Video: Determining the Bond Energy of HCl from Standard Enthalpy of Formation and the Bond Energy of the H₂ and Cl₂

To the nearest kJ/mol, what is the bond energy of HCl? H₂(g) + Cl₂(g) ⇌ 2HCl(g) Δ𝐻_(r) ^(⦵) = −184.7 kJ/mol

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Video Transcript

To the nearest kilojoule per mole, what is the bond energy of HCl?

We’ve been provided with the equation for the reaction between hydrogen and chlorine producing HCl, otherwise known as hydrogen chloride. This symbol here refers to the standard reaction enthalpy change. And it refers to the enthalpy change for the forward reaction. The word enthalpy refers to the heat content of a system at constant pressure. When bonds are broken or formed, the enthalpy of the system will change. This standard symbol is called a plimsoll. It indicates that the reaction enthalpy change was measured with all the components in their standard states at one bar of pressure.

The negative sign in the standard reaction enthalpy change indicates that the enthalpy of the system, that means the reactants and the products, decreases as a result of their reaction. Energy leaves the system moving to the surroundings. And the total enthalpy of the system decreases. The mole here indicates that a certain amount of energy is released when either a mole of hydrogen or chlorine are consumed or two moles of hydrogen chloride are generated.

We’ve also been given a table of bond energies. A bond energy is the average energy required to break each mole of bonds. So the table tells us that it would take 436 kilojoules to break one mole of hydrogen–hydrogen bonds. The question asks us what is the bond energy of HCl. In HCl, there is one hydrogen–chlorine single bond. But its value is not listed in the table. We’re going to need to have to work it out.

We can work this out using the standard reaction enthalpy change and the enthalpies of the two bonds in hydrogen and chlorine. We’re going to have to calculate two more energy terms. The energy of the reactant bonds and the energy of the products bonds. The energy of the reactant bonds indicates the energy we need to go and to put in to break up the reactants. So it’s going to have a positive value since we’re increasing the enthalpy of the system.

Meanwhile, the energy of the product bonds is energy leaving the system as bonds are formed and therefore has a negative value since enthalpy is being lost from the system. This means that the standard reaction enthalpy change is equal to the energy of the reactant bonds minus the energy of the product bonds. Breaking up the reactants means breaking the H–H bond and the Cl–Cl bond. Meanwhile, the formation of the products means the formation of two equivalents of the hydrogen–chlorine bond. Be careful to remember the two from the original equation.

The next thing to do is rearrange the equation. This produces the energy of the hydrogen–chlorine single bond, in terms of the energy of the hydrogen–hydrogen bond, the energy of the chlorine–chlorine bond, and the standard reaction enthalpy change. This is equal to 436 kilojoules per mole plus 243 kilojoules per mole plus 184.7 kilojoules per mole, all divided by two. This gives us a value of 431.85 kilojoules per mole. This value should be positive since all bond energies are positive values. Since the question asks us to give the bond energy of HCl to the nearest kilojoule per mole, our final value for the bond energy of HCl is 432 kilojoules per mole.

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