Video Transcript
Which of the following statements
best describes the enthalpy of fusion, Δ𝐻 fus? (A) The change in enthalpy
resulting from the combining of two atoms to create a molecule. (B) The change in enthalpy
resulting from the energy released by a substance when burned in oxygen. (C) The change in enthalpy
resulting from the release of energy by a substance to change its state from solid
to gas at constant pressure. (D) The change in enthalpy
resulting from the taking in of energy by a substance to change its state from solid
to liquid at constant pressure. (E) The change in enthalpy
resulting from the mixing of two solutions together.
To solve this problem, we need to
carefully read each answer choice and determine if it accurately describes the
enthalpy of fusion. In the provided symbol for the
enthalpy of fusion, the Δ symbol means change, the capital 𝐻 stands for enthalpy,
and the abbreviation “fus” stands for the process called fusion.
The Enthalpy of fusion is an
example of an enthalpy change. An enthalpy change put most simply
is the change in the energy content of a system. The energy of the system decreases
if the system releases energy through an exothermic process. And the energy of the system can
increase if the system absorbs energy through an endothermic process. Enthalpy changes occur in both
chemical reactions and physical processes. Examples of physical processes that
involve enthalpy changes include the formation of solutions and phase changes.
Fusion is another word for a phase
change that we are quite familiar with, melting. Therefore, we can define the
enthalpy of fusion as the enthalpy change that occurs as a result of melting. When an Enthalpy change is measured
for one mole of a substance at standard conditions, it’s referred to as a standard
enthalpy change. And the abbreviation for the
enthalpy change will include a symbol called the plimsoll.
Now that we know that the enthalpy
of fusion is the enthalpy change when a substance undergoes melting, our strategy
when reading the answer choices is to look for clues that the physical change of
melting has occurred.
The change in enthalpy described in
answer choice (A) involves two atoms combining to form a molecule. The formation of a molecule from
separate atoms involves the formation of one or more chemical bonds. The enthalpy change involved in the
formation of a compound from atoms of different elements is called the enthalpy of
formation. Because answer choice (A) describes
the enthalpy change of a chemical reaction, we can eliminate it as an incorrect
answer choice.
In answer choice (B), a change in
enthalpy occurs when a substance is burned in oxygen and energy is released. This is a description of a type of
chemical reaction called combustion. And the enthalpy change that occurs
as a result of a combustion reaction is called the enthalpy of combustion. Because answer choice (B) describes
an enthalpy change involved in a chemical reaction, we can eliminate it.
Now let’s take a look at answer
choice (E). The statement in answer choice (E)
describes an enthalpy change that results from mixing two solutions together to form
a new solution. Mixing together two solutions could
result in a chemical change, but we are not given evidence of that here. With the information given, the
enthalpy change described in answer choice (E) occurs during the physical process of
dissolving one solution into another. Because the physical process
described is not melting, we can eliminate answer choice (E).
When reading answer choices (C) and
(D), we notice that both statements describe a change in the state of a
substance. We’ll need to look more closely at
answer choices (C) and (D) to determine which of them describes the phase change of
melting.
In answer choice (C), the substance
is being converted from a solid to a gas. The name of the phase change that
occurs when a solid is converted directly to a gas is sublimation. Because answer choice (C) describes
the enthalpy of sublimation not the enthalpy of fusion, we can eliminate it.
Finally, in answer choice (D), the
enthalpy change is caused by a substance absorbing energy resulting in a
transformation from the solid phase to the liquid phase. We know that the phase change that
converts a solid to a liquid is called melting. Therefore, answer choice (D) is the
statement that best describes the enthalpy of fusion.
