Second Edition Julia Burdge & Jason Overbyacidic hydrogen atoms. Polyproticacids lose protons in a...

11

Chemistry: Atoms FirstSecond Edition

Julia Burdge & Jason Overby

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 9

Chemical Reactions in Aqueous

Solutions

M. Stacey Thomson

Pasco-Hernando State College

2

Chemical Reactions in Aqueous Solutions9

9.1 General Properties of Aqueous SolutionsElectrolytes and NonelectrolytesStrong Electrolytes and Weak Electrolytes

9.2 Precipitation ReactionsSolubility Guidelines for Ionic Compounds in WaterMolecular EquationsIonic EquationsNet Ionic Equations

9.3 Acid-Base ReactionsStrong Acids and BasesBrønsted Acids and BasesAcid-Base Neutralization

9.4 Oxidation-Reduction ReactionsOxidation NumbersOxidation of Metals in Aqueous SolutionsBalancing Simple Redox EquationsOther Types of Redox Reactions

3

Chemical Reactions in Aqueous Solutions9

9.5 Concentration of SolutionsMolarityDilutionSerial DilutionSolution Stoichiometry

9.6 Aqueous Reactions and Chemical AnalysisGravimetric AnalysisAcid-Base Titrations

4

General Properties of Aqueous Solutions

A solution is a homogenous mixture of two or more substances.

The substance present in the largest amount (moles) is referred to as the solvent.

The other substances present are called the solutes.

A substance that dissolves in a particular solvent is said to be soluble in that solvent.

Unless otherwise noted, for this chapter, solution will refer specifically to an aqueous solution

9.1

5

Electrolytes and Nonelectrolytes

An electrolyte is a substance that dissolves in water to yield a solution that conducts electricity.

An electrolyte undergoes dissociationand breaks apart into its constituent ions.

NaCl(s) Na+(aq) + Cl–(aq)H2O

Ionization is when a molecular compound forms ions when it dissolves

6

Electrolytes and Nonelectrolytes

A nonelectrolyte is a substance that dissolves in water to yield a solution that does not conduct electricity.

The sucrose molecules remain intact upon dissolving.

C12H22O11(s) C12H22O11(aq)H2O

7

Strong Electrolytes and Weak Electrolytes

An electrolyte that dissociates completelyis known as a strong electrolyte.

Water soluble ionic compounds

Strong Acids

Strong Bases

HCl(g) H+(aq) + Cl–(aq)H2O

NaOH(s) Na+(aq) + OH–(aq)H2O

NaCl(s) Na+(aq) + Cl–(aq)H2O

8


9


A weak electrolyte is a compound that produces ions upon dissolving but exists in solution predominantly as molecules that are not ionized.

Weak Acids

Weak Bases

HC2H3O2(l) H+(aq) + C2H3O2 (aq)

–

NH4(aq) + OH–(aq)NH3(g) + H2O(l)

+

10


The double arrow, , denotes a reaction that occurs in both directions.

When both the forward and reverse reactions occur at the same rate, the reaction is in a state of dynamic chemical equilibrium.

11


12

Worked Example 9.1

Strategy Identify each compound as ionic or molecular; identify each molecular compound as acid, base, or neither; and identify each acid as strong or weak. Sucrose and fructose contain no cations and are therefore molecular compounds–neither is an acid or a base. Sodium citrate and potassium citrate contain metal cations and are therefore ionic comopunds. Ascorbic acid is an acid that does not appear on the list of strong acids in Table 9.1, so ascorbic acid is a weak acid.

Sports drinks typically contain sucrose (C12H22O11), fructose (C6H12O6), sodium

citrate (Na3C6H5O7), potassium citrate (K3C6H5O7), and ascorbic acid (H2C6H6O6),

among other ingredients. Classify each of these ingredients as a nonelectrolyte,

a weak electrolyte, or a strong electrolyte.

Solution Sucrose and fructose are nonelectrolytes. Sodium citrate and potassium citrate are strong electrolytes. Ascorbic acid is a weak electrolyte.

13

Think About It Remember that any soluble ionic compound is a strong electrolyte, whereas most molecular compounds are nonelectrolytes or weak electrolytes. The only molecular compounds that are strong electrolytes are the strong acids listed in Table 9.1.

14

Precipitation Reactions

An insoluble product that separates from a solution is called a precipitate.

2NaI(aq) + Pb(NO3)2(aq) PbI2(s) + 2NaNO3(aq)

9.2

15


A chemical reaction in which a precipitate forms is called a precipitation reaction.

16

Solubility Guidelines for Ionic Compounds in Water

Water is a good solvent for ionic compounds because it is a polarmolecule.

The polarity of water results from electron distributions within the molecule.

The oxygen atom has an attraction for the hydrogen atoms’ electrons and is therefore partially negative compared to hydrogen.

+

–The oxygen atom is partially negative

The hydrogen atoms are partially positive

+

17


Hydration occurs when water molecules remove the individual ions from an ionic solid surrounding them so the substances dissolves.

18


Solubility is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature.

19

Worked Example 9.2

Strategy Use the guidelines in Tables 9.2 and 9.3 to determine whether or not each compound is expected to be water soluble.

Classify each of the following compounds as soluble or insoluble in water: (a)

AgNO3, (b) CaSO4, (c) K2CO3.

Solution (a) Soluble, (b) Insoluble, (c) Soluble.

Think About It Check the ions in each compound against the information in Tables 9.2 and 9.3 to confirm that you have drawn the right conclusion.

20

Molecular Equations

In a molecular equation compounds are represented by chemical formulas as though they exist in solution as molecules or formula units.

Na2SO4(aq) + Ba(OH)2(aq) 2NaOH(aq) + BaSO4(s)

Reactions in which cations in two ionic compounds exchange anions are called metathesis or double replacement reactions.

21

Ionic Equations

In an ionic equation compounds that exist completely or predominately as ions in solution are represented as those ions.

In the reaction between aqueous Na2SO4and Ba(OH)2

the aqueous species are represented as follows:

Na2SO4(aq) → 2Na+(aq) + SO4 (aq)

2–

Ba(OH)2(aq) → Ba2+(aq) + 2OH–(aq)

NaOH(aq) → Na+(aq) + OH–(aq)

2Na+(aq) + SO4 (aq) + Ba2+(aq) + 2OH–(aq) 2Na+(aq) + 2OH–(aq) + BaSO4(s)

2–

Na2SO4(aq) + Ba(OH)2(aq) 2NaOH(aq) + BaSO4(s)

22

Net Ionic Equations

An equation that includes only the species that are actually involved in the reaction is called a net ionic equation.

Ions that appear on both sides of the equation are called spectator ions.

Spectator ions do not participate in the reaction.

2Na+(aq) + SO4 (aq) + Ba2+(aq) +2OH–(aq) 2Na+(aq) + 2OH–(aq) + BaSO4(s)

2–

Ba2+(aq) + SO4 (aq) BaSO4(s)2–

23


To determine the molecular, ionic and net ionic equations:

1) Write and balance the molecular equation, predicting the products by assuming that the cations trade anions.

2) Write the ionic equation by separating strong electrolytes into their constituent ions.

3) Write the net ionic equation by identifying and canceling spectator ions on both sides of the equation.

4) If both the reactants and products are all strong electrolytes, all the ions in solution are spectator ions. In this case, there is no net ionic equation and no reaction takes place.

24

Worked Example 9.3

Strategy Predict the products by exchanging ions and balance the equation. Determine which product will precipitate based on the solubility guidelines in Tables 9.2 and 9.3. Rewrite the equation showing strong electrolytes as ions. Identify and cancel spectator ions.

Write the molecular, ionic, and net ionic equations for the reaction that occurs

when aqueous solutions of lead acetate [Pb(C2H3O2)2], and calcium chloride

(CaCl2), are combined.

SolutionMolecular equation:

Pb(C2H3O2)2(aq)+ CaCl2(aq) → PbCl2(s) + Ca(C2H3O2)2(aq)

Ionic equation:Pb2+(aq) + 2C2H3O2

-(aq) + Ca2+(aq) + 2Cl-(aq) → PbCl2(s) + Ca2+(aq) + 2C2H3O2

-(aq)

Net ionic equation:Pb2+(aq) + 2Cl-(aq) → PbCl2(s)

25

Think About It Remember that the charges on ions in a compound must sum to zero. Make sure that you have written correct formulas for the products and that each of the equations you have written is balanced. If you find that you are having trouble balancing an equation, check to make sure you have correct formulas for the products.

26

Acid-Base Reactions

Acids can be either strong or weak.

A strong acid is a strong electrolyte.

9.3

27

Strong Acids and Bases

Strong bases are strong electrolytes (dissociate completely).

Strong bases are the hydroxides of Group 1A and heavy Group 2A.


A weak acid is a weak electrolyte; it does not dissociate completely.

Most acids are weak acids.

HF(aq) H+(aq) + F-(aq) –

28

Brønsted Acids and Bases

An Arrhenius acid is one that ionizes in water to produce H+ ions.

An Arrhenius base is one that dissociates in water to produce OH–

ions.

HCl(g) H+(aq) + Cl–(aq)H2O


29


A Brønsted acid is a proton donor.

A Brønsted base is a proton acceptor.

In these definitions, a proton refers to a hydrogen atom that has lost its electron—also known as a hydrogen ion (H+).

H2O is a Brønsted acid:donates a proton to

become OH–

NH3 is a Brønsted base:accepts a proton to

become NH4+

30


Brønsted acids donate protons to water to form the hydronium ion (H3O+).

hydrogen ion (H+)

proton

hydronium ion (H3O+)

All refer to the same aqueous species

31


A monoprotic acid has one proton to donate.

Hydrochloric acid is an example:

HCl(g) H+(aq) + Cl–(aq)

one equivalent of solvated hydrogen ion

32


A polyprotic acid has more than one acidic hydrogen atom.

Sulfuric acid, H2SO4, is an example of a diprotic acid; there are two acidic hydrogen atoms.

Polyprotic acids lose protons in a stepwise fashion:

H2SO4(aq) H+(aq) + HSO4 (aq)

–

–HSO4(aq) H

+(aq) + SO4 (aq)2–

Step 1:

Step 2:

In H2SO4, the second ionization occurs only to a very small extent.

In H2SO4, the first ionization is strong.

33


Bases that produce only one mole of hydroxide per mole of compound are called monobasic.

Sodium hydroxide is an example:


one equivalent of hydroxide

34


Some strong bases produce more than one hydroxide per mole of compound.

Barium hydroxide is an example of a dibasic base.

Ba(OH)2(s) Ba2+(aq) + 2OH–(aq)H2O

two equivalents of hydroxide

35

Acid-Base Neutralization

A neutralization reaction is a reaction between an acid and a base.

Generally, a neutralization reaction produces water and a salt.

HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)

The net ionic equation of many acid-base reactions is:

H+(aq) + OH–(aq) H2O(l)

acid base water salt

36

Worked Example 9.4

Milk of magnesia, an over-the-counter laxative, is a mixture of magnesium

hydroxide [Mg(OH)2] and water. Because Mg(OH)2 is insoluble in water (see Table

9.3), milk of magnesia is a suspension rather than a solution. The undissolved

solid is responsible for the milky appearance of the product. When acid such as

HCl is added to milk of magnesia, the suspended Mg(OH)2 dissolves, and the

result is a clear, colorless solution. Write balanced molecular, ionic, and net ionic

equations for this reaction.

37

Worked Example 9.4 (cont.)

Strategy Determine the products of the reaction; then write and balance the equation. Remember that one of the reactants, Mg(OH)2, is a solid. Identify any strong electrolytes and rewrite the equation showing strong electrolytes as ions. Identify and cancel the spectator ions.

SolutionMg(OH)2(s) + 2HCl(aq) → 2H2O(l) + MgCl2(aq)

Of the species in the molecular equation, only HCl and MgCl2 are strong electrolytes. Therefore, the ionic equation is

Mg(OH)2(s) + 2H+(aq) + 2Cl-(aq) → 2H2O(l) + Mg

2+(aq) + 2Cl-(aq)

Cl- is the only spectator ion. The net ionic equation is

Mg(OH)2(s) + 2H+(aq) → 2H2O(l) + Mg

2+(aq)

38

Think About It Make sure your equation is balanced and that you only show strong electrolytes as ions. Mg(OH)2 is not shown as aqueous ions because it is insoluble.

39

Oxidation-Reduction Reactions

An oxidation-reduction (or redox) reaction is a chemical reaction in which electrons are transferred from one reactant to another.

Oxidation is the loss of electrons.

Reduction is the gain of electrons.

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Zn metal loses 2 electrons and is oxidized to Zn2+

Zn2+ is called the reducing agent

Cu2+ gains 2 electrons and is reduced to Cu metal

Cu is called the oxidizing agent

9.4

40



41


A redox reaction is the sum of an oxidation half-reaction and a reduction half-reaction.

Oxidation half-reaction: Zn(s) Zn2+(aq) + 2e–

Cu2+(aq) + 2e– Cu(s)

Cu2+(aq) + Zn(s) Zn2+(aq) + Cu(s)

Reduction half-reaction:

Overall redox reaction:


Oxidation (lose 2e–)

Reduction (gain 2e–)

42

Oxidation Numbers

The oxidation number is the charge an atom would have if electrons were transferred completely.

The oxidation number is sometimes called the oxidation state.

H2(g) + F2(g) → 2HF(g)

+1 –100

0 0 +1 –1

Oxidation number:

Total contribution to charge:

N2(g) + 3H2(g) → 2NH3(g)

–3 +100

0 0 –3 +3

Oxidation number:


43

Oxidation Numbers

To assign oxidation numbers:

1) The oxidation number of an element, in its elemental form, is zero.

2) The oxidation numbers in any chemical species must sum to the overall charge on the species.

• must sum to zero for any molecule

• must sum to the charge on any polyatomic ion

• the oxidation number of a monoatomic ion is equal to the charge on the ion

44

Oxidation Numbers

To assign oxidation numbers:

3) Know the elements that nearly always have the same oxidation number.

45

Oxidation Numbers

Assign the oxidation numbers to the elements in the compound KMnO4.

Step 1: Start with the oxidation numbers you know:

Step 2: The numbers in the boxes (total contribution to charge) must sum to zero (KMnO4 is a neutral compound).

K Mn O4

+1

+1

Oxidation number:


+7

+7

–2

–8

46

Oxidation Numbers

Assign the oxidation numbers to the elements in the compound H2SO4.


Step 2: The numbers in the boxes (total contribution to charge) must sum to zero (the chemical species is neutral).

H2 S O4

+1

+2

Oxidation number:


+6

+6

–2

–8

47

Oxidation Numbers

Assign the oxidation numbers to the elements in the ion ClO3.


Step 2: The numbers in the boxes (total contribution to charge) must sum to negative one (the chemical species is a –1 anion).

Cl O3

+5

+5

Oxidation number:


–2

–6

–

–

48

Worked Example 9.5

Strategy For each compound, assign an oxidation number first to the element that appears higher in Table 9.5. Then use rule 2 to determine the oxidation number of the other element.

Determine the oxidation number of each atom in the following compounds and

ion: (a) SO2, (b) NaH, (c) CO32-, (d) N2O5.

Solution(a) O appears in Table 9.5 but S does not, so we assign oxidation number -2 to O. Because there are two O atoms in the molecule, the total contribution to charge by O is 2(-2) = -4. The lone S atom must therefore contribute +4 to the overall charge.

SO2+4

+4

-2

-4

49


Solution(b) Both Na and H appear in Table 9.5, but Na appears higher in the table, so we assign the oxidation number +1 to Na. This means that H must contribute -1 to the overall charge.

NaH

(c) We assign the oxidation number -2 to O. Because there are three O atoms in the carbonate ion, the total contribution to charge by O is -6. To have the contributions to charge sum to the charge on the ion (-2), the C atom must contribute +4.

+1

+1

-1

-1

+4

+4

-2

-6

CO32-

50


Solution(d) We assign the oxidation number -2 to O. Because there are five O atoms in the N2O5 molecule, the total contribution to charge by O is -10. To have the contributions to charge sum to zero, the contribution by N must be +10, and because there are two N atoms, each one must contribute +5. Therefore, the oxidation number of N is +5.

N2O5+5

+10

-2

-10

Think About It Use the circle and square system to verify that the oxidation numbers you have assigned do indeed sum to the overall charge on each species.

51

Oxidation Numbers

Periodic table showing oxidation numbers for each element. The most common are in red.

52

Oxidation of Metals in Aqueous Solutions

In a displacement reaction, an atom or an ion in a compound is replaced by an atom of another element.

Zn(s) + CuCl2(aq) → ZnCl2(aq) + Cu(s)

Zinc displaces, or replaces copper in the dissolved salt.

Zn is oxidized to Zn2+.

Cu2+ is reduced to Cu.

When a metal is oxidized by an aqueous solution, it becomes an aqueous ion.

0 +2 –1 +2 –1 0

0 +2 –2 +2 –2 0

53


The activity series is a list of metals (and hydrogen) arranged from top to bottom in order of decreasing ease of oxidation.

Cu(s) + ZnCl2(aq) → no reac�on

Element Oxidation Half-Reaction

Zinc Zn → Zn2+ + 2e–

Iron Fe → Fe2+ + 2e–

Nickel Ni → Ni2+ + 2e–

Hydrogen H2 → 2H+ + 2e–

Copper Cu → Cu2+ + 2e–

Silver Ag → Ag+ + e–

Gold Au → Au3+ + 3e–

Zn(s) + CuCl2(aq) → ZnCl2(aq) + Cu(s)

Metals listed at the top are called active metals.

Metals listed at the bottom are called noble metals.

An element in the series will be oxidized by the ions of any element that appears below it in the table. I

ncre

asin

g e

ase o

f o

xid

ati

on

Activity Series (partial)

54


Which of the following reactions will occur?

Solution:

?Co(s) + BaI2(aq)

Cu(s) + SnBr2(aq)Sn(s) + CuBr2(aq)

?Sn(s) + CuBr2(aq)

?Ag(s) + NaCl(aq)

No reaction. Cobalt is below barium.Co(s) + BaI2(aq)

No reaction. Silver is below sodium.Ag(s) + NaCl(aq)

Element Oxidation Half-Reaction

Barium Ba → Ba2+ + 2e–

Sodium Na → Na+ + e–

Cobalt Co → Co2+ + 2e–

Tin Sn → Sn2+ + 2e–

Copper Cu → Cu2+ + 2e–

Silver Ag → Ag+ + e–

Gold Au → Au3+ + 3e–

Incre

asin

g e

ase o

f o

xid

ati

on

Activity Series (partial)

55


56

Worked Example 9.6

Strategy Recognize that the salt in each equation (the compound on the reactant side) is a strong electrolyte. What is important is the identity of the metal cation in the salt. If the cation appears lower in the table, the solid metal will be oxidized (i.e., the reactions will occur). If the cation appears higher in the table, the solid metal will not be oxidized (i.e., no reaction will occur).

Using the activity series, predict which of the following reactions will occur, and

for those that will occur, write the net ionic equation and indicate which element

is oxidized and which is reduced: (a) Fe(s) + PtCl2(aq) → ?, (b)

Cr(s) + AuCl3(aq) → ?, (c) Pb(s) + Zn(NO3)2(aq) → ?

Solution(a)The cation in PtCl2 is Pt

2+. Platinum appears lower in Table 9.6 than iron, so Pt2+(aq) will oxidize Fe(s).

Fe(s) + Pt2+(aq) → Fe2+(aq) + Pt(s)

Iron is oxidized (0 to +2) and platinum is reduced (+2 to 0).

57


Solution(b)The cation in AuCl3 is Au

3+. Gold appears lower in Table 9.6 than chromium, so Au3+(aq) will oxidize Cr(s).

Cr(s) + Au3+ (aq) → Cr3+ (aq) + Au(s)

Chromium is oxidized (0 to +3) and gold is reduced (+3 to 0).

(c)The cation in Zn(NO3)2 is Zn2+. Zinc appears higher in Table 9.6 than lead, so

Zn2+(aq) will not oxidize Pb(s).

Think About It Check your conclusions by working each problem backward. For part (b), for example, write the net ionic equation in reverse, using the same products as reactants: Au(s) + Cr3+(aq) → ? Now compare the posi�ons of gold and chromium in Table 9.6 again. Chromium is higher, so chromium(III) ions cannot oxidize gold. This confirms your conclusion that the forward reaction (the oxidation of chromium by gold ions) will occur.

58

Balancing Simple Redox Equations

Redox reactions must have both mass balance and charge balance.

Before adding half-reactions, the electrons must balance.

Cr(s) + Ni2+(aq) → Cr3+(aq) + Ni(s)

Cr(s) Cr3+(aq) + 3e–Oxidation half-reaction:

Ni(s)Ni2+(aq) + 2e–Reduction half-reaction:

59

Balancing Simple Redox Equations

Prior to adding the two half-reactions, balance the electrons.

Step 1: Multiply the oxidation half-reaction by 2

Step 2: Multiply the reduction half-reaction by 3

This is known as the half-reaction method.

Cr(s) Cr3+(aq) + 3e–Oxidation half-reaction:

Ni(s)Ni2+(aq) + 2e–Reduction half-reaction:

2

3

2Cr(s) 2Cr3+(aq) + 6e–Oxidation half-reaction:

3Ni(s)3Ni2+(aq) + 6e–Reduction half-reaction:

3Ni(s) + 2Cr3+(aq)3Ni2+(aq) + 2Cr(s)

60

Worked Example 9.7

Strategy (a) The cation in CaCl2 is Ca2+. Calcium appears higher in Table 9.6

than aluminum, so Ca2+(aq) will not oxidize Al(s). (b) The cation in Pb(C2H3O2)2 is Pb2+. Lead appears lower in Table 9.6 than chromium, so Pb2+(aq) will oxidize Cr(s). (c) The cation in HI is H+. Hydrogen appears lower in Table 9.6 than tin, so H+(aq) will oxidize Sn(s).

Predict which of the following reactions will occur, and for those that will occur,

balance the equation and indicate which element is oxidized and which is

reduced: (a) Al(s) + CaCl2(aq) → ? (b) Cr(s) + Pb(C2H3O2)2(aq) → (c)

Sn(s) + HI(aq) → ?

Solution(a) No reaction.

61


Solution(b)The two half-reactions are represented by the following:

Oxidation: Cr(s) → Cr3+ (aq) + 3e-

Reduction: Pb2+(aq) + 2e- → Pb(s)

In order to balance the charges, we must multiply the oxidation half-reaction by 2 and the reduction half-reaction by 3:

2×[Cr(s) → Cr3+ (aq) + 3e-] = 2Cr(s) → 2Cr3+ (aq) + 6e-

3×[Pb2+(aq) + 2e- → Pb(s)] = 3Pb2+(aq) + 6e- → 3Pb(s)

We can then add the two half-reactions, canceling the electrons on both sides to get

2Cr(s) + 3Pb2+(aq) → 2Cr3+ (aq) + 3Pb(s)

The overall balanced molecular equation is2Cr(s) + 3Pb(C2H3O2)2(aq) → 2Cr(C2H3O2)3(aq) + 3Pb(s)

Chromium is oxidized (0 to +3) and lead is reduced (+2 to 0).

62


Solution(c)The two half-reactions are as follows:

Oxidation: Sn(s) → Sn2+ (aq) + 2e-

Reduction: 2H+(aq) + 2e- → H2(g)

Adding the two half-reactions and canceling the electrons on both sides yieldsSn(s) + 2H+(aq) → Sn2+ (aq) + H2(g)

The overall balanced molecular equation isSn(s) + 2HI(aq) → SnI2(aq) + H2(g)

Tin is oxidized (0 to +2) and hydrogen is reduced (+1 to 0). Reactions in which hydrogen ion is reduced to hydrogen gas are known as hydrogen displacementreactions.

Think About It Check your conclusions by working each problem backward. Write each equation in reverse and compare the positions of the elements in the activity series.

63

Other Types of Redox Reactions

Combination reactions can involve oxidation and reduction.

Hydrogen is oxidized from 0 to +1.

Nitrogen is reduced from 0 to –3.

N2(g) + 3H2(g) → 2NH3(g)

–3 +100

0 0 –3 +3

64


Decomposition can also be a redox reaction.

Na+ is reduced to Na.

H– is oxidized to H2.

NaH(s) → 2Na(s) + 3H2(g)

–1 00+1

+1 0–1 0

65


Disproportionation reactions occur when one element undergoes both oxidation and reduction.

Oxygen in H2O2 (and other peroxides) has an oxidation number of –1.

2H2O2(aq) → 2H2O(l) + O2(g)

–1 0+1+1

+2 +2–2 0

oxidation

reduction

–2

–2

66


Combustion is also a redox process.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

+1 0 +1–4

–4 +2+4 0

–2

–2

+4

+4

–2

–4

67

Concentration of Solutions

Molarity (M), or molar concentration, is defined as the number of moles of solute per liter of solution.

Other common rearrangements:

moles solutemolarity =

liters solution

molL =

M

mol = LM

9.5

68

Worked Example 9.8

Strategy Convert the mass of glucose given to moles, and use the equations for interconversions of M, liters, and moles to calculate the answers.

moles of glucose =

For an aqueous solution of glucose (C6H12O6), determine (a) the molarity of

2.00 L of a solution that contains 50.0 g of glucose, (b) the volume of this

solution that would contain 0.250 mole of glucose, and (c) the number of moles

of glucose in 0.500 L of this solution.

Solution(a)molarity = = 0.139 M

(b)volume = = 1.80 L

(c)moles of C6H12O6 in 0.500 L = 0.500 L×0.139 M = 0.0695 mol

50.0 g180.2 g/mol

0.277 mol C6H12O62.00 L solution

0.250 mol C6H12O60.139 M solution

= 0.277 mol

69

Think About It Check to see that the magnitude of your answers are logical. For example, the mass given in the problem corresponds to 0.277 mole of solute. If you are asked, as in part (b), for the volume that contains a number of moles smaller than 0.277, make sure your answer is smaller than the original volume.

70

Dilution

Dilution is the process of preparing a less concentrated solution from a more concentrated one.

moles of solute before dilution = moles of solute after dilution

71

Dilution

In an experiment, a student needs 1.00 L of a 0.400 M KMnO4solution. A stock solution of 1.00 M KMnO4 is available.

How much of the stock solution is needed?

Solution: Use the relationship that moles of solute before dilution = moles of solute after dilution.

(1.00 M KMnO4)(Lc) = (0.400 M KMnO4)(1.00 L)

Lc = 0.400 L or 400 mLTo make the solution:

1) Pipet 400 mL of stock solution into a 1.00 L volumetric flask.2) Carefully dilute to the calibration mark.

Mc × Lc = Md × Ld

72

Dilution and Serial Dilution

Because most volumes measured in the laboratory are in milliliters rather than liters, it is worth pointing out that the equation can be written as

Mc ×mLc = Md ×mLd

A series of dilutions that may be used to prepare a number of increasingly dilute solutions is called Serial Dilution.

Step 1: Prepare a dilute solution from the stock

Step 2: Dilute a portion of the prepared solution to make a more dilute solution

Step 3: Repeat as needed

73

Worked Example 9.9

Strategy Mc = 12.0 M, Md = 0.125 M, mLd = 250.0 mL

What volume of 12.0 M HCl, a common laboratory stock solution, must be used

to prepare 250.0 mL of 0.125 M HCl?

Solution12.0 M ×mLc = 0.125 M × 250.0 mL

mLc = 0.125 M × 250.0 mL

12.0 M = 2.60 mL

Think About It Plug the answer into Equation 9.4, and make sure that the product of concentration and volume on both sides of the equation give the same result.

74

Worked Example 9.10

Strategy (a) Md =

Starting with a 2.0 M stock solution of hydrochloric acid, four standard solutions

(1 to 4) are prepared by sequential diluting 10.00 mL of each solution to

250.00 mL. Determine (a) the concentrations of all four standard solutions and

(b) the number of moles of HCl in each solution.

Solution(a) Md1 =

Md2 =

Md3 =

Md4 =

2.00 M × 10.00 mL250.00 mL

Mc×mLcmLd

; (b) mol = M×L, 250.00 mL = 2.500×10-1 L

= 8.00×10-2 M

8.00×10-2 M × 10.00 mL250.00 mL

= 3.20×10-3 M

3.20×10-3 M × 10.00 mL250.00 mL

= 1.28×10-4 M

1.28×10-4 M × 10.00 mL250.00 mL

= 5.12×10-6 M

75


Think About It Serial dilution is one of the fundamental practices of homeopathy. Some remedies undergo so many serial dilutions that very few (if any) molecules of the original substance still exist in the final preparation.

Solution(b) mol1 = 8.00×10

-2 M × 2.500×10-1 L = 2.00×10-2 molmol2 = 3.20×10

-3 M × 2.500×10-1 L = 8.00×10-4 molmol3 = 1.28×10

-4 M × 2.500×10-1 L = 3.20×10-5 molmol4 = 5.12×10

-6 M × 2.500×10-1 L = 1.28×10-6 mol

76

Worked Example 9.11

Strategy Use the concentration given in each case and the stoichiometry indicated in the corresponding chemical formula to determine the concentration of the specified ion or compound.

Using square-bracket notation, express the concentration of (a) chloride ion in a

solution that is 1.02 M in AlCl3, (b) nitrate ion in a solution that is 0.451 M in

Ca(NO3)2, and (c) Na2CO3 in a solution in which [Na+] = 0.124 M.

Solution (a) There are 3 moles of Cl- ion for every 1 mole of AlCl3,AlCl3(s) → Al

3+(aq) + 3Cl-(aq)so the concentration of Cl- will be three times the concentration of AlCl3.

[Cl-] = [AlCl3] ×

= ×

= = 3.06 M

3 mol Cl-

1 mol AlCl31.02 mol AlCl3

L3 mol Cl-

1 mol AlCl33.06 mol Cl-

L

77


Solution (b) There are 2 moles of nitrate ion for every 1 mole of Ca(NO3)2,Ca(NO3)2(s) → Ca

2+(aq) + 2NO3-(aq)

so [NO3-] will be twice [Ca(NO3)2].

[NO3-] = [Ca(NO3)2] ×

= ×

= = 0.902 M

2 mol NO3-

1 mol Ca(NO3)2

0.451 mol Ca(NO3)2L

2 mol NO3-

1 mol Ca(NO3)2

0.902 mol NO3-

L

78


Solution (c) There is 1 mole of Na2CO3 for every 2 moles of sodium ion,Na2CO3(s) → 2Na

+(aq) + CO32-(aq)

so [Na2CO3] will be half of [Na+]. (Assume that Na2CO3 is the only source of Na

+

ions in this solution.)

[Na2CO3] = [Na+] ×

= ×

= = 0.0620 M

1 mol Na2CO32 mol Na+

0.124 mol Na+

L1 mol Na2CO3

2 mol Na+

0.0620 mol Na2CO3L

Think About It Make sure that units cancel properly. Remember that the concentration of an ion can never be less than the concentration of its dissolved parent compound. It will always be the concentration of the parent compound times its stoichiometric subscript in the chemical formula.

79

The pH Scale

The acidity of an aqueous solution depends on the concentration of hydronium ions, [H3O

+].

The pH of a solution is defined as the negative base-10 logarithm of the hydronium ion concentration (in mol/L)

In pure water at 25°C, [H3O+] = log1.0 x 10–7

pH = –log(1.0 x 10–7) = 7.00

pH is a dimensionless quantity.

pH = –log[H3O+] [H3O

+] = 10–pH

80

The pH Scale

81

The pH Scale

82

Worked Example 9.11

Strategy Given [H3O+], use pH = –log[H3O

+] to solve for pH.

Determine the pH of a solution at 25°C in which the hydronium ion

concentration is (a) 3.5×10-4 M, (b) 1.7×10-7 M, and (c) 8.8×10-11 M.

Solution(a) pH = –log(3.5×10-4) = 3.46

(b) pH = –log(1.7×10-7) = 6.77

(c) pH = –log(8.8×10-11) = 10.06

Think About It When a hydronium ion concentration falls between two “benchmark” concentrations in Table 16.4, the pH falls between the two corresponding pH values. In part (c), for example, the hydronium ion concentration (8.8×10-11 M) is greater than 1.0×10-11 M but less than 1.0×10-10 M. Therefore, we expect the pH to be between 11.00 and 10.00.

83

Worked Example 9.12

Strategy Given pH, use [H3O+] = 10-pH to calculate [H3O

+].

Calculate the hydronium ion concentration in a solution at 25°C in which the pH

is (a) 4.76, (b) 11.95, and (c) 8.01.

Solution(a) [H3O

+] = 10-4.76 = 1.7×10-5 M

(b) [H3O+] = 10-11.95 = 1.1×10-12 M

(c) [H3O+] = 10-8.01 = 9.8×10-9 M

84

Think About It Think About It If you use the calculated hydronium ion concentrations to recalculate pH, you will get numbers slightly different from those given in the problem. In part (a), for example, −log(1.7×10-5) = 4.77. The small difference between this and 4.76 (the pH given in the problem) is due to a rounding error. Remember that a concentration derived from a pH with two digits to the right of the decimal point can have only two significant figures. Note also that the benchmarks can be used equally well in this circumstance. A pH between 4 and 5 corresponds to a hydronium ion concentration between 1.7×10-4 M and 1.0×10-5 M.

85

Aqueous Reactions and Chemical Analysis

Gravimetric analysis is an analytical technique based on the measurement of mass.

Gravimetric analysis is highly accurate.

Applicable only to reactions that go to completion or have nearly 100 % yield.

9.6

86

Worked Example 9.14

Strategy Using the mass of AgCl precipitate and the percent composition of AgCl, determine what mass of chloride the precipitate contains. The chloride in the precipitate was originallly in the unknown compound. Using the mass of chloride and the mass of the original sample, determine the percent Cl in the compound.

A 0.8633-g sample of an ionic compound containing chloride ions and an

unknown metal cation is dissolved in water and treated with an excess of AgNO3.

If 1.5615 g of AgCl precipitate forms, what is the percent by mass of Cl in the

original sample?

Setup To determine the percent Cl in AgCl, divide the molar mass of Cl by the molar mass of AgCl:

The mass of Cl in the precipitate is 0.2472 × 1.5615 g = 0.3860 g.

34.45 g(34.35 g + 107.9 g)

× 100% = 24.72%

87


Solution The percent Cl in the unknown compound is the mass of Cl in the precipitate divided by the mass of the original sample:

0.3860 g0.8633 g

× 100% = 44.71%

Think About It Pay close attention to which numbers correspond to which quantities. It is easy in this type of problem to lose track of which mass is the precipitate and which is the original sample. Dividing by the wrong mass at the end will result in an incorrect sample.

88

Acid-Base Titrations

Quantitative studies of acid-base neutralization reactions are most conveniently carried out using a technique known as a titration.

A titration is a volumetric technique that uses burets.

The point in the titration where the acid has been neutralized is called the equivalence point.

89


The equivalence point is usually signalled by a color change.

The color change is brought about by the use of an indicator.

Indicators have distinctly different colors in acidic and basic media.

The indicator is chosen so that the color change, or endpoint, is very close to the equivalence point.

Phenolphthalein is a common indicator.

90


Sodium hydroxide solutions are commonly used in titrations.

NaOH solutions must be standardized as the concentrations change over time. (NaOH reacts with CO2 that slowly dissolves into the solution forming carbonic acid.)

The acid potassium hydrogen phthalate (KHP) is frequently used to standardize NaOH solutions.

acidic proton of KHP;

KHP is a monoprotic acid

91

Worked Example 9.15

Strategy Using the mass given and the molar mass of KHP, determine the number of moles of KHP. Recognize that the number of moles of NaOH in the volume given is equal to the number of moles of KHP. Divide moles of NaOH by volume (in liters) to get molarity.

The molar mass of KHP (KHC8H4O4) = [39.1 g + 5(1.008 g) + 8(12.01 g) + 4(16.00 g)] = 204.2 g/mol.

In a titration experiment, a student finds that 25.49 mL of an NaOH solution is

needed to neutralize 0.7137 g of KHP. What is the concentration (in M) of the

NaOH solution?

Solution moles of KHP =

Because moles of KHP = moles of NaOH, then moles of NaOH = 0.003495 mol.

molarity of NaOH =

0.7137 g204.1 g/mol

= 0.003495 mol

0.003495 mol0.02549 L

= 0.1371 M

92


Think About It Remember that molarity can also be defined as mmol/mL. Try solving the problem again using millimoles and make sure you get the same answer.

0.003495 mol = 3.495×10-3 mol= 3.495 mmol

and

3.495 mmol25.49 mL

= 0.1371 M

93

Worked Example 9.16

Strategy First, write and balance the chemical equation that corresponds to the neutralization reaction:

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)

The base and the diprotic acid combine in a 2:1 ratio: 2NaOH ≈ H2SO4. Use the molarity and the volume given to determine the number of millimoles of H2SO4. Use the number of millimoles of H2SO4 to determine the number of millimoles of NaOH. Using millimoles of NaOH and the concentration given, determine the volume of NaOH that will contain the correct number of millimoles.

What volume (in mL) of a 0.203 M NaOH solution is needed to neutralize 25.0

mL of a 0.188 M H2SO4 solution?

94


Think About It Notice that the two concentrations 0.203 M and 0.188 M are similar. Both round to the same value (~0.20 M) to two significant figures. Therefore, the titration of a diprotic acid with a monobasic base of roughly equal concentration should require roughly twice as much base as the beginning volume of acid 2 × 25.0 mL ≈ 46.3 mL.

Solution millimoles of H2SO4 = 0.188 M × 25.0 mL = 4.70 mmol

millimoles of NaOH required = 4.70 mmol H2SO4 ×

volume of 0.203 M NaOH = 9.40 mmol NaOH ×

2 mmol NaOH1 mmol H2SO4

= 9.40 mmol NaOH

1 mL NaOH0.203 mmol NaOH

= 46.3 mL

95

Worked Example 9.17

Strategy Because the acid is monoprotic, it will react 1:1 ratio with the base; therefore, the number of moles of acid will be equal to the number of moles of base. The volume of base in liters is 0.0125 L.

A 0.1216-g sample of a monoprotic acid is dissolved in 25 mL water, and the

resulting solution is titrated with 0.1104 M NaOH solution. A 12.5-mL volume of

the base is required to neutralize the acid. Calculate the molar mass of the acid.

Solution moles of base =

Because moles of base = moles of acid, the moles of acid = 0.00138 mol. Therefore,

molar mass of the acid =

0.0125 L x 0.1104 molL

= 0.00138 mol

0.1216 g0.00138 mol

= 88.1 g/mol

96

Think About It In order for this technique to work, we must know whether the acid is monoprotic, diprotic, or polyprotic. A diprotic acid, for example, would combine in a 1:2 ratio with the base, and the result would have been a molar mass twice as large.

97

Chapter Summary: Key Points9

Aqueous SolutionsElectrolytes and NonelectrolytesStrong and Weak Electrolytes Precipitation ReactionsHydrationSolubility Molecular EquationsIonic EquationsNet Ionic EquationsMetathesisArrheniusAcids and BasesBrønsted Acids and BasesStrong Acids and Bases Acid-Base Reactions

Oxidation-Reduction ReactionsOxidation NumbersBalancing Simple Redox EquationsOther Types of Redox ReactionsConcentration of SolutionsMolarityDilutionpHChemical AnalysisGravimetric AnalysisAcid-Base Titrations

Date post:	18-Feb-2021
Category:	Documents
Upload:	others
View:	14 times
Download:	2 times

Second Edition Julia Burdge & Jason Overbyacidic hydrogen atoms. Polyproticacids lose protons in a...

Documents