Multiple Choice
Using the bond energies in Table 7.2 of your textbook, calculate the approximate enthalpy change for the following reaction: 2 H3C - CH3(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(g). What is the enthalpy change?
8. Thermochemistry
Enthalpy of Formation
Multiple Choice
Which statement about the standard enthalpy of formation (ΔH_f^ext{°}) is true?
A
ΔH_f^ext{°} is the same for all allotropes of an element.
B
ΔH_f^ext{°} measures the energy required to break a compound into its elements.
C
ΔH_f^ext{°} for any element in its most stable form is zero.
D
ΔH_f^ext{°} for all compounds is always positive.
Verified step by step guidance1
Understand that the standard enthalpy of formation, denoted as \(\Delta H_f^{\text{\circ}}\), 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°C).
Recall that the standard enthalpy of formation for any element in its most stable form (its standard state) is defined as zero by convention. This is because forming an element from itself requires no energy change.
Recognize that \(\Delta H_f^{\text{\circ}}\) is not necessarily the same for all allotropes of an element, since different allotropes have different energies and stabilities (for example, graphite and diamond for carbon).
Note that \(\Delta H_f^{\text{\circ}}\) does not measure the energy required to break a compound into its elements; rather, it measures the energy change when forming the compound from its elements.
Understand that \(\Delta H_f^{\text{\circ}}\) for compounds can be either positive or negative depending on whether the formation is endothermic or exothermic, so it is not always positive.
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