4. An increase in temperature produces an increase in the rate of reaction. A rise of 10º C approximately doubles the rate of reaction. When a mixture of substances is heated, the particles move faster. This has two effects. Since the particles are moving faster they will travel greater distance in a given time and so will be involved in more collisions. Also, because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.
5. The rates of some reactions are increased by exposure to light. Light has a similar effect as temperature because it produces heat.
6. A catalyst is a substance, which can alter the rate of a reaction but remains chemically unchanged at the end of the reaction. Catalysts usually speed up a reaction. A catalyst, which slows down a reaction, is called a negative catalyst or inhibitor. Catalysts speed up reactions by providing an alternative pathway for the reaction, i.e. one that has much lower activation energy. More collisions will, therefore, have enough energy for this new pathway.
All this information is relevant to my investigation, as I now know what would happen to the molecules when using different variables. It also makes it easier to decide what variable I am going to use in this circumstance.
I decided to use the temperature of acid as my variable. I used 5 different temperatures of hydrochloric acid. These strengths would determine the rates of reactions. I decided to measure the acid in millilitres. I predicted that the higher the temperature of the acid, the faster the reaction between magnesium ribbon and the hydrochloric acid. This would be because the acid molecules be vibrating faster and ultimately reacting with the magnesium ribbon. I decided that I would do 5 experiments and the different temperatures of hydrochloric acids were:
10 degrees C
20 degrees C
30 degrees C
40 degrees C
50 degrees C
60 degrees C
Before doing the actual experiment I decided to do some preliminary work. These were to tell me the details that I would need to know for my investigation to be successful. I saw from these preliminary investigations that the magnesium ribbon started to react with the hydrochloric acid the moment that I dropped it in. I decided that it would be a good idea to start timing the second that I dropped it in. When the ribbon had been eaten away by the acid, it stopped fizzing. I decided that I would stop timing the second that the fizzing stopped. Prior to doing this experiment I did a few other experiments providing me with the information needed to get the best results for my main experiments. These previous experiments involved different variables; I found the optimum for each and applied them in my main experiments, such as the length of magnesium and the amount of acid. The reason for doing this, is to rectify the effect of the heat given off during the reaction. During the previous experiments when I felt the beaker I noticed that it was warm, therefore It must have been an exothermic reaction. Part of the preliminary testing was to eradicate the effect of this, (Getting the best length of magnesium ribbon and the correct amount of acid to stop any excess heat coming off as much as possible.) If too much heat was given off it might raise the temperature too much, to give an accurate result. I had to be very precise with the temperatures.
In my preliminary work I did several experiments involving different;
0.5 cm
1.0 cm
1.5 cm
2.0 cm
20.0 mm 40.0 mm
25.0mm 45.0 mm
30.0 mm
35.0 mm
The magnesium ribbon had been exposed to dirt and treated so that it didn’t react with the air or moisture in the air around it, therefore, it had to be cleaned before it was used, I discovered that there two methods of doing this, either by sanding it down or dipping it in to some hydrochloric acid and then immediately dipping in some water before doing the experiment. I choose the sandpaper method. I discovered that the magnesium when reacting floats on the surface of the acid; therefore I had to stir it to ensure that it wouldn’t stick to the side of the beaker. Because this may of effected the time of the reaction.
The variables that I kept constant were the length of magnesium, (1.5 cm) and the volume of acid (30.0 mm)
I didn’t use a catalyst and I kept the pressure and light conditions the same. I knew that the gas given off during this reaction between hydrochloric acid and magnesium was hydrogen and this is an extremely explosive gas therefore I tried to keep it away from any naked flames. The average human threshold for pain is 60.0 degrees C. (One of the temperatures that I was heating my acid to.) To ensure safety I tried to keep it as far away from naked flames as possible. Instead of using a smaller 50.0mm beaker I used a larger 100.0mm beaker, making less of a chance of anything spilling. To maintain a constant temperature and also ensure further safety, I made a water bath for my beaker of acid. So as not to burn my hands or spill acid on them, I used a clamp from a clamp stand to pull me beaker out of the water bath.I also learnt from my preliminary experiments that it was sometimes quite difficult to stop timing on the exact moment that the fizzing stopped. I decided therefore that I would carry out each of the 5 experiments three times and find the average time as this would result in a more accurate figure.
When the magnesium ribbon reacts with the hydrochloric acid, magnesium chloride is formed. I wrote down the equation to show this:
Magnesium + Hydrochloric acid = Magnesium Chloride + Hydrogen
Mg + 2HCl = MgCl + H
The equipment I needed for the investigation were:
Magnesium ribbon- 15 pieces, 1.5 cm long
Hydrochloric acid – 30 ml of 1.0 molar
Stop clock Ruler Gauze
Pipette Clamp Sandpaper
Measuring Cylinder Tripod Recording equipment
Thermometer Thermometer
Safety goggles Bunsen Burner
I decided to do 5 experiments, three times each, using all the information that I gained while I was doing my preliminary experiments.
To ensure a safe experiment and working environment I needed to have at least 1 meter squared of working space around me, wear safety goggles at all times when using acid, secure all equipment and make sure that all the equipment were fully functional and not damaged.
To make the experiment a fair test I used the same amount of acid for all experiments, only changing the temperature. I used the same size of magnesium ribbon. I also started the stop clock when the magnesium touched the acid and stopped it when the magnesium stopped fizzing for each experiment. I always washed out the test tubes when an experiment had finished so the different concentration wouldn’t get mixed together causing strange results.
First I measured out the amount of hydrochloric acid using the measuring cylinder. I used a pipette to pour the acid into the measuring cylinder as to be accurate. I needed 30 ml of acid in the cylinder. I then put a thermometer into the. I then got a piece of magnesium ribbon 1.5cm long and dropped it into the acid and started timing the moment that the magnesium ribbon touched the acid solution. When the magnesium ribbon stopped fizzing, I stopped the clock and recorded the number of seconds (rounded up to nearest second) taken for the reaction from start to finish.
I made a table to record my results in. The table is shown below.
To calculate the average time that it took for the magnesium to be eaten away by the acid, I did the following calculation:
Test 1 + Test 2 + Test 3 = Average time
3
As I already have mentioned, I used a measuring cylinder to make the measurements and used a pipette for further accuracy. I did each experiment three times so I would be able to calculate averages and thereby get more accurate results. I recorded the results in seconds instead of minutes in order to obtain more precise results. I used a stop clock instead of a 24-hour clock so I could look at the milliseconds and round it up to the nearest second, which made the results more exact.
I made a graph to show the results.
The graph above supports my original prediction of: the higher the temperature of the acid the faster the rate of reaction because it shows the time difference between the different temperatures of acids. In a higher temperature there are more acid particles to react with the magnesium ribbon and therefore it is eaten away faster.
I conclude that changing one factor does have a significant effect on the rate of reaction as we have seen.
Looking at the set of results obtained, you can clearly see that they all follow the expected pattern. This is pattern suggests that the reaction rate increases when the temperature of the acid increases because if you increase the temperature of the acid you are making the particles move faster which will in turn produce a faster reaction because there will be more collisions between the particles which is what increases the reaction rate.
The evidence I have been able to gather from this investigation seems to lead to a quite firm conclusion. I might not have been able to find the exact speed of the reactions but the pattern seems to be correct as I have repeated readings three times and as it agrees with the information I have researched.
I used the variable of temperature, which seemed to be of a good choice as it would show the results of how more acid molecules reacting with magnesium, would result in a faster reaction.
There will always be ways in which you can improve your investigations and the same thing goes to my investigation.
I found it very hard to measure out the exact number of millilitres for the acid even though I used a pipette and I was also in a hurry. If I was to redo this investigation I would put some more effort into measuring the acid. I could have used the wrong concentration of acid by accident and that would have affected the speed because there would have been fewer or more acid particles to react with the magnesium ribbon. Next time I do this experiment I would try to remember which acid I am using so it doesn’t get mixed up. Every time I washed a test tube or a measuring cylinder, I did not dry it before using it. This may have affected the rate of reaction, as water would dilute the acid. To improve my results, I could dry the test tubes and the measuring cylinder after they are washed to prevent diluted acids. The size and weight of the magnesium would have affected the rate of reaction. The experiment could be improved by measuring, adjusting and weighing the magnesium ribbons so they all are the same size and weight.
In my investigation I used temperature as my variable. To improve my investigation further, I could use other variables such as, surface area, concentration, pressure for gas, and a presence of a catalyst.
These variables would hopefully prove that they all help speed up a chemical reaction.