How increased concentrations of acid affect the rate of reaction between hydrochloric acid and marble.

I performed a preliminary experiment to see if my procedure was efficient and if I could obtain suitable results using the equipment and procedure. The results I obtained were:

These results helped as they show a clear pattern that backs up my prediction. The higher concentrations make the gas syringe get pushed out further and quicker. It also helped as it means are procedure and equipment can be used to perform the real experiment. To make sure the results are correct I will repeat them in the real experiment and I will keep a record of the temperature change in case it affects the results or produces anomalies. It also showed me to use the large marble chips (more dense) than small marble chips because I started with the small chips and the gas syringe was pushed too quickly to get any results so with the more dense chips the reaction time is slowed and so results can be taken.

To back up my prediction I obtained evidence from ‘Encarta Encyclopaedia’:

Increasing the concentration of reactants (the amount dissolved in a given volume of solution) can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate.

Particle size can also affect reaction rate. Marble chips will dissolve in hydrochloric acid more slowly than an equal amount of ground marble, because less surface area is exposed for the acid to attack.

Results:

Analysis:

In this experiment I have found out that in higher concentrations of hydrochloric acid the reaction time was quicker and the gas syringe was pushed out further. In water there is no reaction and in low concentration s of acid the reaction time was slow and the gas syringe was not pushed very far in the 3 minutes allocated. This happened because in high concentrations of acid there are more particles and so the likelihood of collision between the acid and marble particles is higher and so the reaction time is quicker. In low concentrations of acid there are fewer particles of acid and so there is a lower chance of collision and so the reaction time is slower.

There is a rise in the distance the gas syringe was pushed out to over time. In every 30 seconds the gas syringe is pushed out further. This is because the marble and hydrochloric acid is still reacting and so CO2 is still being produced. In the high concentrations of acid more CO2 is being produced and so the gas syringe is pushed out further. Over time the rate of reaction slows down. This is because the acid particles get used up during a collision and so the concentration decreases and the chance of collision lessens. In high concentrations of acid it takes a while for the reaction to stop but in low concentrations of acid it slows down straight away and the reaction stops after only a few minutes.

Evaluation:

The procedure used was suitable and efficient as it was simple and quick to perform and allowed me to take into consideration key factors, which had to remain constant to keep the test fair. Also it allowed me to obtain results that followed a clear pattern in the way that I predicted in my plan. The evidence is likely to be accurate as in the preliminary experiment and in the actual investigation the results were extremely similar and they followed the pattern and trend that was predicted. Also the prediction was backed up by scientific evidence, which shows that what the results should be is what they are. There are no anomalous results as a pattern follows throughout and there are no huge gaps or no changes when there should be. This makes the evidence even more accurate and improves their reliability.

There weren’t many problems encountered in making the test fair, as I was able to ensure the mass of marble chips stayed equal throughout as the scales measured to 3 decimal places and you could obtain the required mass without difficulty. The measuring cylinders were accurate and we could make sure the volumes we wanted were as close as possible. They are never going to be exactly equal as liquid is lost when pouring into the beaker and it is impossible to see by eye whether the exact volume the liquid is at. This, however, is a minor problem and is unlikely to have any affect on the results which would make them anomalous. The main factor, which could have affected the results but was extremely hard to control, was the temperature. We could record the temperature but we could not keep it the same for the whole of the experiment as it would a long time to do and it would be too difficult.

The reliability of the evidence is sufficient to support the conclusion as it follows the predicted pattern and fitted in with the scientific evidence explaining the predicted pattern. The amount of results is enough to see how the trend progresses and how much or how little changes there are as the time went on. This can be seen on a graph of the results and there are no anomalies or patterns, which contradict or don’t agree with the conclusion provided.

If I were to do the experiment again I would improve it to make the results even more reliable and see if the pattern continues over a greater range or the values in between the previously tested values follow or deter away from the trend shown. I would record temperature changes at the start of the experiment and after the experiment and see if an increase in temperature could have meant the reaction rate was quicker than it should have been and account for this when analysing and evaluating the results. Also I would try the experiment with the marble chips with the less surface area and see if the difference in rates continues in a constant pattern or one catches the other up over time. Also I would measure the values in between to give a wider range of results and see whether they agree with the other results obtained. The same conditions would have to be set up though to make a fair comparison.

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