Video Transcript
Which statement about the enthalpy change for the backward reaction in a reversible
reaction is true? (A) The enthalpy change for the backward reaction is less in magnitude than the
enthalpy change for the forward reaction. (B) The enthalpy change for the backward reaction is greater in magnitude than the
enthalpy change for the forward reaction. (C) The enthalpy change for the backward reaction is the same in magnitude as the
enthalpy change for the forward reaction. (D) The enthalpy change for the backward reaction is half the amount of the enthalpy
change for the forward reaction. (E) The enthalpy change for the backward reaction equals the activation energy plus
the enthalpy change for the forward reaction.
To answer this question, we need to know how the enthalpy change for the backward
reaction in a reversible reaction compares to the enthalpy change for the forward
reaction. Let’s remove the answer choices for now while we discuss enthalpy changes.
Enthalpy change, denoted as Δ𝐻, is the net change in the internal energy of a
system. To better understand enthalpy changes in reversible reactions, let’s consider the
following equilibrium reaction. We’ll start by focusing on the forward reaction. The reactants N2 and three H2 have a certain amount of potential energy. Energy must be supplied to break the bonds in these molecules. Energy is released when new bonds form to create the products, two NH3.
The net change in the energy is the enthalpy change. When the potential energy of the products is lower than the potential energy of the
reactants, the change in enthalpy is negative, indicating that there is a net
release of energy during the reaction. For this particular reaction, the change in enthalpy is negative 92 kilojoules per
mole.
Now, let’s examine the backward reaction. In the backward reaction, two NH3 is the reactant. It has the same potential energy regardless of whether it’s the reactant or
product. N2 and three H2 are the products. Like two NH3, these species have the same potential energy regardless of whether they
are reactants or products. As with the forward reaction, energy needs to be supplied in order to break the bonds
in the reactants. Notice that the amount of energy needed to break the bonds in two NH3 is the same
amount of energy that is released when the bonds of two NH3 form.
Energy is released to form the bonds of nitrogen and hydrogen. This is the same amount of energy that is necessary to break the bonds in the forward
reaction. The net change in the energy is the enthalpy change. As the products in the reverse reaction have a higher potential energy than the
reactants, the change in enthalpy is positive, indicating that the reverse reaction
requires a net increase in the energy of the system.
We should be able to see from this example that the enthalpy change for the backward
reaction has the same magnitude as the enthalpy change for the forward reaction but
the opposite sign. So, for this example reaction, the enthalpy change for the backward reaction would be
positive 92 kilojoules per mole.
Now, let’s bring back the answer choices. The statement about the enthalpy change for the backward reaction in a reversible
reaction that is true is answer choice (C). The enthalpy change for the backward reaction is the same in magnitude as the
enthalpy change for the forward reaction.
