EL 8&9: Properties of s-block (mostly G2) compounds, acid-base equilibria

Question 1

Explain why metals can conduct electricity.

Answer 1

Metals consist of a lattice of cations in a sea of delocalised valence electrons, which can conduct.

Question 2

S-block elements are the most reactive metals. What are the trends in their reactivity and melting/boiling points?

Answer 2

• Down group: reactivity increases, melting/boiling points decrease
• Across periods: reactivity decreases, melting/boiling points increase (as 1st ionisation enthalpy increases)

Question 3

What are the products of the reaction of s-block elements with oxygen?

Answer 3

Metal oxides:

• Group 1: M₂O
• Group 2: MO

Question 4

• Write a general equation for the reaction of group 2 elements with water
• Describe the reaction of these elements, from Mg to Ba, with water

Answer 4

M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g)

Reactivity increases down group (not as vigorous as group 1):

• Mg reacts slowly
• Ba reacts rapidly, producing stream of H₂

Question 5

• Describe the trend in thermal stability of the group 2 carbonates
• Give a general equation for their behaviour when heated

Answer 5

Thermal stability increases down group

Decompose, producing metal oxide + carbon dioxide:

MCO₃(s) → MO(s) + CO₂(g)

Question 6

Explain the increase in thermal stability of group 2 carbonates down the group.

Answer 6

• Down group, cations become larger so have lower charge density
• They polarise [negative charge cloud around] carbonate ion to lesser extent
• Compounds become more stable, so decompose at higher temperature

Question 7

Do group 2 oxides and hydroxides form acidic or alkaline solution?

Answer 7

Alkaline

Question 8

State and explain the trend in pH of group 2 hydroxides in water.

Answer 8

• pH / alkalinity increases down group
• Reactivity [with water] increases down group, producing higher [M(OH)₂] which dissociate to form OH^-

Question 9

What is the trend in solubility of group 2:

• hydroxides?
• carbonates?

Answer 9

Hydroxides: solubility increases down group

Carbonates: solubility decreases down group

• Alkalinity increases down group; higher [OH^-] so M(OH)₂ must dissolve + dissociate more
• Thermal stability of carbonates increases down group; so does stability in solution

Question 10

What is an acid?

Answer 10

A compound which dissociates in water to produce H⁺ ions.

Question 11

What is a base?

Answer 11

A compound which reacts with an acid to produce water and a salt.

Question 12

What is an alkali?

Answer 12

A base which dissolves in water to form OH^- ions.

Compounds which don’t contain OH^- ions are alkaline if react with water to produce them.

Question 13

Explain the relationship between alkalis and bases.

Answer 13

Question 14

What are the other ways of denoting:

• acids?
• bases?

Answer 14

• Proton donors
• Proton acceptors

Question 15

What ion is present in acidic solutions? Explain how it is produced and draw its bonding.

Answer 15

Protons dissociate from acid, then react with water to produce oxonium ions.

H⁺ + H₂O → H₃O⁺

Question 16

Write an equation to show that ammonia is alkaline.

Answer 16

Produces hydroxide ions when dissolved in water:

NH₃ (aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH^-(aq)

Question 17

Write an equation to show that carbonate ions are alkaline.

Answer 17

Produce hydroxide ions when dissolved in water:

CO₃^2-(aq) + H₂O(l) ⇌ HCO₃^-(aq) + OH^-(aq)

Question 18

• Write a general ionic equation to show how group 2 oxides react with acids
• Write an equation showing the reaction of magnesium oxide with nitric acid

Answer 18

MO(s) + 2H⁺(aq) → M²⁺(aq) + H₂O(l)

MgO(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂O(l)

Question 19

• Write a general ionic equation to show how group 2 hydroxides react with acids
• Write an equation showing the reaction of magnesium hydroxide with:
  
  • nitric acid
  • sulfuric acid

Answer 19

M(OH)₂ + 2H⁺ → M²⁺ + 2H₂O

Mg(OH)₂(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + 2H₂​O(l)

Mg(OH)₂(s) + H₂SO₄(aq) → MgSO₄(aq) + 2H₂​O(l)

Question 20

‘Sodamide’, NaNH₂, is a white solid made by reacting sodium and ammonia. It reacts with water and is used to dry ammonia.

1. Write an equation for the reaction of sodium with ammonia to form sodamide.
2. Sodamide reacts with small amounts of water as shown: NaNH₂(s) + H₂O(l) → NH₃(g) + NaOH(s). Suggest why this reaction makes sodamide a good drying agent for ammonia.

Answer 20

1. 2Na + 2NH₃ → 2NaNH₂ + H₂
2. Absorbs water*, and NH₃(g) is the only gas formed so easily collected

*Overall: 2Na + 2NH₃ + 2H₂O → 2NH₃(g) + 2NaOH(s)