Multiple Choice
Which factor is most important when predicting trends in the enthalpy of solution for ionic compounds?
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8. Thermochemistry
Enthalpy of Formation
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Which statement about standard enthalpy of formation is true?
A
The standard enthalpy of formation of a compound is always negative.
B
The standard enthalpy of formation is measured for forming mol of a substance from its elements at and atm.
C
The standard enthalpy of formation equals the enthalpy change for decomposing mol of a compound into its elements in their standard states.
D
The standard enthalpy of formation of an element in its standard state is .
Verified step by step guidance1
Understand that the standard enthalpy of formation (\( \Delta H_f^\circ \)) is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (usually 1 atm and 25\(^\circ\)C).
Recall that the standard enthalpy of formation of an element in its most stable form (its standard state) is defined as zero by convention. This is because no formation reaction is needed to form the element from itself.
Recognize that the standard enthalpy of formation can be either positive or negative depending on whether the formation reaction is endothermic or exothermic; it is not always negative.
Note that the standard enthalpy of formation is not measured at 0\(^\circ\)C and 2 atm, but typically at 25\(^\circ\)C (298 K) and 1 atm pressure.
Understand that the standard enthalpy of formation refers to the formation of a compound from its elements, not the decomposition of a compound into its elements; the latter would be the reverse reaction with the opposite enthalpy change.
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