Video Transcript
What standard enthalpy change can
be defined as the enthalpy change when one mole of a substance transforms from a
liquid state to a solid state under standard conditions? (A) Standard enthalpy of
condensation. (B) Standard enthalpy of
fusion. (C) Standard enthalpy of
solidification. (D) Standard enthalpy of
sublimation. (E) Standard enthalpy of
vaporization.
This question is asking us about
standard enthalpy changes or, in other words, the amount of energy required to make
a certain transformation. For example, we can note that it
takes 40.66 kilojoules per mole of liquid water to turn it into steam. Since vaporization is the process
of turning a liquid into a gas, this energy change, 40.66 kilojoules per mole, is
the standard enthalpy of vaporization for water. It’s the energy change required to
make the change of phase happen.
Although it’s not critical to
solving this problem, we should note that different substances have different
standard enthalpy changes. For example, it takes 40.66
kilojoules to turn one mole of liquid water into a gas, but only 31.27 kilojoules to
turn one mole of liquid acetone into a gas. This question is asking us to name
the standard enthalpy change for the transformation from a liquid state to a solid
state. Let’s take a look at the remaining
answer choices and their corresponding phase changes.
Condensation is the name of the
change from a gas to a liquid. While changing from a solid to a
liquid is typically called melting, we refer to the standard enthalpy change for
this process as the standard enthalpy of fusion. Sublimation refers to the
transformation from a solid state to a gaseous state. So the standard enthalpy of
sublimation is the energy required to convert a solid into a gas. None of these four answer choices
matches the transformation described in the question. The correct answer is answer choice
(C), the standard enthalpy of solidification.
Solidification refers to the
process of transitioning from a liquid state to a solid state. As it turns out, the standard
enthalpy of solidification for water is negative 6.01 kilojoules per mole. The negative value in this
situation means that energy is released, rather than absorbed, when this transition
occurs. Since melting is the same process
as solidifying, just in reverse, the standard enthalpy of fusion is the inverse of
the standard enthalpy of solidification. The standard enthalpy of fusion of
water is positive 6.01 kilojoules per mole, signifying that for this direction,
energy is absorbed rather than released.
So, what is the enthalpy change
when one mole of a substance transforms from a liquid state to a solid state under
standard conditions? That’s choice (C), standard
enthalpy of solidification.
