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Chapter 11: Problem 42

The high melting points of ionic solids indicate that a lot of energy must be supplied to separate the ions from one another. How is it possible that the ions can separate from one another when soluble ionic compounds are dissolved in water, often with essentially no temperature change?

Short Answer

Expert verified
When a soluble ionic compound dissolves in water, the water molecules surround the ions and separate them from one another in a process called solvation or hydration. The dissolution has two energy components: energy required to separate the ions (endothermic) and energy released during solvation (exothermic). If these two energies are almost equal, there is minimal temperature change. Thus, the apparent ease of dissolving ionic compounds in water with little temperature change can be attributed to the balance between the endothermic and exothermic processes involved.

Step by step solution

01

Understanding ionic bonds and separation in solid-state

In ionic solids, the positively charged ions (cations) and negatively charged ions (anions) are held together in a lattice structure by strong electrostatic forces that form ionic bonds. To separate these ions, a considerable amount of energy must be supplied, which results in the high melting points of ionic solids.
02

The dissolution process in water

When an ionic compound dissolves in water, the water molecules surround the ions, and the process is called solvation (or hydration, specifically for water). Water molecules are polar; they have a positively charged part (hydrogen atoms) and a negatively charged part (oxygen atom). The positive and negative parts of water molecules interact with the ionic solid's ions, causing the ions to separate from each other and become surrounded by water molecules.
03

The energy balance during dissolution

The dissolution process has two energy components: the energy required to separate the ions from the ionic solid (endothermic) and the energy released when water molecules solvate the ions (exothermic). In some cases, the energy released during solvation is almost equal to the energy needed to separate the ions, resulting in minimal temperature change.
04

Why some ionic compounds dissolve without significant temperature change

The ability of soluble ionic compounds to dissolve in water with almost no temperature change is due to the balance between the endothermic process of breaking ionic bonds and the exothermic process of solvation. Since the energy required to separate the ions from one another is mostly compensated by the energy released during solvation, only a little amount of additional energy is needed (which can come from the surroundings as a negligible temperature change).

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Most popular questions from this chapter

Calculate the solubility of \(\mathrm{O}_{2}\) in water at a partial pressure of \(\mathrm{O}_{2}\) of 120 torr at \(25^{\circ} \mathrm{C}\). The Henry's law constant for \(\mathrm{O}_{2}\) is \(1.3 \mathrm{X}\) \(10^{-3} \mathrm{~mol} / \mathrm{L} \cdot\) atm for Henry's law in the form \(C=k P\), where \(C\) is the gas concentration \((\mathrm{mol} / \mathrm{L})\).

The term proof is defined as twice the percent by volume of pure ethanol in solution. Thus, a solution that is \(95 \%\) (by volume) ethanol is 190 proof. What is the molarity of ethanol in a 92 proof ethanol-water solution? Assume the density of ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), is \(0.79 \mathrm{~g} / \mathrm{cm}^{3}\) and the density of water is \(1.0 \mathrm{~g} / \mathrm{cm}^{3}\)

What mass of sodium oxalate \(\left(\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right)\) is needed to prepare \(0.250 \mathrm{~L}\) of a \(0.100 M\) solution?

Using the following information, identify the strong electrolyte whose general formula is $$ \mathrm{M}_{x}(\mathrm{~A})_{y} \cdot z \mathrm{H}_{2} \mathrm{O} $$ Ignore the effect of interionic attractions in the solution. a. \(\mathrm{A}^{n-}\) is a common oxyanion. When \(30.0 \mathrm{mg}\) of the anhydrous sodium salt containing this oxyanion \(\left(\mathrm{Na}_{n} \mathrm{~A}\right.\), where \(n=1,2\), or 3 ) is reduced, \(15.26 \mathrm{~mL}\) of \(0.02313 M\) reducing agent is required to react completely with the \(\mathrm{Na}_{n}\) A present. Assume a \(1: 1\) mole ratio in the reaction. b. The cation is derived from a silvery white metal that is relatively expensive. The metal itself crystallizes in a body-centered cubic unit cell and has an atomic radius of \(198.4 \mathrm{pm}\). The solid, pure metal has a density of \(5.243 \mathrm{~g} / \mathrm{cm}^{3}\). The oxidation number of \(\mathrm{M}\) in the strong electrolyte in question is \(+3\). c. When \(33.45 \mathrm{mg}\) of the compound is present (dissolved) in \(10.0 \mathrm{~mL}\) of aqueous solution at \(25^{\circ} \mathrm{C}\), the solution has an osmotic pressure of 558 torr.

How would you prepare \(1.0 \mathrm{~L}\) of an aqueous solution of sucrose \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\) having an osmotic pressure of \(15 \mathrm{~atm}\) at a temperature of \(22^{\circ} \mathrm{C} ?\) Sucrose is a nonelectrolyte.
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