Physical Chemistry
9 marks · HardA student investigates the thermal decomposition of ammonium chloride ([1mNH[0m[1m_4[0mCl(s)) in a sealed test tube.
Write a balanced chemical equation (using state symbols) for the thermal decomposition of ammonium chloride.
Explain, in terms of energy change and temperature, whether this decomposition is endothermic or exothermic. Include what you would expect to observe if touching the outside of the test tube during the reaction.
The student seals the test tube and heats it until no more solid disappears. After some time, the temperature is kept constant. Explain, in terms of reversible reactions and dynamic equilibrium, what happens inside the sealed test tube.
List [1mall[0m the substances present in the reaction mixture at equilibrium in the sealed test tube, and explain your answer.
If the student lowers the temperature of the sealed test tube while it is at equilibrium, predict how the position of equilibrium will shift and what visible change, if any, the student might observe. Explain your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) \ce{NH4Cl(s) <=> NH3(g) + HCl(g)} · must show state symbols
(b) The decomposition is endothermic because heat is absorbed from the surroundings. The test tube would feel cooler to the touch as the reaction occurs. · must show mentions heat absorbed, identifies endothermic, correct observation (feels cold/cooler)
(c) Inside the sealed test tube, a reversible reaction occurs between ammonium chloride and the two gases. Dynamic equilibrium is reached when the rate of decomposition (forward reaction) equals the rate of formation of ammonium chloride (reverse reaction). The concentrations of all substances remain constant, even though both reactions continue. · must show explains both forward and reverse reactions, refers to dynamic equilibrium (equal rates), concentrations remain constant
(d) All three: solid \ce{NH4Cl}, \ce{NH3} gas, and \ce{HCl} gas are present, because, at equilibrium in a sealed container, no substances escape and both forward and reverse reactions occur. · must show lists all three substances, relates to sealed system and equilibrium
(e) The position of equilibrium shifts to favour ammonium chloride formation (the exothermic direction), so more solid forms and the solid increases. · must show predicts shift to solid, links to temperature decrease, states more solid forms or increased deposit
Show mark scheme and worked solution
Marking points
A1 (a(i)) · 1 mark · Correct balanced equation with state symbols for the thermal decomposition of ammonium chloride.
B1 (b(i)) · 1 mark · Correctly identifies the reaction as endothermic and explains heat is absorbed.
B2 (b(i)) · 1 mark · Correct observation: states test tube feels cold (or temperature falls).
C1 (c(i)) · 1 mark · Describes both forward and reverse reactions continue in sealed tube.
C2 (c(i)) · 1 mark · Correctly states rates of forward and reverse reactions are equal (dynamic equilibrium).
C3 (c(i)) · 1 mark · Correctly identifies concentrations of substances remain constant at equilibrium.
D1 (d(i)) · 1 mark · Lists all substances present at equilibrium: NH4Cl(s), NH3(g), HCl(g).
D2 (d(i)) · 1 mark · Explains that all remain because sealed system allows equilibrium with no loss of substances.
E1 (e(i)) · 1 mark · Correct prediction and reason: shift towards solid (NH4Cl) and more solid forms as temperature is lowered.
Worked solution
a) The balanced equation with state symbols is:
b) The reaction is endothermic because it absorbs heat from the surroundings. The temperature inside the test tube falls as the reaction occurs, so if you touch the test tube it will feel cool to the touch.
c) In the sealed test tube, both the decomposition of ammonium chloride and the recombination of ammonia and hydrogen chloride gases can occur. Eventually, dynamic equilibrium is established: this means the rate of the forward reaction (decomposition) equals the rate of the reverse reaction (recombination), so the amounts of each substance stay constant even though both reactions are ongoing.
d) At equilibrium, all three are present: solid ammonium chloride, ammonia gas and hydrogen chloride gas. This is because the reaction is reversible and occurs in a sealed container, so none of the substances can escape.
e) Lowering the temperature shifts the equilibrium towards the exothermic direction, so more ammonium chloride solid forms. Visibly, the amount of white solid will increase in the test tube.