Video Transcript
A reaction profile for a chemical
reaction is shown below. Which of the following statements
best describes this reaction profile? (A) The reaction is endothermic,
with a molar reaction enthalpy Δ𝐻 of 70 kilojoules per mole and an activation
energy of 170 kilojoules per mole. (B) The reaction is exothermic,
with a molar reaction enthalpy Δ𝐻 of negative 70 kilojoules per mole and an
activation energy of 170 kilojoules per mole. (C) The reaction is endothermic,
with a molar reaction enthalpy Δ𝐻 of 240 kilojoules per mole and an activation
energy of 70 kilojoules per mole. (D) The reaction is exothermic,
with a molar reaction enthalpy Δ𝐻 of 170 kilojoules per mole and an activation
energy of 240 kilojoules per mole. (E) The reaction is endothermic,
with a molar reaction enthalpy Δ𝐻 of negative 140 kilojoules per mole and an
activation energy of 70 kilojoules per mole.
This question is asking us to
interpret this reaction profile. For a reaction profile, the 𝑥-axis
represents the progress of the reaction, while the 𝑦-axis represents the chemical
energy. So the reaction profile gives us
the energy levels of the reactants and products over the course of the reaction. Each answer choice has three
components. First, is the reaction endothermic
or exothermic? Second, what is the molar reaction
enthalpy or Δ𝐻 shown in the diagram? And third, what is the activation
energy shown in the diagram?
Let’s go through these three pieces
of information one by one to select the correct answer. First, let’s determine if the
reaction is endothermic or exothermic. Looking at the diagram, we can see
that there is an increase in energy as we move from the reactants to the
products. This means that energy is absorbed
by the system. A reaction that absorbs energy is
called an endothermic reaction. So the correct answer will say “The
reaction is endothermic.”
Next, let’s take a look at the
molar reaction enthalpy or Δ𝐻. The molar reaction enthalpy or Δ𝐻
is the change in energy of the reaction per mole as we move from the reactants to
the products. The energy of the reactants at the
beginning of this reaction is 70 kilojoules per mole. The energy of the products at the
end of the reaction is 140 kilojoules per mole. This change is an increase of 70
kilojoules per mole from the beginning to the end. So the molar reaction enthalpy or
Δ𝐻 is 70 kilojoules per mole. This amount is positive 70
kilojoules per mole because it is an increase in energy. Endothermic reactions, where energy
is absorbed, have positive Δ𝐻 values. At the moment, only answer choice
(A) indicates that the reaction is endothermic and has a molar reaction enthalpy of
70 kilojoules per mole.
Let’s look at the activation energy
of this reaction profile to confirm that this is the correct answer. Activation energy, sometimes
written as 𝐸 sub a, is the difference between the initial energy level of the
reactants and the energy level of the high-energy transition state in between the
reactants and products. The activation energy is also the
minimum amount of energy that is needed for the reaction to proceed. In order for this reaction to
proceed, the energy level of the reactants needs to increase from 70 kilojoules per
mole to 240 kilojoules per mole. This will involve an increase of
170 kilojoules per mole of energy, matching the amount specified in answer choice
(A). Choice (A) is the correct
answer.
So which of the following
statements best describes this reaction profile? That’s choice (A). The reaction is endothermic, with a
molar reaction enthalpy Δ𝐻 of 70 kilojoules per mole and an activation energy of
170 kilojoules per mole.
