Chapter 7: Solutions and Colloids

CHE 140

CHAPTER OUTLINE•CHAPTER OUTLINE•7.1 Physical States of Solutions•7.2 Solubility•7.3 The Solution Process•7.4 Solution Concentrations•7.5 Solution Preparation•7.6 Solution Stoichiometry•7.7 Solution Properties•7.8 Colloids•7.9 Dialysis

Solution Terminology•solution - a homogeneous mixture of two or more substances, the composition of which may vary within definite limits.•solute - the substance dissolved in a solution.•solvent - the dissolving medium in a solution.•A solution is a solute dissolved in a solvent.•In the example of the sugar-water mixture, sugar-water is the solution, sugar is the solute, and water is the solvent.

Suspension Terminology•If clay is mixed with water, it will not dissolve. Clay particles will be suspended in the water. This is called a suspension.•suspension - a heterogeneous mixture•Colloids are large suspended particles. They may be homogeneous, but they are not true solutions because the large particles are not dissolved.•Examples : mayonnaise, ketchup, mustard

Electrolytes

•electrolyte - a substance whose water solution conducts electricity•Examples : acids, bases, salts•nonelectrolytes - a substance whose water solution does not conduct electricity

Concentrations of Solutions

•The concentration of a solution depends on the relative amounts of solute and solvent present in a solution. •The terms concentrate and dilute are qualitative. This means they only give "what" is present in a solution and not "how much" is present.

Percent by Mass

•Percent by mass of a solute in solution is the number of grams of solute dissolved in 100g of solution.• mass of solute•% by mass = --------------------------------- x100%• mass solute + solvent •Find the percent by mass of a solution made by dissolving 10g of sodium hydroxide in 90g of water.

Molarity

•Molarity is the number of moles of solute in one liter of solution.• moles of solute •Molarity = -----------------------------• liters of solution•Find the molarity of a solution made by dissolving 20g of sodium hydroxide in 500mL of solution.•How many moles of HCl are present in 0.8 L of 0.5 M HCl solution?

Molality•molality (m) - an expression of moles of solute in a kilogram of solvent• moles of solute•molality = -------------------------------------• mass of solvent (kg)•A solution consists of 17.1g of sucrose (C12H22O11) dissolved in 125g of water. Find the molal concentration of this solution.•Determine the mass of copper(II) sulfate needed to prepare a 0.25-m solution using 250mL of water.•Assignment: Find m for 20g NaOH in 250ml solv

Types of Solutions

•air•coke•cake• rain•alcohol and water•wet sponge•smoke•saltwater•brass

gas in gasgas in liquidgas in solidliquid in gasliquid in liquidliquid in solidsolid in gassolid in liquidsolid in solid

Acids and Bases•Acids, bases, and salts are electrolytes, substances whose water solutions conduct electricity. •They are uniquely related in that acids react with bases to produce salts and water. •The word salt is a derivative of the Latin word sal which means vital compound. •Roman soldiers were paid with salt, sal-ary. Salt has long been used to flavor and preserve food and as medicine.

The Nature of Acids

•Acids are present in many foods, especially fruit. All foods which taste sour have some type of acid in them. Some organic acids include;• ACID FOOD• citric acid lemons• malic acid apples• lactic acid milk• butyric acid rancid butter• acetic acid vinegar

Properties of Acids•1. Acids donate protons in the presence of bases.• HCl monoprotic• H2SO4 diprotic

•H+ Cl- + Na+OH- ® Na+Cl- + H+OH-

•2. Acids contain ionizable hydrogen covalently bonded to a nonmetallic element or polyatomic species.•nonmetal......................HCl• polyatomic species...H3PO4

Properties of Acids

•3. Acids taste sour. NEVER USE THE TASTE TEST!!!•4. Acids affect indicators.• litmus turns red• phenolphthalein remains clear• methyl orange turns red•5. Acids neutralize hydroxides.•HCl + NaOH ® NaCl + H2O

Properties of Acids

•6. Acids react with many metals.•Zn(s) + H2SO4(aq) ® ZnSO4(aq) + H2(g)

•7. Acids react with oxides of metals.•CuO + H2SO4 ® CuSO4 + H2O

•8. Acids react with carbonates.•CaCO3 + 2HCl ® CaCl2 + H2O + CO2

The Nature of Bases

•Bases are often referred to as hydroxides. The hydroxide ion, OH-, is responsible for most basic or alkaline properties. The basicity of a substance is also referred to as the alkalinity.•Name some common household bases.• 1. lye 4. soap• 2. lime 5. antacids• 3. laxatives 6. eggs•base - a proton acceptor

Properties of Bases

•1. Hydroxides of active metals supply hydroxide ions, OH-.•Na+OH- ® Na+(aq) + OH-(aq)•2. Soluble hydroxides have a bitter taste. NEVER USE THE TASTE TEST!!•3. Solutions of hydroxides feel slippery.•4. Soluble hydroxides affect indicators.• red litmus paper turns blue• phenolphthalein turns red• methyl orange turns yellow

Properties of Bases

•5. Hydroxides neutralize acids.• NaOH + HCl ® NaCl + H2O

•6. Hydroxides react with oxides of nonmetals.• CO2 + 2NaOH ® Na2CO3 + H2O

•7. Certain hydroxides are amphoteric; i.e., they take on acid properties in the presence of stronger bases and take on basic properties in the presence of strong acids.•Example: Write the formula for aluminum hydroxide which shows acidic properties.

Salts•A salt is a compound composed of the positive ion of an aqueous base and the negative ion of an aqueous acid. •We usually think of table salt, NaCl, when we hear the word salt. There are actually many kinds of salts. •Our bodies obtain minerals in the form of salts. For example, with your knowledge of the active metal sodium, what would happen if you tried to eat pure sodium metal? • Eating NaCl is much safer!

Chemical Equivalents

•Chemical equivalents are the quantities of substances that have the same combining capacity in chemical reactions. •Chemical equivalents are calculated by dividing the molar mass by the total positive charge on the first element in the formula of the compound.

Calculating Chemical Equivalents

• molar mass•1 equiv = ------------------------------• total positive charge•Determine the chemical equivalents of each of the following.• a. HCl ____________________• b. NaOH __________________• c. H2SO4 __________________

Normality

•Normality, (N), is an expression of gram equivalents of solute per liter of solution. •Before calculating normality, the chemical equivalent must be calculated.• mass solute 1 equiv 1000 mL •N= -------------------- X ---------------- X ------------• vol. soln. eq mass L

Sample Problem

•Find the normality of 300mL of a solution containing 55 grams of calcium chloride.

pH

•The pH of a solution is a numerical representation of the degree of hydronium ion concentration. •The term (from French pouvoir hydrogène, "hydrogen power") is defined as the negative logarithm of the concentration of H+ ions (protons). •pH - the common logarithm of the reciprocal of the hydronium ion concentration• 1•pH = log10 ------------ [H3O+]

pH Scale

•The pH scale is a numerical scale which indicates the acidity or basicity of a substance. A neutral substance, one which has an equal concentration of hydronium and hydroxide ions has a pH of 7 which is half way between 0 and 14.• acid base•0---------------------- 7 --------------------- 14

Approximate pH of Some Common MaterialsApproximate pH of Some Common Materials

• MATERIAL pH

• gastric juice 2.0

• lemons 2.3

• vinegar 2.8

• soft drinks 3.0

• apples 3.1

• tomatoes 4.2

• bananas 4.6

• bread 5.5

• rainwater 6.2

• milk 6.5

• pure water 7.0

• eggs 7.8

• seawater 8.5

• milk of magnesia 10.5

pH of Acids

•The concentrations of acids is usually given in molarity, M. Since molarity is an expression of Moles / L, the hydronium ion concentration can readily be determined.•Find the pH of a 0.001-M HCl solution.

pH of Bases

• 10-14

•[H3O+] = ------------

• [OH-]•Find the pH of a 0.01 - M NaOH solution.•Find the pH of a 0.001 - M Ca(OH)2 solution

AssignmentAssignment

•Find the pH of the following.•1. 0.01- M H2SO4

•2. 0.0001- M H2S

•3. 0.1 - M HCl•4. 0.002 –M NaOH

Acid-Base Titration

•Titration is the progressive addition of an acid to a base to achieve neutralization so the concentration of an unknown solution may be found. •The point at which neutralization occurs is known as the end point. Indicators are used for determination of this point. The previously discussed indicators will be used in our titrations.

Titration Calculations

• VANA = VBNB

• VA = volume of acid used in mL• NA = normality of acid used in N• VB = volume of base used in mL• NB = normality of base used in N

Sample Problem

•A student performed a titration and determined 20mL of a 1.5-N HCl solution was needed to neutralize 35mL of a NaOH solution of unknown concentration. Determine the normality of the unknown solution.