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

everyone in this example we need to express our second entropy in terms of our first entropy. So looking at our second entropy expression, we have four moles of our reactant C, which produces six moles of our product B and two moles of our product A. However in our first entropy expression we have a one mole of A reacting with three moles of B to produce two moles of C. So it looks like our reaction here has flipped. And so in order to write our first entropy in terms of our second entropy we would want to flip the sign of our entropy of our first expression here because we want to go ahead and make our B and A. As products as they are expressed in our second entropy expression here. And so that would flip our reaction so that C. Is a reactant and B and A would be products. We also want to recognize that our moles of B and A need to be doubled and so we would go ahead and multiply this expression by two and so to express our second entropy. In terms of our first entropy we would say that our second entropy is going to equal -2 times our first entropy expression. And so this would be our final answer to complete this example. So I hope that everything I reviewed was clear. If you have any questions, please leave them down below and I will see everyone in the next practice video

Ch.6 - Thermochemistry

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Tro 4th Edition

Ch.6 - Thermochemistry

Problem 77b

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Chapter 6, Problem 77b

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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Identify the stoichiometric relationship between the two reactions. The second reaction (2 A + B → 2 C) can be viewed as doubling the first reaction (A + 1/2 B → C).

Write down the enthalpy change for the first reaction: ΔH₁.

Since the second reaction is essentially twice the first reaction, multiply the enthalpy change of the first reaction by 2 to account for the doubling of the reactants and products.

Express the enthalpy change for the second reaction, ΔH₂, in terms of ΔH₁: ΔH₂ = 2ΔH₁.

Conclude that the enthalpy change for the second reaction, ΔH₂, is twice that of the first reaction, 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 from related reactions.

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

Stoichiometry involves the calculation of reactants and products in chemical reactions based on balanced equations. It allows chemists to determine the proportions of substances involved in reactions, which is essential for relating the enthalpy changes of different reactions to one another, as seen in the given problem.

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Stoichiometry Concept

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Textbook Question

Zinc metal reacts with hydrochloric acid according to the balanced equation: Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g) When 0.103 g of Zn(s) is combined with enough HCl to make 50.0 mL of solution in a coffee-cup calorimeter, all of the zinc reacts, raising the temperature of the solution from 22.5 °C to 23.7 °C. Find ΔH_rxn for this reaction as written. (Use 1.0 g/mL for the density of the solution and 4.18 J/g•°C as the specific heat capacity.)

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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.)

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.

c. A → B + 2 C ΔH₁