Module 5: Equilibrium and Acid Reactions

Which statement describes a chemical system at dynamic equilibrium?

Solid calcium carbonate is heated strongly in an open crucible: CaCO₃(s) → CaO(s) + CO₂(g). Why does this system not reach equilibrium?

How does a static equilibrium differ from a dynamic equilibrium?

A saturated solution of lead(II) iodide sits over excess solid in a sealed flask at 25 °C. A few crystals of lead(II) iodide made with radioactive iodine are added. Hours later the mass of undissolved solid is the same as just after the crystals were added, but the solution is now radioactive. What does this show?

Octane burns in a car engine: 2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂O(g). ΔH is large and negative. Which statement best explains why this reaction goes to completion rather than reaching equilibrium?

Photosynthesis, 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g), has ΔH = +2803 kJ mol⁻¹ and a negative ΔS. Which statement is correct?

For CaCO₃(s) → CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹ and ΔS = +161 J K⁻¹ mol⁻¹. Treating both as constant, above what temperature does ΔG become negative?

The graph shows the rates of the forward and reverse reactions for a reaction started with only reactants in a sealed flask at constant temperature. Which statement is true at 8 minutes?

Hydrogen and iodine gases are sealed in a flask at a constant high temperature: H₂(g) + I₂(g) ⇌ 2HI(g). Using collision theory, why does the rate of the forward reaction fall as the reaction proceeds?

Pink hydrated cobalt(II) chloride, CoCl₂.6H₂O, turns blue when heated. When a few drops of water are added to the cooled blue solid, it turns pink again. What does this show?

A sealed glass tube holds an equilibrium mixture: 2NO₂(g) ⇌ N₂O₄(g), ΔH = −57 kJ mol⁻¹. NO₂ is brown and N₂O₄ is colourless. The tube is placed in hot water. What happens?

An equilibrium mixture of NO₂ (brown) and N₂O₄ (colourless), 2NO₂(g) ⇌ N₂O₄(g), is in a gas syringe at constant temperature. The plunger is pushed in quickly to halve the volume and then held. Viewed through the side of the syringe, how does the colour change?

Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) is at equilibrium. Fe³⁺ is pale yellow, SCN⁻ is colourless and FeSCN²⁺ is deep red. A few crystals of sodium fluoride are added; fluoride ions bind Fe³⁺ ions as the colourless ion FeF²⁺. What change is seen, and why?

A catalyst is added to a gaseous system already at equilibrium, at constant temperature. What happens?

Co(H₂O)₆²⁺(aq) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6H₂O(l). Co(H₂O)₆²⁺ is pink and CoCl₄²⁻ is blue. A purple equilibrium mixture turns blue in a hot water bath and pink in ice water. What does this show?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is at equilibrium in a sealed steel vessel of fixed volume. Argon, which does not react, is pumped in at constant temperature, raising the total pressure. What happens to the amount of ammonia?

H₂(g) + I₂(g) ⇌ 2HI(g) is at equilibrium in a sealed gas syringe held at a high temperature; I₂ is purple and the other gases are colourless. The volume is halved at constant temperature. What is observed?

For an exothermic reaction at equilibrium, which statement explains why raising the temperature favours the reverse reaction?

The graph shows the concentrations in a sealed vessel of fixed volume holding N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. What change was made at 10 minutes?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. Which conditions give the greatest equilibrium yield of ammonia?

Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) is at equilibrium. The mixture is diluted with an equal volume of water at the same temperature. Which statement is correct?

What is the equilibrium expression for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)?

Ethanoic acid ionises in water: CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq). What is the expression for its equilibrium constant?

At a certain temperature, an equilibrium mixture for H₂(g) + I₂(g) ⇌ 2HI(g) contains 0.0120 mol L⁻¹ H₂, 0.00800 mol L⁻¹ I₂ and 0.0700 mol L⁻¹ HI. What is Keq at this temperature?

0.200 mol of N₂O₄ is sealed in an empty 2.00 L flask and heated to a fixed temperature: N₂O₄(g) ⇌ 2NO₂(g). At equilibrium the flask holds 0.0600 mol of NO₂. What is Keq at this temperature?

For CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), Keq = 4.00 at a certain temperature. A mixture at that temperature contains 0.200 mol L⁻¹ CO, 0.300 mol L⁻¹ H₂O, 0.400 mol L⁻¹ CO₂ and 0.500 mol L⁻¹ H₂. Which statement is correct?

The table gives Keq at three temperatures for a gas reaction X(g) + Y(g) ⇌ Z(g). Which conclusion is supported?

At a certain temperature N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has Keq = 0.500. What is Keq for 4NH₃(g) ⇌ 2N₂(g) + 6H₂(g) at the same temperature?

At 25 °C, N₂(g) + O₂(g) ⇌ 2NO(g) has a Keq of the order of 10⁻³¹. What does this value show?

At a certain temperature, H₂(g) + I₂(g) ⇌ 2HI(g) has Keq = 49.0. A flask starts with 0.100 mol L⁻¹ of H₂ and 0.100 mol L⁻¹ of I₂ and no HI. What is [HI] at equilibrium?

Lactic acid is a weak monoprotic acid with Ka = 1.4 × 10⁻⁴ at 25 °C. What is the pH of a 0.200 mol L⁻¹ solution of lactic acid at 25 °C?

Solutions are mixed so that, before any reaction, [Fe³⁺] = 1.00 × 10⁻³ mol L⁻¹ and [SCN⁻] = 2.00 × 10⁻⁴ mol L⁻¹: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). In a colourimeter a 2.00 × 10⁻⁴ mol L⁻¹ FeSCN²⁺ standard reads absorbance 0.600 and the equilibrium mixture reads 0.270; absorbance is proportional to [FeSCN²⁺]. What is Keq?

At 1000 K, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) has Keq = 280. An equilibrium mixture at 1000 K contains 0.0500 mol L⁻¹ SO₂ and 0.0200 mol L⁻¹ O₂. What is [SO₃]?

Sodium chloride dissolves in water. How are the water molecules arranged around the separated ions?

Calcium phosphate dissolves slightly: Ca₃(PO₄)₂(s) ⇌ 3Ca²⁺(aq) + 2PO₄³⁻(aq). What is the expression for Ksp?

Ksp of silver chloride is 1.77 × 10⁻¹⁰ at 25 °C, and its molar mass is 143.35 g mol⁻¹. What mass of silver chloride dissolves in 1.00 L of water at 25 °C?

Ksp of lead(II) iodide is 9.8 × 10⁻⁹ at 25 °C: PbI₂(s) ⇌ Pb²⁺(aq) + 2I⁻(aq). What is [I⁻] in a saturated solution of lead(II) iodide at 25 °C?

Ksp of magnesium hydroxide is 5.61 × 10⁻¹² at 25 °C: Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq). What is the solubility of magnesium hydroxide in 0.0100 mol L⁻¹ sodium hydroxide solution at 25 °C?

50.0 mL of 0.0400 mol L⁻¹ lead(II) nitrate is mixed with 50.0 mL of 0.0400 mol L⁻¹ sodium chloride at 25 °C. Ksp of lead(II) chloride is 1.70 × 10⁻⁵ at 25 °C. Which statement is correct?

Which pair of solutions forms a precipitate when they are mixed?

Barium nitrate solution is mixed with sodium sulfate solution and a white precipitate forms. What is the net ionic equation?

Some Aboriginal communities traditionally prepared cycad seeds, which contain a water-soluble toxin, by crushing them and leaving them in running water for days to weeks before cooking. Why is running water more effective than a still container of water?

At 25 °C, 0.0167 g of calcium fluoride dissolves in 1.00 L of water to give a saturated solution: CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq). Use Ca 40.08 and F 19.00. What is Ksp of calcium fluoride at 25 °C?

Solid ammonium nitrate dissolves readily in water even though the solution becomes colder. Which statement explains why it dissolves?

The table gives ΔH and ΔS for four gas reactions. Treating both as constant, which reaction has a negative ΔG at 298 K but a positive ΔG at 1000 K?

A class models dynamic equilibrium with two beakers. Beaker A starts with 100 mL of water and beaker B is empty. In each cycle, two students act at the same moment: one moves 20% of the water in A into B, and the other moves 30% of the water in B into A (both measured at the start of the cycle). After many cycles the volumes stop changing. Then 50 mL of water is poured into B and the cycles continue. What volumes does the model settle at?

A gas syringe holds an equilibrium mixture at constant temperature: 2NO₂(g) ⇌ N₂O₄(g), Keq = 8.00, with [NO₂] = 0.0200 mol L⁻¹ and [N₂O₄] = 0.00320 mol L⁻¹. The plunger is pushed in to halve the volume and held there until equilibrium returns. What is the ratio of the new equilibrium [NO₂] to the original [NO₂]?

For the decomposition 2HI(g) ⇌ H₂(g) + I₂(g), ΔH is taken as +9 kJ mol⁻¹ and the forward activation energy is 184 kJ mol⁻¹. On a gold surface the forward activation energy falls to 105 kJ mol⁻¹. With the gold surface present, what is the activation energy of the reverse reaction, and what happens to the equilibrium yield of hydrogen if the temperature is raised?

A purple solution is at equilibrium at room temperature: Co(H₂O)₆²⁺(aq) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6H₂O(l). Co(H₂O)₆²⁺ is pink and CoCl₄²⁻ is blue. The solution is diluted with an equal volume of water at the same temperature. Which statement is correct?

A sealed vessel of fixed volume holds an equilibrium mixture at a constant temperature: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), with [CO] = [H₂O] = 0.100 mol L⁻¹ and [CO₂] = [H₂] = 0.200 mol L⁻¹. Steam is injected so that [H₂O] rises at once to 0.400 mol L⁻¹, and the temperature is kept constant. What is [H₂] when equilibrium is re-established?

0.400 mol of ammonia is sealed in an empty 2.00 L vessel and held at a constant high temperature until equilibrium is reached; the vessel then contains 0.0800 mol of nitrogen. What is Keq for N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at this temperature?

A 0.150 mol L⁻¹ solution of a weak monoprotic acid HA has a pH of 2.85 at 25 °C: HA(aq) + H₂O(l) ⇌ A⁻(aq) + H₃O⁺(aq). 10.0 mL of this solution is diluted to 100.0 mL with water at 25 °C. What is the pH of the diluted solution?

A solution at 25 °C contains 0.0100 mol L⁻¹ Ba²⁺ and 0.0100 mol L⁻¹ Ca²⁺. Solid sodium sulfate is added a little at a time, with no change in volume. At 25 °C, Ksp of barium sulfate is 1.08 × 10⁻¹⁰ and Ksp of calcium sulfate is 4.93 × 10⁻⁵. What is [Ba²⁺] at the moment calcium sulfate begins to precipitate?

Excess calcium hydroxide is stirred with water at 25 °C and the mixture is filtered. A 25.00 mL aliquot of the clear filtrate needs 26.40 mL of 0.0200 mol L⁻¹ hydrochloric acid to neutralise its hydroxide ions. What value of Ksp for calcium hydroxide do these results give?

In an insulated cup, 8.00 g of ammonium nitrate (NH₄NO₃, 80.05 g mol⁻¹) dissolves completely in 100.0 g of water and the temperature falls by 6.10 °C. Assume all the heat comes from the 100.0 g of water (specific heat capacity 4.18 × 10³ J kg⁻¹ K⁻¹). Which statement gives ΔH for dissolving ammonium nitrate and a correct conclusion about ΔS?