Advanced Inorganic & Organic Chemistry Core Practicals (Edexcel A Level Chemistry): Flashcards

True.

Bromide ions (Br^-) react with Ag⁺ (aq) to form silver bromide (AgBr), which appears as a cream precipitate. Chloride gives white and iodide gives yellow.

True.

Impurities disrupt the crystal lattice, reducing the energy needed to melt the solid. This lowers the observed melting point and produces a broad melting point range rather than the sharp point seen for a pure substance.

The solution changes from colourless to a pale pink tinge. The purple MnO₄^- (aq) is reduced to near-colourless Mn²⁺ (aq); the pale pink marks the first permanent excess of manganate(VII).

In Core Practical 12, ethanol is added to the copper complex solution to cause the product to crystallise out of solution. The mixture is then cooled in an ice bath.

To test for sulfate ions, acidify the sample with hydrochloric acid, then add aqueous barium chloride. A white precipitate of barium sulfate confirms sulfate ions are present.

In Core Practical 16, the aspirin mixture is heated in a water bath at approximately 60 °C for about 20 minutes before being poured into cold water to crystallise.

In a KMnO₄ / Fe²⁺ titration, the molar ratio of MnO₄^- to Fe²⁺ is 1 : 5

The half-equations give 5e^- per MnO₄^- and 1e^- per Fe²⁺, so five iron(II) ions are needed per manganate ion.

CuSO₄•5H₂O + 4NH₃ → Cu(NH₃)₄SO₄•H₂O + 4H₂O

The copper(II) sulfate pentahydrate reacts with four ammonia ligands in a ligand substitution reaction to form the deep-blue tetraamminecopper(II) sulfate monohydrate complex.

With an aldehyde, Tollens' reagent produces a silver mirror on the test tube wall; Ag⁺ is reduced to Ag by the aldehyde, which is oxidised to a carboxylic acid. A ketone cannot be oxidised under these conditions, so no silver mirror forms.

Using the minimum solvent ensures the product remains in solution only at high temperature. Excess solvent would mean more product stays dissolved even on cooling, reducing the yield of recovered crystals.

An iodine-thiosulfate titration is a redox titration in which sodium thiosulfate (Na₂S₂O₃) reduces iodine (I₂) to iodide ions, used to determine the concentration of an oxidising agent that first oxidised iodide to iodine.

Percentage yield is a measure of the efficiency of a reaction, calculated as:

(actual yield ÷ theoretical yield) × 100

2,4-Dinitrophenylhydrazine (2,4-DNPH) is used to test for the presence of a carbonyl group (aldehyde or ketone). A positive result is a deep-orange precipitate, which can be recrystallised and its melting point used to identify the specific compound.

Melting point analysis is used to assess the purity and confirm the identity of a solid product. The measured value is compared with a data book value; a pure substance has a sharp, well-defined melting point, whereas impure samples show a depressed and broadened range.

Starch is added when the iodine colour has faded to a pale straw yellow. Adding it too early causes starch to bind iodine too tightly, making the endpoint difficult to observe. The blue-black colour then disappears sharply at the true endpoint.

1. The reaction may not go to completion, leaving some product in solution (not crystallising out).

2. Product is lost during transfers between vessels or during filtration and washing steps.

Add dilute hydrochloric acid to the sample. CO₃^2- reacts to produce carbon dioxide gas, which is bubbled through limewater (calcium hydroxide solution). A milky/cloudy precipitate confirms CO₂ and therefore carbonate ions.

Pure aspirin melts at 135 °C. Before measurement the sample must be completely dry and finely powdered — achieved by crushing on filter paper with a spatula, which also absorbs any residual moisture.

In an iodine-thiosulfate titration, 2 moles of S₂O₃^2- (aq) react with 1 mole of I₂ (aq).

Equation: 2S₂O₃^2- (aq) + I₂ (aq) → 2I^- (aq) + S₄O₆^2- (aq)

True.

Ethanol has a low flash point and its vapour ignites readily. All sources of ignition must be removed from the work area when ethanol is in use.

True.

Primary and secondary alcohols are oxidised by Cr₂O₇^2- (aq) / H⁺, reducing the chromium from Cr⁶⁺ (orange) to Cr³⁺ (green). Tertiary alcohols cannot be oxidised and give no colour change.

False.

Repeated recrystallisation increases purity but decreases yield, because some product is lost into solution each time the solid is re-dissolved.

1. Write the two half-equations.

2. Deduce the overall equation and molar ratio.

3. Calculate moles of MnO₄^- from titre and concentration.

4. Use the ratio to find moles of reductant.

5. Scale to original solution volume.

6. Calculate concentration or percentage purity as required.

Cold ethanol dissolves impurities but does not dissolve significant amounts of the copper complex product at low temperature. Water would dissolve the product, reducing the yield, whereas cold ethanol preserves the product while removing unwanted contaminants.

Effervescence is observed as carbon dioxide gas is evolved. The carboxylic acid reacts with Na₂CO₃ (s) in an acid-base reaction, producing CO₂, water, and the sodium salt of the acid.

The impure aspirin is recrystallised by dissolving it in ethanol and then pouring the solution into 40 cm³ cold water. The product is recovered using Buchner filtration.

True.

Impurities reduce the number of moles of the active species (e.g. Fe²⁺) per gram of sample, so the calculated mass of the active species is lower relative to the total sample mass, giving a lower percentage purity.

The molar ratio of CuSO₄ to Cu(NH₃)₄SO₄•H₂O in Core Practical 12 is 1 : 1 so all moles calculations use a 1:1 ratio.

Silver halide precipitates can be further distinguished using ammonia solution: AgCl dissolves in dilute ammonia solution, AgBr dissolves only in concentrated ammonia solution, and AgI is insoluble in ammonia.

A water bath provides gentle, even heating and prevents the temperature from exceeding 100 °C at standard pressure, avoiding decomposition of the product or reagents. It also eliminates a fire risk from heating ethanoic anhydride, which is flammable.