In order to make my experiment fair I will ensure that all of the variables, apart from my independent variable (concentration of the acid), are kept the same. I will do this by testing them each time I do the experiment. I will make sure that the length of magnesium ribbon is kept the same and that the volume of acid I use is the same. I cannot test the exact temperature of the acid each time I do the experiment as this would take too long, therefore all of my experiments will be conducted at room temperature.
I will change my independent variable, which is the concentration of the acid, at least 5 times to ensure that I have a wide range of results.
Preliminary Work.
Before I can begin my main experiment I am going to do some preliminary experiments. I will use these experiments to decide what length of magnesium I will use in my experiment and what volume of acid I will use.
The first piece of preliminary work I will do is to find what length of magnesium ribbon to use. I will do this by picking a range of possible lengths and timing their rate or reaction with the highest possible concentration of ‘x’ which is 2mol/dm³ and the lowest possible concentration, which is 0.5mol/dm³. This will enable me to see which length of magnesium ribbon reacts the best with the extremes of the concentrations of acid.
Apparatus
1 beaker
1 burette
20cm of magnesium ribbon
1 digital stop clock
200ml of acid with a concentration of 2mol/dm³
Pure water
Method
Measure out 20ml of acid using a burette.
Cut the magnesium ribbon into strips of different lengths starting at 1cm and going up by 0.5cms to 3cm.
Make sure that the stop clock has been reset
Put the first length of magnesium ribbon into a beaker with the 20ml of acid. Press start on the stop clock as you do this.
When the magnesium ribbon has completely dissolved in the acid press stop.
Record how long it took for the magnesium ribbon to dissolve
Repeat this with the other lengths of magnesium.
Results
Now that I have the results for the rates of reaction of different lengths of magnesium in the acid of concentration 2mol/dm ³ I will do the same in the acid with a concentration of 0.5mol/dm ³.
Use the same method as used for the concentration of 2mol/dm ³
To use an acid with a concentration of 0.5mol/dm ³ you have to dilute the acid of concentration 2mol/dm ³.
To dilute the acid to 0.5mol/dm ³ you use the formula:
0.5 Χ Volume of acid
2
This gives you the answer of 5.
This tells us that of the 20ml volume of acid you dissolve the magnesium in, 5ml needs to be of acid, and the remainder is 15ml of water.
Now that you know get a concentration of 0.5mol/dm ³ you use it to find the rates of reactions of the different lengths of magnesium ribbon.
Results
From my results of the rate of reaction of different lengths of magnesium ribbon in concentrations of 2mol/dm ³ and 0.5mol/dm ³, I can see that the best length of magnesium ribbon to use is 1.5cm. This is because it didn’t react to quickly with the higher concentration and wasn’t too slow to react with the lower concentration.
I was also going to do a preliminary experiment to decide what volume of acid to use in the experiment but couldn’t do so as I ran out of time. I will therefore use 20ml of solution because this volume seemed to work well in my preliminary work into the length of magnesium ribbon.
Prediction
It is impossible for me to predict the concentration of ‘x’ but I can predict how I think changing the concentration of the acid will affect the rate of reaction.
Particles need to collide in order to react. The rate of reaction depends on how often and how hard the reacting particles collide with each other. In my experiment I will be changing the concentration of the acid. I know that if the concentration of a solution is made more concentrated it means that there are more particles of reactant between the water molecules. This makes collisions between the particles more likely. If the particles are more likely to collide, then they are more likely to react. Therefore an increase in concentration causes an increase in reaction.
I therefore predict that as I dilute the concentration of the acid, the rate of reactions will become longer because there will be less particles colliding and therefore reacting.
Finding the Concentration of ‘x’.
Apparatus
1 beaker
2 burettes
Magnesium ribbon
Hydrochloric acid with a concentration of 2mol/dm ³
1 digital stop clock
Pure water
1 ruler
Method
To begin with I need to find the rate of reaction of the acid with a concentration of ‘x’. To do this put 20ml of the acid into a beaker and add 1.5cm of magnesium ribbon to it. Press start on the stop clock, when the magnesium has completely dissolved, press stop and record the time it took. Repeat this twice and find the average rate of reaction.
Now that I know the rate of reaction of the acid of unknown concentration I have to find the rate of reaction of the concentrations between 0.5mol/dm³
and 2mol/dm³.
Begin with a concentration of 0.5mol/dm³. Find this concentration using the formula: Concentration of acid needed X Volume of solution
2
Using the formula I found that a concentration of 0.5mol/dm³ needs 5ml of acid and 15ml of water.
The next step is to add a magnesium strip of length 1.5cm to the solution, pressing start on the stop clock as you do so. When the magnesium has dissolved completely in the solution press stop. Record the result in a table. Repeat twice and find the average rate of reaction.
Now repeat these steps changing the concentration of the acid each time. Use concentrations ranging from 0.5mol/dm³ to 2mol/dm³. Find the concentrations using the formula.
Results.
Now that I have a set of results for both the rate of reaction of ‘x’ and the rate of reaction of different concentrations I am going to plot the results on a graph. This will let me compare the rate of reaction of ‘x’ with the known concentrations and see if there is an area I need to zoom in on. Figure 1 shows my graph.
p.t.o.
The graph in figure 1 compares the rate of reaction of ‘x’ with the rate of reaction of known concentrations. It shows me that the concentration of ‘x’ must lie between 0.5mol/dm³ and 1mol/dm³.
I am therefore going to zoom in on this area to see if I can find the exact concentration of ‘x’.
Firstly I need to decide on the range of concentrations I will use. I know that it needs to have values between 0.5mol/dm³ and 1mol/dm³. Therefore the values I will use are: 0.5mol/dm³, 0.6mol/dm³, 0.65mol/dm³, 0.7mol/dm³, 0.75mol/dm³, 0.8mol/dm³, 0.85mol/dm³, 0.9mol/dm³, 0.95mol/dm³ and 1mol/dm³.
I then have to find the rate of reaction of each of these concentrations with a magnesium strip of length 1.5cm. You find the different concentrations using the same formula as before.
Repeat the experiment with each concentration twice and record the results in a table.
Results.
I am now going to put these results onto a graph along with the result for the rate of reaction of ‘x’. This will allow me to compare the results to see if the rate of reaction of ‘x’ matches up with the rate of reaction of any of the known concentrations.
Figure 2 shows the graph.
Figure 2 is a graph comparing the rate of reactions of known concentrations with the rate of reaction of ‘x’. The graph shows that as the concentration gets higher, the rate of reaction is faster. By using the graph I worked out the concentration of ‘x’ to be 0.73mol/dm³. I did this by finding the rate of reaction of ‘x’ that I know is 289 secs and then following this across until it reached my line of best fit. This then allowed me to find the concentration of the acid. The black dotted line shows how I did this.
Therefore my conclusion is that the concentration of the unknown acid is 0.73mol/dm³
Evaluation.
I found that the concentration of ‘x’ is 0.73mol dm³ this answer is not as accurate as it could be and my results did not give a firm conclusion.
The equipment I used was very accurate, I used a burette to measure out my solutions, this gives a very precise measurement. I also used a digital stop clock that is more accurate than older ones and is easier to use. I also think that my method was quite accurate, but there are still some improvements I could make. I could use a wider range of values that cover those which occur between the ones I picked. This would give me a more precise answer. I could also of repeated each experiment more so that I had a wider range of values to use to find the average. This would also help to make sue that anomalous results don’t affect my conclusion. Figure 2 shows me that there were quite a few anomalous results in my experiment. The anomalous results are circled in green. This are a result of an inaccurate reading or an error made when diluting the acid.
My results do support the prediction I made. I predicted that as the concentration of the acid increased, the rate of reaction would increase. My experiment showed that this is correct because the higher concentrations had a faster rate of reaction than the lower concentrations did.
Overall I think that my conclusion is backed up by my results but could be more accurate if I had more time to take repeat readings and re- do the readings that gave anomalous results.
