Video Transcript
Considering the diagram, which of
the following equations would Hess’s law predict to be true? (A) △𝐻 one equals △𝐻 two minus
△𝐻 three. (B) △𝐻 two equals △𝐻 three
minus △𝐻 one. (C) △𝐻 one equals △𝐻 two plus
△𝐻 three. (D) △𝐻 two equals △𝐻 three plus
△𝐻 one. (E) △𝐻 three equals △𝐻 one plus
△𝐻 two.
In this question, we’re provided a
reaction cycle consisting of three chemical reactions. The letters provided inside the
boxes represent the substances involved in the chemical reactions. Each reaction also has an
associated enthalpy change represented by △𝐻.
Let’s begin by looking at reaction
number one in the reaction cycle. In this reaction, substance A
reacts with substance B to produce substance C. This chemical reaction is a direct
one-step process for producing substance C. If we could not directly or easily
measure the enthalpy change for reaction one, we could calculate the enthalpy change
by adding together the enthalpy changes of several reactions in an alternative
pathway or indirect route. This idea is summarized by Hess’s
law, which states that the enthalpy change of a reaction is independent of the path
taken. As long as our starting and ending
points are the same, the sum of the enthalpy changes of the alternative pathway will
be equal to the enthalpy change of the direct route.
Let’s make use of the following
table to keep track of the chemical reactions and enthalpy changes of the
alternative path. Our starting point is with the
reactants A and B. We can complete the reaction moving
down that converts A and B into E. Let’s call this reaction reaction
number two. The chemical equation for reaction
number two can be written as A plus B reacts to form E. And the enthalpy change for this
reaction is △𝐻 two.
Next, we can complete the reaction
that converts substance E into substance C. Let’s call this reaction reaction
number three. The chemical equation for this
reaction is written as E reacts to form C, and the enthalpy change is △𝐻
three. The final outcome of these two
reactions is the same as that of reaction number one, the direct route. Product C was produced from
substances A and B, but the process involved two separate steps. So, according to Hess’s law, the
sum of the enthalpy changes of these two reactions is equal to the enthalpy change
of reaction one.
Let’s write an equation to
represent this relationship. We can write △𝐻 one equals △𝐻
two plus △𝐻 three. We can also rearrange this equation
to solve for △𝐻 two or △𝐻 three, which gives us the following two equations. When looking at the answer choices,
the only equation that matches one of the three equations we wrote is answer choice
(C). Considering the provided diagram,
the equation that Hess’s law predicts to be true is △𝐻 one equals △𝐻 two plus
△𝐻 three, or answer choice (C).
