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Ch.8 - Basic Concepts of Chemical Bonding
Brown - Chemistry: The Central Science 14th Edition
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook
All textbooksBrown 14th EditionCh.8 - Basic Concepts of Chemical BondingProblem 123
Chapter 8, Problem 123

Use average bond enthalpies from Table 8.4 to estimate the enthalpies of the following gas-phase reactions: Reaction 1: HF1g2 + H2O1g2 Δ F-1g2 + H3O+1g2 Reaction 2: HCl1g2 + H2O1g2 Δ Cl-1g2 + H3O+1g2 Are both reactions exothermic? How do these values relate to the different strengths of hydrofluoric and hydrochloric acid?

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Step 1: Identify the bonds that are broken and formed in each reaction. In Reaction 1, the bonds broken are H-F and H-O, and the bonds formed are F- and H3O+. In Reaction 2, the bonds broken are H-Cl and H-O, and the bonds formed are Cl- and H3O+.
Step 2: Look up the average bond enthalpies for each of these bonds in Table 8.4. Remember that bond breaking is an endothermic process (requires energy) and bond formation is an exothermic process (releases energy).
Step 3: Calculate the total energy change for each reaction by subtracting the energy required to break the bonds from the energy released when new bonds are formed. This will give you the enthalpy change for each reaction.
Step 4: Determine whether each reaction is exothermic or endothermic. If the enthalpy change is negative, the reaction is exothermic (releases heat). If the enthalpy change is positive, the reaction is endothermic (absorbs heat).
Step 5: Compare the enthalpy changes for the two reactions to determine how they relate to the different strengths of hydrofluoric and hydrochloric acid. The reaction with the more negative enthalpy change is the stronger acid, because it releases more heat when it reacts with water.

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Key Concepts

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

Bond Enthalpy

Bond enthalpy, or bond dissociation energy, is the energy required to break a bond in a molecule in the gas phase. It is a measure of bond strength; stronger bonds have higher bond enthalpies. In chemical reactions, the difference in bond enthalpies of reactants and products can be used to estimate the overall enthalpy change of the reaction, indicating whether it is exothermic or endothermic.

Enthalpy Change (ΔH)

Enthalpy change (ΔH) is the heat content change of a system at constant pressure during a chemical reaction. A negative ΔH indicates that the reaction releases heat to the surroundings, classifying it as exothermic, while a positive ΔH indicates heat absorption, classifying it as endothermic. Estimating ΔH using average bond enthalpies allows chemists to predict the energy dynamics of reactions.

Acid Strength and Ionization

The strength of an acid is determined by its ability to donate protons (H+) in solution, which is influenced by the stability of the resulting ions. Hydrofluoric acid (HF) is a weak acid, while hydrochloric acid (HCl) is a strong acid. The differences in acid strength can be related to the bond strength of the H-X bond (where X is the halogen), with weaker bonds leading to stronger acids due to easier ionization.
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Textbook Question
Ammonia reacts with boron trifluoride to form a stable compound, as we saw in Section 8.7. (a) Draw the Lewis structure of the ammonia–boron trifluoride reaction product.
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Textbook Question

Ammonium chloride, NH4Cl, is a very soluble salt in water. (c) If you dissolve 14 g of ammonium chloride in 500.0 mL of water, what is the molar concentration of the solution?

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Textbook Question
Ammonium chloride, NH4Cl, is a very soluble salt in water.(a) Draw the Lewis structures of the ammonium and chlorideions.
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