39. On heating blue coloured copper(II) nitrate, a black solid, oxygen and a brown gas X are formed.
(a) Write a balanced chemical equation.
(b) Identify the brown gas X.
(c) Name the type of reaction.
(d) What could be the pH range of an aqueous solution of X?
(a) \[\boxed{\;2\,Cu(NO_3)_2\,(s) \;\xrightarrow{\Delta}\; 2\,CuO\,(s) + 4\,NO_2\,(g) + O_2\,(g)\;}\]
(b) X is nitrogen dioxide, \(NO_2\) (brown gas).
(c) Thermal (heat) decomposition; it is also a redox change.
(d) \(NO_2\) dissolves in water forming acids (\(HNO_2\) and \(HNO_3\)), hence the solution is acidic, typically \(\text{pH} \approx 3\text{–}4\) (certainly < 7).
Give characteristic tests for the gases: (a) CO2 (b) SO2 (c) O2 (d) H2.
(a) CO2: Turns freshly prepared lime water milky (\(Ca(OH)_2 + CO_2 \to CaCO_3\downarrow + H_2O\)); in excess, milkiness clears (\(Ca(HCO_3)_2\) forms). Extinguishes a burning splint.
(b) SO2: Pungent smell; decolourises acidified \(KMnO_4\) (purple → colourless) or turns acidified \(K_2Cr_2O_7\) orange → green; turns moist blue litmus red then bleaches.
(c) O2: Rekindles a glowing splint; absorbed by alkaline pyrogallol (solution turns brown).
(d) H2: Burns with a pop sound when a burning/glowing splint is brought near (forms water).
41. What happens when a piece of
(a) zinc is added to copper sulphate solution?
(b) aluminium is added to dilute hydrochloric acid?
(c) silver is added to copper sulphate solution?
Write the balanced chemical equations wherever a reaction occurs.
(a) Zinc being more reactive displaces copper: \(\boxed{Zn + CuSO_4 \to ZnSO_4 + Cu}\). Copper is deposited.
(b) Aluminium reacts with \(HCl\) (after oxide film is disrupted) to give \(AlCl_3\) and \(H_2\): \(\boxed{2Al + 6HCl \to 2AlCl_3 + 3H_2}\). Temperature rises (exothermic).
(c) Silver is less reactive than copper; no displacement occurs. No reaction.
42. What happens when zinc granules are treated with dilute solutions of
(i) \(H_2SO_4\), (ii) \(HCl\), (iii) \(HNO_3\), (iv) \(NaCl\), (v) \(NaOH\)? Write equations wherever applicable.
(i) & (ii) Zinc liberates hydrogen and forms salt: \(Zn + H_2SO_4 \to ZnSO_4 + H_2\); \(Zn + 2HCl \to ZnCl_2 + H_2\).
(iii) With dilute \(HNO_3\), \(H_2\) is not evolved (oxidising acid). Example: \(3Zn + 8HNO_3(dil) \to 3Zn(NO_3)_2 + 2NO + 4H_2O\) (or \(N_2O\) depending on conditions).
(iv) Aqueous \(NaCl\): no reaction (neutral salt) under ordinary conditions.
(v) With \(NaOH\) (zinc is amphoteric): hydrogen is evolved and sodium zincate forms, e.g. \(Zn + 2NaOH + 2H_2O \to Na_2[Zn(OH)_4] + H_2\) (or \(Na_2ZnO_2 + H_2\) in hot concentrated base).
43. A drop of barium chloride solution is added to aqueous sodium sulphite and a white precipitate forms.
(a) Write the balanced equation.
(b) What other name can be given to this reaction?
(c) On adding dilute HCl the precipitate disappears. Why?
(a) \(\boxed{BaCl_2(aq) + Na_2SO_3(aq) \to BaSO_3(s)\downarrow + 2NaCl(aq)}\)
(b) Double displacement / precipitation reaction.
(c) \(BaSO_3\) dissolves reacting with acid to give soluble \(BaCl_2\) and \(SO_2\) gas: \(BaSO_3 + 2HCl \to BaCl_2 + SO_2\uparrow + H_2O\); hence the white ppt disappears.
44. You have two containers made of copper and aluminium and the solutions: dilute \(HCl\), dilute \(HNO_3\), \(ZnCl_2\) and water. In which containers can these solutions be safely kept?