Question
When heated in air, magnesium ribbon reacts with oxygen to form magnesium oxide.
(a) (i) Write a balanced equation for the reaction that occurs. [1]
(ii) Identify a metal, in the same period as magnesium, that does not form a
basic oxide. [1]
(b) The reaction in (a)(i) was carried out in a crucible with a lid and the following data
was recorded:
Mass of crucible and lid = 47.372 ±0.001g
Mass of crucible, lid and magnesium ribbon before heating = 53.726 ±0.001g
Mass of crucible, lid and product after heating = 56.941 ±0.001g
(i) Calculate the amount of magnesium, in mol, that was used. [1]
(ii) Determine the percentage uncertainty of the mass of product after heating. [2]
(iii) Assume the reaction in (a)(i) is the only one occurring and it goes to completion,
but some product has been lost from the crucible. Deduce the percentage yield of
magnesium oxide in the crucible. [2]
(c) When magnesium is burnt in air, some of it reacts with nitrogen to form magnesium
nitride according to the equation:
3 Mg(s) + N2 (g) → Mg3N2 (s)
(i) Evaluate whether this, rather than the loss of product, could explain the yield
found in (b)(iii). [1]
(ii) Suggest an explanation, other than product being lost from the crucible or
reacting with nitrogen, that could explain the yield found in (b)(iii). [1]
(d) The presence of magnesium nitride can be demonstrated by adding water to the
product. It is hydrolysed to form magnesium hydroxide and ammonia.
(i) Calculate coefficients that balance the equation for the following reaction. [1]
(ii) Ammonia is added to water that contains a few drops of an indicator. Identify an
indicator that would change colour. Use sections 21 and 22 of the data booklet. [1]
(iii) Determine the oxidation state of nitrogen in Mg3N2 and in NH3. [1]
(iv) Deduce, giving reasons, whether the reaction of magnesium nitride with water is
an acid–base reaction, a redox reaction, neither or both. [2]
(e) Most nitride ions are 14N3- .
(i) State the number of subatomic particles in this ion. [1]
(ii) Some nitride ions are 15N3-. State the term that describes the relationship between 14N3- and 15N3-. [1]
(iii) The nitride ion and the magnesium ion are isoelectronic (they have the same
electron configuration). Determine, giving a reason, which has the greater
ionic radius. [1]
(iv) Suggest, giving a reason, whether magnesium or nitrogen would have the greater
sixth ionization energy. [1]
(f) Suggest two reasons why atoms are no longer regarded as the indivisible units of matter. [2]
(g) State the types of bonding in magnesium, oxygen and magnesium oxide, and how the
valence electrons produce these types of bonding. [4]
Answer/Explanation
Ans.
Question
Ammonia is soluble in water and forms an alkaline solution:
NH3 (g) + H2O(l) \(\rightleftharpoons\) NH4+(aq) + HO–(aq)
(a) State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory. [1]
(b) Determine the concentration, in mol dm-3, of the solution formed when 900.0 dm3 of
NH3 (g) at 300.0 K and 100.0 kPa, is dissolved in water to form 2.00 dm3 of solution. Use
sections 1 and 2 of the data booklet. [2]
(c) (i) Calculate the concentration of hydroxide ions in an ammonia solution with
pH = 9.3. Use sections 1 and 2 of the data booklet. [1]
(ii) Calculate the concentration, in mol dm-3, of ammonia molecules in the solution
with pH = 9.3. Use section 21 of the data booklet. [2]
(iii) An aqueous solution containing high concentrations of both NH3 and NH4+ acts as
an acid-base buffer solution as a result of the equilibrium:
NH3 (aq) + H+(aq) \(\rightleftharpoons\) NH4+(aq)
Referring to this equilibrium, outline why adding a small volume of strong acid
would leave the pH of the buffer solution almost unchanged. [2]
(d) Magnesium salts form slightly acidic solutions owing to equilibria such as:
Mg2+(aq) + H2O(l) \(\rightleftharpoons\) Mg(OH)+(aq) + H+(aq)
Comment on the role of Mg2+in forming the Mg(OH)+ion, in acid-base terms. [2]
(e) Mg(OH)+ is a complex ion, but Mg is not regarded as a transition metal. Contrast Mg
with manganese, Mn, in terms of one characteristic chemical property of transition
metals, other than complex ion formation. [2]
Answer/Explanation
Ans.
