AnswersChemistryEnthalpy of Reaction — QuizQuestion 10
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Enthalpy of Reaction — Quiz

Question 10 • Chemistry

Consider the intermediate chemical equations. CH sub 4 times g plus 2 cap o sub 2 g rightarrow CO sub 2 times g plus 2 cap h sub 2 cap o g triangle cap h sub 1 is equal to negative 802 kJ 2 cap h sub 2 cap o g rightarrow 2 cap h sub 2 cap o l triangle cap h sub 2 is equal to negative 88 kJ The overall chemical reaction is shown. CH sub 4 times g plus 2 cap o sub 2 g rightarrow CO sub 2 times g plus 2 cap h sub 2 cap o l triangle cap h sub 2 is equal to negative 890 kJ What is the correct enthalpy diagram using the Hess law for this system?

Answer
A
Image with description An enthalpy diagram has 3 arrows. The top line is labeled as C H subscript 4 g plus 2 O subscript 2 g, the baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O l, and the new lower baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O g. Arrow 1, points down from a top line to the baseline and is labeled as negative 88 kilo Joule. Arrow 2, continues pointing downward from arrow 1; it meets a horizontal new lower baseline and is labeled as negative 802 kilo Joule. Arrow 3, points down from the top line, past the baseline to a new lower baseline, and is labeled as negative 890 kilo Joule.
B
Image with description An enthalpy diagram has 3 arrows. The top line is labeled as C O subscript 2 g plus 2 H subscript 2 O l, the baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O g, and the new lower baseline is labeled as C H subscript 4 g plus 2 O subscript 2 g. Arrow 1, points down from the top line to the baseline and is labeled as negative 802 kilo Joule. Arrow 2, continues pointing downward from arrow 1; it meets a horizontal new lower baseline and is labeled as negative 88 kilo Joule. Arrow 3, points down from the top line, past the baseline to a new lower baseline, and is labeled as negative 890 kilo Joule.
C
Image with description An enthalpy diagram has 3 arrows. The top line is labeled as C H subscript 4 g plus 2 O subscript 2 g, the baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O g, and the new lower baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O l. Arrow 1, points upward from the lower baseline to the baseline and is labeled as negative 88 kilo Joule. Arrow 2, continues pointing upward from arrow 1; it meets a top line and is labeled as negative 802 kilo Joule. Arrow 3, points down from the top line, past the baseline to a new lower baseline, and is labeled as negative 890 kilo Joule.
D
Image with description An enthalpy diagram has 3 arrows. The top line is labeled as C H subscript 4 g plus 2 O subscript 2 g, the baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O g, and the new lower baseline is labeled as C O subscript 2 g plus 2 H subscript 2 O l. Arrow 1, points down from the top line to the baseline and is labeled as negative 802 kilo Joule. Arrow 2, continues pointing downward from arrow 1; it meets a horizontal new lower baseline and is labeled as negative 88 kilo Joule. Arrow 3, points down from the top line, past the baseline to a new lower baseline, and is labeled as negative 890 kilo Joule.

Explanation

The first reaction takes CH4(g) + 2 O2(g) to CO2(g) + 2 H2O(g), with an enthalpy change of −802 kJ, so that arrow goes down to the gaseous-water level. Condensing the water to liquid releases another 88 kJ, so the next arrow also goes down, from the gas level to the lower liquid-water level. The total change is −802 + (−88) = −890 kJ, matching the overall reaction. Choice D shows these states and downward arrows with the correct enthalpy changes.

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