Rates of Reaction - Concentration.

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Rates of Reaction – Concentration

The rate of reaction is the rate of a loss of a reactant or the formation of a product during reaction. Although there are many different variables to test, I have chosen to experiment with different concentrations of hydrochloric acid, with sodium thiosulphate as the constant.  

Aim

This experiment tests how differing concentrations affect the rate of a chemical reaction. It will be done using a simple experiment where one substance is constant and the other is varying.

Apparatus 

For each experiment I will need the following;

150 cm3 2 Molar hydrochloric acid

140 cm3 water

250 cm3 sodium thiosulphate

1 conical flask

1 measuring cylinder

1 stop clock

5 pieces of paper

1 pen

Preliminary Work;

Prior to this investigation, I conducted a preliminary experiment to determine the strength of my method in terms of accuracy and precision. I decided to test how the temperature of an experiment affects the rate of reaction. Therefore, I got 50 cm3 sodium thiosulphate and 40cm3 hydrochloric acid diluted with 10cm3 water. After pouring the two mixtures into a conical flask I placed it onto a black cross on a piece of paper and timed the time it took for the precipitate of sulphur produced to make the solution seem so cloudy the cross disappeared. The same experiment was conducted at temperature of 10˚C, 20˚C, 30˚C, 40˚C and 50˚C. The two substances that I have chosen for preliminary work will give me an accurate indication of the time taken for the reaction to occur in the actual investigation, as unlike with most experiments, the chemicals produce the precipitate of sulphur which turns the solution a milky yellow colour which is easy to see clearly. From my previous work, I can see that the range of temperatures was enough to give a clear summary of the investigation; therefore, I am transferring the range of temperature to use as amounts of hydrochloric acid as I feel it gives reliable results.

Method 

After retrieving all the apparatus that is needed to successfully complete the experiment, I will begin with drawing a large cross onto the piece of paper. Then I will measure out five different quantities of both water and hydrochloric acid using a measuring cylinder in the following way;

50 cm3 of hydrochloric acid on its own

45 cm3 of hydrochloric acid + 5 cm3 of water

40 cm3 of hydrochloric acid + 10 cm3 of water

35 cm3 of hydrochloric acid + 15 cm3 of water

30 cm3 of hydrochloric acid + 20 cm3 of water

25 cm3 of hydrochloric acid + 25 cm3 of water

20 cm3 of hydrochloric acid + 30 cm3 of water

15 cm3 of hydrochloric acid + 35 cm3 of water

10 cm3 of hydrochloric acid + 40 cm3 of water

Smaller and larger concentrations were available for me to use but due to preliminary experiments I have performed, I know that the amounts I have chosen are neither too slow nor too fast to record. This is the only variable I will be testing. The experiments are to be completed one at a time.  These solutions will be transferred into the conical flask which sequentially will be positioned onto the cross already drawn. The measuring cylinder will be used again to quantify 50cm3 of sodium thiosulphate to react with the hydrochloric acid solution. The action of pouring the sodium thiosulphate into the conical flask and starting the stop clock will be in sync. Following a varied amount of time, I will be able to witness a milky coloured liquid.

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The reason for this occurrence is because a precipitate of sulphur is formed as the substances react. The stop clock is terminated immediately when the cross is no longer perceptible. The time will then be recorded into a table, along with the results of the other experiment. In this reaction sodium thiosulphate reacts with hydrochloric acid forming salt, sulphur water and sulphur dioxide gas.

Sodium thiosulphate + hydrochloric acid  sodium chloride + sulphur + water + sulphur dioxide

Na2S2O3 + 2HCl → 2NaCl + S + H2O + SO2

Fair test 

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