- Gas Syringe (100 cm 3)
- Rubber Tube
- Conical flask
- Measuring Cylinders (30ml) (x2)
- Measuring Beaker (250ml) (x2)
- Clamp Stand
- Stop Watch
- Syringe (20ml). This is to get as accurate results as possible.
- Scissors
I have carefully selected these instruments to enable myself to acquire the most optimum set of results possible.
Diagram
Key Factors And Variables
In my experiment my variables, which I will control, will be: keeping the volume of liquid, which would be 25ml, the same in all experiments to keep the experiment as fair as possible. I will keep the length of the magnesium ribbon the same to get as accurate results as possible. The concentration of hydrochloric acid I will try and keep at the desired concentration so I will keep the number of anomalies to a minimum.
Key factors also include the room temperature. The temperature of the room will always change and never be exactly the same. This means it will have an affect however small on the experiment. If the temperature is higher then the length experiment will be quicker than if the temperature is lower this will obviously mean the experiment will be slower.
I will try and keep the mass of magnesium the same by keeping the length 10cm’s. This should also keep the surface area almost the same through out the experiments. I cannot control the surface area as well as the mass because the magnesium ribbon will not always have the same thickness throughout its 10cm length. I could control the mass better by using digital scales to keep the mass of each piece of magnesium the same but I chose to control the length of the magnesium and not the mass. Although slight changes in mass could result in anomalous results.
Prediction
I predict that as the concentration of solution increases the rate of reaction increases. This happens according to the collision theory, which states that the rate of reaction depends on how hard and how fast reacting particles collide with each other. If the solution is made more concentrated it means there are more acid particles of the reactant colliding with the magnesium ribbon which makes collisions between the molecules more likely. Hydrogen chloride particles dissociate in water into H+ and Cl- ions. The Cl- ions are ‘spectator’ ions as they have no part in the reaction.
2H+(aq) +2Mg(S) = H2(g) + 2Mg+ (aq))
If I double the concentration of acid, say, from 0.4M to 0.8M then I would expect to see the rate of reaction doubled. This is because there are twice as many acid particles so there would be twice as many collisions between the magnesium particles and the acid particles resulting in the rate of reaction being doubled.
If magnesium is the limiting factor then the volume of gas given off in each experiment should be the same. I predict my most accurate results will be the 0.2M and 0.4M solutions because the rate of reaction will be slower. Gas will be given off slowly and not as quick as the higher concentration solutions. The graph below indicates what I think the graph should look like.
According to my CGP revision guide the more reactive the metal (in my case magnesium ribbon) the faster the reaction will go. The quicker the reaction the more hydrogen gas will be produced. Therefore in the higher concentrations the gas produced will be greater than the gas produced in the more diluted concentrations.
Results
These are the results I took during my experiment. They are set out in an easy to read clear table as shown below. The averages are shown in red.
The table shows as I increase the concentration of the hydrochloric acid the rate of reaction increases. The graph shows the volume of hydrogen gas given off with time taken. The eventual amounts of gas given off are not at the same level. In concentrations 0.4 to 1M the amount of gas given off is very similar and the reaction looks like it had stopped. The differences in volume are likely to be due to small variations in the original mass on Mg added. The 0,2 M concentration had probably not stopped but it is also possible this reaction was limited by the reduced amount of HCl. They are not at the same level because I did not wait for the reaction to totally stop. If I had waited for the reaction to stop I would of expected the graph to look like the second graph below. This is because the amount of magnesium is not my limiting factor but the amount of hydrochloric acid is. If I had left the reaction going I would have expected 0.2M dm –3 to last for 540 seconds This explains why the end amount of hydrogen gas was less and I therefore think my results look adequate. All the results look in the region I would expect them to be in. Such as the 1M (most concentrated solution) finished in the high 90’s which would be normal.
My graph below shows the results I obtained well.
Analysis
From my results I can say that the higher the concentration of the solution the faster the rate of reaction will be. My lines on my graph are smooth and going through all of the points which show no anomalous results. They don’t all finish at the same level either because I did not let the reaction finish (because the reaction would have taken about nine to ten minutes to complete) or because there was insufficient hydrochloric acid to react fully with the Magnesium.
My graph clearly shows that as the concentration of the solution increases so does the rate of reaction.
My results do not follow my prediction. Here is an extract form my prediction: “If I double the concentration of acid, say, from 0.4M to 0.8M then I would expect to see the rate of reaction doubled. This is because there are twice as many acid particles so there would be twice as many collisions between the magnesium particles and acid particles resulting in the rate of reaction being doubled”. I basically said, “If I double the concentration the rate of reaction will be doubled ”. When you look at my results for 0.4M and 0.8M it is quite clear that when the concentration is doubled the volume of gas given off at 10 and 20 seconds in both experiments is greater then predicted. After 20 seconds the reaction is getting close to completion in the 0.8M case. The results I have highlighted below correspond to my point above about my results not agreeing with my prediction. Any of the 0.8M results should be about double of what the 0.4M results according to my prediction are and they are not. This could be due to an observational error or could be due to an effect I do not understand.
From 0-10 seconds the reaction rate is more than doubled from 0.4M to 0.8M because 0.8M is 3.357 times quicker than 0.4M.
From 10-20 seconds 0.4M is 3.076 times slower than 0.8M.
From 20-30 seconds the rate of reaction is how predicted because it is doubled.
From 30-40 seconds 0.4M is 1.653 times slower than 0.8M
From 40-50seconds 0.4M is 1.359 times slower than 0.8M.
As you can see from the calculations above in the earlier stages of the 0.8M experiment the rate of reaction was more than doubled.
My graph shows a positive trend because as time decreases the volume of hydrogen given off increases. I have added best fit lines to the graph.
From these results I am able to draw my first conclusion that concentration increases the rate of reaction also increases due to increasing particle movement of the magnesium and hydrochloric acid therefore collisions are more likely.
Evaluation
As the concentration of hydrochloric acid increases the rate of reaction increases. My evidence is consistent with no obvious anomalies. I think I got no anomalous results because the experiment I carried out was simple and little could go wrong.
I could make a number of improvements to my experiment to acquire even more accurate results. I could of taken more results to get an even accurate average. I could do more in between results. For example I could do concentrations such as 0.1M, 0.2M and so on to
2M covering a wider range of concentrations.
My results are accurate and enable me to come to a reliable conclusion that as concentration of hydrochloric acid the rate of reaction increases.
Changing one of my variables could lead to an increase in reliability, I could change the mass of magnesium which would result in a larger amount of gas given off. I could increase the surface area of magnesium, Increasing surface area should increase the speed that the hydrogen gas isgiven off as well. I could use better equipment in my experiment. I could change the gas syringe or one with clearer readings and one that shows all the readings going up in 5cm 3 instead of one going up in 20cm 3.
