Unit 6 – Practical Skills in Chemistry II (A Level) flashcards

Topic 6: Unit 6 – Practical Skills in Chemistry II

Core Practical 9a: why are samples run into sodium hydrogencarbonate solution?

To quench the reaction: it removes the H⁺ catalyst, stopping the iodination so the iodine left can be titrated with thiosulfate.

Core Practical 9b (iodine clock): how is concentration kept proportional to volume?

Keep the total volume constant by making it up with water, changing only one solution’s volume at a time. Initial rate ∝ 1/t.

Core Practical 10: which graph gives the activation energy?

ln(1/t) (y) against 1/T in K⁻¹ (x). Ea = −gradient × 8.31, then divide by 1000 for kJ mol⁻¹.

Core Practical 11: finding Ka of a weak acid

Calibrate the pH meter with buffers; find the end-point volume V for one portion; add V/2 of alkali to an identical portion; its pH = pKa, so Ka = 10⁻ᵖᴴ.

Core Practical 12: preparing the half-cells

Clean the metals with emery paper; use a potassium nitrate salt bridge (no precipitates with metal ions); read the emf on a high-resistance voltmeter.

Iodate(V) releasing iodine: equation and ratio to thiosulfate

IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O, so IO₃⁻ : S₂O₃²⁻ = 1 : 6.

0.642 g KIO₃ (Mr 214.0) in 250.0 cm³; each 25.0 cm³ portion needs 22.50 cm³ thiosulfate. Its concentration?

KIO₃ per portion = (0.642 ÷ 214.0) ÷ 10 = 3.00 × 10⁻⁴ mol. Thiosulfate = 6 × 3.00 × 10⁻⁴ = 1.80 × 10⁻³ mol. c = 1.80 × 10⁻³ ÷ 0.02250 = 0.0800 mol dm⁻³.

Fe²⁺, Fe³⁺ and Zn²⁺ with NaOH(aq)

Fe²⁺: green precipitate, browning in air. Fe³⁺: red-brown precipitate. Zn²⁺: white precipitate, dissolving in excess to a colourless solution.

A redox titration result differs from the accepted value by more than the total uncertainty. What does that suggest?

A systematic error that apparatus uncertainty cannot explain, such as air oxidising Fe²⁺ before titration (lowering every result).