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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.5 - ThermochemistryProblema 63
Capitolo 5, Problema 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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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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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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Stoichiometry Concept
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