Mg + 2HCL (aq) MgCl2 + H2 (g)
The information in the paragraph that follows was researched from a secondary source, which enabled me to further plan my investigation. According to the collision theory, the more concentrated the reaction the greater the number of collisions between reactant molecules. This explains why the rate of reaction is highest when the reactants are initially mixed together (as their concentration are the highest at this time). As the reaction progresses the amount of each of the reactants decreases, hence the concentration decreases also. As a lower concentration means fewer molecules this means on average fewer molecules have activation energy for the reaction. The activation energy (Ea) is the energy each of the molecules must have in order for bonds to break and for the reaction to initiate. In any sort of solution the activation energy is needed to make the molecules of a substance take part in a chemical reaction. If the reaction has the correct amount of energy and the correct direction the collisions increase, therefore making the reaction speed up.
Prediction:
I predict that as you lower the concentration of the hydrochloric acid the speed of the reaction between the substances will decrease. My reason for this is that there will be less hydrochloric acid molecules to react with the magnesium.
Safety Precautions:
- Always wear safety goggles.
- Clear up spilt liquid.
- Keep the experiment away from the edges of the desk.
- Keep others away from the experiment even if they have safety goggles on.
- Take special care with hydrochloric acid.
Fair Test:
- Use 3cm length of magnesium ribbon for each experiment; ideally the mass of Mg should be the same for each experiment but because of insufficient supplies this was not possible.
- Make sure the temperature of the solution is constant.
- Keep the total volume of the solution fixed.
- Wash apparatus when repeating the experiment to prevent cross contamination.
Experiment 1:
Apparatus:
Hydrochloric acid, water, large measuring cylinder, small measuring cylinder, beaker, magnesium ribbon and stop watch.
Method:
- Collect all apparatus.
- Measure 50ml of hydrochloric acid and pour into a beaker.
- Add a 3cm strip of magnesium (mg).
- Start stopwatch when the magnesium touches the acid.
- Record the time taken for the magnesium to totally disappear.
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Repeat with 45ml of hydrochloric acid and 5ml of water (H20)
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Then repeat with 40ml of hydrochloric acid and 10ml water and so on………… (35ml HCL, 15ml H20), (30ml HCL, 20ml H20).
- Repeat whole experiment twice.
Diagram:
Experiment 2:
Apparatus:
Hydrochloric acid, conical flask, rubber tube, beehive shelf, water, bowl and a test tube.
Method:
- Set up apparatus as shown in the diagram on the following page.
- Measure 3cm of magnesium ribbon.
- Put the magnesium in the conical flask with the correct amount of water and hydrochloric acid.
- Start the stopwatch and then stop it when all the water in the tube has been replaced by gas.
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Repeat the experiment with weaker acid. (45ml HCL, 5ml H20), (40ml HCL, 10ml H2O) and so on……………
- Repeat entire experiment twice.
Diagram:
Observation:
Results table of first experiment:
Repeat:
Average:
Results table for second experiment:
Repeat:
Average:
Analysis:
From the results table and graph I can see a decrease in the rate of reaction between the hydrochloric acid and magnesium because of the decrease in the concentration of acid. This is the same as the prediction I had made before. The graph shows that there is a decrease in the rate of reaction as the concentration of acid becomes weaker; this is represented on the graph by the line curving down from a steep point on the graph to a low.
As my planning had shown as the hydrochloric acid became weaker there were fewer molecules with activation energy, therefore this caused fewer collisions with the two substances. The more often the hydrochloric acid and magnesium (Mg) collide the higher the chance of a reaction between them. I feel if you increase the number of particles in the solution, it is more likely that they will collide, hence causing a quicker and larger reaction.
In the reaction when the magnesium touched the hydrochloric acid it fizzed and produced many bubbles, this happened in all the different concentrations but far more rapidly in the concentrations with a larger quantity of acid. Another method which may have had an effect was if the temperature was raised the reaction could have been much faster. As the temperature increases, molecules have more energy and are therefore likely to move around more. This additional movement promotes collisions and more molecules are also likely to have the activation energy, in the case of magnesium and hydrochloric acid, it is the hydrochloric acid molecules that have to have the activation energy because they are the ones that are moving to collide with the magnesium, break it down and produce magnesium chloride.
From this experiment, I can conclude that my prediction was fully supported by my results and the graphs I have constructed.
Evaluation:
The methods I used to prove that the reaction gets faster between the hydrochloric acid and magnesium when the concentration of hydrochloric acid is large, I felt was appropriate for this sort of level.
Firstly I could have been more accurate with timing this is because I did not know fully when the magnesium ribbon had disappeared; there may have been tiny particles still remaining that cannot be seen by the naked eye. This is where technology like a microscope is needed, you can then find out when the reaction has totally finished. I think this could have effected my results slightly.
Secondly, the concentration in the hydrochloric acid given to us in the bottle varied from a low concentration to a high. I tried to control this by only using the same hydrochloric acid from the bottle given to us. This also could have caused a little change in the end result.
Thirdly, the only option given to us was the measurements of the magnesium, which was 3cm. I did not have an aid from a mass balance, which could have caused a difference; measuring the magnesium ribbon also had an effect, which was that the surface area of it could have been larger or smaller than the previous one. If the magnesium consists of a large surface area the hydrochloric acid would have more area to attack it and break it down. If not then the process would take longer because the hydrochloric acid has less room to break down the (mg) ribbon.
Fourthly, the magnesium given to us was very old, because of this erosion had occurred causing the shine to wear off. Instead of the shine the corrosion covered the magnesium, which could have had an effect when the acid was reacting with the magnesium. To cure this problem I would need the aid of an emery cloth, this would get rid of all the corrosion on the magnesium ribbon and make it shiny once more.
Another way I could prove my prediction was through my follow up experiment, this is where you place the beaker with the correct concentration of hydrochloric acid, place the magnesium in; but before doing so record the weight and once finished record again. The difference will show that the mass decreases, which tells us that the reaction has, took place.
Diagram:
Lastly, to further my investigation I could repeat the experiment more times for much more accuracy. I could also test if the temperature of the hydrochloric acid affects the rate of reaction.
Bibliography:
- Website: schoolforchampions.com
- Book: science for GCSE, Graham Hill.