I used varying lengths of Magnesium strip for the preliminary test; 2cm length, 4cm length and 6cm length.
However I decided to use only the 2cm length in actual experiment to prevent wasting the magnesium strip also the 2cm strip was the quickest giving me more time in the real test.
Prediction
I predict that the higher the concentration of the hydrochloric acid the faster the reaction. I also predict that the rate of reaction should be directly proportional to the concentration of the hydrochloric acid used i.e. if the concentration of the hydrochloric acid doubles so will the rate of the reaction as seen in the diagram below.
The equation for this reaction is as follows:
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Increasing the concentration of the hydrochloric acid decreases the time taken for the reaction to end. The reaction gets faster as the concentration increases. This is because in a reaction with low concentration the particles are spread out and therefore there is a less chance of the particles colliding with each other resulting in fewer successful collisions.
In high concentrations there are more particles closer together therefore there is an increased chance of collisions. There are more successful collisions and therefore the reaction is much faster.
Low concentration High concentration
Other factors which affect the rate of chemical reaction are as follows:
- Temperature- higher the temperature faster the reaction as the particles have more energy to collide with each other.
- Catalyst- Adding a catalyst brings the reactants together quickly and therefore speeds up the reaction.
- Surface area- using smaller particles fives a large surface area and makes the reaction happen faster.
- Pressure- In case of a gases increased pressure brings the particles closer together and therefore increases the reaction rate.
Method
Apparatus List:
- 100ml Beaker
- Measuring cylinders: 10ml and 100ml
- 2 molar Hydrochloric acid
- Water
- Magnesium Strip (2cm)
- Stop Watch
- Ruler
- Scissors
- Goggles
Step by step guide
- Put on goggles and tie back any long hair as well as other safety precautions.
- Cut the magnesium strip in 2cm sections using the ruler, 15 strips should be cut altogether.
- Measure 50ml Hydrochloric acid using a 50ml measuring cylinder and pour accurately each time into the 100ml beaker. This is 100% concentration of the acid.
- Drop the magnesium strip (2cm) into the solution while starting the stopwatch simultaneously as the magnesium falls into the solution.
- When the magnesium strip has completely disappeared stop the watch
- Record the time taken for the Magnesium strip to disappear.
- Repeat steps 2 to 6 changing the concentration of the acid using the table below
- repeat each experiment 3 times for each concentration to check the reliability of results
Safety- Goggles must be warn, because the hydrochloric acid us harmful and must also be handled very carefully. Work in a safe manner.
Fair testing
- Used the same volume f hydrochloric acid for each reaction (50 ml).
- Used same length of magnesium strip (2cm) for each test.
- Same width of magnesium strip, keeping the surface area the same.
- All tests carried out at room temperature.
- Same equipment was used and washed.
- Different concentrations of hydrochloric acid were prepared without contamination.
Reliability
Each test was carried out 3 times to make sure some results gained were not too inaccurate.
The experiment was carried out using 5 different solutions as follows: 100%, 80%, 60%, 40% and 20%.
Results
The results clearly show that as the concentration of the hydrochloric acid increases (from 20% solution to 100%) the time taken for the reaction to be completed decreases. This is as I predicted because the more concentrated the solution the more particles are present leading to more effective collision and a faster reaction.
My results had 2 anonmolous results in them, these were at the 20% concentration and the second reading was not accurate (799 seconds) and at 60% concentration the first reading (212.5 seconds) was also inaccurate, both of these readings were excluded when calculation the average for the results.
From the graph my results show that the initial reaction is very fast as the line of best fit declines rapidly a steep graph shows a fast reaction. Towards the end of the reaction the graph curves and becomes flat showing the reaction is coming to an end as the reactants have got used up, again this is as I had expected. There is a positive correlation between the concentration of hydrochloric acid and the rate of reaction.
This supports my prediction: More ions = more effective collisions = faster reaction
Less ions = fewer effective collisions = slower reaction
From my results I cannot see a direct proportion between the concentration of HCl and the rate of reaction e.g.
40% = 225.50 seconds
80% = 37.30 seconds
If they were directly proportional to each other the time taken at 80% should be half of the time taken at 40% and vice versa.
Evaluation
Overall I think I conducted this experiment as fair as I could have done, considering the time and apparatus I acquired. The controlled variables were kept as equal as they could be and the measurement/volumes kept the same, the apparatus was also washed to prevent contamination or assist in any test. The results I achieved supported my simple prediction (higher concentration, faster reaction time); however they did not support my complex prediction i.e. direct proportion. I got 2 anomalous results but they were not included when calculating the average, I may have got these 2 results for several reasons e.g. fluctuation in the room temperature.
The test could have been carried out using an alternative method, whereby the volume of the hydrogen gas produced could be measured against the time taken at different concentrations.
Some variables could not be controlled due to the environment the test was carried out in e.g. temperature. Furthermore human error could have also affected the test e.g. measuring the solution or timing the reaction. To improve my experiment I could have used more ranges of concentrations e.g. 5%, 10%, 30% etc. I also could have investigated different sizes of magnesium or different surface areas. I could have also used different quantities of the solutions instead of just 50ml such as 120ml, 150ml.
I could also compare this reaction with another one such as sodium thiosulphate; using sodium thiosulphate and hydrochloric acid. Change the concentration of the sodium thiosulphate solution and time how long it takes for an image of a cross drawn under the flask to disappear due to sulphur being precipitated. Another test is using marble chips and acid, measure how long it takes to collect a certain volume of carbon dioxide. A burette could have been used to measure the volume in place of the measuring cylinder.
Overall though I feel this experiment went fairly well and am pleased with my results.
