The aim of this experiment is to determine what affects the rate at which a reaction occurs between sodium thiosulphate and hydrochloric acid.

Chemistry Coursework – Sodium Thiosulphate + Hydrochloric Acid

Aim
The aim of this experiment is to determine what affects the rate at which a reaction occurs between sodium thiosulphate and hydrochloric acid. I intend to find out whether (and to what degree) varying the concentration of sodium thiosulphate has any effect on the rate of reaction between these chemicals. I believe that these variables are linked, and that the concentration of sodium thiosulphate has a direct effect on the rate at which the reaction occurs.

Hypothesis

This is the chemical equation for the reaction:

Na2 S2 O3 (aq) + 2NaCl (aq) + H20 (l) + SO2 (aq) +S (s)

Concentration of a solution refers to the amount of reactant particles that have been dissolved in one unit of that solution. Therefore, the more reactant particles in a solution, the higher the concentration. As you increase the concentration of a solution, there are more reactant particles present in the unit of solution, which is always kept at the same volume. So if there are more reactant particles to the same volume of solution, then the rate of reaction will increase.

The rate of reaction increases because if more reactant particles are in a given volume, then there is a greater potential for reactant particles to collide with other reactant particles. The more collisions in a reaction, then the more collisions will be successful, and perform the reaction. A successful collision is determined by whether or not a collision occurs with enough energy to pass through the activation energy barrier.

By increasing the reactant particles in a given volume, we are increasing the rate of reaction.

On the same token, if you decrease the amount of Sodium Thiosulphate, then the reaction will slow down.

In this experiment, I am keeping all other variables fixed, and as temperature remains constant, only a fixed percentage of the total particles will pass through the activation energy. If the concentration is then increased, the percentage will rise.

Method

In order to research what effect the concentration has on the rate of reaction, I will be measuring the time it takes for the reaction to complete, when various different concentrations of sodium thiosulphate are used. To determine when the reaction is complete, I judge by the color of the solution, as it turns a cloudy white, and ceases to be transparent. Therefore, when the mixture becomes opaque, I can assume that the reaction is complete, and I will stop measuring the time. This will give us a value for the rate at which the reaction occurred, which I can compare for different concentrations. Firstly, I will dilute the sodium thiosulphate by way of ratios. I will be measuring 50cm³ of solution each time, with a burette, but I will vary the volume of sodium thiosulphate compared to the volume of water diluting it, making different concentrations of sodium thiosulphate. I will use volumes of: 50cm³ of sodium thiosulphate with no water, 40cm³ of sodium thiosulphate with 10cm³ of water, and so on down to 10cm³ of sodium thiosulphate with 40cm³ of water. I have found from preliminary tests that these concentrations react at rates which I can measure quite accurately with the equipment I have. Then, I will draw a cross on a piece of paper with a black pen, and place it underneath a conical flask. Into the conical flask, I will pour the 50cm³ of sodium thiosulphate solution and 5cm³ of hydrochloric acid, and start the stop clock. When the mixture has become opaque, that is to say, I cannot see the cross underneath the conical flask, I will stop the clock and record how long it has taken to reach this stage. I will repeat this for each concentration, making sure that the conical flask is fully washed out, because I will be using the same flask every time, so that the flask does not introduce any new variables into the results. I will repeat the whole experiment three times, in order to reduce the effects of any anomalous results which may occur, by using averages to make a conclusion. To make the experiment safe, I will be wearing goggles at all times, in case something unexpected occurs. I will be washing my hands regularly and thoroughly washing the equipment before each reaction to make sure that it is not only a fair test, but that it does not cause something dangerous to happen.

Variables
Other variables which may affect the outcome of this experiment, if not kept constant, are as follows:

Temperature Of Sodium Thiosulphate - I believe that the hotter the temperature, the faster the rate of reaction. This is because when a substance is heated up, each particle has more energy, resulting in more collisions between the free moving particles in a liquid, causing more particles to react, therefore increasing the overall rate of reaction. The second effect this has is that the particles collide with more force, increasing the chance of the particles reacting. I will keep this variable constant so that it does not affect the results by avoiding heating the sodium thiosulphate in any way, and so it will stay at room temperature.

Concentration Of Hydrochloric Acid - The more concentrated the hydrochloric acid, the faster the reaction will be, because there will be more particles in the same volume. The particles will be closer together, colliding more often, therefore increasing the speed of the overall reaction. I will keep the concentration of hydrochloric acid constant, by using it from the same bottle each time, which has a fixed concentration.

Size Of Conical Flask - Although this does not affect the rate of reaction, in that it doesn't affect the chemical reaction itself, but what this variable does affect is our measurement of how long the reaction has taken to complete. I will be judging the time taken for the reaction to finish as the time it takes for the reacted mixture of sodium thiosulphate to go completely cloudy so that I cannot see a cross drawn underneath the flask. If the conical flask is wider then the cloudiness will appear less concentrated, because it will be spread across the larger area of the bottom of the conical flask, and so it will take longer for the cross to disappear, and the results will increase. I will keep this variable constant by using the same conical flask for each experiment.

Catalyst - By using a catalyst, this experiment could be speeded up because catalysts lower the energy needed by the chemicals in order for a reaction to occur. However I will not be using a catalyst in this experiment, and I will make sure that there are no other materials present, apart from those chemicals I want to react. This way, I can be sure that no catalysts are affecting the results in any way.

Cross - The material I use draw the cross underneath the conical flask, which I use in determining when the reaction has finished, could affect how easily the cross is seen. This in turn could affect the results because if the cross is seen more easily, if it was drawn with a thick marker, our measurement of how long the reaction took to complete would be longer and so the rate of reaction would seem slower. Equally, if the cross was hard to see, then it would disappear more quickly and the rate of reaction would seem quicker. I will keep this constant by using the same black pen to draw the cross each time.

Volume Of Sodium Thiosulphate And Hydrochloric Acid - The larger the volume of sodium thiosulphate and/or hydrochloric acid, the greater total number of particles in the solution. This means that to completely react, and go cloudy, it will take longer. I will keep this variable constant by using 50cm³ of sodium thiosulphate solution with 5cm³ of hydrochloric acid each time.

Movement Of Reacting Mixture - If I shake the conical flask while it is reacting, then the reaction may speed up and therefore the rate of reaction would be quicker. Likewise, if I stir it then it will increase the rate of reaction, because there will be a greater chance of the particles colliding with other particles which they can react with because the two types of particles will be more mixed together.

Fair Test

In order to achieve a fair test, a number of measures must be taken to ensure it.

All measurements must be kept constant and accurate as possible, using correct measuring equipment.
The timing of the reaction will have to be kept as accurate as possible, making sure a stopwatch is used, and start and stop times are as accurate as possible.
The experiment should be performed over the time period of the one day. The changes in atmospheric pressure could result in differing concentrations and anomalous results.

There is no way to make this experiment ...

This is a preview of the whole essay

Fair Test

In order to achieve a fair test, a number of measures must be taken to ensure it.

All measurements must be kept constant and accurate as possible, using correct measuring equipment.
The timing of the reaction will have to be kept as accurate as possible, making sure a stopwatch is used, and start and stop times are as accurate as possible.
The experiment should be performed over the time period of the one day. The changes in atmospheric pressure could result in differing concentrations and anomalous results.

There is no way to make this experiment 100% fair, as human judgment is inaccurate, and some slightly anomalous results may occur due to judging whether the water is cloudy yet.

Safety

Safety for pupils teachers, and respect of equipment is required in all classroom experiments, to achieve the most safe environment possible we;

Cleared all obstructions (such as bags and pencil cases)
Wore safety glasses at all times.
Took off any loose clothing and tied back hair.

Apparatus

Conical Flask
3 x measuring cylinders
Stopwatch
2 x Beaker
Piece of paper
Goggles

Method

Prediction
I think that as I increase the concentration of sodium thiosulphate, provided that no other variables affect the results, the rate of reaction will also increase. I believe that this will happen because of the way in which the two chemicals react. When the sodium thiosulphate and hydrochloric acid are placed together in the conical flask, their respective molecules get mixed together, moving freely. When molecules from each chemical collide, they react, forming new compounds. The concentration will increase the rate of reaction because if the concentration is higher, then there will be more molecules of sodium thiosulphate in a given volume, and so it is reasonable to say that it greatly increases the chances of one of those molecules colliding with the hydrochloric acid molecules. This means that more of the molecules will be colliding and reacting within a given time, and so the rate of the reaction has speeded up. However, I do not have enough information on the subject to be able to make a quantitative prediction at this time.

Results
The results which I have collected, without any changes to the planned method, are shown below. I have also plotted them onto a graph (see separate sheet) so that the trend can be seen more easily. The room temperature was found to be 21°C throughout the experiment.

Analysis
The results, and the graph appear to show roughly what I predicted. As the concentration of sodium thiosulphate is increased, then the rate of reaction increases also. In this case, according to the graph, for every 1% increase in concentration, there was a 13/50,000 increase in the rate of reaction. What I believe to be happening is that as the concentration decreases, there are less sodium thiosulphate molecules around in the reacting mixture, meaning that it will take longer for each hydrochloric acid molecule to ‘find’ a sodium thiosulphate for it to react with. Overall, this decreases the rate of reaction because it takes longer for the mixture as a whole to react fully, because of the extra time it takes for the molecules to collide with each other. The results support this theory in that the relationship between concentration and rate of reaction is directly proportional, that is to say that every time the concentration increases by 1%, the rate increases by 13/50,000. This also supports the idea that it takes time for the molecules to ‘find’ each other in order to react, because if the concentration is less, then the molecules of sodium thiosulphate will be more spaced out and there will be more space between themselves and the hydrochloric acid molecules. If the molecules move around in the liquids at the same speed each time (because they have a constant temperature), then the further they have to travel to reach another molecule, the longer it will take for them to reach that molecule and then react.

Evaluation
The results of this experiment appear to be quite accurate and so I can make a definite conclusion. From this, I can speculate that the experiment was conducted well, and that the results have only been affected by other variables by little or no amount. However, looking at the graph, I can see that there are some results, two specifically, that do not follow the general trend, and show a higher or lower rate of reaction than would be expected at that point. I believe that perhaps this was caused by a change in another variable, or an inaccuracy in the measuring of the input variable. Either the reacting mixture was shaken accidentally, moving the molecules around more, increasing their chances of reacting, or more likely, the concentrations of sodium thiosulphate were incorrectly measured, and too much sodium thiosulphate was used. This would reduce the distances between the molecules, and increase the rate of reaction. I think that the only flaws in the experiment were in the measuring of the concentrations, and in the judging of when the reaction had completed. It was very hard to get an exact volume of water/sodium thiosulphate to get the right concentration, and so this may have thrown those three results off the trend. Also, there was not really any sure way of knowing when the reaction had finished. Also, it is quite possible that human judgment might waver slightly when deciding if the reaction has completed yet. If I was to repeat the experiment, I would use a much more accurate way of measuring the volumes of the reactants, and I would find a completely different way of measuring when the reaction has completed.