Investigate the amount of heat evolved when magnesium reacts with dilute acids.
What I am going to do I am going to find out how much heat is given out when magnesium reacts with a variety of dilute acids. In order to make comparisons between the acids, I shall use my results to work out how much heat would be given out if 1 mole of magnesium reacted with an excess of each acid. The acids I shall use are hydrochloric acid, sulphuric acid, nitric acid and ethanoic acid. What I already know Magnesium reacts with most dilute acids to give a magnesium salt and hydrogen. For example: Mg + 2HCl Mg + H2SO4 MgCl2 + H2 MgSO4 + H2
I discovered from an A' level text book (A-level Chemistry by Ramsden) that nitric acid doesn't usually give hydrogen, but may do with magnesium if the acid is cold and dilute. The book didn't say how dilute the acid had to be. Other products may include oxides of nitrogen. I know that some acids are weak and some are strong. A strong acid is one which ionises fully in solution to give hydrogen ions and, for example, chloride ions or sulphate ions. Sulphuric acid, hydrochloric acid and nitric acid are all strong acids. Ethanoic acid is a weak acids. That means that its solution contains a high proportion of molecules that haven't split into ions. For all the reactions which give off hydrogen, the same ionic equation applies - whether the acid is strong or weak: Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)
The key factors to vary, control or take into account The amount of heat given out will depend on the mass of magnesium I use - the more magnesium I use, the more of the above reaction happens. But because I am going to work out the amount of heat evolved per mole of magnesium, I don't need to take exactly the same mass for each experiment. I do, though, need to weigh the magnesium accurately for each experiment.
I am going to use an excess of each acid, so their exact concentrations don't matter. The amount of heat given off will depend only on the amount of magnesium used. I shall use the normal dilute acids found in the lab. I do need to know exactly what volume of acid I am using for each experiment. If I don't know this I can't work out how much heat has been evolved. I am going to measure the temperature rise of the reaction mixture. Because the temperature has gone up, heat will be lost to the surroundings. I shall have to take steps to keep this heat loss as low as possible. Preliminary work I needed to find out what sort of amounts of magnesium and acid to use in my experiments. If I use a small amount of magnesium, it will be more difficult to weigh it accurately and it will only give a small temperature rise which will be difficult to measure accurately. If I use a lot of magnesium, the temperature rise will be so great that there will be major heat losses to the surroundings. I also need to choose a sensible volume of acid to use. It has to be enough to react with all the magnesium, but not so much that a lot of spray escapes from the reaction. I started by using a strip of magnesium ribbon which weighed about 0.1 g, and 50 cm3 of acid (measured roughly with a measuring cylinder). All the magnesium dissolved, and it gave a sensible temperature rise of about 9°C, but it took a very long time using ethanoic acid. That increases the possibility of heat losses to the surroundings. This preliminary experiment showed that my quantities were sensible, but that I needed to speed up the reaction. I tried different forms of magnesium. Magnesium turnings also took quite a long time, but magnesium powder reacted quite quickly with the ethanoic acid, and didn't produce too much spray with the strong acids. I decided to use about 0.1 to 0.15 g of magnesium powder for my experiments. I weighed this in a weighing bottle, but found that when I tipped it into the acid, not all of it came out. So I would need to reweigh the bottle again to see how much I had actually added. Producing...
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