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Ch.5 - Thermochemistry
Chapter 5, Problem 47d

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (d) If the reaction were written to produce H2O(g) instead of H2O(l), would you expect the magnitude of ΔH to increase, decrease, or stay the same? Explain.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. In exothermic reactions, like the combustion of methanol, ΔH is negative, indicating that heat is released. The magnitude of ΔH reflects the energy difference between reactants and products, which can vary depending on the physical states of the products formed.
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Enthalpy of Formation

Phase Changes and Energy

The physical state of a substance (solid, liquid, gas) significantly affects its energy content. For example, converting liquid water (H2O(l)) to water vapor (H2O(g)) requires energy input, known as the enthalpy of vaporization. Therefore, if a reaction produces water in the gaseous state instead of the liquid state, the overall energy change (ΔH) will be influenced by this additional energy requirement.
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Entropy in Phase Changes

Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen to produce heat and light, typically resulting in the formation of carbon dioxide and water. The products' states (gas or liquid) can affect the reaction's enthalpy change. In the case of methanol combustion, producing water vapor instead of liquid water would likely increase the energy released, as more energy is required to form gaseous products.
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Related Practice
Textbook Question

At one time, a common means of forming small quantities of oxygen gas in the laboratory was to heat KClO3: 2 KClO3(s) → 2 KCl(s) + 3 O2(g) ΔH = -89.4 kJ (c) The decomposition of KClO3 proceeds spontaneously when it is heated. Do you think that the reverse reaction, the formation of KClO3 from KCl and O2, is likely to be feasible under ordinary conditions? Explain your answer.

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Textbook Question

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (a) What is the enthalpy change for the reverse reaction?

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Textbook Question

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (b) Balance the forward reaction with whole-number coefficients. What is ΔH for the reaction represented by this equation?

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Textbook Question

Consider the decomposition of liquid benzene, C6H6(l), to gaseous acetylene, C2H2(g): C6H6(l) → 3 C2H2(g) ΔH = +630 kJ (a) What is the enthalpy change for the reverse reaction?

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Textbook Question

Consider the decomposition of liquid benzene, C6H6(l), to gaseous acetylene, C2H2(g): C6H6(l) → 3 C2H2(g) ΔH = +630 kJ (b) What is H for the formation of 1 mol of acetylene?

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Textbook Question

Consider the decomposition of liquid benzene, C6H6(l), to gaseous acetylene, C2H2(g): C6H6(l) → 3 C2H2(g) ΔH = +630 kJ (c) Which is more likely to be thermodynamically favored, the forward reaction or the reverse reaction?

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