Ch.5 - Thermochemistry

Problem 81

Chapter 5, Problem 81

Without doing any calculations, predict the sign of H for each of the following reactions: (a) NaCl1s2¡Na+ 1g2 + Cl-1g2

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Identify the type of reaction: The given reaction involves the dissociation of solid sodium chloride (NaCl) into gaseous sodium (Na+) and chloride (Cl-) ions.

Consider the bond breaking and forming: In this reaction, the ionic bond between Na+ and Cl- in the solid state is broken to form gaseous ions. Breaking bonds requires energy, which is absorbed from the surroundings.

Analyze the state change: The reaction involves a change from a solid state to a gaseous state. Typically, transitioning to a gaseous state from a solid or liquid state involves an increase in entropy and requires energy input.

Assess the overall energy change: Since energy is required to break the bonds in NaCl and to overcome the lattice energy of the solid to form gaseous ions, the reaction is likely endothermic.

Predict the sign of ΔH: Based on the energy considerations and the endothermic nature of the reaction, ΔH (enthalpy change) for the reaction is expected to be positive.

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Key Concepts

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

Enthalpy Change (ΔH)

Enthalpy change (ΔH) refers to the 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). Understanding ΔH is crucial for predicting the energy changes during chemical reactions, including the dissolution of ionic compounds like NaCl.

Lattice Energy

Lattice energy is the energy released when gaseous ions combine to form an ionic solid. It is a measure of the strength of the ionic bonds in a crystal lattice. In the case of NaCl, the lattice energy is significant, and its magnitude influences the overall enthalpy change when NaCl dissociates into Na+ and Cl- ions.

Thermodynamic Favorability

Thermodynamic favorability refers to the likelihood of a reaction occurring based on its energy changes. A reaction is thermodynamically favorable if it results in a decrease in free energy (ΔG < 0). In predicting the sign of ΔH for the dissociation of NaCl, understanding the balance between lattice energy and the energy of the gaseous ions is essential for determining whether the process is favorable.

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Textbook Question

Methanol (CH₃OH) is used as a fuel in race cars. (b) Calculate the standard enthalpy change for the reaction, assuming H₂O(g) as a product.

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Textbook Question

Methanol (CH₃OH) is used as a fuel in race cars. (c) Calculate the heat produced by combustion per liter of methanol. Methanol has a density of 0.791 g/mL.

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Textbook Question

Methanol (CH₃OH) is used as a fuel in race cars. (d) Calculate the mass of CO₂ produced per kJ of heat emitted.

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Open Question

Without doing any calculations, predict the sign of _x001F_H for each of the following reactions: (a) 2 NO2(g) → N2O4(g) (b) 2 F(g) → F2(g) (c) Mg2+(g) + 2 Cl-(g) → MgCl2(s) (d) HBr(g) → H(g) + Br(g)

Textbook Question

Use bond enthalpies in Table 5.4 to estimate H for each of the following reactions: (a)

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Open Question

(a) Use enthalpies of formation given in Appendix C to calculate _x001F_H for the reaction Br2(g) → 2 Br(g), and use this value to estimate the bond enthalpy D(Br–Br). (b) How large is the difference between the value calculated in part (a) and the value given in Table 5.4?