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Ch.7 - Thermochemistry
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.7 - ThermochemistryProblem 81c
Chapter 7, Problem 81c

For each generic reaction, determine the value of ΔH2 in terms of ΔH1.
c. A → B + 2 C ΔH1
1/2 B + C → 1/2 A ΔH2 = ?

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1
Identify the stoichiometry of the reactions given. The first reaction is A → B + 2C with a heat change ΔH₁. The second reaction is 1/2 B + C → 1/2 A with a heat change ΔH₂.
To find ΔH₂, we need to reverse and adjust the stoichiometry of the first reaction to match the second reaction. Reversing the first reaction gives B + 2C → A, and the heat change for this reversed reaction would be -ΔH₁.
Adjust the coefficients of the reversed reaction to match those of the second reaction. Divide all coefficients in the reversed reaction by 2 to get 1/2 B + C → 1/2 A.
Since the coefficients of all substances in the reversed reaction are halved, the heat change for this new reaction will also be half of -ΔH₁, which is -1/2 ΔH₁.
Thus, ΔH₂ for the reaction 1/2 B + C → 1/2 A is -1/2 ΔH₁.

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Key Concepts

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

Hess's Law

Hess's Law states that the total enthalpy change for a chemical 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 enthalpy change for a reaction by combining known enthalpy changes of related reactions.
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Hess's Law

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Understanding ΔH is crucial for predicting the energy changes associated with chemical reactions.

Stoichiometry in Reactions

Stoichiometry involves the quantitative relationships between the reactants and products in a chemical reaction. It allows us to relate the amounts of substances consumed and produced, which is essential for calculating enthalpy changes when reactions are manipulated or combined.
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Related Practice
Textbook Question

Calculate ΔHrxn for the reaction:

5 C(s) + 6 H2(g) → C5H12(l)

Use the following reactions and given ΔH's:

C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g) ΔH = –3244.8 kJ

C(s) + O2(g) → CO2(g) ΔH = –393.5 kJ

2 H2(g) + O2(g) → 2 H2O(g) ΔH = –483.5 kJ

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Textbook Question

For each generic reaction, determine the value of ΔH2 in terms of ΔH1.

a. A + B → 2 C ΔH1

2 C→ A + B ΔH2 = ?

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Textbook Question

For each generic reaction, determine the value of ΔH2 in terms of ΔH1.

b. A + 1/2 B → C ΔH1

2 A + B → 2 C ΔH2 = ?

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Textbook Question

Calculate ΔHrxn for the reaction:

Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)

Use the following reactions and given ΔH's:

2 Fe(s) + 3/2 O2(g) → Fe2O3(s) ΔH = –824.2 kJ

CO(g) + 1/2 O2(g) → CO2(g) ΔH = –282.7 kJ

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Textbook Question

Instant cold packs used to ice athletic injuries on the field contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the endothermic reaction: NH4NO3(s) → NH4+(aq) + NO3– (aq) In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 °C and the final temperature (after the solid dissolves) is 21.9 °C. Calculate the change in enthalpy for the reaction in kJ. (Use 1.0 g/mL as the density of the solution and 4.18 J/g•°C as the specific heat capacity.)

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Textbook Question

Consider the generic reaction:

A + 2 B → C + 3 D ΔH = 155 kJ

Determine the value of ΔH for each related reaction.

a. 3 A + 6 B → 3 C + 9 D

b. C + 3 D → A + 2 B

c. 1/2 C + 3/2 D → 1/2 A + B

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