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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 48b
Chapter 5, Problem 48b

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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1
Identify the given reaction and its enthalpy change: The decomposition of 1 mole of liquid benzene (C6H6(l)) into 3 moles of gaseous acetylene (C2H2(g)) has an enthalpy change (ΔH) of +630 kJ.
Understand the relationship between the moles of benzene and acetylene: From the reaction, 1 mole of benzene produces 3 moles of acetylene.
Calculate the enthalpy change per mole of acetylene produced: Since the total enthalpy change for the reaction is +630 kJ for 3 moles of acetylene, divide this value by 3 to find the enthalpy change per mole of acetylene.
Express the enthalpy change for the formation of 1 mole of acetylene: The result from the previous step gives the enthalpy change (ΔH) for the formation of 1 mole of acetylene from benzene.
Note the sign of ΔH: Since the overall reaction is endothermic (+630 kJ), the enthalpy change per mole of acetylene will also be positive, indicating that energy is absorbed in the formation of acetylene from benzene.

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Key Concepts

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

Thermochemistry

Thermochemistry is the study of the heat energy associated with chemical reactions and physical transformations. It involves understanding how energy is absorbed or released during these processes, which is crucial for calculating enthalpy changes (ΔH). In the given reaction, the positive ΔH indicates that the decomposition of benzene is endothermic, meaning it requires energy input.
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Enthalpy of Formation

The enthalpy of formation (ΔH_f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. This concept is essential for determining the energy changes associated with the formation of products from reactants. In this case, calculating the enthalpy for the formation of acetylene involves using the enthalpy change of the decomposition reaction and stoichiometric relationships.

Stoichiometry

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It allows chemists to calculate the amounts of substances consumed and produced in a reaction based on balanced chemical equations. For the decomposition of benzene to acetylene, stoichiometry helps determine how the enthalpy change relates to the formation of one mole of acetylene from the given reaction.
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Related Practice
Textbook Question

b. What are the units of specific heat?

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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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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

Two solid objects, A and B, are placed in boiling water and allowed to come to the temperature of the water. Each is then lifted out and placed in separate beakers containing 1000 g of water at 10.0 °C. Object A increases the water temperature by 3.50 °C; B increases the water temperature by 2.60 °C. (a) Which object has the larger heat capacity?

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Textbook Question

Consider the combustion of isopropanol, C3H7OH(l), which is the primary component of rubbing alcohol: C3H7OH(l) + 9/2 O2(g) → 3 CO2(g) + 4 H2O(l) ΔH = -2248 kJ (b) Balance the forward reaction with whole-number coefficients. What is ΔH for the reaction represented by this equation?

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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. Now consider the reverse reaction, in which KClO3 is formed from KCl and O2. What is Δ𝐻 for the formation of 19.1 g KClO3 from KCl and O2?

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