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Flashcards · 9701 · AS Topic 7

Equilibria (AS Level) flashcards

16 cards on AS Topic 7, subtopics 7.1 to 7.2. Pick a subtopic or study them all.

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7.1 Chemical equilibria: reversible reactions, dynamic equilibrium

Dynamic equilibrium
In a closed system, the rates of the forward and reverse reactions are equal and the concentrations of all species remain constant (not equal). The reactions have not stopped.
Le Chatelier's principle
When a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose that change.
Effect on the position of equilibrium of increasing pressure, increasing temperature and adding a catalyst
Pressure: shifts towards the side with fewer moles of gas (no effect if equal). Temperature: shifts in the endothermic direction. Catalyst: no shift; equilibrium is reached sooner.
Which change alters the value of Kc or Kp?
Only temperature. Concentration, pressure and catalysts can shift the position but do not change K. For an exothermic forward reaction, raising the temperature decreases K.
Write Kc and Kp for aA + bB ⇌ cC + dD
Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ) Kp = (pCᶜ × pDᵈ) ÷ (pAᵃ × pBᵇ) Solids and pure liquids are left out.
Partial pressure of a gas
Partial pressure = mole fraction × total pressure, where mole fraction = moles of that gas ÷ total moles of gas.
H₂ + I₂ ⇌ 2HI in 1 dm³: 0.50 mol H₂, 0.50 mol I₂, 3.0 mol HI. Calculate Kc.
Kc = 3.0² ÷ (0.50 × 0.50) = 9.0 ÷ 0.25 = 36. No units: moles of gas are equal on each side.
Haber process conditions and why they are a compromise
N₂ + 3H₂ ⇌ 2NH₃ (exothermic), about 450 °C, 200 atm, iron catalyst. Low temperature would give a higher yield but too slowly; high pressure raises yield but costs more in plant and energy. The catalyst does not change the yield.

7.2 Brønsted-Lowry theory of acids and bases

Brønsted–Lowry acid and base
Acid: a proton (H⁺) donor. Base: a proton (H⁺) acceptor.
Identify the conjugate pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
NH₃/NH₄⁺ and H₂O/OH⁻. Each pair differs by one H⁺.
Strong and weak acids
Strong: completely dissociated in water (HCl, HNO₃, H₂SO₄). Weak: only partially dissociated, an equilibrium (e.g. ethanoic acid). Strong/weak is not the same as concentrated/dilute.
Observable differences between a strong and a weak acid of the same concentration
The strong acid has a lower pH, reacts faster with a reactive metal (more free H⁺) and conducts electricity better (more ions).
Neutralisation (ionic equation)
H⁺(aq) + OH⁻(aq) → H₂O(l). The remaining ions form a salt.
Equivalence-point pH for the four acid–base titration combinations
Strong acid + strong base: 7. Strong acid + weak base: below 7. Weak acid + strong base: above 7. Weak acid + weak base: about 7, with no vertical section.
Choosing an indicator: which for which titration?
Its pH range must lie within the vertical section. Methyl orange (3.1–4.4): strong acid + weak base. Phenolphthalein (8.3–10.0): weak acid + strong base. Either: strong + strong. Neither: weak + weak (no sharp change).
Amphoteric
Able to act as either an acid or a base, e.g. water, HCO₃⁻.

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