Ch.9 - Thermochemistry: Chemical Energy

Problem 116b

Chapter 9, Problem 116b

Use the data in Appendix B to find standard enthalpies of reaction in kilojoules for the following processes: (b) 2 H2O2 (aq) → 2 H2O (l) + O2(g)

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insert step 1: Identify the standard enthalpy of formation (ΔH_f^°) for each substance involved in the reaction from Appendix B.

insert step 2: Write the balanced chemical equation for the reaction: 2 H2O2 (aq) → 2 H2O (l) + O2(g).

insert step 3: Use the formula for the standard enthalpy change of the reaction: ΔH_rxn^° = Σ ΔH_f^°(products) - Σ ΔH_f^°(reactants).

insert step 4: Calculate the sum of the standard enthalpies of formation for the products: 2 * ΔH_f^°(H2O (l)) + 1 * ΔH_f^°(O2(g)).

insert step 5: Calculate the sum of the standard enthalpies of formation for the reactants: 2 * ΔH_f^°(H2O2 (aq)).

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Key Concepts

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

Standard Enthalpy of Reaction

The standard enthalpy of reaction (ΔH°) is the heat change that occurs when a reaction takes place under standard conditions (1 atm pressure and a specified temperature, usually 25°C). It is a crucial concept in thermodynamics, allowing chemists to predict whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). This value can be calculated using Hess's law or from standard enthalpies of formation.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows chemists to calculate the enthalpy change for complex reactions by breaking them down into simpler steps for which enthalpy values are known. It is particularly useful when direct measurement of ΔH is difficult.

Enthalpy of Formation

The enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. These values are tabulated for many substances and are essential for calculating the standard enthalpy of reaction using the formula: ΔH° = ΣΔHf°(products) - ΣΔHf°(reactants). Understanding this concept is vital for determining the energy changes associated with chemical reactions.

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Enthalpy of Formation

Related Practice

Textbook Question

Calcualte ΔH° in kilojoules for the synthesis of lime (CaO) from limestone (CaCO3), the key step in the manufacture of cement. CaCO3(s) → CaO(s) + CO2(g) ΔH°f [CaCO3(s)] = -1207.6 kJ/mol ΔH°f [CaO(s)] = -634.9 kJ/mol ΔH°f [CO2(g)] = -393.5 kJ/mol

Use the information in Table 9.2 to calculate ΔH° in kilojoules for the photosynthesis of glucose (C6H12O6) and O2 from CO2 and liquid H2O, a reaciton carried out by all green plants

Use the data in Appendix B to find standard enthalpies of reaction in kilojoules for the following processes: (a) C(s) + CO2(g) → 2 CO(g)

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Textbook Question

Use the data in Appendix B to find standard enthalpies of reaction in kilojoules for the following processes: (c) Fe2O3 (s) + 3 CO (g) → Fe(s) + 3 CO2(g)

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Textbook Question

Isooctane, C8H18, is the component of gasoline from which the term octane rating derives (b) The standard molar heat of combustion of isooctane (l) is -5461 kJ/mol. Calculate ΔH°f for isooctane(l)

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Textbook Question

Isooctane, C₈H₁₈, is the component of gasoline from which the term octane rating derives. (a) Write a balanced equation for the combustion of isooctane(l) with O₂ to yield CO₂(g) and H₂O(l)

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