chapter four - 國立臺北科技大學chpro/chem/chap4.pdf · chapter four: types of chemical...

Chapter Four:

TYPES OF CHEMICALREACTIONS ANDSOLUTION STOICHIOMETRY

p125

Contents

p126

3

4-1 Water, the Common Solvent p127

A polar molecule

Figure 4.1

(left) The water molecule is polar.(right) A space-filling model of the watermolecule.

p128

Figure 4.2

Polar water molecules interact with the positive andnegative ions of a salts in the dissolving process.

p129

4-2 The mature of AqueousSolutions: Strong andWeak Electrolytes

p129

Solvent

Solute

Strong electrolyte

Weak electrolyte

Non-electrolyte

Concentrated

Dilute

7

Dissolution of a Solid in a Liquid

8

Strong electrolytes:

The result of ionizations in water.

p131

Weak Electrolytes p132

10

Weak Electrolytes

p133

Nonelectrolytes p133

12

Electrolyte Behavior

p130

13

Electrolytes

p130

4-3 The Composition ofSolutions

p133

M

M

molaritymoles of soluteliters of solution

HClmoles of HCl

liters of solution3

62

Molarity (M) = moles of soluteper volume of solution in liters:

Ex 4.1 Calculation of Molarityp134

Calculate the molarity of a solution prepared

by dissolving 11.5 g of solid NaOH in enough

water to make 1.50 L of solution.Solution:

Ex 4.5 Concentration and Volumep135

Typical blood serum is about 0.14 M NaCl.

What volume of blood contains 1.0 mg NaCl?

Solution:

Ex 4.6 Solution of KnownConcentration

p136

To analyze the alcohol content of a certain wine, a chemist

needs 1.00 L of an aqueous 0.2 M K2Cr2O7 (potassium

dichromate) solution. How much solid K2Cr2O7 must be

weighed out to make this solution?Solution:

Dilutionp137

19

Dilution

Ex 4.7 Concentration and Volumep138

What volume of 16 M sulfuric acid must be used to

prepare 1.5 L of a 0.1 M H2SO4 solution?

Solution:

p139

p137

23

4-4 Types of chemical Reactionsp140

Types of Solution Reactions

Precipitation reactions

Acid-base reactions

Oxidation-reductions

4-5 Precipitation Reactions p140

p141

Figure 4.15

The reaction of K2CrO4(aq) and Ba(NO3)2(aq). (a) The molecular-level

“picture”of the mixed solution before any reaction has occurred. (b)

The molecular-level “picture”of the solution after reaction has to form

BaCrO4(s). Note: BaCrO4(s) is not molecular. It actually contains Ba2+

and CrO42- ions packed together in a lattice. (c) A photo of the solution

has occurred, showing the solid BaCrO4 on the bottom.

26

Precipitation of Silver Chloride

Table4.1 Simple Rules for Solubility

1. Most nitrate (NO3) salts are soluble.

2. Most alkali (group 1A) salts and NH4+ are

soluble.3. Most Cl, Br, and Isalts are soluble

(NOT Ag+, Pb2+, Hg2

2+)4. Most sulfate salts are soluble (NOT BaSO4,

PbSO4, HgSO4, CaSO4)5. Most OHsalts are only slightly soluble

(NaOH, KOH are soluble, Ba(OH)2,Ca(OH)2 are marginally soluble)

6. Most S2, CO32, CrO4

2, PO43salts are

only slightly soluble.

p144

28

Solubility Rules

p144

Ex 4.8 Predicting Reaction Productsp144

Using the solubility rules in Table 4.1, predict what

will happen when the following pairs of solutions are

mixed. (a) KNO3(aq) and BaCl2(aq); (b)Na2SO4(aq)

and Pb(NO3)2(aq); (c) KOH(aq) and Fe(NO3)3(aq)Solution:

(a)

(b)

(c)

p145

4-6 Describing Reactions in Solution p145

Complete ionic equation:

Spectator ions

Net ionic equation:

p146Three Types of Equations Are Used toDescribe Reactions in Solution

The formula equation gives overall reaction

stoichiometry but not necessarily the actual forms of

the reactants and products in the solution.

The complete ionic equation represents as ions all

reactants and products that are strong electrolytes.

The net equations includes only those components

undergoing a charge. Spectator are not include.

Ex 4.9 Writing Equations for Reactions p146

For each of the following reactions, write the formula

equation, the complete ionic equation, and the net ionic

equation.

a. Aqueous potassium chloride is added to aqueous silver

nitrate to from a silver chloride precipitate plus aqueous

potassium nitrate.

b. Aqueous potassium hydroxide is mixed with aqueous

potassium nitrate.

Solution:p146

35

Notice

The solution with the greatest number of ions is not

necessarily the one in which:

the volume of the solution is the largest.

the formula unit has the greatest number of ions.

Describing Reactions in Solution

1. Molecular equation (reactants and products as

compounds)

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3 (aq)

2. Complete ionic equation (all strong electrolytes

shown as ions)

Ag+(aq) + NO3(aq) + Na+(aq) + Cl-(aq) AgCl(s) + Na+(aq)

+ NO3(aq)

Describing Reactions in Solution (continued)

3. Net ionic equation (show only components thatactually react)

Ag+(aq) + Cl(aq) AgCl(s)

Na+ and NO3are spectator ions.

38

4-7 Stoichiometry of PrecipitationReactions

p147

Ex 4.10 Determining the Mass ofProduct Formed

p147

Calculate the mass of solid NaCl that must be added to

1.50 L of a 0.100 M AgNO3 solution to precipitate all the

Ag+ ions in the form of AgCl.

Solution:

Ex 4.11 Determining the Mass of Product Formedp148

When aqueous solutions of Na2SO4 and Pb(NO3)2 are

mixed, PbSO4 precipitates. Calculate the mass of

PbSO4 precipitates. Calculate the mass of PbSO4

formed when 1.25 L of 0.0500 M Pb(NO3)2 and 2.00

L of 0.050 M Na2SO4 are mixed.Solution:

4-8 Acid-Base Reactions p149

An acid is a proton donor An base is a proton acceptor

Key Titration Terms

Titrant - solution of known concentration

used in titration.

Analyte - substance being analyzed.

Equivalence point - enough titrant added

to react exactly with the analyte.

Endpoint - the indicator changes color

so you can tell the equivalence point has

been reached.

Performing Calculations for Acid-Base Reactions

1. List initial species and predict reaction.

2. Write balanced net ionic reaction.

3. Calculate moles of reactants.

4. Determine limiting reactant.

5. Calculate moles of required reactant or

product.

6. Convert to grams or volume, as required.

44

Proton Transfer

45

Neutralization of a StrongAcid by a Strong Base

Ex 4.12 Neutralization Reactions p150

What volume of a 0.100 M HCl solution is needed to

neutralize 25.0 mL of 0.350 M NaOH?Solution:

Acid-Base Titrations p152

1. The exact reaction between titrant and analyte mustbe known (and rapid)

2. The stoichiometric (equivalence) point must beaccurately.

3. The volume of titrant required to reachstoichiometric point must be accurately

48

Acid-Base Titration

Ex 4.14 Neutralization Titration p153

A Student carries out an experiment to standardize (determine the

exact concentration of) a sodium hydroxide solution. To do this , the

student weighs out a 1.3009-g sample of potassium hydrogen

phthalate (KHC8H4O4, often abbreviated KHP). KHP molar mass

204.22 g/mol) has one acidic hydrogen. The student dissolves the

KHP in distilled water. Adds phenolphthalein as an indicator, and

titrates the resulting solution with the sodium hydroxide solution to

the phenolphthalein endpoint. The difference between the final and

initial buret reading indicates that 41.20 mL of the sodium hydroxide

solution is required to react exactly with the 1.3009 g KHP. Calculate

the concentration of sodium hydroxide solution.

Solution:P153

Ex 4.15 Neutralization Analysis P153

An Environmental chemist analyzed the effluent (the

released waste material) from an industrial process known

to produces the compounds carbon tertrachloride(CCl4)

and benzoic acids (HC7H5O2), a weak acid that has one

acidic hydrogen atom per molecule. A sample this effluent

weighting 0.3518 g was shaken with water, and the

resulting aqueous solution required 10.59 mL of 0.1546 M

NaOH for neutralization. Calculate the mass percent ofHC7H5O2 in the original sample.

Ex 4.15 Solution:p154

4-9 Oxidation-Reduction Reactions p154

Oxidation states

Reaction of Sodium and Chlorine p155

Table4.2 Rules for Assigning OxidationStates

1. Oxidation state of an atom in an element = 0

2. Oxidation state of monatomic element = charge

3. Oxygen = -2 in covalent compounds (except in peroxides

where it = -1)

4. H = +1 in covalent compounds

5. Fluorine = -1 in compounds

6. Sum of oxidation states = 0 in compounds

7. Sum of oxidation states = charge of the ion

p156

Ex 4.16 Assigning Oxidation Statesp157

Assign oxidation states to all atoms in the following.

(a) CO2; (b) SF6; (c) NO3¯Solution:(a)

(b)

(c)

57

React

Find the oxidation states for each of the

elements in each of the following compounds:

K2Cr2O7

CO32-

HClO4

MnO2

PCl5

SF4

Ex 4.17 Oxidation-Reduction Reactions p160

When powdered aluminum metal is mixed with pulverized

iodine crystals and a drop of water is added to help the

reaction get started, the resulting reaction produces a great

deal of energy. The mixture bursts into flames, and a purple

smoke of I2 vapor is produced from the excess iodine. The

equation for the reaction is 2Al(s)+3I2(s) 2AII3(s)

For this reaction, identify the atoms that are oxidized and

reduced, and specify the oxidizing and reducing agents.

Solution: p160

4-10 Balancing Oxidation-ReactionEquations

p162

The Half-Reaction Method for Balancing Oxidation-

Reduction Reaction in Aqueous Solutions

p162

62

Zinc and Iodine

63

Barking Dogs

64

Dry Ice and Magnesium

65

Reactions of Silver

66

Balancing Oxidation-ReductionReactions

Cr2O72-(aq) + SO3

-(aq) Cr3+(aq) + SO42-(aq)

How can we balance this equation?

67

Method of Half Reactions

Cr2O72-(aq) 2Cr3+(aq)

SO3-(aq) + SO4

2-(aq)

How many electrons are involved in each half

reaction?

68

6e- + Cr2O72- (aq) 2Cr3+ (aq)

SO3- (aq) + SO4

2- (aq) + 2e-

How can we balance the oxygen atoms?

Method of Half Reactions(continued)

69

6e- + Cr2O72- (aq) Cr3+ (aq) + 7H2O

H2O +SO3- (aq) + SO4

2- (aq) + 2e-

How can we balance the hydrogen atoms?

Method of Half Reactions(continued)

70

This reaction occurs in an acidic solution.

14H+ + 6e- + Cr2O72-(aq) Cr3+(aq) + 7H2O

H2O +SO3- (aq) SO4

2- (aq) + 2e- + 2H+

How can we balance the electrons?

Method of Half Reactions(continued)

Method of Half Reactions(continued)

14H+ + 6e- + Cr2O72- (aq) Cr3+ (aq) + 7H2O

3[H2O +SO3- (aq) SO4

2- (aq) + 2e- + 2H+]

Cr2O72- (aq) + 3SO3

-(aq) + 8H+(aq) 2Cr3+(aq) +

3SO42-(aq) + 4H2O(l)

72

Half-Reaction Method - Balancingin Base

1. Balance as in acid.

2. Add OHthat equals H+ ions (both sides!)

3. Form water by combining H+, OH.

4. Check elements and charges for balance.

Balance the following oxidation-reduction reactions

that occur in acidic solution.

ClO- (aq) + I- (aq) Cl- (aq) + I3- (aq)

Br- (aq) + MnO4- (aq) Br2 (l)+ Mn2+ (aq)

CH3OH(aq) + Cr2O72-(aq) CH2O(aq) + Cr3+(aq)

React

Ex 4.19 Balancing Oxidation-ReductionReactions (Acidic)

p165

Potassium dichromate (K2Cr2O7) is a bright orange

compound that can be reduced to a blue-violet solution of

Cr3+ ions. Under certain conditions, k2Cr2O7 reacts with

ethyl alcohol (C2H5OH) as follows:

Balance this equation using the half-reaction method.

)()()()()()( 223

522

72 lOHgCOaqCrlOHHCaqOCraqH

p165

76

p166

p166

78

p167

Ex 4.20 Balancing Oxidation-Reductions(Basic)

p167

Silver is sometimes found in nature as large nuggets;

more often it is found mixed with other metals and

their ores. An aqueous solution containing cyanide

ion is often used to extract the silver using the

following reaction that occurs in basic solution:

Balance this equation using the half-reaction method.

)()()()()( 22 aqCNAgaqOaqCNsAg Basic

p167

p168