The second factor that has to be kept the same is the temperature. If the temperature is increased the reaction rate will be reached faster. (Although it’s reached faster the outcome is the same if the temperature is lower.) This happens because the heat energy is transferred into the HCl particles, increasing their speed and allowing them to collide with each other much more frequently, meaning there’s more chance of a reaction-taking place.
I predict that the higher the concentration the faster the rate of reaction. After preliminary tests that were carried out, I can back up my prediction by saying that the higher the concentration, the quicker the reaction rate. I have based this on the ‘Collision Theory’. This simply means that for a reaction to occur the reactant particles must collide. Only a certain fraction of the total collisions cause chemical change; these are called fruitful collisions. The fruitful collisions have sufficient energy (activation energy) at the moment of impact to break the existing bonds and form new bonds, resulting in the products of the reaction. Increasing the concentration of the reactants and raising the temperature bring about more collisions and therefore more fruitful collisions, increasing the rate of reaction.
Apparatus
Test Tubes x5
Test Tube Rack
Stopwatch
Magnesium Ribbon (1cm per piece)
Hydrochloric Acid (5 concentrations)
Goggles
Pipette
Diagrams
Method
Set up apparatus as shown on the previous page & remember to use safety goggles.
Fill a test tube with 10cm3 of HCl, at a concentration of 0.5 mol/dm3 and add a 2cm piece of magnesium ribbon.
(Due to preliminary results I have decided to use 10cm3 of HCl as 15cm3 is far too fast and 5cm3 is rather slow, I also think 2cm is a suitable length for the magnesium ribbon, see below for results.)
Quickly, using a stopwatch, record the time taken for the ribbon to react.
Now, repeat the method for the other four concentrations.
Record and present results accurately e.g. using tables and graphs. I also carried out two repeat readings and worked out the averages as these give more accurate results.
Preliminary work
The preliminary work that I will be conducting is to find out the optimum length of magnesium ribbon and the optimum volume of hydrochloric acid.
To do this I will be measuring out a volume of hydrochloric acid and a length of magnesium ribbon and reacting them together. If there is still some magnesium left over when it has stopped effervescing then I will have to increase the volume of hydrochloric acid.
If the reaction takes too long to finish then I will have to shorten the length of magnesium that I use, however if the reaction is too short then I will have to do the opposite and increase the length of magnesium that I use. The optimum rate that I am trying to find is a reaction that isn’t too short but isn’t too long, so I can get enough results to plot a good graph. I need to find the optimum volume of hydrochloric acid so that it is in excess after the reaction is over.
Apart from trying to find the optimum rate of reaction I also have to find out how to keep the temperature change down. This is because as the reaction is taking place the temperature will rise because the reaction is exothermic, and this could cause my results to be inaccurate as the temperature change will heat up the acid and give the acid particles more energy so they will move faster and collide with the magnesium with greater force causing more successful collisions per second.
Preliminary Results
Below are two tables: in the first one we were trying to decide the amount of HCl to use, and in the second we were trying to discover the most appropriate size of Magnesium ribbon to utilize.
In the given time all of the ribbons completely vanished due to the reaction except for the one in the 1cm3 of HCl.
The section highlighted shows inaccurate results compared to the additional results; these could be caused due to several reasons:
When we carried out the experiment, we may have incorrectly cut the magnesium ribbon.
The Mg ribbon may have been dropped into the mistaken test tube, consisting of the wrong amount of HCl.
There don’t seem to be any anomalous results occurring in the table above therefore this experiment must have been carried out correctly
Results
Analysis
From the results in the table and the graph we can see a steady increase in the rate of reaction as the concentration of the acid decreases therefore, the stronger the concentration the quicker the reaction took place. This complies with my prediction, on which I based the collision theory. Particles are more likely to react if they are moving faster and collisions occur more frequently.
The closer together they are, the more often the ions collide. The more often they collide, the higher the chance of a reaction between the magnesium and the hydrochloric acid. Also because there are more particles in the solution which would increase the likelihood that they would hit the magnesium so the reaction rate would increase.
In my prediction, I said that I felt that the higher the concentration, the faster the reaction would occur, and I found this to be true. The graph gives us a good device to prove that if you increase the number of particles in the solution it is more likely that they will collide more often. In the reaction, when the magnesium hit the acid, it fizzed and produced many bubbles it was silver in colour (which is one of magnesium’s physical properties silvery white metallic element), the activation energy of a particle gets higher with heat, the particles which have to have the activation energy are those particles which are moving, in the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles which have to have the activation energy because they are the ones that are moving and bombarding the magnesium particles to produce magnesium chloride.
The only thing I didn’t mention in my prediction is the saturation point. When the same particles collide with each other (HCL) rather than colliding with the particles of the magnesium. Therefore when they start colliding with particles of the same substance the rate of reaction is slowed down.
HCL Particle
Magnesium Particle
Particles of the same substance colliding (Causes the rate at which the particles react to decrease)
Evaluation
Overall I feel that I was rather precise and accurate in recording measurements. Although there were a few inaccurate results. I obtained two anomalous results. The first one being the 0.5 concentration on the first set of results. This could have happened by mistakenly placing the wrong size magnesium into the test tube or cutting the magnesium to the wrong size.
The second error being the average for 0.5. But this was probably affected by the mistake in the first set of results. These errors both occurred in the first run, when my techniques and practical skills had not been perfected over a large number of experiments!
There is also one main factor that could have been controlled better, the temperature. As the weather changes dramatically every day in Wales the temperature often varied, therefore affecting the reaction times. We could have used Bunsen burners etc. to solve this problem.
We could also have carried out more pre-tests to ensure that everything had been thoroughly tested. This would have given us a wider variety of results.
(Encarta: We could have also used different substances. If there were similar results for these materials then we would be certain that concentration does have an effect on the rate of reaction.)
So, this shows us that the collision theory, on which this experiment and my prediction were based, is correct! Concentration does affect the rate of reaction!
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