Question

Estimate the average S-F bond energy in $S{F_6}$. The standard heat of formation values of $S{F_{6(g)}}$.${S_{(g)}}$ and ${F_{(g)}}$ are 1100, 275 and 80 KJ/mole respectively.

Answer 1

To determine the standard enthalpy of the reaction, standard enthalpy formation of the reactant is added and then the value is subtracted with the sum value of standard enthalpy of formation of product.

Complete answer:

Given,

The standard heat formation value of $S{F_{6(g)}}$ is 1100 KJ/mole.

The standard heat formation value of ${S_{(g)}}$ is 275 KJ/mole.

The standard heat formation value of ${F_{(g)}}$ is 80 KJ/mol.

The reaction is shown below.

$S(g) + 6F(g) \to S{F_6}(g)$

In this reaction, Sulphur hexafluoride gas dissociates to give Sulphur gas and fluorine gas.

The standard enthalpy of reaction is defined as the enthalpy change of the reaction which is calculated by the standard enthalpies of formation for the reactant compound and product.

The formula to calculate the standard enthalpy of the reaction is shown below.

$\Delta {H_{reaction}} = \sum {\Delta H_{fProduct}^0} - \sum {\Delta H_{f\operatorname{Reactant}}^0}$

Where,

$\Delta {H_{reaction}}$ is the standard enthalpy of the reaction.

$\sum {\Delta H_{fProduct}^0}$ is the sum of standard enthalpy formation of the product.

$\sum {\Delta H_{f\operatorname{Reactant}}^0}$ is the sum of standard enthalpy formation of reactants.

To calculate the standard enthalpy of the reaction, substitute the values of standard enthalpy formation of product and standard enthalpy formation of reactant in the above equation.

$\Rightarrow \Delta {H_{reaction}} = 1100-(275 + 6 \times 80)$

$\Rightarrow \Delta {H_{reaction}} = 345KJ/mol$

The average bond energy is calculated as shown below.

$Average\;bond\;energy = \dfrac{{\Delta {H_{reaction}}}}{{Total\;no.of\;bonds}}$

Where,

$\Delta {H_{reaction}}$ is standard enthalpy of reaction.

$\Rightarrow Average\;bond\;energy = \dfrac{{345}}{6}$

$\Rightarrow Average\;bond\;energy = 57.5KJ/mol$

Therefore, the average S-F bond energy in $S{F_6}$ is 57.5KJ/mol.

Note: The change in enthalpy is not dependent upon the specific pathway of the reaction but it depends on the overall energy level of both the reactant and products. Enthalpy is a state function therefore it is additive.