Textbook Question
Methanol, CH3OH, is used as race car fuel.
c. How many kilojoules are released by burning 50.0 g of methanol?
Ch.7 Chemical Reactions: Energy, Rate and Equilibrium
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187
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Table of contents
- Ch.1 Matter and Measurements27
- Ch.2 Atoms and the Periodic Table20
- Ch.3 Ionic Compounds8
- Ch.4 Molecular Compounds23
- Ch.5 Classification & Balancing of Chemical Reactions12
- Ch.6 Chemical Reactions: Mole and Mass Relationships28
- Ch.7 Chemical Reactions: Energy, Rate and Equilibrium64
- Ch.8 Gases, Liquids and Solids24
- Ch.9 Solutions52
- Ch.10 Acids and Bases25
- Ch.11 Nuclear Chemistry22
- Ch.12 Introduction to Organic Chemistry: Alkanes57
- Ch.13 Alkenes, Alkynes, and Aromatic Compounds47
- Ch.14 Some Compounds with Oxygen, Sulfur, or a Halogen50
- Ch.15 Aldehydes and Ketones45
- Ch.16 Amines42
- Ch.17 Carboxylic Acids and Their Derivatives57
- Ch.18 Amino Acids and Proteins82
- Ch.19 Enzymes and Vitamins63
- Ch.20 Carbohydrates44
- Ch.21 The Generation of Biochemical Energy65
- Ch.22 Carbohydrate Metabolism45
- Ch.23 Lipids42
- Ch.24 Lipid Metabolism33
- Ch.25 Protein and Amino Acid Metabolism24
- Ch.26 Nucleic Acids and Protein Synthesis45
McMurry8th EditionFundamentals of General, Organic, and Biological ChemistryISBN: 9780134015187Not the one you use?Change textbook
Chapter 7, Problem 72b
For the evaporation of water, H2O(l) → H2O(g), at 100°C, ∆H = +9.72 kcal/mol (+40.7 kJ/mol).
b. How many kilojoules are released when 10.0 g of H2O(g) is condensed?
Verified step by step guidance1
Step 1: Understand the problem. The condensation of water is the reverse of evaporation. Since evaporation absorbs energy (endothermic, ∆H = +40.7 kJ/mol), condensation releases the same amount of energy (exothermic, ∆H = -40.7 kJ/mol).
Step 2: Convert the mass of water (10.0 g) to moles. Use the molar mass of water (H₂O), which is approximately 18.02 g/mol. The formula to calculate moles is: moles = mass / molar mass.
Step 3: Calculate the energy released during condensation. Multiply the number of moles of water by the enthalpy change for condensation (∆H = -40.7 kJ/mol). The formula is: energy released = moles × ∆H.
Step 4: Ensure the correct sign for the energy change. Since condensation is exothermic, the energy released will be negative, indicating that energy is leaving the system.
Step 5: Double-check the units and ensure the final answer is in kilojoules (kJ). If necessary, confirm that all intermediate calculations are consistent with the given data.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy of Vaporization
The enthalpy of vaporization is the amount of energy required to convert a unit mass of a substance from liquid to gas at a constant temperature and pressure. For water, this value is positive when vaporizing, indicating energy absorption. Conversely, during condensation, the same amount of energy is released, which is crucial for calculating energy changes in phase transitions.
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02:30
Endothermic & Exothermic Reactions
Molar Mass of Water
The molar mass of water (H2O) is approximately 18.02 g/mol. This value is essential for converting between grams of water and moles, allowing for the calculation of energy changes based on the number of moles involved in the phase change. Understanding molar mass is fundamental in stoichiometry and thermodynamic calculations.
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01:10Calculating Molar Mass
Energy Conversion
Energy conversion refers to the process of changing energy from one form to another. In this context, it involves converting the energy associated with the phase change of water from gas to liquid, quantified in kilojoules. Recognizing how to convert between different energy units (e.g., kcal to kJ) is vital for accurate calculations in thermodynamics.
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Related Practice
Textbook Question
Sketch an energy diagram for a system in which the forward reaction has Eact = +25 kcal/mol (+105 kJ/mol) and the reverse reaction has Eact = +35 kcal/mol (+146 kJ/mol).
a. Is the forward process endergonic or exergonic?
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Textbook Question
Ammonia reacts slowly in air to produce nitrogen monoxide and water vapor:
NH3(g) + O2(g) ⇌ NO(g) + H2O(g) + Heat
b. Write the equilibrium equation.
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Textbook Question
Magnetite, an iron ore with formula Fe3O4, can be reduced by treatment with hydrogen to yield iron metal and water vapor.
d. This reaction has K = 2.3 × 10-18. Are the reactants or the products favored?
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Textbook Question
Magnetite, an iron ore with formula Fe3O4, can be reduced by treatment with hydrogen to yield iron metal and water vapor.
a. Write the balanced equation.
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Textbook Question
For the production of ammonia from its elements, ∆H = -22 kcal/mol(-19 kJ/mol).
b. How much energy (in kilocalories and kilojoules) is involved in the production of 0.700 mol of NH3?