(a)(1) Define each of the following terms: I. deliquescence; II. efflorescence.
(ii) Give one example I substance that undergoes each of the processes in (a)(i).
(b) Outline how three colourless gases suspected to be ethane, ethene and ethyne could be distinguished in the laboratory.
(c)(i) What is water crystallization?
(ii) Hydrated calcium chloride (CaCl\(_2\) = 111, xH\(_2\)O) contains 49.32% water of cystallization. Calm the value of x. [ (CaCl\(_2\) = 111, H\(_2\)O = 18 ]
(d) Name one calcium compound used (i) in the manufactur cement; (ii) as a dessicant; (iii) in the production of plaster of Paris (POP);
(iv) to neutralise acidic soils
(e) A concentrated solution of sodium chloride was electrolysed using graphite electrodes.
(i) State the ions present in the solution
(ii) Name the products at the I. anode, II. cathode.
(iii) Give the by-product of the electrolysis.
(a)(i) Definitions
- I. Deliquescence: the process by which a substance absorbs moisture (water vapour) from the atmosphere and dissolves in it to form a solution.
- II. Efflorescence: the process by which a hydrated crystalline substance loses part or all of its water of crystallization to the atmosphere and becomes powdery.
(a)(ii) Examples
- Deliquescence: sodium hydroxide (\(NaOH\)) or calcium chloride (\(CaCl_2\)).
- Efflorescence: washing soda, \(Na_2CO_3\cdot 10H_2O\) (or \(Na_2SO_4\cdot 10H_2O\)).
(b) Distinguishing ethane, ethene and ethyne
Pass each gas separately through bromine water: ethane does not decolourize it (no reaction), while ethene and ethyne both decolourize the red-brown bromine water. To separate ethene from ethyne, pass each of the two remaining gases through ammoniacal silver trioxonitrate(V) solution (Tollens' reagent): ethyne gives a white precipitate (silver dicarbide/acetylide), while ethene gives no precipitate. Thus ethane gives no change with bromine water, ethene decolourizes bromine water but gives no precipitate, and ethyne decolourizes bromine water and gives a white precipitate.
(c)(i) Water of crystallization
Water of crystallization is the definite/fixed number of water molecules chemically combined with each formula unit of a substance in its crystalline (hydrated) form.
(c)(ii) Value of x in \(CaCl_2\cdot xH_2O\)
Water is 49.32% of the hydrate, so anhydrous \(CaCl_2\) is \(100 - 49.32 = 50.68\%\).
\[x = \frac{49.32/18}{50.68/111} = \frac{2.740}{0.4566} = 6\]
Therefore \(x = 6\); the hydrate is \(CaCl_2\cdot 6H_2O\).
(d) Calcium compounds
- (i) Manufacture of cement: calcium trioxocarbonate(IV), \(CaCO_3\) (limestone).
- (ii) As a desiccant: anhydrous calcium chloride, \(CaCl_2\).
- (iii) Production of plaster of Paris: calcium tetraoxosulphate(VI), \(CaSO_4\cdot\tfrac{1}{2}H_2O\) (from gypsum).
- (iv) To neutralise acidic soils: calcium oxide (quicklime, \(CaO\)) or calcium hydroxide (slaked lime).
(e) Electrolysis of concentrated sodium chloride solution (graphite electrodes)
- (i) Ions present: \(Na^+\), \(H^+\), \(Cl^-\) and \(OH^-\) (from the salt and from water).
- (ii) Products: I. Anode: chlorine gas (\(Cl_2\)); II. Cathode: hydrogen gas (\(H_2\)).
- (iii) By-product: sodium hydroxide (\(NaOH\)) solution.
(a)(i) Definitions
- I. Deliquescence: the process by which a substance absorbs moisture (water vapour) from the atmosphere and dissolves in it to form a solution.
- II. Efflorescence: the process by which a hydrated crystalline substance loses part or all of its water of crystallization to the atmosphere and becomes powdery.
(a)(ii) Examples
- Deliquescence: sodium hydroxide (\(NaOH\)) or calcium chloride (\(CaCl_2\)).
- Efflorescence: washing soda, \(Na_2CO_3\cdot 10H_2O\) (or \(Na_2SO_4\cdot 10H_2O\)).
(b) Distinguishing ethane, ethene and ethyne
Pass each gas separately through bromine water: ethane does not decolourize it (no reaction), while ethene and ethyne both decolourize the red-brown bromine water. To separate ethene from ethyne, pass each of the two remaining gases through ammoniacal silver trioxonitrate(V) solution (Tollens' reagent): ethyne gives a white precipitate (silver dicarbide/acetylide), while ethene gives no precipitate. Thus ethane gives no change with bromine water, ethene decolourizes bromine water but gives no precipitate, and ethyne decolourizes bromine water and gives a white precipitate.
(c)(i) Water of crystallization
Water of crystallization is the definite/fixed number of water molecules chemically combined with each formula unit of a substance in its crystalline (hydrated) form.
(c)(ii) Value of x in \(CaCl_2\cdot xH_2O\)
Water is 49.32% of the hydrate, so anhydrous \(CaCl_2\) is \(100 - 49.32 = 50.68\%\).
\[x = \frac{49.32/18}{50.68/111} = \frac{2.740}{0.4566} = 6\]
Therefore \(x = 6\); the hydrate is \(CaCl_2\cdot 6H_2O\).
(d) Calcium compounds
- (i) Manufacture of cement: calcium trioxocarbonate(IV), \(CaCO_3\) (limestone).
- (ii) As a desiccant: anhydrous calcium chloride, \(CaCl_2\).
- (iii) Production of plaster of Paris: calcium tetraoxosulphate(VI), \(CaSO_4\cdot\tfrac{1}{2}H_2O\) (from gypsum).
- (iv) To neutralise acidic soils: calcium oxide (quicklime, \(CaO\)) or calcium hydroxide (slaked lime).
(e) Electrolysis of concentrated sodium chloride solution (graphite electrodes)
- (i) Ions present: \(Na^+\), \(H^+\), \(Cl^-\) and \(OH^-\) (from the salt and from water).
- (ii) Products: I. Anode: chlorine gas (\(Cl_2\)); II. Cathode: hydrogen gas (\(H_2\)).
- (iii) By-product: sodium hydroxide (\(NaOH\)) solution.
