Solutions Part 1.PDF

Copyright 2011 Pearson Education, Inc.

Chapter 12Solutions

Chemistry: A Molecular Approach, 2nd Ed.Nivaldo Tro

Roy KennedyMassachusetts Bay Community College

Wellesley Hills, MA

Copyright 2011 Pearson Education, Inc.Tro: Chemistry: A Molecular Approach

2

TYPES OF SOLUTIONS AND

SOLUBILITY


Solutions• Homogeneous mixtures

composition may vary from one sample to anotherappears to be one substance, though really

contains multiple materials

• Most homogeneous materials we encounter are actually solutionse.g., air and seawater

3Tro: Chemistry: A Molecular Approach, 2/e


Solutions• When table salt is mixed with water, it seems to

disappear, or become a liquid – the mixture is homogeneous the salt is still there, as you can tell from the taste, or simply

boiling away the water

• Homogeneous mixtures are called solutions

• The component of the solution that changes state is called the solute

• The component that keeps its state is called the solvent if both components start in the same state, the major

component is the solvent



Examples of Solutions



Intermolecular Forces and the Solution Process

• Energy changes in the formation of most solutions also involve differences in attractive forces between the particles

• For the solvent and solute to mix you must overcome1. all of the solute–solute attractive forces

2. some of the solvent–solvent attractive forces both processes are endothermic

• At least some of the energy to do this comes from making new solute–solvent attractions which is exothermic



Intermolecular Attractions



Solution Interactions



Solubility• There is usually a limit to the solubility of one

substance in anothergases are always soluble in each othertwo liquids that are mutually soluble are said to be

miscible alcohol and water are miscibleoil and water are immiscible

• The maximum amount of solute that can be dissolved in a given amount of solvent is called the solubility

• The solubility of one substance in another varies with temperature and pressure



Will It Dissolve?• Chemist’s Rule of Thumb –

Like Dissolves Like• A chemical will dissolve in a solvent if it has a similar structure

to the solventwhen the solvent and solute structures are similar, the solvent

molecules will attract the solute particles at least as well as the solute particles are attracted to each other

• Polar molecules and ionic compounds will be more soluble in polar solvents

• Nonpolar molecules will be more soluble in nonpolar solvents



Classifying Solvents



Example 12.1a: Predict whether the following vitamin is soluble in fat or water

Vitamin C

The four OH groups make the molecule highly polar and it will also H-bond to water.

Vitamin C is water soluble.

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Water is a polar solvent. Fat is mostly made of nonpolar molecules.

Tro: Chemistry: A Molecular Approach, 2/e


Example 12.1b: Predict whether the following vitamin is soluble in fat or water

Vitamin K3

The two C=O groups are polar, but their geometric symmetry suggests their pulls will cancel and the molecule will be nonpolar.

Vitamin K3 is fat soluble.

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Water is a polar solvent. Fat is mostly made of nonpolar molecules.

Tro: Chemistry: A Molecular Approach, 2/e


Practice – Decide if the following are more soluble in hexane, C6H14, or water

nonpolar moleculemore soluble in C6H14

polar moleculemore soluble in H2O

nonpolar part dominantmore soluble in C6H14



Practice – Explain the solubility trends seen in the table below



Practice – Explain the solubility trends seen in the table below

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These alcohols all have a polar OH part and a nonpolar CHn part.

As we go down the table the nonpolar part gets larger, but the amount of OH stays the same.

We therefore expect that the solubility in water (polar solvent) should decrease and the solubility in hexane (nonpolar solvent) should increase, and it does.Tro: Chemistry: A Molecular Approach, 2/e


Solution Equilibrium• The dissolution of a solute in a solvent is an

equilibrium process

• Initially, when there is no dissolved solute, the only process possible is dissolution

• Shortly after some solute is dissolved, solute particles can start to recombine to reform solute molecules – but the rate of dissolution >> rate of deposition and the solute continues to dissolve

• Eventually, the rate of dissolution = the rate of deposition – the solution is saturated with solute and no more solute will dissolve



Solution Equilibrium



Solubility Limit• A solution that has the solute and solvent in

dynamic equilibrium is said to be saturated if you add more solute it will not dissolve the saturation concentration depends on the temperature

and pressure of gases

• A solution that has less solute than saturation is said to be unsaturatedmore solute will dissolve at this temperature

• A solution that has more solute than saturation is said to be supersaturated



Adding a Crystal of NaC2H3O2 to a Supersaturated Solution



Factor affecting solubility. 1. Temperature Dependence of Solubility of

Solids in Water

• Solubility is generally given in grams of solute that will dissolve in 100 g of water

• For most solids, the solubility of the solid increases as the temperature increaseswhen Hsolution is endothermic

• Solubility curves can be used to predict whether a solution with a particular amount of solute dissolved in water is saturated (on the line), unsaturated (below the line), or supersaturated (above the line)



Solubility Curves



Purification by Recrystallization

• One of the common operations performed by a chemist is removing impurities from a solid compound

• One method of purification involves dissolving a solid in a hot solvent until the solution is saturated

• As the solution slowly cools, the solid crystallizes out, leaving impurities behind



50 g KNO3 in 100 g H2O @ 34 ºC

50 g KNO3 in 100 g H2O @ 50 ºC

50 g KNO3 in 50 g H2O @ 50 ºC

100 g NH4Cl in 200 g H2O @ 70 ºC

saturated

unsaturated

supersaturated

unsaturated

Practice – Decide if each of the following solutions is saturated, unsaturated, or supersaturated

100 g NH4Cl in 150 g H2O @ 50 ºCsupersaturated



2. Temperature Dependence of Solubility of Gases in Water

• Gases generally have lower solubility in water than ionic or polar covalent solids because most are nonpolar moleculesgases with high solubility usually are actually

reacting with water

• For all gases, the solubility of the gas decreases as the temperature increasesthe Hsolution is exothermic because you do not need

to overcome solute–solute attractions


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