Hess’s Law (CIE AS Chemistry)

Propane gas is commonly used as a fuel for outdoor cooking. It can be produced in a number of ways, including from the addition reaction of propyne gas with hydrogen. Propyne has the formula CHCCH₃ and includes a triple bond. 

Construct an equation for the formation of propane from propyne. Include state symbols.

Table 2.1 lists the relevant enthalpy of combustion data for the formation of propane from propyne. 

Table 2.1

Calculate the enthalpy change in kJ mol^-1 for the formation of propane from propyne. Show all working. 

Suggest why a calculated bond enthalpy value is often different to the data book value.

Enthalpy changes of combustion can be determined using calorimetry or calculated using Hess cycles.

Fig. 3.1 shows the equipment required to determine the enthalpy of combustion of 2-methylpropan-2-ol, C₄H₉OH.

Fig. 3.1

Table 3.1 shows the results of the experiment.

Table 3.1

Calculate the enthalpy change of combustion, ΔH_c, of 2‐methylpropan‐2‐ol. Include the sign and units.

The standard enthalpy change of combustion, ΔH_c^θ of 2-methylpropan-2-ol can be calculated using the data shown in Table 3.2.

Table 3.2

The value for ΔH_c^θ obtained in part (a) is much less exothermic than ΔH_c^θ calculated in (b)(iii).

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

Define the term standard enthalpy of combustion, ΔH^ϴ_c.

Construct an equation for the complete combustion of propanol, CH₃CH₂CH₂OH (l).

Construct a Hess’s Law cycle for the complete combustion of propanol.

Use the enthalpy of formation data given in Table 4.1, to calculate the enthalpy change of the reaction, ΔH^ϴ_r.

Table 4.1