1.5 Chemical Energetics

The enthalpy change of solution for ammonium chloride can be measured using calorimetry. 12.04 g of ammonium chloride is dissolved in 125.0 cm³ of water at 19.5 °C. 

The enthalpy of solution of ammonium chloride is +15.1 kJ mol^-1. Determine the energy change, in J, when 12.04 g of ammonium chloride is dissolved in 125.0 cm³ of water.

Use your answer to part (a) to determine the change in temperature, in °C, when the ammonium chloride is dissolved.

Use your answer to part (b) to determine the final temperature, in °C, of the solution during the reaction.

Determine the enthalpy of hydrogenation of propene using the data in Table 3.1.

Table 3.1

Enthalpy of hydrogenation, ΔH_r = ................................. kJ mol^–1

Use the data in Table 3.1 to suggest, with a reason, whether the polymerisation of propene is exothermic or endothermic

Carbon, hydrogen and ethene each burn exothermically in an excess of air.

Use the data to calculate the standard enthalpy change of formation, ∆H^θ_f , in kJ mol^–1, of ethene at 298 K.

∆H^θ_f = ................................. kJ mol^–1

The apparatus shown in Fig. 2.1 can be used to determine the enthalpy of combustion of butan-1-ol, C₄H₉OH (M_r = 74.12 gmol^-1).

Fig. 2.1

[2]

An experiment was carried out and the follow measurements were recorded in Table 2.1

Table 2.1

(Specific heat capacity of water = 4.18 J g^-1K^-1)

Use the data to calculate the enthalpy of change, q, for the reaction. Give your answer to two significant figures.

q = ........................................... kJ

Determine the enthalpy of combustion of butan-1-ol using Table 2.1 and your answer to part (b).  

Give your answer to three significant figures.

ΔH = ..................... kJ mol^-1

The standard enthalpy of combustion of butan-1-ol is -332.42 kJ mol^-1. 

[1]

Complete the following Hess' Law energy cycle relating butane to enthalpies of formation and combustion.

• enthalpy of formation, ΔH^ϴ_f
• enthalpy of combustion, ΔH^ϴ_c

On your diagram:

• include the relevant species in the two empty boxes,
• label each enthalpy change with its appropriate symbol,
• complete the remaining two arrows showing the correct direction of enthalpy change

Use the data in Table 3.1 to calculate the enthalpy of combustion of butane, ΔH^θ_c

Table 3.1

ΔH^θ_c  (C₄H₁₀) = ............................................kJ mol^-1

Using the values given in Table 3.2, calculate the enthalpy of combustion of butane.

Table 3.2

ΔH^θ_c  (C₄H₁₀) = ............................................kJ mol^-1

Suggest, with a reason, which of the two values of enthalpy of combustion from parts (b) and (c) is likely to be more accurate.

Two common oxides of nitrogen are nitrogen monoxide, NO, and nitrogen dioxide, NO₂.

[1]

• The reaction of N₂ with O₂

• The reaction of NO with O₂

[2]

Molecules of NO₂ can be formed by the reaction between N₂ and O₂. The bond between the N and O atoms in NO₂ is a double covalent bond.

The enthalpy change of reaction for this reaction is +497 kJ mol^–1. Calculate the bond enthalpy, in kJ mol^–1, of the N=O bond. Use relevant data from Table 4.2.

Table 4.2

Bond enthalpy of the N=O bond = ............................. kJ mol^–1
[2]

The boiling points of carbon dioxide and nitrogen dioxide are -78.5 ^oC and 21 ^oC respectively.

Suggest a reason for the difference.

Write an equation for the enthalpy of formation of nitrogen monoxide, NO.

Propane gas is used widely as a fuel which can be be used in camping gas stoves. The enthalpy of combustion for propane is -2219.2 kJ mol^-1. 

What is the minimum mass of fuel is needed to bring 150 cm³ of water at 21 °C to its boiling point?

Show your working.

Suggest why more propane may be required than calculated in part (a).

Using Fig. 5.1, construct a labelled reaction pathway diagram for the reaction in part (a) including the activation energy.

Fig. 5.1