AQA A Level Chemistry Paper 1 November 2020 question paper 7405/1

This question is about enthalpy changes.

Figure 1 shows a Born–Haber cycle for the formation of strontium chloride, SrCl₂

Table 1 shows some thermodynamic data.

Use the data in Table 1 to calculate a value for the electron affinity of chlorine.

State why there is a difference between the theoretical and experimental values.

Table 3 shows enthalpy of hydration values for ions of some Group 1 elements.

Explain why the enthalpy of hydration becomes less exothermic from Li⁺ to K⁺

Calcium bromide dissolves in water.

Table 4 shows some enthalpy data.

Use the data in Table 4 to calculate the enthalpy of hydration, in kJ mol^–1, of bromide ions.

This question is about the isotopes of chromium.

Give the meaning of the term relative atomic mass.

A sample of chromium containing the isotopes⁵⁰Cr,⁵²Cr and⁵³Cr has a relative atomic mass of 52.1

The sample contains 86.1% of the⁵²Cr isotope.

Calculate the percentage abundance of each of the other two isotopes.

State, in terms of the numbers of fundamental particles, one similarity and one difference between atoms of⁵⁰Cr and⁵³Cr

The sample of chromium is analysed in a time of flight (TOF) mass spectrometer.

Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

A⁵³Cr⁺ ion travels along a flight tube of length 1.25 m

m = mass of the ion / kg

Calculate the time, in s, for the⁵³Cr⁺ ion to travel down the flight tube to reach the detector.

This question is about Period 3 elements.

Figure 2 shows the second ionisation energies of some elements in Period 3.

Draw a cross (x) on Figure 2 to show the second ionisation energy of silicon.

Identify the element in Period 3, from sodium to argon, that has the highest second ionisation energy.

Give an equation, including state symbols, to show the process that occurs when the second ionisation energy of this element is measured.

If you were unable to identify the element you may use the symbol Q in your equation.

Equation

Explain why the atomic radius decreases across Period 3, from sodium to chlorine.

Identify the element in Period 3, from sodium to chlorine, that has the highest electronegativity.

Phosphorus burns in air to form phosphorus(V) oxide. Give an equation for this reaction.

Propanoic acid (C₂H₅COOH) is a weak acid.

The acid dissociation constant (K_a) for propanoic acid is 1.35 × 10^–5 mol dm^–3

State the meaning of the term weak acid.

Give an expression for the acid dissociation constant for propanoic acid.

A student dilutes 25.0 cm³ of 0.500 mol dm^–3 propanoic acid by adding water until the

Calculate the pH of this diluted solution of propanoic acid.

Give your answer to 2 decimal places.

A buffer solution with a pH of 4.50 is made by dissolving x g of sodium propanoate (C₂H₅COONa) in a solution of propanoic acid. The final volume of buffer solution is 500 cm³ and the final concentration of the propanoic acid is 0.250 mol dm^–3

Calculate x in g For propanoic acid, K_a = 1.35 10^–5 mol dm^–3

Some reactions of the [Al(H₂O)₆]³⁺(aq) ion are shown.

Give the formula of the white precipitate B.

State one other observation when Na₂CO₃(aq) is added to a solution containing

Give an equation for this reaction.

Equation

Give the formula of the complex ion C.

State one condition needed for the formation of C from [Al(H₂O)₆]³⁺(aq) and NaOH(aq).

Give an equation for this reaction.

Equation

Deduce the formula of the complex ion A.

Explain, with the use of an equation, why a solution containing [Al(H₂O)₆]³⁺ has a pH ˂7

Equation

Methanol can be manufactured in a reversible reaction as shown.

Figure 3 shows how the partial pressures change with time at a constant temperature.

Draw a cross (x) on the appropriate axis of Figure 3 when the mixture reaches equilibrium.

At equilibrium the mixture contains 0.120 mol of carbon monoxide.

Calculate the partial pressure, in kPa, of hydrogen in the equilibrium mixture.

Give an expression for the equilibrium constant (K_p) for this reaction.

State the units.

Some more carbon monoxide is added to the mixture in Question 06.2. The new mixture is allowed to reach equilibrium at temperature T.

State the effect, if any, on the partial pressure of methanol and on the value of K_p

Effect on partial pressure of methanol

State the effect, if any, of the addition of a catalyst on the value of K_p for this equilibrium. Explain your answer.

The melting point of XeF₄ is higher than the melting point of PF₃

Explain why the melting points of these two compounds are different.

In your answer you should give the shape of each molecule, explain why each molecule has that shape and how the shape influences the forces that affect the melting point.

A student does an experiment to determine the percentage by mass of sodium chlorate(I), NaC l O, in a sample of bleach solution.

Method:

• Dilute a 10.0 cm³ sample of bleach solution to 100 cm³ with distilled water.

• Transfer 25.0 cm³ of the diluted bleach solution to a conical flask and acidify using sulfuric acid.

• Add excess potassium iodide to the conical flask to form a brown solution

• Add 0.100 mol dm^–3 sodium thiosulfate solution (Na₂S₂O₃) to the conical flask from a burette until the brown solution containing l₂(aq) becomes a colourless solution

The student uses 33.50 cm³ of sodium thiosulfate solution.

The density of the original bleach solution is 1.20 g cm^–3

The equations for the reactions in this experiment are

Use all the information given to calculate the percentage by mass of NaC l O in the original bleach solution.

Give your answer to 3 significant figures.

The total uncertainty from two readings and an end point error in using a

What is the total percentage uncertainty in using the burette in this experiment?

Tick (✓) one box.

This question is about sodium halides.

State what is observed when silver nitrate solution is added to sodium fluoride solution.

State one observation when solid sodium chloride reacts with concentrated sulfuric acid.

Give an equation for the reaction.

State the role of the chloride ions in the reaction.

Equation

Give an equation for the redox reaction between solid sodium bromide and concentrated sulfuric acid.

Explain, using oxidation states, why this is a redox reaction.

Equation

State what is observed when aqueous chlorine is added to sodium bromide solution.

Give an ionic equation for the reaction.

Ionic equation

Methanol is formed when carbon dioxide and hydrogen react.

Table 5 contains enthalpy of formation and entropy data for these substances.

Use the equation and the data in Table 5 to calculate the Gibbs free-energy change (ΔG), in kJ mol^–1, for this reaction at 890 K

Figure 4 shows how the Gibbs free-energy change varies with temperature in a different gas phase reaction.

The straight line graph for this gas phase reaction has been extrapolated to zero Kelvin.

Use the values of the intercept and gradient from the graph in Figure 4 to calculate the enthalpy change (ΔH), in kJ mol^–1, and the entropy change (ΔS), in J K^–1 mol^–1, for this reaction.

State what Figure 4 shows about the feasibility of the reaction.

This question is about a glucose–oxygen fuel cell.

When the cell operates, the glucose (C₆H₁₂O₆) molecules react with water at the negative electrode to form carbon dioxide and hydrogen ions.

Oxygen gas reacts with hydrogen ions to form water at the positive electrode.

Deduce the half-equation for the reaction at the negative electrode.

Deduce the half-equation for the reaction at the positive electrode.

Give the equation for the overall reaction that occurs in the Glucose–oxygen fuel cell.

The negative electrode is made of carbon and the positive electrode is made of platinum.

Give the conventional representation for the glucose–oxygen fuel cell.

State what must be done to maintain the EMF of this fuel cell when in use.