5.3 Bond Enthalpy

Methane reacts violently with fluorine to form carbon tetrafluoride and hydrogen fluoride

Formulate the equation for this reaction. 

Use your answer to part a) and section 10 of the data booklet to calculate the following:

A student suggested that one reason for the high reactivity of fluorine is a weak F−F bond. State whether the student is correct. Justify your answer. 

Sketch a labelled energy diagram for the reaction of methane and fluorine.

Hydrazine has the formula N₂H₄ and is used as a rocket fuel (e.g. for the Apollo moon rockets). It burns in the following reaction for which the enthalpy change is -583 kJ mol^-1.

N₂H₄ (g) + O₂ (g) → N₂ (g) + 2H₂O (g)

Sketch the Lewis structure of hydrazine, N₂H₄.

Use section 11 of the Data booklet and the information in part a) to deduce the bond enthalpy, in kJ mol^-1, for the N-N bond.

Outline why the value of enthalpy of reaction calculated from bond enthalpies is less accurate.

Ozone prevents UV radiation emitted from the Sun reaching the surface of the Earth. Draw the resonance Lewis structures of ozone. 

By using equations, state the environmental impact of CFCs on the ozone layer. 

The destruction of ozone is a significant environmental issue as ozone can absorb frequencies of ultraviolet radiation that oxygen can not. 

Explain with reference to the structure and bonding of oxygen and ozone why this occurs. 

Enthalpy changes can be found using bond enthalpy data. Some bond enthalpy values are shown below in Table 1.

Table 1

 The balanced equation for the reaction between methane and propane is

CH₄ (g) +  CH₃CH₂CH₃ (g) → CH₃CH₂CH₂CH₃ (g) + H₂ (g)

Use the equation and bond enthalpy data to calculate the enthalpy change for the above reaction.                                  

Define the term average bond enthalpy.

Enthalpy changes can be found using bond enthalpy data. Some bond enthalpy values are shown below in Table 2. 

Table 2

The balanced equation for the combustion of ethene is

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

Use the equation and bond enthalpy data to calculate the enthalpy of combustion of ethene.

Bond enthalpies can be found using Hess’s Law or from experimental data.

Outline the difference between the two ways of finding bond enthalpy.

Alkanes can be used as fuels in internal combustion engines. When sufficient oxygen is present, they undergo complete combustion reactions.

Write an equation for the enthalpy of combustion of butane.

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

Table 1 below contains bond enthalpy data for the reaction shown in part (a).

Table 1

Using the data in Table 1 and the equation in part (a), calculate the enthalpy change of combustion of butane.

In the absence of sufficient oxygen, butane will undergo incomplete combustion.

Write an equation for the incomplete combustion of butane.

Use the energy level diagram to determine the activation energy, E_a, for the given reaction in Figure 1.

Figure 1

Ethene can be hydrated via the following reaction:

C₂H₄ (g) + H₂O (g) → C₂H₅OH (g)

Table 1

Use the data in Table 1 to calculate the enthalpy change for the hydration of ethene.

Explain why the value to your answer to part (b) is different from the data book value for the hydration of ethene.

C₂H₄ (g) + H₂O (g) → C₂H₅OH (g)

Table 2 below has some enthalpy data for a different chemical reaction. Hydrazine, N₂H₄ can react with hydrogen peroxide in an exothermic reaction, as shown below.

N₂H₄(g) + 2H₂O₂ → N₂(g) + 4H₂O(g)                          ΔH^ϴ_r = -789 kJ mol^-1

The structure of hydrazine is shown in Figure 1.

Figure 1

Table 2

Using the reaction equation and the data in the table above, calculate the value of the N-H bond in hydrazine.

The bond enthalpies, in kJmol^-1, of oxygen-oxygen single and double bonds are shown below in Table 1.

Table 1

Predict, with a reason, the bond enthalpy of the oxygen-oxygen bond in ozone, O₃.

Ozone is formed in the upper atmosphere in a two-step process as shown below:

            Step A:            O₂ →  2O^•
            Step B:            O₂    +     O^•    →   O₃

Ozone is naturally lost through the decomposition of ozone:

            Step C:            O₃   → O₂    +     O^•    

Draw an enthalpy level diagram to represent the three steps A, B and C shown in part (b). Clearly label the position of oxygen, ozone, and the oxygen radical.

What can be deduced from the fact that ozone absorbs UV radiation in the region of 340 nm and molecular oxygen in the region of 242 nm?