Multiple Choice
Which of the following expressions correctly calculates the change in enthalpy (ΔH) for a reaction using standard heats of formation?
8. Thermochemistry
Enthalpy of Formation
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Choose the reaction that illustrates the standard enthalpy of formation (ΔHf°) for Ca(NO2)2.
A
Ca(NO2)2(aq) → Ca2+(aq) + 2 NO2^-(aq)
B
Ca2+(aq) + 2 NO2^-(aq) → Ca(NO2)2(aq)
C
Ca(NO2)2(s) → Ca(s) + N2(g) + 4 O2(g)
D
Ca(s) + N2(g) + 2 O2(g) → Ca(NO2)2(s)
Verified step by step guidance1
Understand the concept of standard enthalpy of formation (ΔHf°), which is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.
Identify the elements in their standard states for the compound Ca(NO2)2. Calcium (Ca) is a solid, nitrogen (N2) is a diatomic gas, and oxygen (O2) is a diatomic gas.
Write the balanced chemical equation for the formation of one mole of Ca(NO2)2 from its elements in their standard states: Ca(s) + N2(g) + 2 O2(g) → Ca(NO2)2(s).
Compare the given reactions to the definition of the standard enthalpy of formation. The correct reaction should show the formation of the compound from its elements in their standard states.
Select the reaction that matches the criteria for the standard enthalpy of formation, which is: Ca(s) + N2(g) + 2 O2(g) → Ca(NO2)2(s).
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