Group II

Q1 Do you expect the second ionisation energy of group II to follow the same pattern?

Yes for the same reasons, distance from the nucleus and shielding?

Q2 What do you think the trend in reactivity is down group II? Explain your answer

Reactivity will increase. As you move down the group the outer electrons become easier to remove, so the metals are more reactive

a) Describe, for magnesium to barium the trend in i) Metallic radii

ii) 1st ionisation energies

Increases

Decreases

b) Explain the trends that you have described in a.

i) The radius increases as additional shells of electrons are added going down the group from magnesium to barium.

ii) the electron removed is further away from the nucleus and shielded by more inner filled shells of electrons. It therefore takes less energy to remove this electrons as we descend group II

Experiment shows reactivity INCREASES down groupas the outer electrons become easier to remove,because, (even though proton number increases)the outer e- are further from the attracting nucleusand more shielded from the attracting nucleus.

Increasingly easy to ionise

increasingly reactive

M = Mg, Ca, Sr or Ba with Oxygen

2M(s) + O2(g) 2MO(s)

Remember! The oxides, MO, are ionic compounds, M2+O2-

M has been oxidised O2 has been reduced- loses electrons - gains electrons

M M2+ + 2e- O2 + 4e- 2O2-

or oxidation state increases

or oxidation state decreases

M(0) M(+2) O(0) O(-2)

M is the reducing agent O2 is the oxidising agent

Group 2 metals become INCREASINGLY strong reductants down the group.

a) Write an equation, including state symbols, for the burning of strontium in oxygen.

b) Describe what you might observe during this reaction.

c) Identify the element which is oxidised and the element which is reduced. Explain your answer in terms of electron transfer and oxidation states.

d) Explain the increasing reactivity of the Group II metals going down the group.

2Sr(s) + O2(g) 2SrO(s)

The metal burns with a red flame and a white solid is formed.

Sr has been oxidised

M M2+ + 2e-

M(0) M(+2)

O2 has been reduced

O2 + 4e- 2O2-

O(0) O(-2)

Two electrons are lost from each metals atom in a reaction. Down the group, the first ionisation energies decrease from magnesium to barium. Consequently, the reactivity of the metals increase down the group as less energy is required to remove he two outer electrons,.

(2) M = Ca, Sr or Ba with waterCa(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)

Sr(s) + 2H2O(l) Sr(OH)2(aq) + H2(g)

Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)

Alkaline pH (>7) caused by OH-(aq)

M(OH)2(s) + aq M2+(aq) + 2OH-

(aq)

Note : Ca(OH)2 NOT very soluble less alkaline than others

M has been oxidised H2O has been reduced- loses electrons - gains electrons

M M2+ + 2e- H2O + e- ½H2 + OH-

or oxidation state increases or oxidation state decreases

M(0) M(+2) H(+1) H(0)

M is the reductant H2O is the oxidant

White metal oxides - ionic – M2+ (group 2) and O2- ions

White metal hydroxide – ionic M2+ (group 2) and OH- ions

Add water

Group II Oxides with Water

Group 2:

MO(s) + H2O(l) M2+(aq) + 2OH-

(aq)

Ionic Equations for Group II Hydroxide Precipitations

Mg2+(aq) + 2OH-

(aq) Mg(OH)2(s)

Ca2+(aq) + 2OH-

(aq) Ca(OH)2(s)

Sr2+(aq) + 2OH-

(aq) Sr(OH)2(s)

Ba2+(aq) + 2OH-

(aq) no precipitate

Trend to remember!

Solubility of group II hydroxides INCREASE down group II.

OH-(aq) alkaline pH – value depends on hydroxide solubility

Suggest why calcium hydroxide is not used in indigestion remedies.

Calcium hydroxide is more soluble in water than magnesium hydroxide. Calcium hydroxide therefore produces a solution with a much higher pH – around pH 10 or even higher. People strongly dislike the taste of solutions of high pH.

Group 2 Aqueous Ions + NaOH(aq)

Mg 2+(aq) Ca 2+

(aq) Sr 2+(aq) Ba 2+

(aq)

NaOH(aq) NaOH(aq) NaOH(aq) NaOH(aq)

+ OH-(aq) + OH-

(aq) + OH-(aq) + OH-

(aq)

No ppt.V. light white ppt.

Light white ppt.

V. V. light white ppt.

Group 2 hydroxides INCREASE in solubility

All undergo thermal decomposition to give the corresponding white metal oxide, MO(s)

and CO2 gas.

MCO3 MO + CO2

N.B. Increasingly MORE difficult to decompose DOWN group

Thermal decomposition of group II carbonates

Strontium carbonate decomposes when heated.

a)What are these type of reactions called?

b) Write a balanced chemical equation for this reaction.

c)Compare the ease of decomposition of strontium carbonate with that of calcium carbonate and barium carbonate.

SrCO3 SrO + CO2

Strontium carbonate is harder to decompose than calcium carbonate but easier to decompose than barium carbonate.

Thermal decomposition

Uses of group II metal hydroxides

Calcium hydroxide, Ca(OH)2, is used by farmers and gardeners as ‘lime’ to neutralise acid soils

Magnesium hydroxide, Mg(OH)2,is used in ‘milk of magnesia’ to relieve indigestion. It works by neutralising any excess acid in the stomach.

Explain why magnesium oxide has an extremely high melting point, with reference to its structure and bonding.

MgO is giant ionic. The bonding is strong, giving a very high melting point.