Multiple Choice
Given the thermochemical equation CH_4(g) + 2 O_2(g) → CO_2(g) + 2 H_2O(l); ΔH = -890 kJ, what is the enthalpy change (ΔH) when 530 g of methane (CH_4) is combusted?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
Which of the following best describes the difference between the heat (q) of a reaction and the reaction enthalpy (ΔH_{rxn})?
A
q and ΔH_{rxn} both refer to the change in temperature of the system, not the heat exchanged.
B
ΔH_{rxn} refers to the heat exchanged at constant volume, while q is the heat exchanged at constant pressure.
C
q is always equal to ΔH_{rxn} for all reactions.
D
q refers to the heat exchanged under any conditions, while ΔH_{rxn} is the heat exchanged at constant pressure.
Verified step by step guidance1
Understand that heat (q) is a general term for the energy transferred due to temperature difference, and it can occur under various conditions such as constant volume or constant pressure.
Recognize that reaction enthalpy (\$\(\Delta\) H_{rxn}\$) specifically refers to the heat exchanged during a reaction carried out at constant pressure.
Recall the thermodynamic relationship that at constant pressure, the heat exchanged (q_p) is equal to the change in enthalpy, so \$q_p = \(\Delta\) H_{rxn}\$.
Note that at constant volume, the heat exchanged (q_v) is related to the change in internal energy (\$\(\Delta\) U\$), not enthalpy, so \$q_v \(\neq\) \(\Delta\) H_{rxn}\$ in general.
Conclude that \$q\$ can refer to heat exchanged under any conditions, but \$\(\Delta\) H_{rxn}\$ is specifically the heat exchanged at constant pressure, making the last statement the best description.
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