Ch.6 - Thermochemistry

Tro - Chemistry: A Molecular Approach 4th Edition

Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook

Tro 4th Edition

Ch.6 - Thermochemistry

Problem 79

Chapter 6, Problem 79

Calculate ΔH_rxn 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

Verified step by step guidance

Identify the given reactions and their respective enthalpy changes (ΔH). For the first reaction: 2 Fe(s) + 3/2 O2(g) → Fe2O3(s) with ΔH = -824.2 kJ. For the second reaction: CO(g) + 1/2 O2(g) → CO2(g) with ΔH = -282.7 kJ.

Write the target reaction for which you need to calculate ΔHrxn: Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g).

Reverse the first given reaction to match the products and reactants of the target reaction. The reversed reaction is: Fe2O3(s) → 2 Fe(s) + 3/2 O2(g). Reversing the reaction changes the sign of ΔH, so ΔH becomes +824.2 kJ.

Multiply the second given reaction by 3 to balance the number of CO and CO2 in the target reaction: 3[CO(g) + 1/2 O2(g) → CO2(g)]. This also multiplies the ΔH by 3, giving 3(-282.7 kJ) = -848.1 kJ.

Add the modified reactions from steps 3 and 4 to get the target reaction. Add their ΔH values to find the ΔHrxn for the target reaction: ΔHrxn = +824.2 kJ + (-848.1 kJ).

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Was this helpful?

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 (ΔHrxn) for a reaction by using known ΔH values from related reactions, making it essential for solving problems involving reaction enthalpies.

Standard Enthalpy of Formation

The standard enthalpy of formation (ΔHf°) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. This concept is crucial for calculating ΔHrxn, as it provides a reference point for the energy changes associated with the formation of reactants and products in a chemical reaction.

Stoichiometry in Thermochemistry

Stoichiometry in thermochemistry involves using the coefficients from a balanced chemical equation to relate the amounts of reactants and products to their respective enthalpy changes. Understanding stoichiometry is vital for accurately calculating ΔHrxn, as it ensures that the enthalpy changes are applied in proportion to the quantities of substances involved in the reaction.

Recommended video:

Guided course

01:16

Stoichiometry Concept

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

Write an equation for the formation of each compound from its elements in their standard states, and find ΔH°_rxn for each in Appendix IIB. b. MgCO₃(s)

1906

views

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 = ?

1909

views

Textbook Question

Consider the generic reaction:

A + 2 B → C + 3 D ΔH = 155 kJ

Determine the value of ΔH for each related reaction.