Download Lesson 11.1 properties of solutions

Lesson 11.1 Properties of Solutions
Suggested Reading

Zumdahl Chapter 11 Section11.2
Essential Questions

What are the basic properties of solutions?
Learning Objectives


List the steps of solution formation.
Define and calculate ∆Hsoln and ∆Hhyd.
Introduction
There are various practical reasons for preparing solutions. For example,
most chemical reactions are run in solutions. Also, solutions have particular
properties that make them useful. In this lesson we will look at some of these
basic properties, in brief. In future chemistry courses, you will likely learn a
great deal more about solutions.
Types of Solutions
When sodium chloride dissolves in water, the resulting uniform dispersion of
ions in water is called a solution. In general, a solution is a homogenous
mixture of two or more substances consisting of ions or molecules. However,
solutions may exist in any of the three states of matter. The term solute and
solvent refer to the components of a solutions. In most cases, the solute is the
gas or solid dissolved in in a liquid, but in some cases the solute is the
component in the smaller amount. The solvent is the liquid in which the solute
has been dissolved, but may also be the component in the greater amount.
Gaseous Solutions
In general, nonreactive gases or vapors can mix in all proportions to give a
gaseous mixture. Fluids that mix with or dissolve in each other in all
proportions are said to be miscible fluids. Gases are thus miscible. If two fluids
do not mix but form layers, they are said to be immiscible.
Liquid Solutions
Most liquid solutions are obtained by dissolving a gas, liquid, or solid in some
liquid. Soda water, for example, consists of a solution of carbon dioxide gas in
water. Ethanol, C2H5OH, in water is an example of liquid-liquid solution.
Seawater contains both dissolved gases (from air) and solids (mostly NaCl).
It is also possible to make a liquid solution by mixing two solids together.
Potassium-sodium alloy is an example of this. Both potassium and sodium are
solid metals at room temperature, but a liquid solution results when the
mixture contains 10% to 50% sodium.
Solid Solutions
Solid solutions are also possible, alloys of gold and silver are examples of this
as are old-fashioned dental fillings, which are an alloy of mercury and silver.
Solubility
The amount of substance that will dissolve in a solvent depends on both the
substance and the solvent. We describe the amount that dissolves in terms of
solubility.
A saturated solution is a solution that is in equilibrium with respect to a
particular dissolved substance. No more solute can dissolve at this
point. Solubility is the amount of a substance that dissolves in a given
quantity of solvent (such as water) at a given temperature to give a saturated
solution. For example, the solubility of NaCl in water is 36.0 g/100mL at 20∘C.
If you mix 30.0 g os NaCl with 100 mL of water, all of the crystals will dissolve
and you would have an unsaturated solution.
An unsaturated solution is a solution that is not in equilibrium with respect to
a given dissolved substance and in which more of the substance can
dissolve.
Sometimes it is possible to obtain a supersaturated solution, a solution
that contains more dissolved substance than a saturated solution does.
How does this happen? For example, the solubility of sodium thiosulfate,
Na2S2O3 in water at 100∘C is 231 g/100mL. But at room temperature it is
only 50 g/100mL. Suppose you prepare a solution saturated with sodium
thiosulfate at 100∘C. You might expect some of the sodium thiosulfate to
precipitate out as the solution cools. In fact, if the solution is slowly cooled
to room temperature, this does not occur. Instead the result is a solution in
which 231 g of sodium thiosulfate is dissolved in 100 mL. of cold water,
compared with the 50 g you would normally expect.
The solubilities of substances in one another vary widely. You might find a
substance miscible in one solvent but nearly insoluble in another. As a
general rule, "like dissolves like." That is, similar substances dissolve one
another. For example, oil is miscible in gasoline because both substances
are nonpolar hydrocarbons. On the other hand, oil is immiscible in water,
which is a polar substance. The solubility of one substance in another can
be explained in terms of two factors. One is the natural tendency of
substances to mix. However, if the process of dissolving one substance in
another involved nothing more than mixing, you would expect all
substances to be completely soluble in one another. You know this is not
always the case, so we must consider a second factor.
The second factor that limits solubility is the relative forces of attraction
between species. Suppose there are strong forces of attraction between
solute species and strong attractions between solvent species, but week
attractions between solute and solvent species. In that case, the mixing is
not energetically favorable and the solute and solvent species do not mix
(Recall that breaking strong bonds to make weak bonds requires an input
of energy). Thus, the solubility of a solute in a solvent depends on a
balance between the natural tendency for the solute and solvent to mix and
the tendency for a system to have the lowest energy possible. By
determining the enthalpy of solution we can see whether or not mixing is
energetically favorable. If the enthalpy of solution is exothermic then mixing
will occur, and if it is endothermic than it will not.
Enthalpy of Solution
The enthalpy change of solution refers to the overall amount of heat which is
released or absorbed during the dissolving process (at constant pressure).
The enthalpy of solution can either be positive (endothermic) or negative
(exothermic). The enthalpy of solution is commonly referred to as ΔHsolution.
To help understand the solution process it is easiest to think of it as
happening in three steps, each with its own change in enthalpy. The enthalpy
change associated with the solution formation (enthalpy of solution), is then
the sum of the three steps. The final value for the enthalpy of solution can
either be endothermic or exothermic, depending on how much energy is
required or given off in each step.
∆Hsoln = ∆H1 + ∆H2 + ∆H3
∆H1
- In the first step the solute is separated into its components. The
enthalpy of this process is called ΔH1. This is an endothermic process
because energy is required for this reaction, so ΔH1 > 0.
∆H2
- Second, the solvent molecules separate from each other. The
enthalpy of this process is called ΔH2. Like the first step, this reaction is
endothermic and ΔH2 > 0 because energy is required to break the forces
between the molecules.
∆H3
- Third, step the solute and solvent mix. The mixing of these two will
create a solution. The enthalpy of this process is called ΔH3. This is an
exothermic reaction, because energy is given off as the two substances bond
together; therefore, ΔH3 < 0.
The enthalpy of hydration is similar to the enthalpy of solution, but applies only
to ions. Enthalpy of hydration, ∆Hhyd, of an ion is the amount of energy
released when one mole of an ion dissolves in a large amount of water
forming a dilute solution. Basically, the terms ∆H2 + ∆H3 from above are combined,
so
∆Hhyd = ∆H2 + ∆H3. Thus, the heat of solution is ∆Hhyd + ∆H1.
HOMEWORK: Book questions pg. 519 questions
9,12,13,14,15,17,20,27,37,39,45,49