For example, reacting a substance with 2M (2 moles per litre/2 mol.dm-3) acid will double the speed of reacting the same substance with 1M acid.
- SURFACE AREA increases the rate of reaction.
Breaking a solid up into smaller pieces will increase its surface area. This will expose more particles to the solution, and will mean that more successful collisions will occur, therefore increasing the rate of reaction.
This picture illustrates the above concept.
- Using a CATALYST increases the rate of reaction.
A catalyst is a substance that speeds up a reaction without taking part in the reaction itself. It works by providing a surface for the reacting particles to stick to, where they can collide. None of the catalyst is used up, and this method increases the rate of reaction by lowering the required activation energy.
My Experiment
I have decided to do an experiment to investigate how concentration affects the rate of reaction.
Method:
-
Take 20cm3 of 2M hydrochloric acid in a conical flask.
- Set up a clamp and stand to hold a gas syringe.
- Collect 3 large (9-12mm diam.) marble chips (calcium carbonate) and record the mass.
- Place the marble chips into the conical flask, place the bung of the gas syringe on it and start the stopclock.
- Check and record the amount of carbon dioxide being released into the gas syringe every 15 seconds for 3 minutes.
- Do the same experiment for:
16cm3 of hydrochloric acid and 4cm3 of water
12cm3 of hydrochloric acid and 8cm3 of water
8cm3 of hydrochloric acid and 12cm3 of water
4cm3 of hydrochloric acid and 16cm3 of water
- Now repeat each of the above experiments.
List of apparatus:
- Conical flask
- Gas syringe
- Clamp and stand
- Measuring cylinder
- Stopclock
- Marble chips (calcium carbonate)
- Hydrochloric acid
Diagram of apparatus:
Safety
There are a number of safety points to consider while doing this experiment:
- Wear goggles at all times to avoid splashes into the eye
- Tie hair back to prevent it getting in the way
- Clear up spillages immediately to ensure that nothing is damaged
- Place apparatus in the middle of the bench to avoid anything being knocked or spilt
Fair Test
There a number of points to consider to ensure that the experiment is a fair test:
- Keep temperature constant
- Keep size of marble chips constant
- Only change volumes of hydrochloric acid and water, using appropriate measuring cylinders
-
Keep volume of ‘acid + water’ constant – 20cm3
- Measure accurately
- Keep gas syringe perpendicular to conical flask
Prediction
I predict that with increasing concentration, the rate of reaction will also increase.
The evidence for this prediction is the Collision Theory. This states that concentration (also known as pressure) increases the number of reactant particles in a given volume, which makes collisions between reactants more likely. This will consequently increase the number of collisions, thus increasing the rate of reaction.
Also, previously in Science, I have done a similar experiment using two concentrations of hydrochloric acid. The results are given below:
This is also evidence for my prediction and is a secondary source of information.
Observations
My first experiment:
My repeated experiment:
All of my results fit the expected pattern. Therefore, I will not repeat any of the experiments further. I did not obtain any anomalous results (results that do not fit the pattern). However, if I had, I would have repeated the specific experiment and hopefully obtained a better measurement. If any anomalous results had occurred, they could be caused by a number of factors: human error in measuring concentration, timing the experiment, or measuring the amount of carbon dioxide; an alteration in room temperature; or an alteration in the temperature of the tap water.
Analysing
A graph of my first experiment:
A graph of my repeated experiment:
From my results and graphs, I can see that as you increase concentration, the rate of reaction also increases. This is because concentration increases the number of reactant particles in a given volume, which makes collisions between reactants more likely. This will consequently increase the number of collisions, thus increasing the rate of reaction.
All of my results proved this fact. There were no anomalous results, meaning that the pattern formed through this experiment is valid. My background knowledge and the principles of the Collision Theory also support this statement.
My results agreed with my preliminary prediction; that increased concentration causes the rate of reaction to also be increased.
Evaluation
My experiment was designed to investigate the effect of concentration on the rate of reaction. It was relatively straightforward, as it used a simple method and basic equipment. The results all fitted the expected pattern, even though this use of a simple method could lead to errors. Using a more sophisticated method and more complex equipment would reduce the chance of errors, and therefore reduce the chance of anomalies occurring. Anomalies can be caused by a number of things: human error in measuring concentration, timing the experiment, or measuring the amount of carbon dioxide; an alteration in room temperature; or an alteration in the temperature of the tap water.
Further tests using different apparatus or different methods could be used to verify the results, and hopefully eliminate the chance of any anomalies. One way of doing this could be to use a mass balance instead of a gas syringe and bung. This would, however, increase the chance of human error, as an extra calculation would be required to establish the amount of carbon dioxide lost from the conical flask. An alternative method could be to use magnesium and a mass balance instead of calcium carbonate and a conical flask.
I could improve my experiment by doing a number of things:
- Change the timescale over which I conduct the experiment. This would give a wider picture of the overall trend of the results.
- Use a different acid for conducting the experiment. This would help to determine whether my conclusion is appropriate for all acids, and therefore, more rates of reaction.
- Repeat the experiment more times. This improves accuracy, emphasises anomalies and helps to give a wider picture of the overall trend.
The secondary sources I have used are my previous experiment results in Science (page 7), and various diagrams from ‘EDEXCEL Modular Science: The Revision Guide – Book One’ (pages 3 and 4).
