Groups 1 & 2 - Inorganic (4)

Question 1

Explain the trend in ionisation energies down group 2 (4)

Answer 1

1. Ionisation energy decreases down group 2
2. Because the outer electrons (additional shell) become further from the nucleus and experience smaller electrostatic attraction with the nucleus going down the group
3. Although the nuclear charge increases, it is compensated by the increase in shielding electrons
4. Overall, less energy is needed to overcome the weaker forces holding the outer electron to the atom going down the group

Question 2

Explain the trend in reactivity down group 2 (5)

Answer 2

1. Reactivity increases going down the group
2. Because the outer electrons become further from the nucleus (additional shell) and experience smaller electrostatic attraction with the nucleus going down the group
3. Although the nuclear charge increases, it is compensated by the increase in shielding electrons
4. Therefore, less energy is needed to overcome the weaker forces holding the outer electron to the atom going down the group
5. Therefore, sum of the first and second ionisation energies decreases down the group, making it easier to remove the two outer electrons which increases reactivity

Question 3

Explain the trend of solubility of Group 2 hydroxides (7)

Answer 3

1. Solubility Increases down the group
2. Going down the group the ionic radius of the group 2 cation increases
3. Because of this, the distance between the centres of mass of the cation and anion increase and since a hydroxide ion is relatively small, the change in lattice enthalpy down the group is significant
4. Because lattice enthalpy is dependant on the length of an ionic bond; a shorter bond having a higher lattice enthalpy, an increase in ionic bond length causes a decrease in the lattice (dissociation) enthalpy (less endothermic).
5. An increase in the size of the group 2 cation causes the hydration enthalpy to decrease (less exothermic) as the larger the size of the ions, the lower (less endothermic) the hydration enthalpy is
6. However, the lattice (dissociation) enthalpy decreases (becomes less endothermic) faster than the hydration enthalpy as a square factor is involved.
7. Therefore, going down the group the hydration enthalpy becomes higher (more exothermic) than the lattice enthalpy and it becomes more energetically favourable for the group 2 hydroxide to become hydrated

Question 4

How does a group 2 metal react with Oxygen, Chlorine, and Water?

Answer 4

1. Group 2 metal + Oxygen = Group 2 oxide
2. Group 2 metal + chlorine = Group 2 chloride
3. Group 2 metal + water = group 2 hydroxide + hydrogen

Question 5

Explain the trend in solubility of group 2 Sulphates (5)

Answer 5

1. solubility decreases down the group
2. Going down the group the ionic radius of the group 2 cation increases
3. But because a sulphate ion is relatively large, the increase in ionic radius of the cation produces a less significant decrease (less endothermic) in lattice enthalpy down the group.
4. Increase in the size of the group 2 cation decreases (less endothermic) the hydration enthalpy in the same proportions if the anion were relatively small
5. Therefore, going down the group, the hydration enthalpy decreases (become less endothermic) faster than the lattice enthalpy. So it becomes less energetically favourable for the group 2 sulphate to become hydrated.

Question 6

What are the reactions of

i) group 2 oxide and water
ii) group 2 oxide and hydroxides with dilute acids

Answer 6

i) Group 2 oxide + water = Group 2 hydroxide

ii) Group 2 oxide/hydroxide +acid = salt + water (neutralisation reaction)

Question 7

Give the equations for the decomposition of (grp 1 & 2) nitrates (both greater & lesser decomposition)

Answer 7

1. (lesser) Metal nitrate = metal nitrite + oxygen

2. (greater) Metal nitrate = metal oxide + nitrogen dioxide (brown fumes) + oxygen

Question 8

Explain how thermal decomposition of a carbonate ion takes place (6)

Answer 8

1. A cation attracts the delocalised electrons of the carbonate ion, polarising it
2. The polarisation causes the bond length of the O-C bond to extend, weakening the bond
3. Upon the addition of heat, energy is supplied breaking the weaker bond first (as opposed to all the bonds which will cause a change in state) causing CO2 to be formed and the metal oxide resulting in decomposition
4. The ease of decomposition (less energy needed to break the weaker bond) depends on the polarising power of the cation and the polarisability of the anion
5. Polarising power is proportional to the charge density of the cation. Both a smaller radius and a larger charge result in a larger charge density and polarising power
6. The polarisability of the anion increases when it has a larger radius and a larger charge

Question 9

Describe the experimental procedure for testing the decomposition of Nitrates and Carbonates (4)

Answer 9

1. Place the same mass of each carbonate and nitrate separately in series of test tubes
2. Fix a delivery tube to the test tube and clamp in a stand. The tube will lead to an upside down water-filled test tube (with marker of same length) in a water bath
3. Light Bunsen and measure time taken for gas evolved to reach mark in test tube. Also note presence of brown gas (NO2) with nitrates.
4. Repeat with remaining Carbonate (or Nitrates) making sure distance of Bunsen flame to test tube is consistent

Question 10

Explain the trend in decomposition of group 1 nitrates (6)

Answer 10

1. Ease of decomposition decreases down Grp 1
2. As ionic radius of the group 1 cation increases down the group whereas charge remains same, reducing the polarising power.
3. Meanwhile, polarisability of nitrate anion remains same
4. This means that the grp 1 cation is less able to distort the N-O bond of the nitrate anion (weakening it less) so more energy is needed to break the bond for decomposition.
5. Only Li undergoes greater decomposition (releases brown NO₂ gas) Others undergo lesser decomposition (no brown gas)
6. This is because Li has a small enough ionic radius and a high enough polarising power despite having only a 1+ charge

Question 11

Explain the trend in decomposition of group 2 nitrates. Compare with group 1 (4)

Answer 11

1. All group 2 nitrates undergo greater decomposition (releasing brown NO₂ gas) as they have a larger 2+ charge and thus a high enough polarising power (compared to grp 1)
2. Ease of decomposition decreases down the group as ionic radius of the Grp 2 cation increases whereas charge remains same.
3. Polarisability of nitrate anion remains same
4. This decreases the polarising power so that the Grp 2 cation distorts the N-O bond of the nitrate anion less (weakening it less) and so more energy is needed to break the bond for decomposition.

Question 12

Explain trend in decomposition of group 1 carbonates (4)

Answer 12

1. Only Li carbonate decomposes when heated
2. This is because Li has a small ionic radius with a 1+ charge to have a sufficient polarising power
3. Li distorts the C-O bond in the carbonate anion (weakening it) so that energy supplied by heat can break the bond.
4. All other Grp 1 carbonates do not undergo decomposition as they have too large of a ionic radius and a too little polarising power

Question 13

Explain the trend in decomposition of Group 2 Carbonates. Compare with group 1. (4)

Answer 13

1. All group 2 carbonates decompose on heating. A larger 2+ charge (compared to a 1+ charge of Grp 1) allows for a sufficient polarising power for decomposition.
2. Ease of decomposition decreases down the group as ionic radii of Grp 2 cation increases. Whereas charge remains same. Leading to a lower polarising power
3. The polarisability of the carbonate anion remains same.
4. Therefore, the Grp 2 cation distorts the C-O bond in the carbonate anion less (weakening it less) and so more energy is needed to break the bond.