A chemical reaction is the process by which atoms or groups of atoms are redistributed, resulting in a change in the molecular composition of substances. The reaction rate is how quickly a reaction occurs and this can be affected by four different factors: ¡¤changing the surface area of any solids involved
¡¤changing the concentration of reactants in solution
¡¤changing the temperature at which it is carried out
¡¤the use of a catalyst
I am going to investigate into exactly how one of these variables affects the reaction rate and I believe the most suitable variable to change is the concentration of the solution as all necessary equipment is available within the school chemistry lab, and five results can be obtained easily and within the time allowance. I will be using the following reaction in my investigation:
Calcium carbonate + Hydrochloric acid Calcium chloride + Carbon dioxide + Water CaCO3 (s) + 2HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
I have chosen to use these substances because research has shown that they react well together. This will mean that I will be able to have quick and efficient experiments.
Aim: I wish to test how the concentration of the hydrochloric acid affects the rate of a reaction
In order to fulfil this aim I have to come up with a suitable method that will produce reliable data so that a valid and accurate conclusion can be drawn. Carbon dioxide seems an obvious product from the reaction to use to calculate the rate, as it is common scientific knowledge that the faster the reaction, the more carbon dioxide should be produced. The test for the presence of carbon dioxide is that it turns transparent limewater cloudy and I could use this idea to base my experiment on. Previously we have studied the effect of changing the concentration of a solution on the reaction rate between sodium thiosulphate (Na2S203) and dilute hydrochloric acid (HCl), where we placed the reaction vessel over a pencilled cross drawn on a piece of paper. We then timed how long it took before we could no longer see the cross as the reaction had caused the mixture to turn cloudy. I believe this method could be adapted for my experiment; however I would have to use two conical flasks; one where the reaction would take place, and one containing the limewater with a delivery tube connecting the two. Bungs in the conical flasks will prevent the carbon dioxide escaping. It is essential that all other independent variables except the concentration of the solution remain constant throughout the investigation so that the experiment is fair and an accurate conclusion can be drawn. If more than one independent variable is changed at a time, it is impossible to work out which one is causing the change in the reaction rate. Therefore the size of the calcium carbonate, amount of hydrochloric acid and the temperature at which the investigation shall be carried out at shall be kept constant throughout my experiment. I have decided to use 50ml of acid but it should give me a relatively quick reaction, so the experiment will take only a short amount of time. For the same reason I shall use 10ml of limewater so it does not take very long to turn cloudy. The experiment will be carried out at room temperature which is approximately 20¢ªC and I shall use small marble chips of 2-5ml. I shall test 5 different concentrations of hydrochloric acid and these are 0.5 molar, 1 molar, 2 molar, 3 molar and 4 molar. To enable me to gain reliable results, I shall test each concentration three times and then take an average of these results to plot on a graph. This should hopefully give me a clear result of what effect the concentration of acid has on the reaction rate. Before my main experiment I will perform a trial experiment to...