EL8 - Group 1 and Group 2 Chemistry

Question 1

Group 2 elements react with water and oxygen to form what?

Answer 1

BASES.

Question 2

What do group 2 elements react with water to form?

What is the trend (in terms of reactivity with water) and why?

Answer 2

METAL HYDROXIDES
e.g. Sr (s) + 2H2O (l) –> Sr(OH)2 (aq) + H2 (g)

Trend:
Reactivity INCREASES with water.
(There is no reaction with Be)

This is because the atom gets larger and the electron is further from the nucleus.
This means it is easier to remove and so it’s more reactive.

Question 3

What do group 2 elements react with oxygen to form? What do these look like?

Then what happens when they react with water?

What is the trend?
(incl. alkilinity)

Answer 3

METAL OXIDES
e.g. 2Mg (s) + O2 (g) –> 2MgO (s)

Group 2 oxides are WHITE SOLIDS

______

THEN

oxides react readily with water to make HYDROXIDES
which DISSOCIATE to form OH- ions.

Magnesium oxide reacts very slowly and the hydroxide barely dissolves!
This is because of its low reactivity,
as Mg is higher up in the group.
This means that OH- don’t really dissolve,
so we don’t really form very strong alkaline solutions.

This means they become MORE STRONGLY ALKALINE as we go DOWN the group as the hydroxides become more soluble.

______

Question 4

What are the group 2 solubility trends as carbonates and as hydroxides?

Why? i.e. What’s the rule?

Answer 4

Group 2 hydroxides and carbonates have opposite solubility as we go down the group.

AS CARBONATES: DECREASING
AS HYDROXIDES: INCREASING

___

Rule:
If the anion (neg) has a double charge they become less soluble going down.
e.g. CO3-2 is double charged.

If the anion is a single charge, it becomes more soluble going down.
e.g. OH- is singularly charged.

Question 5

What is the
thermal decomposition point trend
in group 2 carbonates?

Why?

Answer 5

Temperature for decomposition point INCREASES as you go down.

This is because the thermal stability increases going down,
because charge density decreases.

Question 6

What is meant by the charge density of carbonates?

How does this affect decomposition point and trend across group 2 carbonates?

Answer 6

CHARGE DENSITY is a measure of the CONCENTRATION of CHARGE on the cation.

The smaller the +2 ion, the higher the charge density.

Cations with a higher charge density (at the top) can distort or POLARISE the negative charge cloud around the carbonate ion.
This makes it less stable and easier to break up on heating.

Question 7

What is the trend for group 2 in terms of alkalinity of hydroxides after reacting with water?

Answer 7

Group 2 elements react with water to form metal hydroxides and hydrogen gas.

e.g. Sr (s) + 2H2O (l) –> Sr(OH)2 (aq) + H2 (g)

The ions produced (in the metal hydroxide)
consist of a metal ion (e.g. Sr2+ (aq))
and a hydroxide ion (2OH- (aq)).

The OH- ions make it an ALKALINE SOLUTION.

(Forming alkaline solutions is typical of metal oxides and hydroxides, in contrast to non-metals whose oxides are usually acidic.)

The most strongly alkaline oxides and hydroxides are those at the bottom of the group.

_____________

The oxides and hydroxides react with acids to form salts.

This neutralising effect is used by farmers when they put lime (calcium hydroxide) on their fields to neutralise soil acidity.

Question 8

First ionisation of Ca (g)?

Second ionisation?

Answer 8

FIRST:
Ca (g) –> Ca+ (g) + e-

SECOND:
Ca+ (g) –> Ca2+ (g) + e-

________

Each of the second and subsequent ionisation processes involves the removal of an electron from a positive ion.