rate of reaction

According to the collision theory, there are five factors, which affect the rate of a reaction:

1. The temperature.

The reaction rate generally gets affected to double or triple by temperature above room                         temperature.

2. The concentrations.

Dilute acid has a lower chance of successful collisions as it has a lower amount of acid particles. An acid, which is highly concentrated, has more particles and will therefore result in a faster rate of reaction. When the particles collide the reaction becomes faster. Therefore, the rate of reaction will increase if the concentration of an acid increases. This is also is backed up with the collision theory.

             Low concentration of particles                       High concentration of particles

 

              Low pressure or concentration                          High pressure or concentration

3. The pressure of gaseous reactants.

When the pressure increases, you squeeze the molecules together; therefore you will increase the frequency of collisions between them.

4. The surface areas open to the elements.

In view of the fact, that most reactions depend on the reactants coming into contact, the surface exposed proportionally affects the rate of the reaction.

5.  The catalyst.

Catalysts can speed up or slow down the rate of a reaction without being used up in the process.

Method

1. We measure 10cm3 of hydrochloric acid and 25cm3 of Sodium Thiosulphate in separate clean measuring cylinders.
2. We draw a black cross on a white piece of paper, and place a flask on the centre of the cross.
3. First we add the Sodium Thiosulphate to the flask, and then add the hydrochloride.
4. Immediately, we start the stopwatch.
5. When the mixture has turned sufficiently cloudy so that the cross can no longer be seen, the stopwatch will be stopped and the time will be recorded.  
6. The whole experiment is repeated for eight more times with different concentrations.

Fair test

In support of my result to be valid, the experiment has to be accurate and a fair one.

• First of all, it is important to ensure that the measuring cylinders for the hydrochloric acid and thiosulphate will not be mixed up. 

•  I will keep the volume of sodium thiosulphate at a regular amount throughout the experiment; otherwise this would make the results inaccurate.
• I must make sure that the conical flask is rinsed well and clean before use; otherwise it could affect the results of the experiment.
• I will make sure, that I add the sodium thiosulphate and the hydrochloric acid in the conical flask at the same order as usual.
• I will laminate the cross and make sure that I keep the same one throughout the experiment.
• I must start the stopwatch at the exact time as we put the sodium thiosulphate and hydrochloric acid into the conical flask.

 

Reliability

To make sure that my results are reliable, my measurements have to be precise and accurate.

• I can do this by repeating the experiment 3 times.
• I will be using the same measurement each time for deciding when the cross has disappeared.
• I will also use a digital stopwatch, as it is more accurate than a manual stopwatch.
• I will ensure to do the same experiment on the same day and in the same lesson so the temperature stays the same at 20°C and doesn’t affect my results.

Prediction

I predict that the less concentrated the hydrochloric acid is, the slower the cross will disappear. This is because in a more concentrated solution there are more hydrochloric acid molecules, which means that there will be more collisions between the hydrochloric acid molecules and the Sodium Thiosulphate molecules. This means that the rate of reaction will increase if the concentration of reactants is increased.

This graph shows that the concentration is proportional to the rate of reaction, which is shown by the straight line. This emphasises that as the concentration increases the rate of reaction increases too.

This graph shows that the concentration and time taken for the reaction to occur will be inversely proportional. This is because as the concentration increases, time taken for the reaction to occur decreases.

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Preliminary work

Before I started my experiment, I carried out an investigation to find the best volume of sodium thiosulphate, by varying it and keeping the hydrochloric acid constant at 10ml. I also changed the time to seconds as this would make my results more accurate.  

The following results were obtained:

 

I have chosen to use 10ml of sodium thiosulphate, as this will produce more accurate results. This is because using 5ml is too slow to react, and using 15 and 20 are too fast.

 

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Results

Sodium thiosulphate was kept constant at 10ml and the concentration of HCL was being varied. I have also changed the time to seconds here to make my results more accurate.

Conclusion

From my results, I can see that as the concentration of HCL acid increases, the time taken for the cross to disappear decreases. This indicates that the rate of reaction becomes faster. I also know this by looking at my graph which shows a straight line. This proves that the concentration is proportional to the rate of reaction (1/Time).

Also by looking at my graph over the relationship between the concentration and the average time, I can see that the curve shows very fast reaction at the start of the experiment, but slows down as we use more concentration of acid.

This tells us that increasing the concentration would lead to more collisions and therefore the rate of reaction would occur faster. For example, by doubling the amount of concentration, it would double the amount of reactant particles. This would lead to more collisions taking place, and therefore the cross will disappear quicker.  

My results of my experiment support my prediction of the rate of reaction increasing, when the concentration of HCL acid is increased too.  

Evaluation

I carried my experiment the way I intended to, and my results were accurate enough to prove the prediction. The points on my graph gave the accepted straight-line and curve to give an idea of a relationship; which means I did not get any anomalous results. However, they could be improved to drop on a straighter line as some points didn’t drop on the line as it usually would.

I could do this by:

• Doing the experiment more times, to give more accurate results.
•  Measuring the solution more accurately.
• Taking a more precise reading when deciding, that the cross has disappeared.  
• Using dry conical flasks for each experiment.

I would be more precise with these points, as this would again improve accuracy.