The lattice enthalpy of sodium chloride, Δ H ° for NaCl ( s ) → Na + ( g ) + Cl − ( g ) is 787 kJ/mol; the heat of solution in making up 1 M NaCl( aq ) is +4.0 kJ/mol. From these data, obtain the sum of the heats of hydration of Na + and Cl − . That is obtain the sum of Δ H ° values for Na + ( g ) → Na + ( a q ) Cl − ( g ) → Cl − ( a q ) If the heat of hydration of Cl − is −338 kJ/mol, what is the heat of hydration of Na + ?

General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
Publisher: Cengage Learning
ISBN: 9781305580343

General Chemistry - Standalone boo...

11th Edition
Steven D. Gammon + 7 others
Publisher: Cengage Learning
ISBN: 9781305580343

Solutions

Chapter
Section
Chapter 12, Problem 12.133QP
Textbook Problem
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The lattice enthalpy of sodium chloride, ΔH° for NaCl ( s ) → Na + ( g ) + Cl − ( g ) is 787 kJ/mol; the heat of solution in making up 1 M NaCl(aq) is +4.0 kJ/mol. From these data, obtain the sum of the heats of hydration of Na+ and Cl−. That is obtain the sum of ΔH° values for Na + ( g ) → Na + ( a q ) Cl − ( g ) → Cl − ( a q ) If the heat of hydration of Cl− is −338 kJ/mol, what is the heat of hydration of Na+?

Expert Solution
Interpretation Introduction

Interpretation:

Sum of heat of hydration of Na+ and Cl has to be calculated.  Heat of hydration of Na+ ion alone has to be calculated.

Concept Introduction:

• The energy released when gaseous state ions of unlike charges that are infinitely farther apart combine to form a stable ionic solid is called Lattice energy.  Conversely, the energy required to break the electrostatic force of attraction between the ions of unlike charges in the ionic solid and revert them to gaseous state is also termed as Lattice energy of an ionic solid.
• Hydration energy refers to the quantity of energy released when one mole of ion gets hydrated at constant pressure.  Hydration energy of an ion depends upon its ionic size.

Explanation of Solution

Given that lattice enthalpy of sodium chloride is 787 kJ/mol. heat of solution of aqueous NaCl is +4 kJ/mol.   lattice enthalpy is energy released when one mole of NaCl is formed. Therefore, Sum of heat of hydration of Na+ and Cl corresponds to sum of lattice energy of NaCl and heat of solution of NaCl and is calculated as follows –

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