Skip to main content
Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Not the one you use?Change textbook
All textbooksBrown 15th EditionCh.5 - ThermochemistryProblem 43
Chapter 5, Problem 43

Consider the following reaction: 2 Mg(s) + O₂(g) → 2 MgO(s) ΔH = -1204 kJ (b) Calculate the amount of heat transferred when 3.55 g of Mg(s) reacts at constant pressure. (d) How many kilojoules of heat are absorbed when 40.3 g of MgO(s) is decomposed into Mg(s) and O₂(g) at constant pressure?

Verified step by step guidance
1
Step 1: Calculate the moles of Mg(s) reacting. Use the molar mass of Mg (24.31 g/mol) to convert 3.55 g of Mg to moles.
Step 2: Use the stoichiometry of the reaction to determine the heat transferred. The reaction shows that 2 moles of Mg release 1204 kJ of heat. Use this ratio to find the heat released for the moles of Mg calculated in Step 1.
Step 3: Calculate the moles of MgO(s) decomposed. Use the molar mass of MgO (40.31 g/mol) to convert 40.3 g of MgO to moles.
Step 4: Use the stoichiometry of the reverse reaction to determine the heat absorbed. Since the decomposition is the reverse of the given reaction, the same amount of heat (1204 kJ) is absorbed for 2 moles of MgO. Use this ratio to find the heat absorbed for the moles of MgO calculated in Step 3.
Step 5: Summarize the results. For part (b), state the heat transferred when 3.55 g of Mg reacts. For part (d), state the heat absorbed when 40.3 g of MgO decomposes.

Key Concepts

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

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced equation. It allows us to determine the amount of substance consumed or produced in a reaction, using mole ratios derived from the coefficients in the balanced equation. In this case, it helps calculate the heat transferred when a specific mass of magnesium reacts.
Recommended video:
Guided course
01:16
Stoichiometry Concept

Enthalpy Change (ΔH)

Enthalpy change (ΔH) represents 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 calculating the heat transfer associated with the reactions of magnesium and magnesium oxide.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is essential for converting between grams and moles, allowing for stoichiometric calculations in chemical reactions. In this problem, knowing the molar mass of magnesium and magnesium oxide is necessary to determine the amount of heat transferred based on the mass of the substances involved.
Recommended video:
Guided course
02:11
Molar Mass Concept
Related Practice
Textbook Question

Ozone, O3(g), is a form of elemental oxygen that plays an important role in the absorption of ultraviolet radiation in the stratosphere. It decomposes to O2(g) at room temperature and pressure according to the following reaction: 2 O3(g) → 3 O2(g) ΔH= -284.6 kJ b. Which has the higher enthalpy under these conditions, 2 O3(g) or 3 O2(g)?

5
views
Textbook Question

When solutions containing silver ions and chloride ions are mixed, silver chloride precipitates Ag+(aq) + Cl-(aq) → AgCl(s) H = -65.5 kJ (a) Calculate H for the production of 0.450 mol of AgCl by this reaction. (b) Calculate H for the production of 9.00 g of AgCl. (c) Calculate H when 9.25⨉10-4 mol of AgCl dissolves in water.

861
views
Textbook Question

Without referring to tables, predict which of the following has the higher enthalpy in each case: (c) 1 mol H2(g) and 0.5 mol O2(g) at 25 °C or 1 mol H2O(g) at 25 °C

2
views
Textbook Question

Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (d) How many kilojoules of heat are released when 38.5 g of CH4(g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?

458
views
Textbook Question

Without referring to tables, predict which of the following has the higher enthalpy in each case: (b) 2 mol of hydrogen atoms or 1 mol of H2

2
views
Textbook Question

Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (b) Calculate the amount of heat transferred when 24.0 g of CH3OH(g) is decomposed by this reaction at constant pressure.

719
views
1
rank