Group 2: Alkaline earth metals
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I have been given the question "a student predicted that the rate of hydrochloric acid and any metal in Group 2 would be directly proportional to the number of shells occupied by the electrons in an atom of the metal...... Your method should involve measurements of the gas produced in the reaction."
I understand the relevant chemical knowledge for Group 2, but I am confused on the set up of the equipment and how to do the calculations as all I know is the Mr. If you could just start me off on how to go about equations and the right experiment that I need to be doing, I should be able to get going.
210208
Corrie writes ......
The balanced equation for the reaction of a Group 2 metal, say M, and HCl, to form the metal chloride and hydrogen gas, should be the same for all the metals in the Group. You should be able to balance it or look it up:
M(s) + ?HCl(aq) -> ?MCl2 + ?H2(g)
Therefore, in theory, the same amount (in moles) of each Group 2 metal should produce the same volume of gas if it reacts with excess acid. You will need to calculate what this excess amount should be, once you have decided how much metal to use.
Although the volumes of hydrogen produced from the same amount of each metal should be the same, the rate at which gas will be produced will, as your plan notes, differ depending on the reactivity of the metal (which is linked to the number of shells). [One other complicating factor here, because a solid is involved, will be the surface area of the metal, which will also vary from metal to metal for the same amount used.]
So you need ways to measure the rate at which hydrogen is produced, once the acid and metal have been put together (for some of these metals the reaction will be very vigorous and you are unlikely to be carrying it out). The simplest way to measure a gas that is not very soluble in water is to collect it over water in an upturned measuring cylinder of appropriate size and time how long it takes to reach a certain mark. Then the rate is proportional to 1/time, i.e. the shorter the time, the higher the rate.
'Appropriate size' means you must have some idea before you start how much hydrogen will be produced so you can choose a measuring cylinder of the right size. How much hydrogen will depend on the amount (mol) of metal you start with and the equation above, which links the metal and the hydrogen in a simple (mol) ratio, 1:1, 2:1, 1:2 etc depending on your balanced equation.
Another, more sophisticated, method is to use a gas syringe, if these are available. They usually have a volume of 100 cm3, so you would aim to produce about that much hydrogen gas, and time how long it takes to for, say 60 cm3, to be collected in the syringe.
Yet another way would be to let the gas bubble out of delivery tube under water and count the number of bubbles in a given time. My guess is that in some cases the rate of bubbling might be too fast for accurate counting - but I suppose you could dilute the acid until the rate was manageable.
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updated: 21 February 2008
