Textbook Question
Order these compounds in order of decreasing carbon–carbon bond length: HCCH, H2CCH2, H3CCH3.
Ch.10 - Chemical Bonding I: The Lewis Model
Chapter 10, Problem 82
Ethanol is a possible fuel. Use average bond energies to calculate ΔHrxn for the combustion of ethanol. CH3CH2OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)
Verified step by step guidance1
Step 1: Write the balanced chemical equation for the combustion of ethanol.
Step 2: Identify and list all the bonds broken in the reactants. For ethanol (CH3CH2OH), consider C-H, C-C, C-O, and O-H bonds. For O2, consider the O=O bond.
Step 3: Identify and list all the bonds formed in the products. For CO2, consider C=O bonds. For H2O, consider O-H bonds.
Step 4: Use the average bond energies to calculate the total energy required to break the bonds in the reactants.
Step 5: Use the average bond energies to calculate the total energy released when forming the bonds in the products. Then, calculate \( \Delta H_{\text{rxn}} \) by subtracting the total energy of bonds formed from the total energy of bonds broken.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Bond Energies
Bond energies refer to the amount of energy required to break a bond between two atoms in a molecule. In the context of chemical reactions, the total energy required to break bonds in the reactants must be compared to the energy released when new bonds are formed in the products. This concept is crucial for calculating the enthalpy change (ΔHrxn) of a reaction.
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01:50
Bond Energy
Enthalpy Change (ΔHrxn)
Enthalpy change (ΔHrxn) is the heat change associated with a chemical reaction at constant pressure. It can be calculated using the formula ΔHrxn = Σ(Bond Energies of Reactants) - Σ(Bond Energies of Products). A negative ΔHrxn indicates that the reaction is exothermic, meaning it releases heat, which is typical for combustion reactions like that of ethanol.
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02:34Enthalpy of Formation
Combustion Reaction
A combustion reaction is a chemical process in which a substance reacts rapidly with oxygen, producing heat and light. In the case of ethanol, the reaction with oxygen produces carbon dioxide and water. Understanding the stoichiometry of the reaction and the products formed is essential for accurately calculating the enthalpy change and assessing the energy output of the fuel.
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02:24Combustion Apparatus
Related Practice
Textbook Question
Which of the two compounds, H2NNH2 and HNNH, has the strongest nitrogen-nitrogen bond, and which has the shorter nitrogen-nitrogen bond.
Textbook Question
Hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds. Use average bond energies to calculate ΔHrxn for the hydrogenation reaction. H2C=CH2(g) + H2(g) → H3C–CH3(g)
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Textbook Question
Ethane burns in air to form carbon dixode and water vapor.
2 H3C¬CH3( g) + 7 O2( g)¡4 CO2( g) + 6 H2O( g)
Use average bond energies to calculate ΔHrxn for the reaction.
Textbook Question
In the Chemistry and the Environment box on free radicals in this chapter, we discussed the importance of the hydroxyl radical in reacting with and eliminating many atmospheric pollutants. However, the hydroxyl radical does not clean up everything. For example, chlorofluorocarbons—which destroy stratospheric ozone—are not attacked by the hydroxyl radical. Consider the hypothetical reaction by which the hydroxyl radical might react with a chlorofluorocarbon: OH(g) + CF2Cl2(g) → HOF(g) + CFCl2(g) Use bond energies to explain why this reaction is improbable. (The C–F bond energy is 552 kJ/mol.)