To investigate factors which change the rate of reaction between sodium thiosulphate and hydrochloric acid.

GCSE Science

Devesh Parekh

INVESTIGATING FACTORS WHICH CHANGE THE RATE OF REACTION

Planning

Aim

To investigate factors which change the rate of reaction between sodium thiosulphate and hydrochloric acid.

The reaction

The reaction that will take place between sodium thiosulphate and hydrochloric acid is as follows:

Na2S2O3 (aq) + 2HCl (aq) ➔ 2NaCl (aq) + H2O (l) + SO2 (aq) +S (s)

In this experiment, a solid insoluble substance, sulphur is produced (so the initially clear solution eventually turns cloudy). This is useful as we can use this to estimate the rate of reaction by timing how long the reaction takes by timing how long it takes for a black cross to be obscured by the solution. As the rate of reaction is defined as the amount of sulphur produced (a constant) divided b the time it takes for the sulphur to obscure the cross, the rate of reaction can be measured.

Variables

There are several factors, which can alter the rate of a reaction. These are:

Temperature: An increase in temperature will make the reaction occur faster. This happens because the particles of that substance move faster when it is heated. Because the particles move faster, they would collide more often, making the reaction rate faster. Also see collision theory below.

Concentration: The increase in concentration, the greater and faster the reaction. This is because if the solution is more concentrated, it will increase the number of molecular collisions in a given time.

Surface area: If the solid reactant is crushed up into smaller pieces, it will increase the total surface area, which means there are more particles to react with the other reactant in the solution, thus increasing the rate of reaction. There are no solid reactants in the reaction.

Catalyst: This is a substance, which increases the rate of a reaction, without being changed or used up.

From the variables above, I will need to choose one independent variable, which I will investigate in the experiment. I have chosen temperature as this variable as it is quite easy to control.

How does changing temperature change the rate of reaction?

For a chemical reaction to take place, some bonds in the reactants must be broken. The colliding particles must have enough energy to break these bonds. This minimum amount of energy is called the activation energy. Only the very fastest moving particles have enough energy to break bonds.

In gases, liquids and in solution, the particles move at a range of speeds. Some are moving very slowly and others are moving very fast. To react, particles must collide with enough energy and in the correct orientation for bonds to be broken.

Increasing the temperature of the reactants makes the particles inside them move faster and with more force. This means that the particles have more energy than in a cold solution. The particles then hit each other more often (this speeds up that rate of reaction) but also with more force (this ensures that the activation energy is met in all collisions thus speeding up the rate of reaction even more).

I therefore predict that a higher temperature will increase the rate of reaction. It is generally recognised that an increase in temperature of 10oC roughly doubles the ...

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I therefore predict that a higher temperature will increase the rate of reaction. It is generally recognised that an increase in temperature of 10oC roughly doubles the rate of reaction. Therefore the graph for the results could look lik it does below:

Preliminary experiment

I will have to do a preliminary experiment at room temperature to find out the optimum conditions for the reaction to take place in (for example, the volume and concentration of the two reactants; sodium thiosulphate and hydrochloric acid).

The two reactants were measured out in separate measuring cylinders (25ml of sodium thiosulphate and another quantity of hydrochloric acid). The concentration of the sodium thiosulphate was kept constant, however the concentration of hydrochloric acid was changed. The sodium thiosulphate was then poured into a test tube which had a black cross on the back of it. Next the hydrochloric acid was added to the test tube and the stopwatch started. When the solution turned so cloudy that the black cross was completely obscured, the stopwatch was stopped and the time recorded.

The results are as follows:

The results can be summarised in a graph as follows:

From the results we can tell that the best volume and concentration of acid to use would be 3ml of 0.2 molar HCl as it gives us a good time (around 1.5 minutes). A long time like this is required, as when higher temperatures are used, the rate of reaction will increase dramatically so if the reaction only takes 10 seconds at room temperature, at 60oC, it is likely to take around 0.625 seconds, which is a time that cannot be measured accurately. Therefore, to make the test more accurate, the rate of reaction at room temperature should be around 1.5 minutes.

For the main experiment, the method used in this experiment would be suitable, but a Bunsen burner and water bath will also be needed to heat the reactants up. A range of temperatures will need to be tested (18oC - 65oC).

Main Experiment

Apparatus

Sodium thiosulphate
Hydrochloric acid (0.2mol/dm3)
400ml beaker (and water to act as a water bath)
Test tube with black cross on the back
Thermometer
Gauze
Tripod
Bunsen Burner
Heat-proof mat

Diagram

The apparatus was set up as below:

Method

The apparatus was set up as above and the water in the beaker and sodium thiosulphate in the test tube was heated up to the desired temperature using a Bunsen burner, or left alone to measure 16oC (tap water temperature).
Once the sodium thiosulphate had reached the correct temperature, the Bunsen burner was turned off, 3ml of 0.2 molar hydrochloric acid was added, and the solution was stirred vigorously with the thermometer for 5 seconds. The stopwatch was also started.
When the solution became so cloudy that the black cross at the back of the test tube was not visible, the stopwatch was stopped. The time was recorded
The experiment was repeated for 18oC, 25oC, 35oC, 45oC, 55oC and 65oC. For each temperature, triplicates were done to make the results more accurate.

Safety

Hydrochloric acid is only slightly corrosive but safety goggles were still worn. Sulphur dioxide was produced in the reaction so care was taken not to inhale the gas.

Results

The results can be represented in a graph as follows:

The results can also be plotted in a graph, which shows temperature against the time taken to obscure the cross. As rate of reaction is defined as:

and the amount of sulphur produced is constant, the rate of reaction can also be defined as:

This means that rate of reaction is the reciprocal of time. The graph is below:

Conclusion

From the results it is possible to conclude that there is a strong positive correlation between temperature and rate of reaction. This means that as temperature increases, the rate of reaction also speeds up. This supports my hypothesis. This speed up roughly increases at a steady rate. This is because temperature is proportional to the rate of reaction as follows:

The evidence found in this experiment supports the idea of the Collision Theory where higher temperature make the rate of reaction speed up because there are more collisions between reacting molecules per a second. Therefore, it is more like that a molecule will hit another molecule in the correct place, and with enough force (as to exceed the molecules activation energy) so that the bond will break and the two molecules react and eventually form new bonds.

A popular theory is that a 10oC rise in temperature roughly doubles the rate of reaction. Using a graph we can see if the results obtained support this theory.

From the graph above we can see that our results roughly show that a 10oC rise in temperature roughly doubles the rate of reaction as the two lines (one representing the results obtained and the other representing a line showing a rate of reaction which doubles every 10oC) are very close together. The fact that the two lines are not touching can tell us that either:

The results obtained in the experiment were inaccurate.
The statement “The rate of reaction doubles with a 10oC rise in temperature” is inaccurate.

Evaluation

The experiment was done fairly accurately, however, there are some small “lumps” in the curve of the graph, which suggest that perhaps the results obtained were not completely accurate. Inaccuracies in the results could have occurred for several reasons:

The point at which the black cross was obscured may have changed according to human error. As the point at which the black cross gets obscured represents the place where a certain volume of sulphur is produced, if the time it takes for this point to occur is not taken accurately, the results will be inaccurate too. The point at which the black cross was obscured may have changed according to human error. As a result, the time could only be read to the nearest one second, which reduced the experiments accuracy.

To fix this, a light gate could have been used where once the solution became a certain opaqueness, the timer on the light gate would be stopped and an accurate reading (correct to 100th of a second) would be obtained. This method could be adopted in an extension experiment.

The quantities of reactant may not have been measured accurately. For example, only 3ml of hydrochloric acid was used and this tiny quantity is nearly impossible to measure accurately using conventional measuring cylinders. Using inaccurate volumes of reactant can either speed up the rate of reaction (if too much reactant is used) or slow it down (if too little reactant is used).

To fix this problem, a burette could be used to measure the quantities of acid and sodium thiosulphate out with more accuracy.

The temperature of the reactants could only be kept to a certain degree of accuracy as the Bunsen Burner heated water baths were hard to control.

This could be fixed by using an electronic water bath. Heating up the hydrochloric acid to the desired temperature before it is added to the sodium thiosulphate, so that the net temperature of the reactants is not lowered when the acid is poured into the test tube.

It was also hard to keep a constant stirring rate as the method of stirring was not decided upon in the planning.

This could be fixed by using an electronic magnetic stirrer which would keep the test tube still but have a fast, efficient and constant stirring speed to produce more accurate results.

The experiment can be extended and made more useful by

Increasing the number of temperatures measured (for example, every 5oC not 10oC) and perhaps increasing the range of temperatures measured (within safety bounds). This will give a more reliable curve and a larger range of results to see if the curve changes in much higher temperatures.
Using a more reliable/accurate method (as suggested above – use measuring equipment with a higher degree of accuracy and a light gate to eradicate human error.
Investigate other variables such as concentration of acid, volume of acid, etc.