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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.5 - ThermochemistryProblema 81
Capitolo 5, Problema 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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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.
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02:34
Enthalpy of Formation

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.
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Lattice Energy

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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01:18
First Law of Thermodynamics
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