Ch.9 - Thermochemistry: Chemical Energy

Problem 12

Chapter 9, Problem 12

A table of standard enthalpies of formation (ΔH°f) gives a value of −467.9 kJ/mol for NaNO3(s). Which reaction has a ΔH° value of −467.9 kJ? (a) Na+ (aq) + NO3−(aq) → NaNO3(s) (b) Na(s) + N(g + O3(g) → NaNO3(s) (c) Na(s) + 1/2 N2(g) + 3/2 O2(g) → NaNO3(s) (d) 2 Na(s) + N2(g) + 3 O2(g) → 2 NaNO3(s)

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Identify the definition of standard enthalpy of formation (ΔH°f), which is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

Examine each reaction option to determine if it represents the formation of NaNO3(s) from its elements in their standard states. The standard states are Na(s) for sodium, N2(g) for nitrogen, and O2(g) for oxygen.

Recognize that the correct reaction must produce exactly one mole of NaNO3(s) from its constituent elements.

Check the stoichiometry of each reaction to ensure it aligns with the formation of one mole of NaNO3(s) from elemental sodium, nitrogen, and oxygen.

Select the reaction that correctly represents the formation of one mole of NaNO3(s) from its elements in their standard states, which will have a ΔH° value of −467.9 kJ.

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Key Concepts

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

Standard Enthalpy of Formation (ΔH°f)

The standard enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics, allowing chemists to predict the energy changes associated with chemical reactions. A negative ΔH°f value indicates that the formation of the compound is exothermic, releasing energy.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to achieve the reaction. This principle allows for the calculation of enthalpy changes for complex reactions by summing the enthalpy changes of individual steps. It is particularly useful when direct measurement of a reaction's enthalpy change is difficult.

Stoichiometry in Chemical Reactions

Stoichiometry involves the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It is essential for determining the relationships between the quantities of substances involved in a reaction. Understanding stoichiometry helps in identifying which reaction corresponds to a given enthalpy change, as it allows for the proper interpretation of coefficients in the balanced equations.

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Textbook Question

A 25.0 g piece of granite at 100.0°C was added to 100.0 g of water of 25.0°C, and the temperature rose to 28.4°C. What is the specific heat capacity of the granite? (The specific heat capacity for water is 4.18 J/(g•°C).) (LO 9.10) (a) 0.563 J/(g•°C) (b) 1.53 J/(g•°C) (c) 0.992 J/(g•°C) (d) 0.794 J/(g•°C)

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Textbook Question

When 12.5 g of NH4NO3 is dissolved in 150.0 g of water of 25.0 °C in a coffee cup calorimeter, the final temperature of the solution of 19.7 °C. Assume that the specific heat of the solution is the same as that of water, 4.18 J/(g•°C). What is the ΔH per mol of NH4NO3? (LO 9.10) NH4NO3 (s) → NH4+ (aq) + NO3−(aq) ΔH = ? (a) +3.60 kJ (b) +23.0 kJ (c) +21.3 kJ (d) −3.60 kJ

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Textbook Question

Calculate the enthalpy change for the reaction C(s) + 2 H2(g) → CH4(g) ΔH = ? Given the enthalpy values for the following reactions CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH = −890.4 kJ C(s) + O2(g) → CO2(g) ΔH = −393.5 kJ H2(g) + 1/2 O2(g) → H2O (g) ΔH = −285.8 kJ (a) −1569.7 kJ (b) +211.1 kJ (c) −1855.5 kJ (d) −74.7 kJ

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Textbook Question

What is ΔH for the explosion of nitroglycerin? (LO 9.14) 2 C3H5(NO3)3(l) → 3 N2(g) + 1/2 O2(g) + 6 CO2(g) + 5 H2O(g) (a) −315.0 kJ (b) −4517 kJ (c) −3425 kJ (d) −3062 kJ

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Open Question

The boiling point of a substance is defined as the temperature at which liquid and vapor coexist in equilibrium. Use the heat of vaporization ΔHvap = 30.91 kJ/mol and the entropy of vaporization ΔSvap = 93.2 J/K·mol to calculate the boiling point (°C) of liquid bromine.

Open Question

Consider the following endothermic reaction of gaseous AB3 molecules with A2 molecules (LO 9.16, 9.17). Identify the true statement about the spontaneity of the reaction. (a) The reaction is likely to be spontaneous at high temperatures. (b) The reaction is likely to be spontaneous at low temperatures. (c) The reaction is always spontaneous. (d) The reaction is never spontaneous.