Results
Using our pilot study we have decided to:
- Use 10 grams marble chips
- Use 100ml solution
- Use a range of 10% to 100% concentration of acid, going up in tens
- To measure how much gas is collected every 20 seconds for 60 seconds
We decided to use these weights and volumes because they produced a good range of results. A range of 10% to 100% is large and we will be able to obtain a large number of readings to eventually produce a reliable and precise graph. We will measure the volume of gas produced every 20 seconds so that we can work out the rate soon after the reaction starts and produce a graph which show hoe much gas was collected versus time for each concentration.
Aim:
To find out the effect of concentration on the rate of a reaction between hydrochloric acid and marble chips (CaCO3)
The reaction we will be investigating is:
Hydrochloric + Calcium → Calcium + Water + Carbon
Acid Carbonate Chloride Dioxide
2HCl + CaCO3 → CaCl2 + H2O + CO2
Prediction:
I predict that as the concentration of the hydrochloric acid increases the rate of reaction will also increase. This means more carbon dioxide gas will be produced per second as the concentration of the solution increases. This is because if there is a higher concentration there will be more molecules of reactant particles per unit volume, and therefore more frequent collisions. Effective collisions should also be more frequent although the percentage of effective collisions will not change. I predict the graph showing concentration versus rate of reaction will be a straight line, for example:
I predict the graph showing time versus volume of gas released will look like this:
This is because I think the reaction will get slower as time passes because the concentration of the acid would decrease during the reaction as the acid would get used up.
Method
Apparatus
- Conical Flask
- Container
- Measuring Cylinders
- Delivery Tube with Bung
- 2M Hydrochloric Acid
- Distilled Water
- Marble Chips
- Stopwatch
Safety
Since we will be using 2M acid which is quite strong we must wear goggles to protect our eyes. We must also be careful not to spill any acid.
We decided to change our method of the pilot study slightly for the final experiment to obtain more results. This is the method we will use for our experiment.
- We will measure the Hydrochloric acid and distilled water into a conical flask to give the desired concentration. We will use a range from 10% to 100% going up in 10s. This is so we can measure the reaction rates for various concentrations of acid.
- We will then half fill the container and completely fill the measuring cylinder with water. We will turn the measuring cylinder upside down in the container without allowing any gas in. This is so that as the gas from the reaction collected, water in the measuring cylinder would be displaced. No gas should be let into the measuring cylinder before the reaction starts so the results are more accurate.
- We will measure 10g of marble chips on a top pan balance. In our pilot study we found this produced a good range of results.
- We will place one end of the delivery tube under the measuring cylinder. Then, we will tip the marble chips into the conical flask and immediately place the bung securely into the top of the conical flask. This is so the gas from the reaction will immediately collect in the measuring cylinder and the results will be more accurate.
- We will then measure the volume of gas collected every 20 seconds for 60 seconds. From this we can work out the rate of the reaction.
- We will repeat the experiment three times and take an average. This is to try and obtain more reliable and accurate results, and eliminate anomalous results.
- We will work out the rate of reaction at the beginning of the experiment for each concentration. We will work out the rate at 20s because it is only at the beginning of the reaction that the concentration is actually what we intend because the acid gets used up in the reaction and the concentration decreases.
We will use small (100ml) measuring cylinders to measure the volume of acid and distilled water to make up our different solutions as this is far more accurate than a larger measuring cylinder. For the lower concentrations we will also use a small measuring cylinder to collect the gas from the reaction as this is more accurate. However for the middle concentrations we will have to use a big measuring cylinder and for the most concentrated solutions we will probably have to use two measuring cylinders. As we are conducting the experiment we will decide when to switch to big cylinders. We will make up the different concentrations of acid by using 2M acid and diluting it with distilled water. For example to make a solution with a 30% concentration we will use 30ml acid and 70ml water.
Fair Testing
To ensure a fair test there should only be one variable throughout the experiment, the concentration of the acid. Therefore all other variables which affect reaction rates must be constant. So, the particle size and temperature must be constant. Pressure only affects reacting gases so that does not need to be considered here. No other substances should be put into the reaction which may affect the rate, for example tap water, which may have impurities in it. The volume of the acid solution should always remain the same, 100ml. We will always use 2M hydrochloric acid and distilled water to dilute it.
The rate of each reaction is calculated by working out the gradient of the graph at 20 seconds for each concentration. To work out the gradient you must divide the y value by the x value at a certain point. Therefore you must divide the volume of gas produced divided by 20 seconds. We measured the rate of the reaction for each concentration at 20 seconds because this is near the beginning of the experiment so the concentration of the acid would have been almost the same as what was measured out at the start of the experiment. The table below shows the rate of the reaction for each concentration after 20 seconds.
From my results and graphs I can conclude that as the concentration of the acid increases the rate of the reaction also increases. This is because in a more concentrated solution of hydrochloric acid there are more hydrochloric acid particles than in a dilute solution. Therefore there will be more frequent collisions between hydrochloric acid particles and calcium carbonate particles. If the frequency of collisions increases then the frequency of effective collisions with the necessary activation energy and in the right direction to produce a reaction will also increase. If the frequency of the effective collisions increases then the rate of the reaction increases as well.
This supports my earlier prediction that as the concentration increased the rate of the reaction would also increase. Our results clearly show this: for a 10% concentrated solution the rate of the reaction was 0.0335cm³/s and for a 100% concentrated solution the rate was 3.225cm³/s. However, graphs I produced from our results undermine my prediction. The graph showing the volume of gas collected versus time showed the reaction was faster at 60 seconds than at 20 seconds because the graph was steeper at 60 seconds, but I predicted it would be faster at the beginning. Carbon dioxide gas, which was produced from the reaction, is soluble in water. So at the beginning of the reaction the carbon dioxide may have jut dissolved into the water instead of collecting in the measuring cylinder. This may explain why the reaction was slower at the beginning and got faster as the water became saturated with carbon dioxide. It may also be because the marble chips got smaller through the reaction. The final graph, showing the rate of the reaction versus the concentration, was actually a curve whereas I predicted a straight line. I thought the rate of reaction would increase steadily, however in our experiment the increase in rate rose as the concentration increased. This may once again be because the carbon dioxide dissolved into the water until it was saturated, but in the higher concentrated solutions there was less water for the carbon dioxide gas to dissolve into, so it collected in the measuring cylinder quicker. I did not take this into account when I formed my prediction.
Overall I think our experimental procedure was quite good, although our results did not quite follow the trend we expected from our background knowledge and other sources. These inaccuracies may have been caused by things which we could not control. The experiment was carried out over several different days. The room temperature may have changed on the different days and caused a change in the rate of the reaction. We used marble chips for our experiment but these chips were different sizes and would have therefore had different surface areas so this would have affected the reaction rate as well. It was also very difficult to measure the marble chips to 10 grams exactly using a top pan balance. Therefore there were slight inaccuracies each time we did an experiment. It was very hard to push the bung into the conical flask immediately, so some gas would have escaped. Some gas was also lost because for the higher concentrations we had to change the measuring cylinders collecting the gas when they were filled up. The reaction rate should have been faster at the beginning because the concentration of the acid would be higher at first before it gets used up in the experiment. However, from our graphs, it appeared the reaction was faster at 60 seconds than at 20 seconds. As I explained before in my analysis it was probably because the carbon dioxide, which was produced from the experiment, dissolved in the water in the solution, and only some of the gas produced collected in the measuring cylinder. If we had used calcium carbonate powder rather than marble chips we would have virtually eliminated the problem of different sized particles and it would have been far easier and accurate to weigh. We could also have used bigger measuring cylinders to collects the gas produced from the reactions with more concentrated acid. There is no way we could have prevented the carbon dioxide dissolving in the water unless we used a lower volume of solution so less water would be used. There was one anomalous result when the concentration was 90%. This is indicated on the graph showing the rate of the reaction versus concentration. The actual point shows the rate was higher than the line of best fit indicates. This may have been because of an inaccuracy when weighing the marble chips, or we may have measured the acid and water inaccurately. Apart from this all our results seem to be quite accurately measured and recorded as they are all either on or very close to the line of best fit. Overall the results are sufficient to support a firm conclusion that as the concentration increases the reaction rate also increases. However, due to all the problems I explained above they are not reliable enough to support a conclusion that the rate of the reaction accelerates as the concentration increases. I should like to carry out other experiments which may provide more reliable evidence to support or undermine this later conclusion, for example using powdered calcium carbonate and larger measuring cylinders. It would also be interesting to see how long it takes for the reaction to finish and whether the rate of the reaction for each concentration would increase until the reaction stopped.
