Variables
A variable is anything which affects the results of an experiment, for example, the variable could affect the speed the particles move or the amount of energy needed in a reaction. For this experiment, there are many variables we could use. For example:
Concentration of acid: This would affect the speed of the reaction. This would happen due to the fact that is the concentration is high in a substance, or the amount of reactant particles is high, more collisions will occur.
Surface area of marble chips: If the surface area, not the mass, of the marble chips is large, which means that the pieces are very small, more reactant particles will be available for the reaction.
Temperature: The speed of a reaction can depend on the temperature at which the reaction takes place. If the temperature is high, more kinetic energy will be given to the reactant particles so they will move faster and with more force, increasing the probability of a collision and the speed of the reaction.
Use a catalyst: A catalyst something which is added to a chemical reaction that stays chemically unchanged throughout the reaction. The catalyst combines with a reactant, making it more ready to react with other reactants. This means that the acceleration of the reaction is increased but the end product is not any different to the same reaction without a catalyst.
The variable we have chosen to use is Concentration of acid.
Apparatus
I will use all of the following apparatus in my experiment:
- Water
- Stop watch
- Conical flask
- Calcium carbonate chips
- Measuring cylinder
- Water tank
- Glass tube
- Hydrochloric acid
Method
In this experiment I will keep the temperature constant, at 24ºC. I will keep the size and mass of the chips constant, at 3 grams. I will keep the volume of the acid at a constant 50ml. Finally I will not add a catalyst. The only variable will be the concentration of the acid. We will perform this experiment in the following steps:
1. I will the tank with water.
2. Fill the measuring cylinder with water, making sure that there are no air bubbles present.
3. Turn the measuring cylinder upside down, making sure that the open end is in the water at all times so no water escapes.
4. Add the marble chips to a conical flask containing 50ml of acid (at the chosen molar for this experiment)
5. Link the flask containing the reactants by placing it under the measuring cylinder and in the bung at the top of the conical flask.
6. As the Co2 is produced, the water will be forced out of the cylinder and we will be able to measure the speed of the reaction.
7. Repeat 3 times, recording the volume of CO2 every ten seconds.
8. Work out mean average for every 10 seconds with the acid at 1, 0.8, 0.6, 0.4 and 0.2 molar.
The final reaction should look something like the below diagram:
Measuring cylinder
Water glass tube
CO2 HCl
Tank Marble chips
Results
1 molar 0.8 molar
Analysis
At first glance the lines seem to have a high correlation meaning an accurate experiment, but to prove that there is a correlation I must try the following test:
If my prediction is true the volume of CO2 must be directionally proportional to the strength of the acid. This means if I take any result of my experiment, double it, and I should have the correct result for an experiment using twice as strong acid. To test this I will choose my result for 80 seconds on 0.4 molar acid. The volume of CO2 is 38ml. To prove my prediction right the volume of CO2 after 80 seconds using 0.8 molar acid would have to be 76ml or very near.
The volume on my graph is 80ml. Taking the several imperfections (explained below) into account, I can say that my prediction was right.
Evaluation
Though the results are clear enough to prove my prediction and conclusion, there are several ways I could have improved the experiment.
1. I could have made sure the surface area of the chips was always the same as this might have made a small imperfection on my results.
2. The first few seconds of the experiment were inaccurate as a large amount of gas escaped as I was fastening the bung to the conical flask, if I was to repeat the experiment I would need to find a way of adding the chips without the loss of gas.
3. There were some impurities in the water added to the acid which could have affected the results.
4. In all the reactions the amount of result recorded were restricted by the volume of the measuring cylinder and the amount of time set (100 seconds). This means I cannot prove my prediction that after a certain amount of time all the reactants would run out and the reaction would end. If I were to repeat the experiment I would set larger boundaries for the results.
5. The measuring of the CO2 was inaccurate as it took several seconds to record results and the accuracy of the readings was inaccurate. To get an accurate reading the C02 would have to be measured electronically.
Conclusion
From the above results I can say that the concentration of acid is directly proportional to the rate of reaction. This is because there are more reactant particles in a concentrated solution so the possibility of a reaction is increased.
By Joe Howard
