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Ch.9 - Chemical Bonding I: The Lewis Model
Tro - Chemistry: A Molecular Approach 4th Edition
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Tutti i libri di testoTro 4th EditionCh.9 - Chemical Bonding I: The Lewis ModelProblema 105a
Capitolo 9, Problema 105a

If hydrogen were used as a fuel, it could be burned according to this reaction: H2(g) + 1/2 O2(g) → H2O(g) Use average bond energies to calculate ΔHrxn for this reaction.

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1
Identify the bond energies involved in the reaction. You will need the bond energy for the H-H bond in H2 and the O=O bond in O2. Additionally, consider the bond energy for the O-H bonds formed in H2O.
Write down the bond energies: Bond energy of H-H, Bond energy of O=O, and Bond energy of O-H. Note that the O-H bond energy will be used twice since each water molecule has two O-H bonds.
Calculate the total energy required to break the bonds in the reactants. This is done by adding the bond energy of H-H and half the bond energy of O=O (since only half a mole of O2 is used in the reaction).
Calculate the total energy released when the bonds in the products are formed. Multiply the bond energy of O-H by two (as there are two O-H bonds in each H2O molecule).
Determine ΔHrxn by subtracting the total energy of bond formation (step 4) from the total energy required to break the bonds in the reactants (step 3). The formula to use is ΔHrxn = Energy of bonds broken - Energy of bonds formed.

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Bond Energies

Bond energies refer to the amount of energy required to break a bond between two atoms in a molecule. In chemical reactions, bond energies are crucial for calculating the overall energy change, as they help determine how much energy is absorbed or released when bonds are broken and formed.
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Bond Energy

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is a measure of the total heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Calculating ΔH for a reaction involves subtracting the total bond energies of the products from the total bond energies of the reactants.
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Enthalpy of Formation

Stoichiometry of the Reaction

Stoichiometry involves the quantitative relationships between the reactants and products in a chemical reaction. In the given reaction, the coefficients indicate the molar ratios of hydrogen and oxygen reacting to form water, which is essential for accurately calculating the total bond energies involved and thus determining the ΔH for the reaction.
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Stoichiometry Concept
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Domanda del libro di testo

If hydrogen were used as a fuel, it could be burned according to this reaction: H2(g) + 1/2 O2(g) → H2O(g) Which fuel yields more energy per gram?

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Some theories of aging suggest that free radicals cause certain diseases and perhaps aging in general. As you know from the Lewis model, such molecules are not chemically stable and will quickly react with other molecules. According to certain theories, free radicals may attack molecules within the cell, such as DNA, changing them and causing cancer or other diseases. Free radicals may also attack molecules on the surfaces of cells, making them appear foreign to the body's immune system. The immune system then attacks the cells and destroys them, weakening the body. Draw Lewis structures for each free radical implicated in this theory of aging. d. CH3OO (unpaired electron on terminal oxygen)

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Free radicals are important in many environmentally significant reactions (see the Chemistry in the Environment box on free radicals in this chapter). For example, photochemical smog— smog that results from the action of sunlight on air pollutants— forms in part by these two steps:

The product of this reaction, ozone, is a pollutant in the lower atmosphere. (Upper atmospheric ozone is a natural part of the atmosphere that protects life on Earth from ultraviolet light.) Ozone is an eye and lung irritant and also accelerates the weathering of rubber products. Rewrite the given reactions using the Lewis structure of each reactant and product. Identify the free radicals.

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Domanda del libro di testo

Some theories of aging suggest that free radicals cause certain diseases and perhaps aging in general. As you know from the Lewis model, such molecules are not chemically stable and will quickly react with other molecules. According to certain theories, free radicals may attack molecules within the cell, such as DNA, changing them and causing cancer or other diseases. Free radicals may also attack molecules on the surfaces of cells, making them appear foreign to the body's immune system. The immune system then attacks the cells and destroys them, weakening the body. Draw Lewis structures for each free radical implicated in this theory of aging. c. OH

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Domanda del libro di testo

If hydrogen were used as a fuel, it could be burned according to this reaction: H2(g) + 1/2 O2(g) → H2O(g) Which fuel yields more energy per mole?

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Domanda del libro di testo

If hydrogen were used as a fuel, it could be burned according to this reaction: H2(g) + 1/2 O2(g) → H2O(g) Use average bond energies to calculate ΔHrxn for the combustion of methane (CH4).

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