12
QuizMultiple Choice

Energy in Chemical Reactions — Test

Question 12 • Chemistry

Consider these intermediate chemical equations. cap c times s plus cap o sub 2 times g rightarrow CO sub 2 times g triangle cap h sub 1 is equal to negative 393 point 5 kJ 2 CO g plus cap o sub 2 times g rightarrow 2 CO sub 2 g triangle cap h sub 2 is equal to negative 566 point 0 kJ 2 cap h sub 2 cap o g rightarrow 2 cap h sub 2 g plus cap o sub 2 times g triangle cap h sub 3 is equal to 483 point 6 kJ The overall chemical equation is C(s)+H_2​O(g)→CO(g)+H_2​(g) To calculate the final enthalpy of the overall chemical equation, which step must occur?

Answer
A
Multiply the first equation by three, and triple the enthalpy. Then, add.
B
Divide the third equation by two, and double the enthalpy. Then, add.
C
Reverse the second equation, and change the sign of the enthalpy. Then, add.
D
Reverse the first equation, and change the sign of the enthalpy. Then, add.

Explanation

To produce CO from CO₂, the second equation must be reversed, changing its enthalpy from −566.0 kJ to +566.0 kJ. Then divide that reversed equation by 2, and divide the third equation by 2, so the CO₂, O₂, and H₂O amounts cancel to give C(s) + H₂O(g) → CO(g) + H₂(g). Thus, the required step listed is reversing the second equation; the enthalpy sign changes when an equation is reversed.

AI-written and checked against the verified answer.

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