Question

Explain how you would calculate the total change in bond energy for the reaction H2+Cl2->2HCI. How would you know if the reaction was endothermic or exothermic?

Answer 1

To calculate the total change in binding energy we must apply the following equation:

`\Delta H=\Delta H_{f(\text{Reagents)}}-=\Delta H_{f(\text{Products)}}`

This equation tells us that the energy change will be equal to the sum of the potential energy of the product bonds minus the sum of the potential energy of the reagent bonds.

Now, let's calculate each term separately.

`\begin{gathered} \Delta H_{f(\text{Reagents)}}=n_(H2)*\text{Energy bond H-H + }n_(Cl2)\text{Energy bond Cl-Cl} \\ \Delta H_{f(\text{Reagents)}}=1molH_2*432(kJ)/(mol)+1molCl_2*239(kJ)/(mol) \\ \Delta H_{f(\text{Reagents)}}=671kJ \end{gathered}`
`\begin{gathered} \Delta H_{f(\text{Products)}}=n_{\text{HCl}}* EnergyBond\text{ H-Cl} \\ \Delta H_{f(\text{Products)}}=2molHCl*427(kJ)/(mol) \\ \Delta H_{f(\text{Products)}}=854kJ \end{gathered}`

So, the change in bond energy will be:

`\begin{gathered} \Delta H=\Delta H_{f(\text{Reagents)}}-=\Delta H_{f(\text{Products)}} \\ \Delta H=671kJ-854kJ=-183kJ \end{gathered}`

We have a negative value in the result, when this happens it means that the reaction is exothermic, that is to say, that it releases heat and the energy of the products is greater than that of the reagents.

When we have a positive value the reaction will be endothermic, this means that it needs energy.