Video Transcript
Which of the following statements
best defines the standard enthalpy of formation, 𝛥𝐻 plimsoll symbol 𝑓? (A) It is the change in enthalpy
for the decomposition of one mole of a compound to its elements, under standard
conditions, and with all elements in their standard state. (B) It is the change in enthalpy
for the reaction of one mole of a compound with one mole of another compound to form
a mole of products, under standard conditions, and with all elements in their
standard state. (C) It is the change in enthalpy
for the formation of one mole of a compound from its elements, under standard
conditions, and with all elements in their standard state. (D) It is the change in enthalpy
for the formation of elements from one mole of a compound, under standard
conditions, and with all elements in their standard state. Or (E) it is the change in enthalpy
for the formation of one mole of crystal from a saturated solution, under standard
conditions, and with all elements in their standard state.
This question is asking us to
choose which of the statements provides the best definition for the standard
enthalpy of formation. The formation process is referring
to a substance being formed from its constituent elements. An example would be the formation
of carbon dioxide from carbon and oxygen. The standard enthalpy of formation
in this case is how much energy is released or absorbed when one mole of carbon
dioxide is formed from carbon and oxygen. The formation of carbon dioxide
from one mole of carbon and one mole of oxygen releases 393.5 kilojoules of
energy. We can therefore state that the
standard enthalpy of formation for carbon dioxide is minus 393.5 kilojoules per
mole, where the negative sign indicates energy is released.
Let us now revisit our choices for
this question. Statement (A) refers to the
decomposition of a substance rather than formation. Thus, statement (A) is not the
answer to this question. Statement (B) does involve the
formation of a new compound. However, the statement refers to
this compound being formed from another compound and not its constituent
elements. For this reason, statement (B) is
not the answer to this question.
Statement (C), “It is the change in
enthalpy for the formation of one mole of a compound from its elements, under
standard conditions, and with all elements in their standard state,” seems to
correctly describe the standard enthalpy of formation. However, we can confirm this by
checking statements (D) and (E).
Statement (D) involves the
formation of elements from a compound. This is the opposite of the
definition for the standard enthalpy of formation. Thus, statement (D) is
incorrect. Statement (E) talks about the
change in enthalpy for the formation of one mole of crystal from a saturated
solution. This statement refers to the
standard enthalpy of crystallization and not of formation. Thus, statement (E) is
incorrect.
Therefore, we can confirm that the
statement that best describes the enthalpy of formation is statement (C). It is the change in enthalpy for
the formation of one mole of a compound from its elements, under standard
conditions, with all elements in their standard state.
