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Ch.5 - Thermochemistry
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 72
Chapter 5, Problem 72

Acetylene (C2H2(g)) is used for welding because oxyacetylene is the hottest burning common fuel gas. Using standard enthalpies of formation, calculate the quantity of heat produced when 10 g of acetylene is completely combusted in air under standard conditions.

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1
Identify the balanced chemical equation for the combustion of acetylene: \( 2 \text{C}_2\text{H}_2(g) + 5 \text{O}_2(g) \rightarrow 4 \text{CO}_2(g) + 2 \text{H}_2\text{O}(l) \).
Determine the standard enthalpies of formation (\( \Delta H_f^\circ \)) for each substance involved in the reaction: \( \text{C}_2\text{H}_2(g) \), \( \text{O}_2(g) \), \( \text{CO}_2(g) \), and \( \text{H}_2\text{O}(l) \). Note that the \( \Delta H_f^\circ \) for \( \text{O}_2(g) \) is zero because it is in its elemental form.
Use the formula for the standard enthalpy change of the reaction: \( \Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants}) \). Substitute the values from step 2 into this formula to find \( \Delta H_{rxn}^\circ \).
Calculate the moles of acetylene (\( \text{C}_2\text{H}_2 \)) using its molar mass: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \).
Determine the heat produced by multiplying the moles of acetylene by the \( \Delta H_{rxn}^\circ \) per mole of acetylene combusted.

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Key Concepts

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

Standard Enthalpy of Formation

The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics, as it allows for the calculation of heat changes in chemical reactions, particularly combustion. Each substance has a specific enthalpy of formation value, which can be found in tables and is essential for determining the overall heat released or absorbed during a reaction.
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Combustion Reaction

A combustion reaction is a chemical process in which a substance reacts rapidly with oxygen, producing heat and light. In the case of acetylene, the reaction with oxygen produces carbon dioxide and water. Understanding the stoichiometry of the combustion reaction is vital for calculating the amount of heat released, as it involves balancing the chemical equation and determining the moles of reactants and products involved.
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Heat of Combustion

The heat of combustion is the amount of energy released when a substance is completely burned in oxygen. It is typically expressed in kilojoules per mole (kJ/mol) and is derived from the standard enthalpies of formation of the reactants and products. This value is essential for calculating the total heat produced during the combustion of a specific mass of a substance, such as acetylene, allowing for practical applications in fields like welding and energy production.
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Related Practice
Textbook Question

For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up ΔH°f for each substance in Appendix C. (a) NO2(g) (b) SO3(g) (c) NaBr(s) (d) Pb(NO3)2(s).

Textbook Question

Write balanced equations that describe the formation of the following compounds from elements in their standard states, and then look up the standard enthalpy of formation for each substance in Appendix C: (a) CH3OH(l)

Textbook Question

Using values from Appendix C, calculate the value of H for each of the following reactions: (a) CaO(s) + 2 HF(g) → CaF2(s) + H2O(g) (b) Fe2O3(s) + 3 C(s) → 2 Fe(s) + 3CO(g) (c) 2 CO(g) + 2 NO(g) → N2(s) + 2 CO2(g) (d) 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2Og)

Textbook Question

Complete combustion of 1 mol of acetone (C3H6O) liberates 1790 kJ: C3H6O(l) + 4 O2(g) → 3 CO2(g) + 3 H2O(l) ΔH° = -1790 kJ Using this information together with the standard enthalpies of formation of O2(g), CO2(g), and H2O(l) from Appendix C, calculate the standard enthalpy of formation of acetone.