Video Transcript
NO2 is a brown gas that can exist
in equilibrium with colorless dinitrogen tetroxide, as shown in the following
reaction. Two NO2 gas is in equilibrium with
N2O4 gas. Δ𝐻 equals negative 57.2 kilojoules
per mole. Is the enthalpy change of the
forward reaction exothermic or endothermic? Which ampoule in the picture is at
the highest temperature? (A) The forward reaction is
exothermic, and the ampoule on the left is the hottest. (B) The forward reaction is
exothermic, and the ampoule on the right is the hottest. (C) The forward reaction is
endothermic, and the ampoule on the left is the hottest. (D) The forward reaction is
endothermic, and the ampoule on the right is the hottest.
There are two parts to this
problem, so let’s focus on the first question. Enthalpy change represented by Δ𝐻
is the change in the internal energy of a system. A negative enthalpy change
indicates that there is a net release of energy over the course of the reaction. This type of reaction is
exothermic. A positive enthalpy change
indicates that there is a net increase in the energy of the system. This type of reaction is
endothermic. We can see in the question that
we’ve been provided with the enthalpy change. When Δ𝐻 is written after an
equilibrium reaction, it indicates the change in enthalpy for the forward
reaction. As this value is negative, we know
that the forward reaction is exothermic. With this information, we can
eliminate answer choices (C) and (D).
Now we can focus on the second
question. In this question, we want to know
which of the ampoules shown is at the highest temperature. To answer this question, we need to
consider Le Chatelier’s principle. Le Chatelier’s principle states
that for a dynamic equilibrium if the conditions change, the equilibrium position
will move to counteract the change. So, if the temperature is
increased, the equilibrium position will move to decrease the temperature. We know that the forward reaction
is exothermic and energy is released, typically in the form of heat. The reverse reaction is the exact
opposite. It is endothermic, meaning that
energy, typically heat, is absorbed.
When the temperature is increased,
the system will counteract the change by absorbing the added heat. Thus, the reverse reaction is
favored and the equilibrium shifts towards the reactants. So, as the temperature increases,
the concentration of nitrogen dioxide will increase and the concentration of
dinitrogen tetroxide will decrease. We are told in the question that
nitrogen dioxide is a brown gas, while dinitrogen tetroxide is colorless. If the concentration of NO2
increases as temperature increases, then the color of the ampoule should get
increasingly more brown with rising temperature. So, the ampoule on the right must
be the hottest.
In conclusion, with regards to the
equilibrium between nitrogen dioxide and dinitrogen tetroxide, the forward reaction
is exothermic and the ampoule on the right is the hottest, answer choice (B).
