Apparatus
Method
The apparatus was set up as below.
Stickers were attached to the pipettes, measuring cylinders and beakers; one set was for nitric acid, one for sulphuric acid and the other for sodium hydroxide. 20cm3 of 2M nitric acid was measured out and then 20cm3 of 2M sodium hydroxide, then a 10cm strip of magnesium strip was placed in a boiling tube, folded back halfway. The measured volume of nitric acid was rapidly added to the magnesium and the stopclock started once the nitric acid was fully added to the boiling tube. Once absolutely all of the magnesium had reacted away, leaving a clear liquid, the stopclock was stopped. The measured volume of sodium hydroxide was then added to the boiling tube.
The mixture was then placed in the conical beaker and titrated with 2M nitric acid with methyl red as the indicator, which has a clear colour change from yellow to red, once red has been achieved the alkali has been neutralised and the volume of nitric acid required recorded.
Thus the time taken until the end of the reaction is known. From this judgements were made about when to stop the reaction in order to back-titrate and discover how far the reaction has gone in, say, 30 seconds. If the total reaction took under three minutes then the reaction would be halted and back-titrated every 30 seconds, if longer than three minutes then every 60 seconds. The reaction was halted by adding the measured volume of sodium hydroxide into the reaction of nitric acid and magnesium after the time period had elapsed, say 30 seconds from the start of the reaction. If the total time for reaction was, say, 1:42, then the reaction would be stopped and back-titrated at 0:30, 1:00 and 1:42, the end of the reaction. Note that the sodium hydroxide was added rapidly as soon as the time period had elapsed.
This whole process was then repeated with different concentrations of nitric acid, all other variables are constant. The nitric acid was diluted from its original 2M by the table below.
The entire process was then repeated with sulphuric acid in place of nitric acid.
Then the whole experiment was repeated at 50°C. This was achieved by placing all reactants; acid, sodium hydroxide and magnesium in separate boiling tubes and submersing them in a water bath set at 50°C. There they were left to heat for five minutes, and then the acid added to the magnesium and the boiling tube placed back in the water bath. The titration was performed as before.
Dilution Table
Start with 2M nitric or sulphuric acid, depending on experiment. Always make up to 20cm3. Water used is distilled water.
Risk Assessment
Clearly I will be dealing with strong concentrated acids and alkalis and so care must be taken not to allow contact with skin or eyes. Care must also be taken against spills so as to prevent slip hazards.
Standard lab procedures will be followed, for example clearing away all unnecessary equipment, wearing lab coat and safety goggles.
Justification of Method
The equipment used to handle and measure the nitric acid, sulphuric acid and sodium hydroxide were kept separate to ensure no contamination occurred. As molar calculations will form part of my analysis it is imperative that exact values of reactants are measured, and that substances are not inadvertently mixed.
The stopclock was not started until all of the acid had been added and not stopped until the reaction had finished, or the sodium hydroxide added to the metal-acid reaction until the designated time period had elapsed, to ensure that any error is constant and systematic and that the same conditions were applied in all experiments.
Much thought was put into the method of finding the rate of the reaction, not only over the whole reaction but also after certain time periods, such as after 30 seconds and so on. The method chosen is back-titration, which I have described in my method. This involves stopping the metal-acid reaction before it has completed by adding an excess of alkali (sodium hydroxide), which reacts immediately with all the remaining acid. This solution, which is now alkaline, is then titrated with acid until it is neutral. And so, as all volumes and concentrations are known, the amount of acid used in the titration to neutralise the excess of alkali will then tell us how much excess alkali was present. This will then tell us how much acid was present when the metal-acid reaction was halted. This will tell us how far the reaction has gone.
The experiment was performed with both nitric and sulphuric acid to give a wider range of results and to see how the two acids compare, and if conclusions can be made about magnesium-acid reactions in general, or simply on an acid-by-acid basis. The experiment was repeated at 50°C in order to give a wider range of results and to analyse what effect temperature has on rate. A wide range of concentrations of nitric and sulphuric acid were used in order to analyse the effect of concentration on rate.
ANALYSIS OF RESULTS
Looking first at the tables of results in the appendix, there are many things that we can note. First we notice that whilst the time and rate of each reaction clearly varies the total number of moles of each acid, considering sulphuric and nitric experiments separately, that reacted with the magnesium remains fairly constant. This is of course expected, as the mass of magnesium reacted in each experiment was constant, then a constant number of moles of each acid will react with it, regardless of the concentration, as long as we consider the entire reaction to its end point. Fewer, in fact half, as many moles of sulphuric acid will react with the same amount of magnesium as nitric acid, due to the ratios of the reactions below:
Mg + 2HNO3 ➔ Mg(NO3)2 +H2
Mg + H2SO4 ➔ MgSO4 +H2
HNO3 + NaOH ➔ NaNO3 + H2O
H2SO4 +2NaOH ➔ Na2SO4 +2H2O
EVALUATION
Sources of Error
The 10cm3 measuring cylinders used to measure out the volumes of acid and base were accurate to ±0.1cm3.
The burette used in the titration was accurate to ±0.05 cm3.
There is human error involved in judging the end-point of the titration, though I chose methyl red as my indicator as it gives a very clear colour change and was very careful in my method it is possible that I did not measure the exact end-point. It is also likely that such an error would not be systematic and so could affect the results randomly.
Improvements
If I were to perform this investigation again with unlimited resources, I would first use larger quantities of all reactants in order to reduce possible error and to make conclusions firmer.
I would investigate a wider range of acids, acid concentrations and temperatures in order to draw firmer conclusions about the trends and relationships I have identified.
