(a) In the laboratory preparation of dry chlorine... | WAEC SSCE - Chemistry - 2020

Question 1 Report

(a) In the laboratory preparation of dry chlorine gas, state the: I. reagents used; II. drying agent III. the mode of collection.

(i) Write the equation for the preparation of chlorine gas. (iii) Write an equation to show how chlorine reacts with hot concentrated NaOH.

(b)i). Name the main raw materials used for the extraction of iron in the blast furnace. (ii) Write the equations of the reactions taking place in the blast furnace.

(iii) What is the name given to the iron obtained directly from the blast furnace? (iv) State why the iron named in 4(b)(iii) have a relatively low melting point?

(c) The following equation represents one of the reaction steps involved in the contact process: \(2\mathrm{SO}_2 + \mathrm{O}_2 \;\text{⇌}\; 2\mathrm{SO}_3 \quad \Delta H = -395.7\ \text{kJ mo-l}\)

(i) Why is the \(\mathrm{SO}_3\) produced during the reaction not dissolved directly in water to form \(\mathrm{H}_2\mathrm{SO}_4\)? (ii) Why is the \(\mathrm{H}_2\mathrm{SO}_4\) regarded as a heavy chemical? (iii) State the property exhibited by tetraoxosulphate (VI) acid in each of the following reaction equations.

I. \(\mathrm{Pb}(\mathrm{NO}_3)_2 + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{PbSO}_4 + 2\mathrm{HNO}_3\) (d) Write a balanced chemical equation for the reaction between propanol and sodium

Answer

(a) Laboratory preparation of dry chlorine

I. Reagents: concentrated hydrochloric acid and manganese(IV) oxide (\(MnO_2\)). (Warm together.)
II. Drying agent: concentrated tetraoxosulphate(VI) acid (\(H_2SO_4\)).
III. Mode of collection: upward delivery (downward displacement of air), because chlorine is denser than air.

(i) Equation for preparation:

\[MnO_{2(s)} + 4HCl_{(aq)} \to MnCl_{2(aq)} + Cl_{2(g)} + 2H_2O_{(l)}\]

(iii) Chlorine with hot concentrated NaOH:

\[3Cl_{2(g)} + 6NaOH_{(aq)} \to 5NaCl_{(aq)} + NaClO_{3(aq)} + 3H_2O_{(l)}\]

(b)(i) Main raw materials for iron extraction in the blast furnace

Iron ore (haematite, \(Fe_2O_3\)), coke (carbon), limestone (\(CaCO_3\)) and hot air (oxygen).

(b)(ii) Reactions in the blast furnace

\[C + O_2 \to CO_2\]\[CO_2 + C \to 2CO\]\[Fe_2O_3 + 3CO \to 2Fe + 3CO_2\]\[CaCO_3 \to CaO + CO_2\]\[CaO + SiO_2 \to CaSiO_3\ (\text{slag})\]

(b)(iii) The iron obtained directly from the blast furnace is called pig iron (cast iron).

(b)(iv) Pig iron has a relatively low melting point because it contains a high proportion of impurities, chiefly about 4% carbon, which lowers its melting point.

(c)(i) Why SO3 is not dissolved directly in water

Dissolving \(SO_3\) directly in water is violently exothermic and produces a dense, corrosive mist (fog) of tetraoxosulphate(VI) acid that is difficult to condense and escapes. Instead the \(SO_3\) is absorbed in concentrated \(H_2SO_4\) to form oleum, which is later diluted safely.

(c)(ii) \(H_2SO_4\) is regarded as a heavy chemical because it is manufactured and used industrially in very large quantities (tonnage).

(c)(iii) In \(Pb(NO_3)_2 + H_2SO_4 \to PbSO_4 + 2HNO_3\), the tetraoxosulphate(VI) acid acts as an acid (a less volatile acid displacing the more volatile trioxonitrate(V) acid and forming an insoluble sulphate).

(d) Propanol with sodium

\[2C_3H_7OH + 2Na \to 2C_3H_7ONa + H_{2}\]

(sodium propoxide and hydrogen gas).

Read lesson note on Chemical Combination (JAMB)

Answer Details