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How does concentration affect the rate of a reation In this experiment I will have to look at how concentration affects the rate of reaction. I will have to decide which concentrations to change, and the reaction that I will be performing. Prediction The reaction I have chosen is: Sodium + Hydrochloric ==> Sulphur + Sodium + Sulphur + Water Thiosulphate + Acid Chloride Dioxide Na2S2O3 (aq) + HCL (aq) ==> S(s) + 2NaCl (aq) + SO2 (g) + H2O (l) There are many different things I can change which will affect the rate of reaction, so I will have to do preliminary experiments to find out ideal quantities for my final experiment.

Below are some variables I can change for my experiments: o Temperature o Concentration of Sodium Thiosulphate o Concentration of Hydrochloric Acid o Volume of Sodium Thiosulphate o Volume of Hydrochloric Acid o If I stir the experiment or let it stand For this reaction I will have to decide a suitable quantity of Sodium Thiosulphate and Hydrochloric acid during my preliminary investigations, then I will have to decide whether I will be changing the concentration of the Hydrochloric Acid or the Sodium Thiosulphate.


The reaction takes place relatively quickly and relies on the naked eye. I will be having to time how long it takes for an image below the conical flask in which the reaction taking place, gets entirely obscured by the build up of sulphur. I predict that the more concentrated the Sodium Thiosulphate, the faster it will react. This is because there are more molecules and more Sodium Thiosulphate, so they are more likely to collide making the reaction faster. I also think that when the concentration is lower, the reaction will take place slower.

In the low concentration, 50% concentration on the Sodium Thiosulphate, you can see that although there are the same amount of molecules, there is water which the Hydrochloric acid will also ‘bump’ into. This means that the Hydrochloric acid is less likely to ‘bump’ the Sodium Thiosulphate molecules and thus, make the reaction slower. Activation Energy is the amount of energy required for a reaction to take place. As a reaction takes place two molecules approach each other and then repel, the energy required to force the two particles to collide is activation energy, the particles then get close enough to rearrange the electron shells.

Using a catalyst lowers the amount of activation energy required for the electrons to be rearranged. As shown below. I predict that the graph for my main experiment will look like this: Method For the preliminaries I will be testing how the amount of hydrochloric acid affects the rate of reaction and how the concentration of the hydrochloric acid affects the rate of reaction. First of all I will have to draw a picture on a piece of white paper and place it underneath the conical flask. I will have to get a conical flask and put 50cm of Sodium Thiosulphate into the conical flask. Then I will put 5cm Hydrochloric Acid into the flask.

Then I will time how long it takes for the image of the picture to be completely obscured by the build up of sulphur. Then I will repeat the experiment using different quantities of hydrochloric acid and then using different concentrations of hydrochloric acid. I will change the concentration of the hydrochloric acid by instead of 20cm, have 18cm Hydrochloric Acid and 2cm of water. And changing it until eventually I have a very low concentration, but keeping the volume the same. I have ensured that this experiment will be a fair test by using the same batch of Sodium Thiosulphate and Hydrochloric Acid.

I washed out the flasks after each experiment and dried thouroughly. I also used the same temperature of room and made sure the experiment was conducted in the same place to ensure fair light. I am also repeating results to ensure no unusual readings were made. Preliminary Investigation 1 For the first preliminary investigation I tested how much hydrochloric acid I would need. I tested three different volumes of hydrochloric acid, 5cm, 15cm and 30cm. The results are below. HCL (cm) Na2S2O3 (cm) Time Taken (Seconds).

I chose 5cm even though it was the slowest reaction time; it was easier to see when the image was obscured as the image ‘faded’ slower. Preliminary Investigation 2 For the second preliminary investigation I chose to test the concentration of the hydrochloric acid. I tested using 20cm of hydrochloric acid at different concentrations, 1. The results are below. HCL (cm) Water (cm) Na2S2O3 (cm) Time Taken (Seconds) .

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Kylie Garcia

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