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.
