- The surface area of calcium carbonate used in the experiment.
Hydrochloric acid particles can collide only with calcium carbonate particles on the surface of the marble chip. Therefore, if the marble chip is crushed into a powder, there will be a larger surface area and therefore more collisions.
I have chosen the concentration of hydrochloric acid as the variable that I will use for the investigation. The other factors will have to be kept constant in order for the experiment to be a fair test. The temperature shouldn’t vary too much throughout the investigation but it may be difficult to keep the same surface area of the marble chips. To overcome this problem I could grind up the marble chips into a powder or select marble chips of similar size and surface area.
Preliminary Experiment
A preliminary experiment was performed to help determine the set up of the experiment. These were the results gained:
The preliminary experiment helped to determine:
- The concentrations of hydrochloric acid to use.
This is the factor that is to be the variable. The hydrochloric acid that is supplied has a concentration of 4M. I decided to do five runs each with five different diluted concentrations of acid. These concentrations were 4M, 3.2M, 2.4M, 1.6M and 0.8M.
Initially I intended to use 100ml of diluted acid but the stronger concentrations were far too strong meaning that the rate of reaction was too fast to be realistically recorded as shown above in the results obtained from the preliminary work.
It was then chosen to use 50ml of diluted acid. This meant that there wasn’t as much acid so the rate of reaction could now be realistically recorded.
- The mass of marble chips to use.
I decided to use 5g of marble chips accurate to within 1g. 5g of marble chips crushed into a powder was attempted to be used but the rate of the reaction was far too fast due to the large surface area. Therefore, 5g of intact marble chips of relatively the same size and surface area were used but the rate of the reaction was still too fast so 5g of large marble chips of relatively the same surface area were used and this slowed down the rate of reaction considerably so that it was realistically recordable.
The gas released during the experiment was first attempted to be recorded in an upside down measuring cylinder filled with water.
The gas released during the experiment was then attempted to be recorded by attaching a syringe and then as gas was released, the syringe was pushed along a scale indicating how much gas was released during the experiment.
The syringe method was chosen for the experiment because it is easier to set up but just as accurate.
- How long the experiment should last.
The experiment was initially to last ten minutes with the amount of gas collected recorded every one minute. However, the results went off the scale before the ten minutes were over. Therefore, it was decided to record the time for every 10cm3 of gas that was collected. The scale goes up to 100cm3 so 10 sets of results would be collected.
Apparatus
The apparatus used in the experiment was:
Conical flask, glass tube with bung, syringe, clamp, stand, marble chips and 4M hydrochloric acid.
Method
The apparatus was set up as shown in the diagram. 5g of large marble chips with relatively the same surface area were weighed out to within 1g accuracy and placed into the conical flask with 50ml of the diluted acid as decided in the preliminary experiment. The bung was then immediately put onto the conical flask and the stopwatch started. This meant that the gas produced from the reaction travelled along the glass tube to the syringe where it pushed the plunger along the scale indicating how much gas was released. As in the preliminary experiment, the time was recorded for every 10cm3 of gas that was collected. The scale goes up to 100cm3 so 10 times were recorded.
This experiment was performed with the 5 concentrations of acid that were chosen during the preliminary experiment:
Each concentration of acid was experimented with twice to get two sets of results to average together in order to eliminate any anomalous results.
Prediction
I predict that the higher the concentration of hydrochloric acid, the quicker the rate of the reaction will be. This is because in order for the calcium carbonate and hydrochloric acid particles to react together they must collide with each other. The higher the concentration of acid, the more particles of acid there are in the solution. Therefore it is more likely for a successful collision between an acid particle and a calcium carbonate particle to occur and so the rate of reaction will be higher.
I also predict that the reactions with all the concentrations will start off quickly and then slow down as time goes on because as the reaction occurs, more and more acid particles and calcium carbonate particles will react together, reducing the number of these particles. With there being fewer particles to react, collision between these particles will become less and less frequent so the rate of reaction should decrease as time goes on.