Flashcards · 9202 · Topic 8

The rate and extent of chemical change flashcards

26 cards on Topic 8, subtopics 3.8.1 to 3.8.4. Pick a subtopic or study them all.

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3.8.1 Rate of reaction

Ways to calculate a mean rate of reaction, with units: Mass of reactant used ÷ time (g/s); volume of gas formed ÷ time (cm³/s); moles ÷ time (mol/s). For a time to a visible change, 1 ÷ time (s⁻¹).
A gas syringe reads 21 cm³ at 20 s and 57 cm³ at 50 s. Mean rate over this interval?: (57 − 21) ÷ (50 − 20) = 36 ÷ 30 = 1.2 cm³/s
Collision theory and activation energy: Particles react only when they collide with sufficient energy. Activation energy is the minimum energy particles need for a reaction to happen.
Why does raising the temperature increase the rate?: Particles move faster, so they collide more frequently and more energetically. A greater proportion of collisions have at least the activation energy.
How do concentration, pressure and surface area increase the rate?: Concentration and pressure: more particles in the same volume. Surface area: more particles exposed. Each gives more frequent collisions — but not more energetic ones.
Catalyst: what it does and why industry uses one: Alters the rate of a reaction and is not used up (same mass at the end); it does not change the amount of product. Industry: faster production, lower temperatures and pressures (less energy), and it can be reused, reducing costs.
A product curve levels off. What does that mean, and what does a faster run with the same reactants look like?: The reaction has stopped: a reactant is used up. A faster run starts steeper, levels off sooner, at the same height.

3.8.2 Factors affecting equilibrium

Equilibrium (9202 definition): In a closed system, the forward and backward reactions occur at exactly the same rate. Both reactions continue; the amounts stay constant but are usually not equal.
Effect of raising the temperature on an equilibrium: The yield of the endothermic direction increases; the yield of the exothermic direction decreases. (Lowering the temperature does the opposite.)
Effect of increasing the pressure on an equilibrium of gases: It favours the reaction producing the fewest molecules of gas. If both sides have the same number of molecules, the yield does not change.
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). Effect of increasing the pressure?: 1 molecule on the left, 2 on the right, so the backward reaction is favoured: more PCl₅ remains.
Does a catalyst change the yield at equilibrium?: No. It speeds up the forward and backward reactions equally, so equilibrium is reached sooner but the amounts are the same.
Four things industry balances when choosing conditions: Yield, rate, cost and safety. For an exothermic forward reaction a low temperature gives the best yield but is too slow, so a compromise temperature is used.

3.8.3 Production of ammonia and sulfuric acid

Haber process: equation, raw materials and conditions: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Nitrogen from the air; hydrogen from natural gas or other sources. Iron catalyst, about 450 °C, about 200 atmospheres.
Haber process: how is ammonia separated, and what happens to unreacted gases?: Cooling: ammonia condenses at a much higher temperature than nitrogen or hydrogen, so it liquefies and is removed. Unreacted nitrogen and hydrogen are recycled.
Why is about 450 °C used in the Haber process?: The forward reaction is exothermic, so a lower temperature would give a higher yield but would be too slow. 450 °C is a compromise.
Why not use a pressure much higher than 200 atmospheres in the Haber process?: Higher pressure gives a higher yield (4 → 2 molecules) and faster rate, but needs stronger, more expensive equipment and more energy, and is more dangerous.
Why are fertilizers made from ammonia needed?: Crops need nitrogen to make proteins, and most cannot take it from the air; harvesting removes nutrients; the population is rising and farmland is limited, so fertilizers maximise food yields.
The three stages of the contact process: 1. S + O₂ → SO₂ 2. 2SO₂ + O₂ ⇌ 2SO₃ (exothermic) 3. H₂O + SO₃ → H₂SO₄
Contact process stage 2: conditions, and why atmospheric pressure is used: Vanadium(V) oxide catalyst, around 450 °C, atmospheric pressure. The yield is already high, so the extra cost of high pressure is not justified.

3.8.4 Redox reactions

Oxidation and reduction in terms of oxygen: Oxidation: gain of oxygen by a substance. Reduction: loss of oxygen from a substance.
Oxidation and reduction in terms of electrons: Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).
In Fe₂O₃ + 3CO → 2Fe + 3CO₂, what is oxidised and what is reduced?: Fe₂O₃ loses oxygen: reduced. CO gains oxygen: oxidised.
Zinc in lead(II) nitrate solution: ionic equation and half equations: Zn + Pb²⁺ → Zn²⁺ + Pb Zn → Zn²⁺ + 2e⁻ (oxidation) Pb²⁺ + 2e⁻ → Pb (reduction)
Is MgO + 2HCl → MgCl₂ + H₂O a redox reaction?: No. The ions only change partners; no particle gains or loses electrons and nothing gains oxygen. Neutralisation, precipitation and carbonate decomposition are not redox.
Fe²⁺ → Fe³⁺ + e⁻: oxidation or reduction?: Oxidation — an electron is lost, so the ion becomes more positive.