Energetics (Edexcel International AS Chemistry)

Magnesium reacts with sulfuric acid in an exothermic reaction.

Write an equation for the reaction.
Include state symbols in your answer.

A student carried out an experiment to determine the enthalpy change of the reaction.

A sample of 0.50 g of magnesium powder was added to 25 cm³ of 0.20 mol dm⁻³ sulfuric acid.

Calculate the number of moles of magnesium and of sulfuric acid that reacted.
Justify your answer.

(2)

(2)
ΔT = ..............................................................

Calculate the standard molar enthalpy change for the reaction, using your answers to (b) and (c)(ii).

Include a sign and units in your answer.

[Specific heat capacity of solution = 4.18 J g^–1 °C^–1]

Enthalpy changes of combustion can be determined using calorimetry or calculated using Hess cycles. Apparatus for a calorimetry experiment is shown.

A sample of 2‐methylpropan‐2‐ol was burned in a spirit burner and used to heat 75 g of water. The results are shown.

(1)

(4)

The standard enthalpy change of combustion, Δ_c, can be calculated using standard enthalpy changes of formation.

Compound	∆f/ kJ mol−1
2‐methylpropan‐2‐ol	–359
carbon dioxide	–394
water	–286

State why no ∆_fvalue has been given for oxygen.

(2)

The value for Δ_cH obtained in part (a)(ii) is much less exothermic than Δ_ccalculated in (b)(iii).

Suggest two reasons for this other than non‐standard conditions.

This question is about ethanoic acid and some related salts.

A test to confirm the presence of an aqueous acid is adding a small amount of solid sodium carbonate to the solution.
Describe two observations you would see in this test.

Sodium ethanoate is a component of reusable hand warmers.

In use, a supersaturated solution of sodium ethanoate recrystallises to form solid hydrated sodium ethanoate, releasing energy.

CH₃COONa (aq) + 3H₂O (l) → CH₃COONa.3H₂O (s)      Δ_rH = −19.7 kJ mol⁻¹

A hand warmer has a mass of 63.2 g and forms 20.1 g of hydrated sodium ethanoate on recrystallisation.

Calculate the maximum temperature reached by the hand warmer if its initial temperature is 5.0 °C.

[Specific heat capacity of the hand warmer = 3.0 J °C⁻¹ g⁻¹]

Ammonium ethanoate, CH₃COONH₄(s), is used to control the pH of foods.
It can be formed by the reaction of pure ethanoic acid, CH₃COOH(l), with ammonium carbonate, (NH₄)2CO₃(s).

Calculate the standard enthalpy change for this reaction by completing the Hess cycle and using the data shown.

Ammonium carbonate, (NH₄)₂CO₃ , is an ingredient in cleaning solutions for camera lenses.

These are aqueous solutions which contain no more than 1.8 g of ammonium carbonate in 100 cm³ of solution.

Calculate the maximum concentration, in mol dm⁻³, of ammonium carbonate in such a solution.