Textbook Question
Which of the two compounds, H2NNH2 and HNNH, has the strongest nitrogen-nitrogen bond, and which has the shorter nitrogen-nitrogen bond.
Ch.10 - Chemical Bonding I: The Lewis Model
Chapter 10, Problem 83
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.
Verified step by step guidance1
Identify the bonds broken in the reactants: In ethane (C2H6), there are C-C and C-H bonds. In O2, there are O=O bonds.
Identify the bonds formed in the products: In CO2, there are C=O bonds. In H2O, there are O-H bonds.
Use the average bond energies to calculate the total energy required to break the bonds in the reactants. This involves multiplying the number of each type of bond by its bond energy and summing them up.
Use the average bond energies to calculate the total energy released when the bonds in the products are formed. Again, multiply the number of each type of bond by its bond energy and sum them up.
Calculate the enthalpy change (ΔH_rxn) for the reaction using the formula: ΔH_rxn = (Total energy of bonds broken) - (Total energy of bonds formed).
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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 chemical reactions, the total energy required to break the bonds in the reactants is 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, as it allows us to estimate the energy changes associated with bond breaking and forming.
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01:50
Bond Energy
Enthalpy Change (ΔHrxn)
The enthalpy change (ΔHrxn) of a reaction is the difference in energy between the products and the reactants, indicating whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). It can be calculated using the formula ΔHrxn = Σ(Bond Energies of Reactants) - Σ(Bond Energies of Products). Understanding this concept is essential for predicting the energy dynamics of the combustion of ethane.
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02:34Enthalpy of Formation
Combustion Reaction
A combustion reaction is a chemical reaction that typically involves a hydrocarbon reacting with oxygen to produce carbon dioxide and water, releasing energy in the form of heat and light. In the case of ethane, the balanced equation shows that it reacts with oxygen to produce carbon dioxide and water vapor. Recognizing the characteristics of combustion reactions is important for applying bond energy calculations to determine the overall energy change.
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Related Practice
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
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)
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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.)
Textbook Question
Write an appropriate Lewis structure for each compound. Make certain to distinguish between ionic and molecular compounds. b. ClF5