Open Question
Can you use an approach similar to Hess’s law to calculate the change in internal energy, _x001F_E, for an overall reaction by summing the _x001F_E values of individual reactions that add up to give the desired overall reaction?
Ch.5 - Thermochemistry
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
Verified step by step guidance1
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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Enthalpy 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, 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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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
Consider the following hypothetical reactions: A → B ΔHI = +60 kJ B → C ΔHII = -90 kJ (a) Use Hess’s law to calculate the enthalpy change for the reaction A → C.
Textbook Question
Consider the following hypothetical reactions: A → B ΔHI = +60 kJ B → C ΔHII = -90 kJ (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hess's law applies.
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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)
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Textbook Question
From the enthalpies of reaction H2(g) + F2(g) → 2 HF(g) ΔH = -537 kJ C(s) + 2 F2(g) → CF4(g) ΔH = -680 kJ 2 C(s) + 2 H2(g) → C2H4(g) ΔH = +52.3 kJ Calculate H for the reaction of ethylene with F2: C2H4(g) + 6 F2(g) → 2 CF4(g) + 4 HF(g)
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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)
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