(a) Mention one oxide in each case, which (i) used in bleaching (ii) is a redish-brown gas (iii) reacts with NaOH and also with HCI; (iv) dissolves in water...
Question 1 Report
(a) Mention one oxide in each case, which
(i) used in bleaching
(ii) is a redish-brown gas
(iii) reacts with NaOH and also with HCI;
(iv) dissolves in water to give a solution with pH greater than 7;
(v) oxidizes hot, concentrated HCI to chlorine
(b)(i) State three methods that can be used to removo hardness in a sample of water that contains calcium hydrogentrioxocarbonate (IV).
(ii) Explain with the aid of appropriate equation, why it is not advisable to build a house with limestone in an environment polluted by sulphur (IV) oxide.
(c)(i) List two compounds of potassium which yield oxygen when heated strongly
(ii) Calculate the amount (in moles) of gas which occupies 250 cm\(^3\) at s.t.p. [1 mole of gas occupies 22.4 dm\(^3\) at s.t.p]
(iii) If 250 cm\(^3\) of a gas at s.t.p. is heated to 27°C at constant pressure, calculate its new volume.
(iv) Explain in terms of the collision theory what happens as a gas is heated at constant pressure.
Answer Details
(a) (i) Used in bleaching: sulphur(IV) oxide, SO2. (ii) Reddish-brown gas: nitrogen(IV) oxide, NO2. (iii) Amphoteric (reacts with NaOH and HCl): aluminium oxide, Al2O3 (or ZnO). (iv) Dissolves in water to give pH greater than 7: sodium oxide, Na2O (or CaO). (v) Oxidizes hot concentrated HCl to chlorine: manganese(IV) oxide, MnO2.
(b)(i) Three methods to remove hardness due to Ca(HCO3)2 (temporary hardness): boiling; adding slaked lime (Clark's method); adding sodium trioxocarbonate(IV) (washing soda).
(ii) Sulphur(IV) oxide pollution forms acid with rain water (SO2 + H2O → H2SO3, and this is oxidized to H2SO4). This acid attacks limestone (CaCO3), wearing the building away: CaCO3 + H2SO3 → CaSO3 + H2O + CO2. The limestone is eroded, so it is unwise to build with it there.
(c)(i) Two potassium compounds that yield oxygen on strong heating: potassium trioxochlorate(V), KClO3, and potassium trioxonitrate(V), KNO3 (or KMnO4).
(ii) Amount of gas in 250 cm3 at s.t.p.:
\[ n=\frac{0.250}{22.4}=0.0112\ \text{mol} \](iii) 250 cm3 at s.t.p. (273 K) heated to 27°C (300 K) at constant pressure:
\[ V_2=250\times\frac{300}{273}=274.7\approx 275\ \text{cm}^3 \](iv) On heating at constant pressure the molecules gain kinetic energy and move faster, colliding more frequently and more forcefully; the gas expands (volume increases) so that the number of collisions per unit area of the walls, and hence the pressure, stays constant.
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