OCR A Level Chemistry A Unified chemistry November 2020 question paper H432/03

This question is about organic chemistry.

This part is about two practical techniques used in organic preparations.

Complete the missing labels on the diagram and name the technique.

Name of technique:........

Draw a labelled diagram to show apparatus set up for filtration under reduced pressure (vacuum filtration).

This part is about amines.

Amine A is a liquid at room temperature and pressure.

When vaporised, 0.202 g of the amine produces 72.0 cm³ of gas at 1.00 × 10⁵ Pa and 100 °C. The¹³C NMR spectrum of amine A has 3 peaks.

Determine the molecular formula of A and suggest a possible structure for amine A.

Molecular formula of A........

Structure of A

Propane, C₃H₈, (boiling point −42 °C) is used as ‘camping gas’. A student plans to determine the enthalpy change of combustion of propane, _cH (C₃H₈), by two methods.

The student first carries out an experiment using the apparatus below.

Determine the enthalpy change of combustion of propane, in kJ mol⁻¹.

Give your answer to 3 significant figures.

_cH (C₃H₈) =........ kJ mol⁻¹

The student finds that the experimental enthalpy change _cH (C₃H₈) is much less exothermic than the accurate standard enthalpy change _cH (C₃H₈) in databases.

One reason could be that the student’s experiment had not been carried out under standard conditions.

Suggest two other reasons for this difference in enthalpy change.

A student carries out an investigation to identify two metals, M and X, by two different methods.

In Stage 1, a redox reaction takes place between M and HC l (aq), forming hydrogen and a solution containing M in the +2 oxidation state.

Write an overall equation, with state symbols, for this reaction. Write half-equations for the oxidation and reduction processes.

Overall equation........

Oxidation half-equation........

Reduction half-equation........

In Stage 1, suggest two observations that would confirm that all of metal M has reacted.

In Stage 3, write the ionic equation for the reaction taking place in the titration.

Metal M can be identified following the steps below.

1. The amount, in mol, of excess HC l (aq) that remains after the reaction of M with HC l (aq).

2. The amount, in mol, of HC l (aq) that reacted with M.

3. The identity of metal M.

Analyse the results to identify metal M.

The student is provided with the carbonate of an unknown metal, X₂CO₃.

The student measures the mass loss when the X₂CO₃ is reacted with an excess hydrochloric acid. The equation is shown below.

X₂CO₃(s) + 2HCl (aq) → 2XCl (aq) + CO₂(g) + H₂O(l)

The reaction is carried out using this method:

Step 1 Add 100 cm³ HCl (aq) to a conical flask and weigh.

Step 2 Add X₂CO₃ to the conical flask and immediately reweigh.

Step 3 After 5 minutes, reweigh the conical flask and contents.

Calculate the amount, in mol, of CO₂ released in the reaction.

After analysing the results, the student was told that their molar mass of X₂CO₃ was incorrect.

The student evaluated the experiment for possible reasons for the incorrect result.

The student wondered whether the reaction was complete when the mass was recorded after 5 minutes (Step 3).

How could the student modify the experimental procedure to be confident that the reaction was complete?

The student finds out that carbon dioxide is slightly soluble in water.

State and explain how the solubility of CO₂ would affect the calculated molar mass of X₂CO₃.

State and explain how increasing the temperature affects the position of this equilibrium and whether the forward reaction is exothermic or endothermic.

Plot a graph of ln K_p against using the axes provided on the opposite page. Use your graph and equation 5.1 to determine H, in kJ mol⁻¹, for this equilibrium.

Give your answer to 3 significant figures.

H =........ kJ mol⁻¹

This question is about two different types of acid found in organic compounds, carboxylic acids and sulfonic acids, as shown in Fig. 6.1.

Complete Table 6.1 to predict bond angles a and b and name the shapes which makes these bond angles in the functional groups of carboxylic acids and sulfonic acids.

Ethanoic acid, CH₃COOH, and methanesulfonic acid, CH₃SO₂OH, are both monobasic acids. The pK_a values are shown in the table.

A student suggests that 1.0 mol dm⁻³ CH₃SO₂OH should have a lower pH value than 1.0 mol dm⁻³ CH₃COOH.

Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH₃SO₂OH with water and explain, with reasons, whether the student is correct.

Label the conjugate acid–base pairs: A1, B1 and A2, B2.

Carboxylic acids and sulfonic acids both form esters.

Sulfonic acid esters can be hydrolysed by aqueous alkali. The equation shows the alkaline hydrolysis of a sulfonic acid ester.

CH₃SO₂OCH₃ + OH⁻ → CH₃SO₂O⁻ + CH₃OH

In the 3 boxes below, add curly arrows to show the mechanism for this reaction.

In the first box, the hydroxide ion acts as a nucleophile.