Rates of Reaction Coursework

Prediction

(Na2 S2 O3 + 2HCL ---> 2NaCl + SO2 + S + H2O)

This is the equation for Sodium Thiosulphate reacting with 2 Molar of Hydrochloric Acid. There are a number of variables in the experiment which must be controlled in order for the experiment to be a fair test.

These include temperature, which must be kept constant throughout the experiment or the results would not match. The experiment will be performed at room temperature (Around 25 degrees). Another variable is volume of Hydrochloric acid, which must be kept the same as well. For this experiment, I will be using 5cm of HCL. If all of these variables are controlled, then the experiment will be a fair test. However, there will be a degree of human error because there is no definate point at which the cross disappears, it relies on human judgement.

My prediction is that as the concentration of Sodium Thiosulphate decreases, the rate of reaction will decrease accordingly and the reaction will take longer to complete. This is the basic principle of the Particle Theory. The particle theory is that if there are less atoms to 'collide' into each other and react, then the reaction will be slower than if there were more atoms to collide.

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Therefore, if the concentration, so number of atoms if lower, then the amount of collisions between Sodium Thiosulphate and Hydrochloric Acid will be less frequent. Also, when the concentration of Sodium Thiosulphate is higher, the water will get cloudy faster due to the fact that there will be less water available to disrupt the reaction.

As the reaction takes place, the amount of Hydrochloric Acid and Sodium Thiosulphate decreases, which therefore decreases the probability of the two atoms hitting each other, reacting. The reaction will slow down, because if two Thiosulphate atoms or two HCL atoms collide, then nothing will happen. This is the particle theory.

Method

Equipment

Beakers
Measuring cylinders
2 molar Hydrochloric acid
0.15 Molar Sodium Thiosulphate
Distilled Water
Stop Clock

We wanted to change the concentration of Sodium Thiosulphate and Hydrochloric Acid, but without changing the overall quantities. To do this, the sodium thiosulphate and water were mixed at different ratios, with a constant amount of acid (5cm3)

A large cross was marked on a piece of white paper and placed on a flat surface e.g. bench. 50cm of sodium thiosulphate was measured using a suitable measuring cylinder, and poured into a dry 100cm conical beaker and placed over the cross. 5cm of hydrochloric acid was also measured in a measuring cylinder. With a stop clock ready, the hydrochloric acid was added to the sodium thiosulphate in a conical beaker and the clock started. The cross was observed through the solution and the time when the cross disappeared from view was noted in a results table.

The beaker was then emptied, washed out with tap, then distilled water, and then the base was dried if needed. The above procedure was then repeated, but with varying amounts of sodium thiosulphate and distilled water. The solution of thiosulphate and water remained at 50cm . The solution was again placed in the conical beaker, the acid was added and the clock started. The time taken for the cross to disappear was again noted.

The experiment was repeated to obtain additional results. Each ratio of sodium thiosulphate to water was carried out twice, and the results averaged to get a more accurate result. The total solution of the thiosulphate, hydrochloric acid and water always equalled 55cm .

Obtaining Evidence