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Trends in Group 2
Foundation ChemistrySemester 1
Dr Joanne Nicholson
http://teachable.net/res.asp?r=1492
Trends in Melting Point
Explanation• The Group 2 elements are all metals with metallic bonding, so
you expect their melting points to be high. In metallic bonding, metal cations in a metal lattice are attracted to delocalised electrons.
Going down Group 2:
• The number of delocalised electrons remains the same.
• The charge on each metal cation stays the same at 2+.
• The ionic radius increases.
• So the attraction between the delocalised electrons and the metal cations decreases.
One possible explanation is that beryllium and magnesium have different metallic structures from the other elements in the group:
• Beryllium and magnesium have a hexagonal close-packed structure;
• Calcium, strontium and barium have a cubic structure
Notice that the melting point for magnesium is anomalously low. Notice that the melting point for magnesium is anomalously low.
Good News!
You will see that there is no obvious pattern in the boiling points - implying that there is no simple
pattern in the strengths of the metallic bonds.
You will see that there is no obvious pattern in the boiling points - implying that there is no simple
pattern in the strengths of the metallic bonds.
The Reactivity of Group 2 Elements
Reactions with Water
Beryllium has no reaction with water
or steam even at red heat.
Water
Steam
This is a more complete reaction!
Very clean magnesium has a very slight reaction with cold water. The reaction soon stops because the magnesium hydroxide
formed is almost insoluble in water and forms a barrier on the magnesium preventing further
reaction.
These all react with cold water with increasing
vigour to give the metal hydroxide and hydrogen.
The hydroxides aren't very soluble, calcium
hydroxide formed shows up mainly as a white precipitate. You get less precipitate as
you go down the Group because more of the
hydroxide dissolves in the water.
The hydroxides aren't very soluble, calcium
hydroxide formed shows up mainly as a white precipitate. You get less precipitate as
you go down the Group because more of the
hydroxide dissolves in the water.
Ca(OH)2 is called slaked lime and is commonly
used in farming to counteract
acidity of soil!
ConclusionConclusion
The group 2 metals become more reactive as you move down the
group!
To Understand it we need to know about Enthalpy!
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.
If you calculate the enthalpy change for the possible reactions between beryllium or magnesium and
steam, you come up with these answers:
NOTE that the reaction with Beryllium DOESN’T
actually happen!Beryllium has a strong
resistant layer of oxide on its surface this makes it very
un-reactive.
NOTE that the reaction with Beryllium DOESN’T
actually happen!Beryllium has a strong
resistant layer of oxide on its surface this makes it very
un-reactive.
Both reactions are exothermic and give out a lot of
heat!
Also there is very little difference in the amount of energy given out by the group 2 elements so this
doesn’t explain the reactivity trend!
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. If there is a high activation energy
barrier, the reaction will take place very slowly, if at all.
The activation energy for a reaction is the minimum amount
of energy which is needed in order for the reaction to take place. If there is a high activation energy
barrier, the reaction will take place very slowly, if at all.
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 formation of the ions from the original metal involves various stages all of which require the input of energy.
These stages involve the input of:
The formation of the ions from the original metal involves various stages all of which require the input of energy.
These stages involve the input of:
1) Atomisation energy of the metal. This is the energy needed to break the bonds holding the atoms together in the metallic lattice.
2) The first + second ionisation energies. These are necessary to convert the metal atoms into ions with a 2+ charge.
Ionisation energies fall as you go down the Group. Because it gets easier to form the ions,
the reactions will happen more quickly.
The effect of energy-absorbing stages as you move down Group 2
The reactions with oxygen
Metals burn in oxygen to form a simple metal oxide.
• Reactivity decreases as you move down the group•This is because the 1st and 2nd ionisation energies decrease.• This is due to shielding and distance!
CaO is called quick lime and is used in farming to counteract soil acidity! It is made commercially by the
thermal decomposition of limestone!
.
Formation of peroxidesStrontium 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.
In each case, you will get a mixture of the metal oxide and the
metal nitride.
The reactions with air
They all react with
nitrogen to
produce nitrides.
The Explanations
There are no simple patterns!There are no simple patterns!
How reactive a metal is depends on how fast the reaction happens - not the overall amount of
heat evolved!Even though the
activation energy falls as you go down the Group.................
The effect of the fall in the activation energy is masked by other factors - oxide layers on
the metals, and the impossibility of controlling how much heat is
supplied in order to get the metal to start burning.
Why do some metals form peroxides on heating in oxygen?
The peroxide ion, O22- looks like
this:The covalent bond between the two oxygen atoms is relatively weak.
Do not form peroxides!
Do form peroxides!
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 a high charge density (because they are so small) so 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?
All group 2 metals combine with it to produce nitrides, X3N2, containing X2+ and N3- ions.
Nitrogen is fairly un-reactive because of the very large amount of energy needed to break the triple bond joining the two atoms in the nitrogen molecule, N2.
This reaction needs a lot of energy to break all of the bonds concerned!
• Energy is evolved when the ions come together to produce the crystal lattice - lattice enthalpy.
• 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.
