Calcium carbonate reacts with dilute hydrochloric acid according to the equation below

Safety

Whilst undertaking this experiment, I must wear goggles and tie my hair back. I must wear goggles to protect my eyes, as the hydrochloric acid is highly corrosive, and could damage my eyes. I must also be careful not to spill any of the liquid during the experiment because of the dangers it will cause to both myself, and those working around me.

Preliminary work

The aim of the experiment was to find out how much gas is given off by marble chips when they react with hydrochloric acid. This would show if the concentration of hydrochloric acid affects the speed of reaction.

Results of investigation

Conclusion

As the percentage of hydrochloric acid is increased, the amount of gas produced by the reaction increases. At 100% there was more gas produced than at all of the other percentages below this. The results are consistent with the collision theory. The rate of the chemical reactions in this experiment depended on the concentration of hydrochloric acid. The higher the percentage of hydrochloric acid the more collisions between the molecules there were, because there are more molecules. The more collisions there were between the molecules, the faster the rate of reaction, (the faster the gas was given off, the more gas given off). Increasing the concentration increases the pressure, making the molecules more ‘squashed up’ together; this means there is going to be more collisions. As the concentration of hydrochloric acid is increased, the rate of reaction increases.

The results of this investigation showed me that there was a definite link between the concentration of hydrochloric acid and the amount of gas given off. The preliminary work shows that there is a link, which may be directly proportional. This also shows the importance of taking a variety of results and repeating experiments to overcome anomalies.  

Results

I took results for 10 different concentrations of hydrochloric acid. I then repeated the experiment twice.  I noted the starting weight of the marble chips at the beginning of each experiment because it could not always be make exactly 10.00g. The masses, and therefore the change in mass were recorded to 2 decimal places, to ensure accuracy.

Concentration 73.0g dm-3

Concentration 65.7g dm-3

Concentration 58.4g dm-3

Concentration 51.1g dm-3

Concentration 43.8g dm-3

Concentration 36.5g dm-3

Concentration 29.2g dm-3

Concentration 21.9g dm-3

Concentration 14.6g dm-3

Concentration 7.3g dm-3

Graphs

I will now draw graphs for each of the results.

On each of these graphs I will calculate the initial rate of reaction. I will do this for the first 15 seconds, because this is when the concentration of acid is most accurate.

The rate of reaction is calculated by dividing the change in mass by the number of seconds it changed in. therefore, I will divide each figure by 15.

I must now draw a graph showing the concentration of acid against the average rate of reaction. The correlation between the two, will tell me if it is a first order reaction. I will draw a line of best fit through the points to see what the general trend is.

The graph that plots average rate of reaction against the concentrations shows that the rates of reactions for the different concentrations vary, with some lower concentrations having higher rates of reaction. However, by drawing a line of best fit through the plotted points, I was able to establish a general trend.

Conclusion

By conducting this investigation, I have been able to prove that the reaction between calcium carbonate and hydrochloric acid is a first order reaction. The line of best fit on the rate of reaction against concentration graph shows that the reaction is directly proportional. These results support, and are consistent with the prediction that I had made before I did the experiments.

The results are consistent with the collision theory. The rate of reaction in this experiment depended on the concentration of hydrochloric acid. The higher the concentration of hydrochloric acid the more collisions there were between molecules, because there were more molecules to collide. The more collisions there were between the molecules, the faster the rate of reaction. Increasing the concentration increases the pressure, making the molecules more ‘squashed up’ together; this means there is going to be more collisions. As the concentration of hydrochloric acid is increased, the rate of reaction increases. Therefore, the rate of reaction is proportional to the concentration of acid.

In this case, the experiment has shown, that although there are some anomalous results, the reaction between calcium carbonate and hydrochloric acid is directly proportional to the concentration of acid. This shows that it is a first order reaction.

Evaluation

The results that I have collected are fairly accurate and reliable because they are consistent with the prediction that I made. The results I collected were accurate enough that I could come to a reasoned conclusion with evidence to show that the conclusion I reached was correct. However, within each set of results for the different concentrations, there was some variation as to the change in mass, and the rate of reaction. This created anomalous results, which could be eliminated by redoing the experiment under more controlled conditions.

The anomalous results may have occurred for many reasons, however, during this experiment the temperature conditions were a major issue. The experiment was conducted on several different days, when the temperatures varied greatly. One day the experiment was carried out whilst it was snowing. This caused a massive temperature drop, and also a change in the atmospheric pressure. When the temperature increased, the molecules would have moved around faster. This means that there would have been more collisions between the molecules, increasing the rate of reaction. However, when it got colder, it would have been the opposite. The molecules would have moved around slower, causing fewer collisions, therefore the rate of reaction would have been slower.

The method I used meant that the size of the marble chips varied. Although I weighed about 10g of marble chips, it was often impossible to ensure they weighed exactly the same amount. By trying to make the weight of the marble chips 10g, I changed the size of the marble chips, using some big and some small. Using marble chips of different sizes meant that they did not have the same surface area. This will have affected the results I have collected, as there would have been different amounts of surface area upon which the molecules could collide. This is a limitation of the method, and therefore makes the reliability of my method questionable.  

If I were to conduct this experiment again, I would try to improve these aspects of the experiment, to eliminate anomalous results. I would aim to conduct the experiment under more controlled conditions. I would carry it out on the same day, to ensure that the temperature was as similar as possible for each of the experiments. Also, as well as weighing the marble chips, I would try to find a way to ensure that the surface area of the chips was similar.