Ch.9 - Thermochemistry: Chemical Energy

Problem 38b

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Chapter 9, Problem 38b

The reaction of A with B to give D proceeds in two steps: (1) A + B → C ΔH° = -20 kJ (2) C + B → D ΔH° = +50 kJ (3) A + 2B → D ΔH° = ? (b) What is the value of ΔH° for the overall reaction A + 2 B → D ΔH° = ?

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Identify the given reactions and their enthalpy changes. The first reaction is A + B → C with an enthalpy change ΔH° = -20 kJ. The second reaction is C + B → D with an enthalpy change ΔH° = +50 kJ.

Understand that the overall reaction A + 2B → D can be obtained by adding the two given reactions. This means you sum up the left-hand sides and the right-hand sides of the reactions respectively.

Add the reactions: (A + B) + (C + B) = C + (D). Simplify this to get A + 2B = D, which matches the overall reaction provided.

Add the enthalpy changes for the individual steps to find the overall enthalpy change ΔH° for the reaction A + 2B → D. This is done by adding ΔH° = -20 kJ and ΔH° = +50 kJ.

The sum of the enthalpy changes will give you the ΔH° for the overall reaction A + 2B → D. This is the value you need to find.

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Key Concepts

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

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH° indicates an exothermic reaction, where heat is released, while a positive ΔH° indicates an endothermic reaction, where heat is absorbed. Understanding ΔH° is crucial for predicting the energy changes associated with chemical reactions.

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 us to calculate the overall ΔH° for a reaction by adding the ΔH° values of the steps involved, making it essential for solving multi-step reactions.

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It is important for determining how much of each substance is consumed or produced. In this context, understanding stoichiometry helps in balancing the overall reaction and applying Hess's Law correctly to find the overall ΔH°.

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