1.6 Energetics

The equations below show the formation of sulfur oxides from sulfur and oxygen.

S (s) + O₂ (g) → SO₂ (g)           ∆H_f^⦵ = –297 kJ mol^–1

S (s) + 1½O₂ (g) → SO₃ (g)     ∆H_f^⦵ = –395 kJ mol^–1

What is the enthalpy change of reaction, ∆H_f^⦵, of 2SO₂ (g) + O₂ (g) → 2SO₃ (g)?

The reaction of hydrochloric acid with sodium hydroxide reached a maximum temperature of 297.4 K.

Given the following data, how much thermal energy was evolved during this reaction?

Initial temperature of 25.0 cm³ hydrochloric acid = 17.6 C

Initial temperature of 25.0 cm³ sodium hydroxide = 18.5 C 

The specific heat capacity of water is 4.18 J K^-1 g^-1.

The temperature rise when 0.1 MJ of heat energy is absorbed by 2.5 kg of solid aluminium is 44.4 C.

What is the specific heat capacity of aluminium?

An iron cube, with a mass of 0.5 kg, increases in temperature by 12.7 K when heated.

The specific heat capacity of iron is 0.448 J K^-1 g^-1.

What is the enthalpy change for heating the iron cube?

The table shows the standard enthalpy of formation, ΔH_f^θ, for some of the substances in the reaction.

4FeS₂ (s) + 11O₂ (g) → 2Fe₂O₃ (s) + 8SO₂ (g)    ΔH_r^θ = -3312 kJ mol^-1 

What is the standard enthalpy of formation, in kJ mol^-1, for SO₂ (g)?

When completing experiments to measure enthalpy change for exothermic processes, which of the following is not a reason for the experimental data to be lower than the expected value?

Heptathiepane, S₇, can undergo complete combustion to form sulfur dioxide.

S₇ (s) + 7O₂ (g) → 7SO₂ (g)

Which enthalpy changes can be used to describe this reaction?

Consider the reactions

C₂H₄ (g) + 2O₂ (g) → 2CO (g) + 2H₂O (g)            ∆H = −758 kJ mol⁻¹

2C (s) + 2H₂ (g) → C₂H₄ (g)                                  ∆H = +52 kJ mol⁻¹

2C (s) + O₂ (g) → 2CO (g)                                     ∆H = −222 kJ mol⁻¹

What is the enthalpy of formation of water?

Butane can be produced by the hydrogenation of buta-1,3-diene.

C₄H₆ (g) + 2H₂ (g) → C₄H₁₀ (g)   ΔH = -248 kJ mol^-1

Some mean bond enthalpy data is given in the table.

Use the information in the table and the stated enthalpy change to calculate the bond enthalpy for the C=C bond in buta-1,3-diene.

The combustion of ethanol (C₂H₅OH) is increasingly being used to fuel cars. 

The enthalpy change of combustion of hydrogen is –286 kJ mol^-1

The enthalpy change of combustion of carbon is –394 kJ mol^-1

The enthalpy change of combustion of ethanol is – 1367 kJ mol^-1

Using the enthalpy of combustion data, what is the enthalpy change for the formation of ethanol?

An experiment was carried out to determine the approximate value for the molar enthalpy change of reaction between hydrochloric acid and sodium hydroxide.. 

• 50.0 cm³ of 2.00 mol dm^–3 hydrochloric acid was placed in a polystyrene beaker of negligible heat capacity
• The temperature of the acid was recorded as 19.0 C
• 75.0 cm³ of 1.50 mol dm⁻³ sodium hydroxide at the same temperature was quickly added and the solution stirred
• The temperature rose to 33.0 C

Assume that both solutions have a density of 1.0 g cm^–3 and a specific heat capacity of 4.18 J K^–1 g^–1.

Which value for the molar enthalpy change of reaction is correct? 

The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation.

4NH₃ (g) + 5O₂ (g)  4NO (g) + 6H₂O (g)

Using the following standard enthalpy change of formation data, what is the value of the standard enthalpy change for this reaction?

Nitrogen reacts with fluorine according to the following equation.

N₂ (g)  +  3F₂ (g)  →  2NF₃ (g)   ΔH_r = -246 kJ

The bond energy of  N≡N is 945 kJ mol^-1 and F–F is 159 kJ mol^-1.

What is the bond energy of the N–F bond?

The complete combustion of ethene, C₂H₄, is shown in the equation below.

C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (l)

Using the information in the table, what is the enthalpy change of combustion of ethene?

Titanium can be extracted by heating the mineral rutile, TiO₂, with carbon according to the following equation.

TiO₂ (s) + 2C (s) → Ti (s) + 2CO (g)

The standard enthalpy changes of formation of TiO₂ (s) and CO (g) are –890 kJ mol^-1 and –105 kJ mol^-1 respectively.

What is the standard enthalpy change for the extraction of titanium?

Use the following information to answer this question.

The value in kJ mol⁻¹ for the enthalpy of combustion of butane is:

The table below discusses three types of enthalpy change:

‘+’ means that this type of standard enthalpy change can only have positive values,

‘–’ means that this type of standard enthalpy change can only have negative values,

‘+ / –’ means that either positive or negative values are possible.

Which row is correct?

A student carried out an experiment to determine the enthalpy change for the combustion of ethanol, using the following method:.

• Measure the mass of a spirit burner containing ethanol.
• Place 100.0 cm³ of water in a glass beaker above the spirit burner. 
• Record the initial temperature of the water.
• Light the spirit burner and allow it to heat the water for a few minutes. 
• Extinguish the flame.
• Record the final temperature of the water.
• Measure the new mass of the spirit burner.

Using the following results obtained by the student, how much of the heat energy produced, in kilojoules, by the burning of ethanol went into the water?

The specific heat capacity of water is 4.18 J K⁻¹ g⁻¹.