What factors affect the rate of a chemical reaction?

Plan

I am going to investigate the reaction between sodium thiosulphate and dilute hydrochloric acid.

The rate of this reaction can be changed by a number of things:

The temperature
The use of a catalyst in the reaction
The surface area (Of solid reactants)
The concentration of the reactants

I will investigate the effect of varying the concentration of Sodium thiosulphate solution in a reaction with Hydrochloric Acid.

In my preliminary tests I found that a mixture of Sodium thiosulphate and water totalling 50cm³ mixed with 10cm³ of Hydrochloric acid worked at the right type of speed for the time I have to do my experiment in, so these are the amounts I will use in the experiment.

When sodium thiosulphate and hydrochloric acid are mixed together, yellowish sulphur is formed in the liquid, making the clear liquids cloud over. This is the equation for the reaction:

Sodium Thiosulphate + Hydrochloric acid Sodium chloride + Sulphur Chloride

+ Sulphur + Water

Na S O + 2HCl 2NaCl + SO + S + H O

I will measure the rate of this reaction by placing 10cm³ of hydrochloric acid in a conical flask. Then I place the conical flask over a piece of paper with an ‘X’ marked on it. Next I will add the 50cm³ mixture of thiosulphate and water into the conical flask and start a stop clock. I will measure the amount of time it takes to make it so you can’t see the ‘X’ by looking down through the solution. I will repeat this for all the different concentrations of thiosulphate and water, and record the time for each one. For each concentration I will also re-test it 2 more items, so I can get the average time from the 3 results giving a more accurate result.

The equipment I will need is:

2 Measuring Cylinders
1 Conical Flask
1 Stop Watch
1 Piece of paper with cross on it
1 Pipette
Sodium Thiosulphate Solution
Hydrochloric Acid
Water

Below is how I will lay out the experiment:

These are the different concentrations I will use:

In the experiment I will keep these things the same:

The temperature (Room temperature)
The volume of Hydrochloric Acid
The combined volume of Water and Sodium Thiosulphate
The equipment

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The equipment I will need is:

2 Measuring Cylinders
1 Conical Flask
1 Stop Watch
1 Piece of paper with cross on it
1 Pipette
Sodium Thiosulphate Solution
Hydrochloric Acid
Water

Below is how I will lay out the experiment:

These are the different concentrations I will use:

In the experiment I will keep these things the same:

The temperature (Room temperature)
The volume of Hydrochloric Acid
The combined volume of Water and Sodium Thiosulphate
The equipment

This is the to make it a fair test. Also if there are any very odd results I will redo them to make sure that the mean result of the 3 tests will not be changed a lot by an odd result.

During the experiment I will follow the normal laboratory rules, which include:

The wearing of safety goggles to protect my eyes from chemical splashes.
Standing up to conduct the experiment, therefore reducing the risk of tripping and spilling chemicals.
Taking care when handling chemicals, particularly Hydrochloric acid and Sodium Thiosulphate because they are irritants.
Taking care when using glassware to prevent injury.

Looking at my plans, I can now make a prediction about what will happen as I change the concentration of sodium thiosulphate.

I predict that as the concentration rises, the faster the reaction between Sodium Thiosulphate and Hydrochloric Acid will occur.

I know that if there are more successful collisions between the molecules in the mixture the faster the chemicals will react. I also know from a secondary source (Lansdale Science Revision Guide) that the higher the concentration of a chemical the more molecules there are. If there are more molecules in the mixture there will be more successful collisions, meaning the mixture will react quicker as the concentration rises.

Results

Below are the results I have gained from the experiment:

To get these results I used the experiment explained in my plan.

To help stop human error in the experiment, the person timing the experiment looked for the disappearance of the cross. If two people had done the separate jobs there would have been a time lapse between seeing the cross disappear and telling the other person to stop the clock and then eventually stopping the clock.

I used the 2 measuring cylinders and pipette in my equipment list to measure out the Sodium Thiosulphate, Water and Hydrochloric acid as accurately as possible. Measuring carefully to the bottom of a meniscus.

Now that I have these results I can start to analyse them.

Analysis

I have produced a graph of my results and have drawn on a curve of best fit. (See next page).

By looking at my graph and results I can clearly see that the more concentrated the sodium thiosulphate is, the quicker the rate of reaction time will be.

I have come to this conclusion because my results give conclusive evidence that as the amount of Sodium Thiosulphate decreases and the amount of water increases in the solution there are fewer atoms to collide and therefore less successful collisions causing chemical change, so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.

My results support the prediction I made because I said:

‘I predict that as the concentration rises, the faster the reaction between Sodium Thiosulphate and Hydrochloric Acid will occur.’

I believe I was correct and a secondary source (The Double Science Award Chemistry Book) states that the reaction time will be faster with a more concentrated solution because, the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up.

Evaluation

My experiment went exactly the way I planned it without any mistakes.

I kept the conditions the same that I said I would, used the same equipment all the way through, and to make sure I got an average result I did the experiment 3 times.

No results showed up as odd so I didn’t need to repeat any of the results because of it being odd.

All though I tried to make the experiment as fair as possible, there could be a lot of human error in this experiment. This is when measuring the liquids and looking for an end point in the reaction. The end point of the experiment is when the cross is not visible anymore though the mixture, but this can’t be done exactly because it can depend on the person’s eyesight etc.

To make it more efficient with no human error a light gate can be used to detect when the light doesn’t pass through any longer, and it can also use the data to produce graphs and tables on a computer. I could also use more precise measurement tools.

To improve the experiment I could go on to try and do lots more different concentrations. I could change other things like, temperature or by adding a catalyst, and see what effects it would have on the reaction rate. If I had time I could go on and do those things but I only had enough time to do the experiment that I have done.