Amount of gas produced in experiment: I could take readings by collecting the gas in a gas syringe, but again a small volume could result in inaccuracies.
However I have chosen to measure:
How long the magnesium takes to dissolve: This way I can achieve accurate results with a safe experiment, which will clearly show any changes to reaction rate, by timing the reaction for each different concentration using an accurate stopwatch, preferably one that readings to 2.d.p.
Key factor: I have chosen to use concentration because I can accurately get different concentrations of hydrochloric acid, by diluting it with water, accurately measured from a burette, which will give me a wide range of accurate results, which are reliable and reproducible.
Hypothesis:
I predict that the higher the concentration is, the quicker the rate of reaction. I have drawn this conclusion from my knowledge of the ‘collision theory’. The collision theory is based on the ides that for a chemical reaction to take place, it is necessary for reacting particles to collide with each other with enough energy to make or break bonds with other particles- a successful collision. If they don’t have enough energy, they will simply bounce away, off each other, which is an unsuccessful collision. By increasing the concentration, the higher the number of acid particles there will be for the magnesium to react with. This will mean a higher number of collisions per second. This will increase the number of successful collisions per second, and therefore the rate of reaction.
This is further explained in this quote from ‘Chemistry in Focus’-
‘ When a solid reacts, only the particles on it’s surface are available for collision for reaction. If the concentration is increased more particles will available for collision. Thus the frequency of collision increases and so does the rate of reaction’
This quote basically sums up my prediction using the ‘collision theory’ idea- that as there are more particles to react on in a higher concentration, the rate of reaction will increase. However, the quote goes on to explain why, in terms on equations, this happens-
rate of reaction = collision frequency x activation energy
This formula shows us why the rate of reaction changes- when we increase the concentration we also increase the collision frequency that will directly affect the rate of reaction, as it is equal to the collision frequency x the activation energy. Using this formula as a basis I think that I can also say that if I halve the concentration I will decease the rate of reaction by half. This is because there will be half as many particles, and therefore less successful collisions. This will decrease the collision frequency and therefore the rate of reaction.
I have also based my prediction on preliminary work involving changing the concentration. In this experiment I looked at the reaction between sodium thiosulphate solution and hydrochloric acid. When they reacted they form a sulphur precipitate, which turns the solution cloudy. We therefore placed a cross beneath the flask and timed how long it took for the cross to disappear, and repeated the experiment using different concentrations of sodium thiosulphate solution:
The results were as follows:
These results back-up my hypothesis because, as shown, as the concentration increases so does the rate of reaction. As these results are reliable, having used accurate equipment and showing a clear pattern with no anomalous results, I think it is reasonable to say that my experiment will have similar results.
Plan:
I intend to react a chosen length of magnesium ribbon with a chosen concentration of hydrochloric acid. I will measure the rate of reaction by timing how it takes the magnesium ribbon to dissolve in the hydrochloric acid. I will proceed as follows:
I shall begin the test by looking at the rate of reaction between a 2cm length of magnesium ribbon and 50cm of hydrochloric acid. Firstly I will fill a conical flask with hydrochloric acid. This should be measured out using the thinnest measuring cylinder available, so that I can get an accurate measurement, to the nearest mm . I will use a thermometer to check that the beginning temperature of the acid. I must record this and the starting temperatures of all the repeats, as temperature will affect the rate of reaction, and mean my results are unreliable. Ideally they will all start at the same temperature. I will then measure out 2cm of magnesium ribbon, using a ruler to achieve an accurate length to the nearest half mm. It is necessary for me to achieve accuracy when measuring out the acid and ribbon, so that I can have the most reliable results possible. I will then clean the magnesium ribbon with sandpaper- this will remove any oxide etc. that has formed on the surface. Allowing that to remain will mean my results aren’t reliable, as not all the magnesium will be able to come into contact with the acid, and therefore react with it. I will then drop the magnesium ribbon into the acid, and, at the same time, start the stop clock. The stop clock will be stopped as soon as the magnesium ribbon has dissolved. The reaction must be watched closely, so that I can stop the watch as soon as the ribbon dissolves. Again this is to make sure the results are accurate. I will then repeat the experiment using weaker concentrations of acid that have been diluted with water. The concentrations will be as follows: -
-(50cm HCl and 0cm water)
-40 cm HCl and 10cm water
-30 cm HCl and 20 cm water
-20cm HCl and 30cm water
-10 cm HCl and 40cm water
Again the magnesium will be measured using a ruler to the nearest half mm and the acid measured to the nearest mm using a measuring cylinder, and then added to the flask. I will then add the water to the flask using a burette, which will allow me to be accurate to half a mm , and achieve more reliable results. I will also mix up the solution, to make sure it is completely mixed up, using a glass rod.
This experiment will give me 5 results, a large enough range for me to achieve an accurate reliable graph. I will then repeat the experiment, to make sure that my results are repeatable and reliable. (As noted in my preliminary work) The results will then be recorded in a table, and then onto a graph to be compared. I will record the rate of reaction using the formula-
Rate of reaction = 1/Time taken for magnesium to dissolve.
Diagram:
Safety: In order to have a safe experiment I will have to do the following thing:
- Wear goggles and lab coat at all times, so I am protected if the acid splashes etc.
- The acid I use will not have a concentration higher than 1M, which will mean I am not handling any acids that are very strong and potentially dangerous.
- Dispose safely all regents and laboratory chemicals.
- Care when returning all glassware etc.
- Care in using equipment -glassware, which would be dangerous when broken, as would the thermometer (mercury) and the acid, which could burn you.
Fair Test: I must make sure that I have a fair test, as otherwise my results would have been effected my other factors, and therefore be unreliable. These are all factors that I must keep the same in order to ensure that I have a fair test:
- Starting temperature of the acid- as temperature effects the rate of reaction it would not be fair if some tests had a higher starting temperature than others, as this would mean that the results would be affected and unreliable. Therefore I must measure the starting temperature of the acid solution before each test, to make sure that they are the same.
- Volume of acid solution- the volume of the acid solution must remain at 50 cm for example: 50cm HCL to 0cm water or 30cm HCl to 20cm water otherwise there may not be as many particles in total, which would change the reaction rate, but not because of the concentration. This would make my results unreliable.
- Surface area and length of magnesium- the length must remain the same i.e.) at 2cm because otherwise the rate of reaction would be affected and my results unreliable.
- Clean magnesium- otherwise not all magnesium would be exposed to acid solution, and this would effect the rate of reaction, as the surface area of magnesium exposed would be different, and my results unreliable.
- Stir the acid solution- so water and acid particles mixed up i.e.) not all one area, and the magnesium particles areable to collide fairly.
Results:
