grp 2 elements

Question 1

describe the trend of reactivity of grp 2 elements as reducing agents down the group.

Answer 1

reactivity as reducing agents increase.

explanation 1 - east of losing electrons based on ionisation energies:
- no. of electronic shells inceases which leads to an increase in screening effect and each successive element has its valence electrons located in a shell with a higher principal quantum number, n.
- hence, valence electrons are increasingly further away from the nucleus and are less strongly attracted to the nucleus, despite the increase in nuclear charge.
- smaller amount of energy needed to remove the valence electrons
- thus, sum of first and second ionisation energies of grp 2 elements decrease down the group.
- increase ease of atoms losing electrons to form cations = oxidation
- reducing power and hence reactivity increase

explanation 2 - ease of losing electrons based on standard electrode potential, E, value:
- the reducing power and hence reactivity of grp 2 elements increases down the group as reflected by the increasingly negative E value down the group
- Grp 2 metals are more easily oxidised down the group and hence better reducing agents down the group.

Question 2

what is standard electrode potential value?

Answer 2

it is the measure of the tendency for the species, M^n+ to undergo reduction

the more positive the E value is, the greater is the tendency from M^n+(aq) to be reduced to M(s)

conversely the more negative the E value is, the greater is the tendency for M(s) to be oxidised to M^N+(aq).

Question 3

describe reaction of Be with O2

Answer 3

1. forms a protective layer of BeO after heating in air which prevents further reaction
2. Eq:
   - 2Be(s) + O2(g) = 2BeO(s)

Question 4

describe reaction of Mg with O2

Answer 4

1. burns with an intense brilliant white flame to form white solid MgO.
2. Eq:
   - 2Mg(s) + O2(g) = 2MgO(s)

Question 5

describe reaction of Ca with O2

Answer 5

1. burns with a brick-red flame to form white solid CaO.
2. Eq:
   - 2Ca(s) + O2(g) = 2CaO(s)

Question 6

describe reaction of Sr with O2.

Answer 6

1. burns with a crimson flame to form white solid SrO.
2. Eq:
   - 2Sr(s) + O2(g) = 2SrO(s)

Question 7

describe reaction of Ba with O2

Answer 7

1. burns with a pale green flame to form white solid BaO.
2. Eq:
   - 2Ba(s) + O2(g) = 2BaO(s)

Question 8

how do grp 2 elements react with O2?

Answer 8

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

redox reaction.

Question 9

how do grp 2 elements react with water

Answer 9

redox reaction:
- M(s) + 2H2O(l) = M(OH)2(s/aq) + H2(g)

Question 10

how does Be react with water?

Answer 10

no reaction with cold water or steam

Question 11

how does Mg react with water or steam?

Answer 11

1. very slow reaction with cold water, but reacts more rapidly with steam
2. Mg(OH)2 decomposes at high temperatures to MgO, hence Mg(OH)2 is not formed with steam.
3. Eq:
   - (slow) Mg(s) + 2H2O(l) = Mg(OH)2(s) + H2(g)
   - (rapid) Mg(s) + H2O(g) = MgO(s) + H2(g).

Question 12

how do Ca, Sr and Ba react with water/steam?

Answer 12

1. reaction with cold water gets more vigorous down the group
2. Eq:
   - Ca(s) + 2H2O(l) = Ca(OH)2(s) + H2(g)
   - Sr(s) + 2H2O(l) = Sr(OH)2 (aq) + H2(g)
   - Ba(s) + 2H2O(l) - Ba(OH)2(aq) + H2(g)

Question 13

how does solubility of grp 2 metal hydroxides vary down the group?

Answer 13

1. increases down the group
   - but grp 2 metal hydroxides are not very soluble.

Question 14

how does thermal stability of grp 2 carbonates change down the group? Explain.

Answer 14

1. increases down the group
   - carbonates of grp 2 elements decompose on heating to form stable oxides(s) and CO2(g).
   - decomposition temperature of grp 2 carbonates also increase down the group.

explanation:
- ionic radius of cation increases
charge density of cation decreases (since charge remains the same)
- polarising power of cation decreases.
- extent of distortion/polarisation of electron cloud of CO32- decreases.
- extent of weakening of covalent bonds with the CO32- decreases.
- thermal stability of compounds increases, hence decomposition temperature increases.

Question 15

describe thermal decomposition of grp 2 nitrates.

Answer 15

1. follow same trend of thermal stability of grp 2 carbonates.
2. decompose on heating to give metal oxides, nitrogen dioxide and oxygen.

Question 16

describe thermal decomposition of grp 2 hydroxides.

Answer 16

1. follow same trend of thermal stability of carbonates
2. decompose to give metal oxides and steam, H2O(g).

Question 17

how is Li similar to Mg?

Answer 17

1. lithium compounds undergo thermal decomposition in a similar way as magnesium compounds, giving similar products.
2. for comparison:
   - other grp 1 nitrates decompose to find nitrites and oxygen, but lithium nitrate decompose to fome oxide, brown NO2(g) and oxygen.

Question 18

how do grp 2 oxides react with water?

Answer 18

1. all grp 2 oxides(except BeO) react with water to form hydroxides
2. general Eq:
   - MO(s) + H2O(l) = M(OH)2(aq).
3. reaction of grp 2 oxides with water increases with vigour down the group.

Question 19

describe reaction of BeO with water

Answer 19

1. does not react with water due to very high magnitude of lattice energy of BeO
2. pH of sol.: 7 (solid does not dissolve)

Question 20

describe reaction of MgO with water.

Answer 20

1. reacts less vigorously to form weakly basic solution due to high magnitude of LE of MgO (more difficult to break ionic lattice structure)
2. MgO only slightly soluble in water
3. Eq:
   - MgO(s) + H2O(l) <=> Mg(OH)2(aq)
4. pH of resulting solution: 9 (solid sparingly soluble)

MgO has very low solubility with water means MgO forms very little hydroxides

Question 21

describe reaction of CaO, SrO and BaO with water.

Answer 21

1. reacts vigorously with water to produce an increasingly more alkaline solution.
2. solubility of solid increases from CaO to BsO .
3. Eqs:
   - CaO(s) + H2O(l) = Ca(OH)2(aq)
   - SrO(s) + H2O(l) = Sr(OH)2(aq)
   - BaO(s) + H2O(l) = Ba(OH)2(aq)
4. pH of resulting solution: increases from 11 to 13.

Calcium hydroxide is only slightly soluble in limewater but barium hydroxide is a very soluble base which can be used in acid-base titration.

Question 22

how does solubility of grp 2 hydroxides change down the grp?

Answer 22

it increases down the group.

explanation:
- the more exothermic the enthalpy change of solution is, the more soluble the salt will be. this is deduced by the eq: Hsol = Sum of (Hhyd) - L.E.
- down the group, the magnitudes of both Hhyd(M2+) and L.E. (M(OH)2) decrease due to increasing ionic radius of M2+.
- for grp 2 hydroxides, the magnitude of L.E. decreases more significantly than that of Hhyd, causing Hsol. to become more negative/less positive down the grp, resulting in solubility increasing down the group.

Hhyd is calculated using charge density and L.E. is calculated by using (Q1 x Q2)/ (R1 + R2)

Question 23

how does solubility of grp 2 sulfates change down the grp?

Answer 23

it decreases.

explanation:
- for grp 2 sulfates, the magnitude of Hhyd decreases more significantly compared to that of L.E.
- Hence, Hsol. becomes less negative/more positive down the group, resulting in the solubility to decrease down the group.