Flashcards · J248 · Topic 3

Topic C3 – Chemical Reactions flashcards

35 cards on Topic 3, subtopics C3.1 to C3.4. Pick a subtopic or study them all.

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C3.1 Introducing Chemical Reactions

Which elements exist as diatomic molecules?: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ (the Group 7 elements plus hydrogen, nitrogen and oxygen).
Formula of iron(III) oxide and of magnesium nitrate: Fe³⁺ and O²⁻: lowest common multiple 6, so Fe₂O₃. Mg²⁺ and two NO₃⁻: Mg(NO₃)₂.
The four state symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Pure water is (l); a precipitate is (s).
Half equation for chloride ions forming chlorine: 2Cl⁻ → Cl₂ + 2e⁻ (atoms and charges both balance).
Why might a balance reading change during a reaction in an open container?: Falls: a gas product escapes (e.g. CO₂ from an acid and a carbonate). Rises: a gas from the air joins on (e.g. oxygen forming a metal oxide). Mass is still conserved.
Ionic equation for barium chloride solution + sodium sulfate solutionHigher: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Na⁺ and Cl⁻ are spectator ions.
The mole and the Avogadro constantHigher: One mole contains exactly 6.02214076 × 10²³ elementary entities (classically, as many as atoms in 12 g of carbon-12). Avogadro constant ≈ 6.02 × 10²³ per mole. amount = mass ÷ Mr.
Mass of one methane molecule (Mr 16)Higher: 16 ÷ (6.02 × 10²³) = 2.66 × 10⁻²³ g
2H₂ + O₂ → 2H₂O with 0.30 mol H₂ and 0.12 mol O₂. Which is limiting, and how much water forms?Higher: 0.30 mol H₂ needs 0.15 mol O₂, but only 0.12 mol is present, so oxygen is limiting. Water = 2 × 0.12 = 0.24 mol.

C3.2 Energetics

Exothermic vs endothermic (in terms of the surroundings): Exothermic: energy transferred from the chemicals to the surroundings; their temperature rises. Endothermic: energy transferred from the surroundings to the chemicals; their temperature falls.
Examples of exothermic and endothermic reactions: Exothermic: combustion, neutralisation, many metals + dilute acids. Endothermic: thermal decomposition of a carbonate, hydrogencarbonate + citric or ethanoic acid.
Profile: reactants 140, peak 200, products 90 kJ/mol. Activation energy and energy change?: Activation energy = 200 − 140 = 60 kJ/mol. Energy change = 90 − 140 = −50 kJ/mol, so exothermic.
A candle needs a match to light. Does that make burning endothermic?: No. The match only supplies the activation energy. Overall more energy is released than taken in, so it is exothermic.
Measuring a temperature change in a polystyrene cup: main systematic error and how to reduce it: Heat transfer to or from the room makes every measured change too small. Insulate the cup and use a lid.
Bond breaking and bond making in energy calculationsHigher: Breaking bonds takes in energy; making bonds releases it. Energy change = bonds broken − bonds made. Negative = exothermic.
2H₂O₂ → 2H₂O + O₂. O–H 464, O–O 146, O=O 498 kJ/mol. Energy change?Higher: Broken: 4 × 464 + 2 × 146 = 2148. Made: 4 × 464 + 498 = 2354. Change = 2148 − 2354 = −206 kJ/mol (exothermic).
Why does a value from bond energies differ from a measured one?Higher: Bond energies are average values from many compounds, so the calculation gives only an estimate.

C3.3 Types of Chemical Reactions

Oxidising agent and reducing agent: Oxidising agent: oxidises something else and is itself reduced. Reducing agent: reduces something else and is itself oxidised.
NiO + H₂ → Ni + H₂O: what is reduced, and what is the reducing agent?: Nickel(II) oxide loses oxygen, so it is reduced (the oxidising agent). Hydrogen gains oxygen, so it is oxidised and is the reducing agent.
Redox in terms of electrons: Mg + Zn²⁺ → Mg²⁺ + ZnHigher: OIL RIG: oxidation is loss, reduction is gain of electrons. Mg → Mg²⁺ + 2e⁻ (oxidised). Zn²⁺ + 2e⁻ → Zn (reduced).
What ions do acids and alkalis form in water?: Acids form H⁺ ions (HCl → H⁺ + Cl⁻). Alkalis form OH⁻ ions (LiOH → Li⁺ + OH⁻). An alkali is a soluble base.
Balanced equation: calcium carbonate + nitric acid: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂
Making a pure, dry salt from an insoluble base (PAG C7): Warm the acid; add the base in excess, stirring; filter off the unreacted solid; heat the filtrate to boil off part of the water and leave to crystallise; filter and dry the crystals.
Strong vs weak, and concentrated vs dilute (acids)Higher: Strong/weak: degree of ionisation (hydrochloric, nitric, sulfuric are fully ionised; ethanoic, citric, carbonic partly). Concentrated/dilute: amount of acid in a given volume.
pH falls from 6 to 3. How does the H⁺ concentration change?Higher: Each fall of 1 pH unit is 10 times more H⁺: 10 × 10 × 10 = 1000 times greater.
One advantage of a pH probe over universal indicator: Any one: more precise; not affected by the colour of the solution; can record pH continuously; no subjective colour matching.

C3.4 Electrolysis

Electrolysis and electrolyte: Electrolysis: decomposition of a liquid (molten or aqueous) by passing a direct current through it. Electrolyte: an ionic substance, melted or in solution, that carries the current and is broken down.
Why must an ionic compound be molten or dissolved to be electrolysed?: In the solid, the ions are locked in the lattice and cannot move. Melted or dissolved, the ions are free to move and carry charge.
Products of electrolysing molten sodium chloride with inert electrodes: Sodium at the cathode (negative); chlorine at the anode (positive).
Rules for the products of an aqueous solution (inert electrodes): Cathode: the metal only if it is less reactive than hydrogen (Cu, Ag); otherwise hydrogen. Anode: a halogen if Cl⁻, Br⁻ or I⁻ is present; otherwise oxygen.
Products of electrolysing NaCl(aq) and CuSO₄(aq) with inert electrodes: NaCl(aq): hydrogen at the cathode, chlorine at the anode; sodium hydroxide is left. CuSO₄(aq): copper at the cathode, oxygen at the anode; sulfuric acid is left and the blue colour fades.
Half equation for oxygen forming at the anode from an aqueous solution: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (charge 4− on each side).
Why do hydrogen and oxygen form in a 2 : 1 volume ratio?: Each H₂ needs 2 electrons and each O₂ releases 4, so twice as many hydrogen molecules form as oxygen molecules.
Inert vs non-inert anode in copper sulfate solution: Graphite gives oxygen and a fading colour; copper dissolves and the colour stays.
In electrolysis, what moves through the liquid and what moves through the wires?: Ions move through the liquid; electrons move through the wires.