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Ch.5 - Thermochemistry
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 63
Chapter 5, Problem 63

Calculate the enthalpy change for the reaction P4O6(s) + 2 O2(g) → P4O10(s) given the following enthalpies of reaction: P4(s) + 3 O2(g) → P4O6(s) ΔH = -1640.1 kJ P4(s) + 5 O2(g) → P4O10(s) ΔH = -2940.1 kJ

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1
Identify the target reaction: P_4O_6(s) + 2 O_2(g) → P_4O_10(s).
Write the given reactions: 1) P_4(s) + 3 O_2(g) → P_4O_6(s) with ΔH = -1640.1 kJ, 2) P_4(s) + 5 O_2(g) → P_4O_10(s) with ΔH = -2940.1 kJ.
Reverse the first reaction to match the formation of P_4O_6(s) as a reactant: P_4O_6(s) → P_4(s) + 3 O_2(g), changing the sign of ΔH to +1640.1 kJ.
Add the reversed first reaction to the second reaction to cancel out P_4(s) and some O_2(g), resulting in the target reaction.
Calculate the enthalpy change for the target reaction by adding the ΔH values of the modified reactions: ΔH = (+1640.1 kJ) + (-2940.1 kJ).

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Key Concepts

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

Enthalpy of Reaction

Enthalpy of reaction, or ΔH, is the heat change associated with a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Understanding the enthalpy changes for specific reactions is crucial for calculating the overall enthalpy change in a multi-step reaction.
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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, regardless of the pathway taken. This principle allows chemists to calculate the enthalpy change for a reaction that may not be easily measured directly by using known enthalpy changes from related reactions.
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Hess's Law

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced chemical equations. It is essential for determining the proportions of substances involved in a reaction, which is necessary when applying Hess's Law to find the overall enthalpy change for a reaction involving multiple steps.
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Related Practice
Textbook Question

A 1.800-g sample of phenol (C6H5OH) was burned in a bomb calorimeter whose total heat capacity is 11.66 kJ/°C. The temperature of the calorimeter plus contents increased from 21.36 to 26.37 °C. b. What is the heat of combustion per gram of phenol?

Textbook Question

Under constant-volume conditions, the heat of combustion of benzoic acid (C6H5COOH) is 26.38 kJ/g. A 2.760-g sample of benzoic acid is burned in a bomb calorimeter. The temperature of the calorimeter increases from 21.60 to 29.93 °C. c. Suppose that in changing samples, a portion of the water in the calorimeter were lost. In what way, if any, would this change the heat capacity of the calorimeter?

Textbook Question

Consider the following hypothetical reactions: A → B ΔH = +30 kJ B → C ΔH = +60 kJ (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hess's law applies.

Textbook Question

From the enthalpies of reaction 2 C(s) + O2(g) → 2 CO(g) ΔH = -221.0 kJ 2 C(s) + O2(g) + 4 H2(g) → 2 CH3OH(g) ΔH = -402.4 kJ Calculate ΔH for the reaction CO(g) + 2 H2(g) → CH3OH(g)

Textbook Question

The concentration of alcohol 1CH3CH2OH2 in blood, called the 'blood alcohol concentration' or BAC, is given in units of grams of alcohol per 100 mL of blood. The legal definition of intoxication, in many states of the United States, is that the BAC is 0.08 or higher. What is the concentration of alcohol, in terms of molarity, in blood if the BAC is 0.08?

Textbook Question

Given the data N2(g) + O2(g) → 2 NO(g) ΔH = +180.7 kJ 2 NO(g) + O2(g) → 2 NO2(g) ΔH = -113.1 kJ 2 N2O(g) → 2 N2(g) + O2(g) ΔH = -163.2 kJ use Hess's law to calculate ΔH for the reaction N2O(g) + NO2(g) → 3 NO(g)