Multiple Choice
For the freezing of water, represented by the equation H2O(l) → H2O(s), what are the signs of ΔH (enthalpy change) and ΔS (entropy change)?
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8. Thermochemistry
Thermochemical Equations
Multiple Choice
Which one of the following reactions would produce the largest amount of heat per mole of hydrogen?
A
H2(g) + Br2(l) → 2 HBr(g) ΔH = -72.8 kJ
B
H2(g) + 1/2 O2(g) → H2O(g) ΔH = -241.8 kJ
C
2 H2(g) + O2(g) → 2 H2O(l) ΔH = -571.6 kJ
D
H2(g) + Cl2(g) → 2 HCl(g) ΔH = -184.6 kJ
Verified step by step guidance1
Identify the enthalpy change (ΔH) values given for each reaction and note that these values correspond to the heat released or absorbed per the reaction as written.
Determine the amount of hydrogen gas (H2) involved in each reaction equation to understand how many moles of H2 correspond to the given ΔH values.
Calculate the heat released per mole of H2 by dividing the given ΔH by the number of moles of H2 in the balanced reaction equation. For example, if the reaction uses 2 moles of H2, divide ΔH by 2.
Compare the heat released per mole of H2 for each reaction to find which reaction produces the largest amount of heat per mole of hydrogen.
Remember that a more negative ΔH per mole of H2 indicates a larger amount of heat released, so the reaction with the most negative value per mole of H2 is the one that produces the largest heat per mole of hydrogen.
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