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

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

In a calorimetric experiment 2.50 g of a fuel is burnt in oxygen. 30 % of the energy released during the combustion is absorbed by 500 g of water, the temperature of which rises from 25 °C to 68 °C. 

The specific heat capacity of water is 4.2 J g^–1 K^–1.

What is the total energy released per gram of fuel burnt?

Which equation correctly shows how the bond energy for the covalent bond Y-Z can be calculated by dividing ΔH by n?

The diagram shows the skeletal formula of cyclobutane.

The enthalpy change of formation of cyclobutane is +75.1 kJ mol^–1, and the enthalpy change of atomisation of graphite is +712 kJ mol^–1.

The bond enthalpy of C–H is 390 kJ mol^–1 and of H–H is 429 kJ mol^–1.

What is the average bond enthalpy of the C–C bond in cyclobutane, rounded to the nearest whole number?

Some bond energy values are listed below.

These bond energy values relate to the following four reactions.

What is the correct order of enthalpy changes of the above reactions from most negative to most positive?

A student calculated the standard enthalpy change of formation of propane, C₃H₈, using a method based on standard enthalpy changes of combustion.

He used correct values for the standard enthalpy change of combustion of propane (–2220 kJ mol^–1) and hydrogen (–286 kJ mol^–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol^–1.

What did he use for the standard enthalpy change of combustion of carbon?

Given the following enthalpy changes,

                            I₂ (s) → I₂ (g)                          ∆H^Ɵ = +38 kJ mol^–1

                            I₂ (g) + 3Cl₂ (g) → 2ICl₃ (s)    ∆H^Ɵ = –214 kJ mol^–1

What is the correct value for ∆H_f^Ɵ of iodine trichloride, ICl₃ (s)?

Using the following information:

                       CO (g) +  O₂ (g)   → CO₂ (g)            ∆H^Ɵ  = –283 kJ mol^–1

                       H₂ (g) +  O₂ (g)   → H₂O (I)              ∆H^Ɵ  = –286 kJ mol^–1

                       H₂O (g)   → H₂O (I)                              ∆H^Ɵ = –44 kJ mol^–1

What is the enthalpy change, ∆H^Ɵ, for the following reaction? 

CO₂ (g) + H₂ (g) → CO (g) + H₂O (g)

The conversion of graphite into diamond is an endothermic reaction. 

(ΔH = +3 kJ mol^–1).

C (graphite)  →  C (diamond)

Propanol has the molecular formula C₃H₆O. Use the following information to calculate the enthalpy change for the formation of propanol?

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 propanol is – 1816 kJ mol^-1

An experiment was carried out to determine the approximate value for the molar enthalpy change of neutralisation. 

75 cm³ of 3.00 mol dm^–3 hydrochloric acid was placed in a polystyrene beaker of negligible heat capacity. Its temperature was recorded, and then 75 cm³ of 3.00 mol dm⁻³ potassium hydroxide at the same temperature was quickly added, and the solution stirred. 

The temperature rose by 14 °C. The resulting solution may be considered to have a specific heat capacity of 4.2 J g^-1 K^-1.

Which calculation below 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 ∆H_f^⦵ for this reaction?

The standard enthalpy change ∆H^⦵ for the below reaction is -1776 kJ.

2N (g) + 6F (g) → 2NF₃ (g)

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

The complete combustion of ethyne, C₂H₂, is shown in the equation below.

H−C≡C−H + 2½ O═O → O−H−H + 2O═C═O

Using the average bond enthalpies given in the table, what is the enthalpy change of combustion of ethyne?

Compound R into compound T, it was found that the reaction proceeded by way of compound S, which could be isolated. The following steps were involved.

R → S;   ΔH is positive

S → T;   ΔH is negative

Which reaction profile fits these data?

Titanium occurs naturally as the mineral rutile, TiO₂. One possible method of extraction of titanium is to reduce the rutile by heating with carbon.

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 of the extraction of titanium?

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

The standard enthalpy change of formation of carbon dioxide is –382 kJ mol^–1. 

The standard enthalpy change of formation of water is –275 kJ mol^–1.

The standard enthalpy change of formation of ethanol is −266kJ mol^–1. 

What is the standard enthalpy change of combustion of ethanol?

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?