Enthalpy changes

This question is about carbon monoxide and carbon as reducing agents in industry.

Iron(III) oxide, Fe₂O₃, can be reduced by carbon monoxide to form iron.

Some standard enthalpy change of formation, _fH, and standard entropy, S, values are shown in the table.

Substance	Fe2O3(s)	CO(g)	Fe(s)	CO2(g)
Δf H / kJ mol–1	–824	–111	0	–394
S / J K–1 mol–1	87	198	To calculate in part (a)(iv)	214

Explain why the standard enthalpy change of formation of Fe has a value of 0 kJ mol^–1.

Calculate the enthalpy change, _rH, for the forward reaction in Equilibrium 17.1.

H =........ kJ mol^–1

Explain why the standard entropy of CO₂ is greater than the standard entropy of Fe₂O₃.

The standard entropy change, S, for the forward reaction in Equilibrium 17.1 has a value of +15 J K^–1 mol^–1.

Use data from the table to calculate the standard entropy, S, of Fe.

S =........ J K^–1 mol^–1

Carbon can also reduce Fe₂O₃(s) to iron as shown in Equilibrium 17.2.

The forward reaction is only feasible at high temperatures.

• Show that the forward reaction is not feasible at 100 °C.

• Calculate the minimum temperature, in K, for the forward reaction to be feasible.

Give the minimum temperature to the nearest whole number.

Steam can be reduced by carbon monoxide in a reversible reaction.

At 700 K the value of K_p is 8.13.

Explain why K_p for Equilibrium 17.3 has no units.

An equal number of moles of steam and carbon monoxide are left at 700 K in a sealed container until equilibrium is reached.

At equilibrium, both H₂ and CO₂ have a partial pressure of 211 kPa.

Calculate the partial pressures of H₂O and CO at equilibrium.

At a second temperature the value of K_p for Equilibrium 17.3 is 129.

Explain whether the second temperature is higher or lower than 700 K.

The equation for the reaction of sulfuric acid with potassium hydroxide is shown below.

25 cm³ of 1.00 mol dm^–3 H₂SO₄ is reacted with excess KOH. The energy given out is 2.8 kJ.

What is the enthalpy change of neutralisation, in kJ mol^–1?

A –56

B –70

C –112

D –224

Your answer

This question is about different areas of chemistry.

Why do all average bond enthalpies have a positive value?

Determine the C–H bond enthalpy, in kJ mol^–1, using the information above.

C–H bond enthalpy =........ kJ mol^–1

Hydrogen gas is being considered as a household fuel to replace methane.

The enthalpy change of formation, _fH, for H₂O(l) is –285.8 kJ mol^–1.

Determine the energy released when 60.0 m³ of hydrogen is used as a household fuel at RTP.

Give your answer to 3 significant figures and in standard form.

Compound A is a chloride of a Period 3 element.

A student carries out the 2 steps below to find the formula of compound A.

Step 1 The student adds 5.00 10^–4 mol of compound A to water. A colourless solution is formed.

Step 2 The colourless solution reacts with exactly 60.0 cm³ of 2.50 10^–2 mol dm^–3 AgNO₃(aq) to form a white precipitate.

Write an ionic equation, with state symbols, for the reaction in Step 2.

Determine the formula of compound A.

Compound B, shown below, is an antiviral medicine.

What is the molecular formula of compound B ?

How many chiral carbon atoms are there in one molecule of compound B ?

This question is about energy changes.

Magnesium reacts with aqueous silver nitrate, AgNO₃(aq) as shown below.

A student adds an excess of magnesium to 100.0 cm³ of 0.400 mol dm^–3 AgNO₃(aq). The initial temperature is 20.0 °C.

Determine the maximum temperature reached in this reaction. Give your answer to 3 significant figures.

Assume that the specific heat capacity and density of the solution are the same as for water, and that there are no heat losses.

The student wants to repeat the experiment, but there is not enough AgNO₃(aq) left to use another 100.0 cm³ portion.

The student decides to modify the method by adding an excess of magnesium to 50.0 cm³ of 0.400 mol dm^–3 AgNO₃(aq).

Predict, with reasons, how this modification would affect the maximum temperature reached. Assume that there are no heat losses.

Nitric acid is manufactured from ammonia in a multi-stage process. The equation for the first stage in this process is shown in Reaction 17.1.

Some standard enthalpy changes of formation are shown in the table.

Compound	f H / kJ mol–1
NH3(g)	–46
H2O(l)	–286

Explain the term enthalpy change of formation.

Calculate the standard enthalpy change of formation, _fH, of NO(g).

_fH of NO(g) =........ kJ mol^–1

Carbon disulfide, CS₂, reacts with dinitrogen oxide, N₂O, as shown in Reaction 17.2.

Standard entropies, S, are shown in the table.

Substance	CS2(l)	N2O(g)	S8(s)	CO2(g)	N2(g)
S/ J K–1 mol–1	151	220	256	214	192

Explain the term entropy.

The free energy change, G, of Reaction 17.2 is –2672 kJ mol^–1 at 25 °C.

Calculate the enthalpy change, H, of Reaction 17.2, in kJ mol^–1.

H =........ kJ mol^–1

A student concludes that Reaction 17.2 is feasible at all temperatures.

Explain whether the student is correct or not.

This question is about acids, bases and buffers.

Sodium hydroxide, NaOH, is a strong base.

Calculate the pH of 0.140 mol dm^–3 NaOH(aq) at 298 K.

Give your answer to 2 decimal places.

Sulfuric acid reacts with sodium hydroxide as shown in Equation 22.1. Equation 22.1 H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l) H₁ This is a neutralisation reaction.

A student carries out an experiment to determine the enthalpy change H₁ and uses this value to deduce the enthalpy change of neutralisation, _neutH.

The student measures out two solutions:

• 25.0 cm³ of 1.60 mol dm^–3 H₂SO₄(aq)

• 55.0 cm³ of 1.50 mol dm^–3 NaOH(aq) (an excess).

The temperature of each solution is the same.

The student mixes the two solutions. The temperature increases by 13.0 °C.

Show that NaOH is in excess.

Calculate the enthalpy change, H₁, for Equation 22.1, and deduce the value for the enthalpy change of neutralisation, ∆_neutH, in kJ mol^–1.

Assume that the densities of all solutions and the specific heat capacity, c, of the reaction mixture are the same as for water.

enthalpy change, H₁ =........ kJ mol^–1

enthalpy change of neutralisation, _neutH =........ kJ mol^–1

The student repeats the experiment using 50.0 cm³ of 1.60 mol dm^–3 H₂SO₄ and 110.0 cm³ of 1.50 mol dm^–3 NaOH.

Predict the increase in temperature.

Explain your reasoning.

* Nitrous acid, HNO₂, is a weak Brønsted–Lowry acid with a pK_a value of 3.34 at room temperature.

HNO₂ can be prepared by reacting N₂O₃ with water. HNO₂ is the only product.

A chemist makes up a buffer solution by the following method.

Step 1 The chemist weighs a sample of N₂O₃. Water is then added to form 100 cm³ of 0.500 mol dm^–3 HNO₂(aq).

Step 2 The chemist adds 100 cm³ of 0.150 mol dm^–3 NaOH(aq) to the 100 cm³ solution of 0.500 mol dm^–3 solution of HNO₂(aq). The resulting solution is made up to 1.00 dm³.

Explain why a buffer solution forms in Step 2. Determine the pH of this buffer solution and the mass of N₂O₃ that was used in Step 1.

Radical reactions are responsible for the catalysed breakdown of the ozone layer.

The overall equation is shown below.

The molar gas volume in the ozone layer is approximately 2.5 m³ mol⁻¹.

What is the energy released, in kJ, during the breakdown of 1.0 m³ of ozone in the ozone layer?

A 56.8

B 113.6

C 355

D 710

Your answer

Which enthalpy change(s) is/are endothermic?

1 The bond enthalpy of the C–H bond

2 The second electron affinity of oxygen

3 The standard enthalpy change of formation of magnesium

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer

A student carries out two experiments in the laboratory based on succinic acid (butanedioic acid), (CH₂COOH)₂.

Succinic acid is esterified by ethanol, C₂H₅OH, in the presence of an acid catalyst to form an equilibrium mixture.

The equilibrium constant, K_c, for this equilibrium can be calculated using the amounts, in moles, of the components in the equilibrium mixture, using expression 5.1.

A student carries out an experiment to determine the value of K_c for this equilibrium.

• The student mixes together 0.0500 mol of succinic acid and 0.150 mol of ethanol, with a small amount of an acid catalyst.

• The mixture is allowed to reach equilibrium.

• The student determines that 0.0200 mol of succinic acid are present in the equilibrium mixture.

Which technique could be used to determine the equilibrium amount of succinic acid?

Write the equation for the equilibrium reaction that takes place.

Draw the skeletal formula of the ester present in the equilibrium mixture.

K_c is the equilibrium constant in terms of equilibrium concentrations.

Why can expression 5.1 be used to calculate K_c for this equilibrium?

Calculate the value of K_c for this reaction.

Show your working.