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REACTIONS OF THE GROUP 2 ELEMENTS WITH AIR OR OXYGEN
This page looks at the reactions of the Group 2 elements – beryllium, magnesium, calcium, strontium and barium – with air or oxygen. It explains why it is difficult to observe many tidy patterns.
The reactions with oxygen
Formation of simple oxides
On the whole, the metals burn in oxygen to form a simple metal oxide.
Beryllium is reluctant to burn unless it is in the form of dust or powder. Beryllium has a very strong (but very thin) layer of beryllium oxide on its surface, and this prevents any new oxygen getting at the underlying beryllium to react with it.
“X” in the equation can represent any of the metals in the Group.
It is almost impossible to find any trend in the way the metals react with oxygen. It would be quite untrue to say that they burn more vigorously as you go down the Group.
To be able to make any sensible comparison, you would have to have pieces of metal which were all equally free of oxide coating, with exactly the same surface area and shape, exactly the same flow of oxygen around them, and heated to exactly the same extent to get them started. It can’t be done!
What the metals look like when they burn is a bit problematical!
Formation of peroxides
Strontium and barium will also react with oxygen to form strontium or barium peroxide.
Strontium forms this if it is heated in oxygen under high pressures, but barium forms barium peroxide just on normal heating in oxygen. Mixtures of barium oxide and barium peroxide will be produced.
The strontium equation would look just the same.
The reactions with air
The reactions of the Group 2 metals with air rather than oxygen is complicated by the fact that they all react with nitrogen to produce nitrides. In each case, you will get a mixture of the metal oxide and the metal nitride.
The general equation for the Group is:
The familiar white ash you get when you burn magnesium ribbon in air is a mixture of magnesium oxide and magnesium nitride (despite what you might have been told when you were first learning Chemistry!).
Trying to pick out patterns in the way the metals burn
There are no simple patterns. It would be tempting to say that the reactions get more vigorous as you go down the Group, but it isn’t true.
The overall amount of heat evolved when one mole of oxide is produced from the metal and oxygen shows no simple pattern:
If anything, there is a slight tendency for the amount of heat evolved to get less as you go down the Group.
But how reactive a metal seems to be depends on how fast the reaction happens – not the overall amount of heat evolved. The speed is controlled by factors like the presence of surface coatings on the metal and the size of the activation energy.
You could argue that the activation energy will fall as you go down the Group and that will make the reaction go faster. The activation energy will fall because the ionisation energies of the metals fall.
In this case, though, the effect of the fall in the activation energy is masked by other factors – for example, the presence of existing oxide layers on the metals, and the impossibility of controlling precisely how much heat you are supplying to the metal in order to get it to start burning.
Why do some metals form peroxides on heating in oxygen?
Beryllium, magnesium and calcium don’t form peroxides when heated in oxygen, but strontium and barium do. There is an increase in the tendency to form the peroxide as you go down the Group.
The peroxide ion, O22- looks llike this:
The covalent bond between the two oxygen atoms is relatively weak.
Now imagine bringing a small 2+ ion close to the peroxide ion. Electrons in the peroxide ion will be strongly attracted towards the positive ion. This is then well on the way to forming a simple oxide ion if the right-hand oxygen atom (as drawn below) breaks off.
We say that the positive ion polarises the negative ion. This works best if the positive ion is small and highly charged – if it has a high charge density.
Ions of the metals at the top of the Group have such a high charge density (because they are so small) that any peroxide ion near them falls to pieces to give an oxide and oxygen. As you go down the Group and the positive ions get bigger, they don’t have so much effect on the peroxide ion.
Barium peroxide can form because the barium ion is so large that it doesn’t have such a devastating effect on the peroxide ions as the metals further up the Group.
Why do these metals form nitrides on heating in air?
Nitrogen is often thought of as being fairly unreactive, and yet all these metals combine with it to produce nitrides, X3N2, containing X2+ and N3- ions.
Nitrogen is fairly unreactive because of the very large amount of energy needed to break the triple bond joining the two atoms in the nitrogen molecule, N2.
When something like magnesium nitride forms, you have to supply all the energy needed to form the magnesium ions as well as breaking the nitrogen-nitrogen bonds and then forming N3- ions. All of these processes absorb energy.
This energy has to be recovered from somewhere to give an overall exothermic reaction – if the energy can’t be recovered, the overall change will be endothermic and won’t happen.
Energy is evolved when the ions come together to produce the crystal lattice. This energy is known as lattice energy or lattice enthalpy.
The size of the lattice energy depends on the attractions between the ions. The lattice energy is greatest if the ions are small and highly charged – the ions will be close together with very strong attractions. In the whole of Group 2, the attractions between the 2+ metal ions and the 3- nitride ions are big enough to produce very high lattice energies.
When the crystal lattices form, so much energy is released that it more than compensates for the energy needed to produce the various ions in the first place. The excess energy evolved makes the overall process exothermic.
This is in contrast to what happens in Group 1 of the Periodic Table (lithium, sodium, potassium, rubidium and caesium). Their ions only carry one positive charge, and so the lattice energies of their nitrides will be much less.
Lithium is the only metal in Group 1 to form a nitride. Lithium has by far the smallest ion in the Group, and so lithium nitride has the largest lattice energy of any possible Group 1 nitride. Only in lithium’s case is enough energy released to compensate for the energy needed to ionise the metal and the nitrogen – and so produce an exothermic reaction overall.
In all the other cases in Group 1, the overall reaction would be endothermic. Those reactions don’t happen, and the nitrides of sodium and the rest aren’t formed.
REACTIONS OF THE GROUP 2 ELEMENTS WITH WATER.
This page looks at the reactions of the Group 2 elements – beryllium, magnesium, calcium, strontium and barium – with water (or steam). It uses these reactions to explore the trend in reactivity in Group 2.
Beryllium reacts with steam at high temperatures (typically around 700°C or more) to give white beryllium oxide and hydrogen. (Equation just like the magnesium one below.)
Magnesium burns in steam to produce white magnesium oxide and hydrogen gas.
Very clean magnesium ribbon has a very slight reaction with cold water. After several minutes, some bubbles of hydrogen form on its surface, and the coil of magnesium ribbon usually floats to the surface. However, the reaction soon stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium preventing further reaction.
Calcium, strontium and barium
These all react with cold water with increasing vigour to give the metal hydroxide and hydrogen. Strontium and barium have reactivities similar to lithium in Group 1 of the Periodic Table.
Calcium, for example, reacts fairly vigorously with cold water in an exothermic reaction. Bubbles of hydrogen gas are given off, and a white precipitate (of calcium hydroxide) is formed, together with an alkaline solution (also of calcium hydroxide – calcium hydroxide is slightly soluble).
The equation for the reactions of any of these metals would be:
The hydroxides aren’t very soluble, but they get more soluble as you go down the Group. The calcium hydroxide formed shows up mainly as a white precipitate (although some does dissolve). You get less precipitate as you go down the Group because more of the hydroxide dissolves in the water.
Summary of the trend in reactivity
The Group 2 metals become more reactive towards water as you go down the Group.
Explaining the trend in reactivity
Beryllium as a special case
There is an additional reason for the lack of reactivity of beryllium compared with the rest of the Group. Beryllium has a strong resistant layer of oxide on its surface which lowers its reactivity at ordinary temperatures. However, the oxide layer breaks up above 750°C and exposes the beryllium metal surface below it, and so the protection then fails.
Looking at the enthalpy changes for the reactions
The enthalpy change of a reaction is a measure of the amount of heat absorbed or evolved when the reaction takes place. An enthalpy change is negative if heat is evolved, and positive if it is absorbed. That’s really all you need to know for this section!
If you calculate the enthalpy change for the possible reactions between beryllium or magnesium and steam, you come up with these answers:
Notice that both possible reactions are strongly exothermic, giving out almost identical amounts of heat. However, the magnesium reaction is much faster. The explanation for the different reactivities must lie somewhere else.
Similarly, if you calculate the enthalpy changes for the reactions between calcium, strontium or barium and cold water, you again find that the amount of heat evolved in each case is almost exactly the same – in this case, about -430 kJ mol-1.
The reason for the increase in reactivity must again lie elsewhere.
Looking at the activation energies for the reactions
The activation energy for a reaction is the minimum amount of energy which is needed in order for the reaction to take place. It doesn’t matter how exothermic the reaction would be once it got started – if there is a high activation energy barrier, the reaction will take place very slowly, if at all.
When Group 2 metals react to form oxides or hydroxides, metal ions are formed.
The formation of the ions from the original metal involves various stages all of which require the input of energy – contributing to the activation energy of the reaction. These stages involve the input of:
After this, there will be a number of steps which give out heat again – leading to the formation of the products, and overall exothermic reactions.
The graph shows the effect of these important energy-absorbing stages as you go down Group 2.
Notice that the ionisation energies dominate this – particularly the second ionisation energies. Ionisation energies fall as you go down the Group. Because it gets easier to form the ions, the reactions will happen more quickly.
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