IGCSE Chemistry 0620: Reversible Decomposition of Ammonium Chloride

A small amount of white ammonium chloride solid is placed in the closed end of a long test tube. The tube is held at a slight angle and heated strongly at the closed end, while the open end of the tube remains much cooler.

(a) Describe what would be observed at the heated end of the tube, and further along the tube in the cooler region, as heating continues. [2]

(b) Ammonium chloride decomposes on heating to form two colourless gases, ammonia and hydrogen chloride. Write a balanced symbol equation, including state symbols and the $\rightleftharpoons$ symbol, for this reaction. [2]

(c) Explain, in terms of the forward and reverse reactions taking place in different parts of the tube, why this experiment provides evidence that the decomposition of ammonium chloride is reversible. [2]

Part (a): Observations along the tube

Ammonium chloride decomposes on strong heating into two gases. Since both gases are colourless, no visible smoke or coloured fumes form inside the tube. Instead:

At the heated end, the white solid gradually disappears, because it is decomposing into ammonia gas and hydrogen chloride gas.
Further along the tube, in the cooler region, a new deposit of white solid forms on the inside of the glass, some distance from the flame.

Part (b): Writing the symbol equation

The equation must show:

The correct formula for ammonium chloride, ammonia and hydrogen chloride.
Correct balancing (the equation is already balanced 1:1:1, since one formula unit of $\text{NH}_4\text{Cl}$ contains exactly the atoms needed for one $\text{NH}_3$ and one $\text{HCl}$ ).
State symbols showing a solid decomposing into two gases.
The $\rightleftharpoons$ symbol, since the change can go in either direction.

$\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}$

Checking atoms on each side: left-hand side has 1 N, 4 H, 1 Cl; right-hand side has $(1\text{ N} + 3\text{ H}) + (1\text{ H} + 1\text{ Cl}) = 1\text{ N}, 4\text{ H}, 1\text{ Cl}$ . The equation balances.

Part (c): Why this shows the reaction is reversible

Near the flame, the temperature is high, so the forward reaction (solid ammonium chloride breaking down into ammonia and hydrogen chloride gases) happens much faster than the reverse reaction. This is why the white solid disappears at the hot end.

As the two gases spread away from the flame into the cooler part of the tube, conditions there favour the reverse reaction instead: the ammonia and hydrogen chloride gas particles collide and recombine to re-form solid ammonium chloride. This is why a new white deposit appears further along the tube.

Because both the forward reaction (at the hot end) and the reverse reaction (at the cool end) of the very same chemical system are observed happening, this experiment provides direct evidence that the decomposition of ammonium chloride is a reversible reaction, not a one-way change.

Final answers

(a) White solid disappears at the hot end; a new white solid forms in the cooler part of the tube.
(b) $\text{NH}_4\text{Cl(s)} \rightleftharpoons \text{NH}_3\text{(g)} + \text{HCl(g)}$
(c) The forward reaction occurs at the hot end (solid $\to$ gases) and the reverse reaction occurs at the cool end (gases $\to$ solid); seeing both directions happen is evidence the reaction is reversible.