(a) Explain the differences in the reactions of... | WAEC SSCE - Chemistry - 1989 (Essay)

Question 1 Report

(a) Explain the differences in the reactions of zinc with dilute trioxonitrate (V) acid and zinc with dilute hydrochloric acid.

(b) Write equations to illustrate how ammonia gas can be converted into trioxonitrate (V) acid.

(c) Calculate the mass of sodium trioxonitrate (V) produced when 30.0g of pure sodium hydroxide reacts with 100cm\(^3\) of 1.00 M trioxonitrate (V) acid. (H =1, N = 14, 0 = 16, Na = 23)

(d) Write the equations for the decomposition by heat of:

(i) sodium trioxonitrate(V);

(ii) copper (II) trioxonitrate (V);

(iii) mercury (II) trioxonitrate (V);

Answer

(a) Zinc with dilute HNO₃ vs dilute HCl

Dilute hydrochloric acid is a non-oxidizing acid. Because zinc lies above hydrogen in the activity series, it displaces hydrogen ions as hydrogen gas: \[Zn_{(s)} + 2HCl_{(aq)} \rightarrow ZnCl_{2(aq)} + H_{2(g)}\]

Trioxonitrate(V) acid (nitric acid) is a strong oxidizing acid. The nitrate ion, rather than the hydrogen ion, is reduced, so hydrogen gas is not evolved; instead oxides of nitrogen (or ammonium salt with very dilute acid) are formed, for example: \[3Zn + 8HNO_3 \rightarrow 3Zn(NO_3)_2 + 2NO + 4H_2O\]

(b) Ammonia to trioxonitrate(V) acid (Ostwald process)

\[4NH_3 + 5O_2 \xrightarrow{Pt} 4NO + 6H_2O\]

\[2NO + O_2 \rightarrow 2NO_2\]

\[4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3\]

(c) Mass of sodium trioxonitrate(V)

\[NaOH + HNO_3 \rightarrow NaNO_3 + H_2O\]

Moles of HNO₃ = \(1.00 \times \frac{100}{1000} = 0.100\,mol\). Moles of NaOH = \(\frac{30}{40} = 0.75\,mol\).

The acid is the limiting reagent, so moles of NaNO₃ = 0.100 mol. Molar mass of NaNO₃ = 23 + 14 + 48 = 85 g mol^-1.

\[\text{mass} = 0.100 \times 85 = 8.5\,g\]

(d) Decomposition by heat

(i) \[2NaNO_3 \rightarrow 2NaNO_2 + O_2\]
(ii) \[2Cu(NO_3)_2 \rightarrow 2CuO + 4NO_2 + O_2\]
(iii) \[2Hg(NO_3)_2 \rightarrow 2Hg + 4NO_2 + O_2\]

Read lesson note on Acids, Bases And Salts (JAMB)

Answer Details