was successful; however, if I was to do it again a monitored temperature would be more appropriate. A way to tackle this problem would be to use a water bath to place the flask into which would keep the solution at a constant temperature.

Apart from the results taken for 24cm of sodium thiosulphate, I think the quality of my results is at a high level. As the results show on the “Line of Best Fit” graph, all of the results are very close. This indicates that the teat was a fair one, and the measurements of the solutions were accurate (this is probably a result of using a pipette).

A way to improve my experiment further would be to conduct it using computerised equipment, and not by human judgement. This could be done by having a light sensor underneath the flask, so when the solution becomes cloudy and no light reaches the sensor the timer can be stopped at the exact instance it should be.

When looking back at my results, I think that I did take enough results, and I think sufficient evidence for this can be gathered from them.