Multiple Choice
What is the standard enthalpy of formation (ΔH_f^ext{°}) for CH_4(g) in kJ/mol?
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8. Thermochemistry
Enthalpy of Formation
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Which statement is true about the standard enthalpy change of a reaction, , and standard enthalpies of formation, ?
A
is independent of the physical states of reactants and products.
B
can be calculated as the sum of for products minus the sum of for reactants, each multiplied by their stoichiometric coefficients.
C
For any element in its standard state, is always less than .
D
is always positive because bond breaking requires energy.
Verified step by step guidance1
Understand that the standard enthalpy change of a reaction, \(\Delta^{\circ}H_{rxn}\), represents the heat absorbed or released during a reaction under standard conditions.
Recall that the standard enthalpy of formation, \(\Delta^{\circ}H_f\), is defined as the enthalpy change when one mole of a compound forms from its elements in their standard states.
Recognize that \(\Delta^{\circ}H_{rxn}\) can be calculated using the standard enthalpies of formation of the reactants and products by applying Hess's Law, which states that the total enthalpy change is the sum of enthalpy changes for individual steps.
Write the formula for the standard enthalpy change of the reaction as:
\(\Delta^{\circ}H_{rxn} = \sum \nu_p \Delta^{\circ}H_f (products) - \sum \nu_r \Delta^{\circ}H_f (reactants)\)
where \(\nu_p\) and \(\nu_r\) are the stoichiometric coefficients of products and reactants respectively.
Note that the physical states of reactants and products affect the values of \(\Delta^{\circ}H_f\) and thus \(\Delta^{\circ}H_{rxn}\), so \(\Delta^{\circ}H_{rxn}\) is not independent of physical states; also, \(\Delta^{\circ}H_f\) for elements in their standard states is defined as zero, not less than zero, and \(\Delta^{\circ}H_{rxn}\) can be positive or negative depending on the reaction.
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