Chem 101 Lecture 13

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CHEM 101Introduction to University Chemistry I

Dr Mark Tonge780-497-5191

5-132D

September 2011

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Types of Chemical Reactions

LECTURE 13: OVERVIEW & LEARNING OUTCOMES

After attending this session you should be able to:

recognise the important classes of chemical reactions,such as precipitation, acid-base, redox

make predictions on the outcomes of simple precipitationreactions

Reading: Chapter 4, sections 4.4 to 4.7

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4.4 FUNDAMENTAL CLASSES OF CHEMICAL REACTIONS

Chemical reactions in aqueous solution can be subdividedas follows:

Precipitation Reactions

Acid-Base Reactions

Redox Reactions: oxidation reduction

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4.5 PRECIPITATION REACTIONS

Precipitation Reactions are a sub-grouping of MetathesisReactions: double-displacement reactions where cations

and anions are exchangedPrecipitation reactions are driven by the formation of a stableprecipitate, and are important in:

Chemical manufacturing, egpharmaceutical synthesis

Analytical measurement, eg chloride ions in seawater

Cleanup and removal of contaminants, egwastewatertreatment

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Some reactions in aqueous solution give riseto insoluble products that precipitate ( ) from

solution:

KCl(aq) + AgNO3(aq) AgCl( ) + KNO3(aq)

Na2CrO4(aq) + BaCl2(aq) BaCrO4( ) + 2NaCl(aq)

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Solubility is not always easy to predict, but can be aided byapplying some simple solubility rules in the following order:

1 salts of ammonium and group 1 cations are usually soluble

2 nitrates, acetates and perchlorates are usually soluble

3 silver, lead and mercury I salts are usually insoluble

4 chlorides, bromides and iodides are usually soluble

5 carbonates, phosphates, sulphides, selenides, chromates,oxides and hydroxides are usually insoluble

6 sulphates are usually soluble, except for those of calcium,strontium and barium

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CompoundPredictedSolubility

SolubilityRule(s)

ObservedSolubility

MgCl2

Mg(OH)2

BaSO4

FeSO4

AgNO3

AgCl

NaOH

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SolubilityRule(s)

ObservedSolubility

MgCl2

Mg(OH)2

BaSO4

FeSO4

AgNO3

AgCl

NaOH

soluble

insoluble

insoluble

soluble

soluble

insoluble

soluble

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SolubilityRule(s)

ObservedSolubility

MgCl2

Mg(OH)2

BaSO4

FeSO4

AgNO3

AgCl

NaOH

soluble

insoluble

insoluble

soluble

soluble

insoluble

soluble

4

5

6

6

2>3

3>4

1>5

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SolubilityRule(s)

ObservedSolubility

MgCl2

Mg(OH)2

BaSO4

FeSO4

AgNO3

AgCl

NaOH

soluble

insoluble

insoluble

soluble

soluble

insoluble

soluble

4

5

6

6

2>3

3>4

1>5

soluble

insoluble

insoluble

soluble

soluble

insoluble

soluble

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Complete the following equations and identify any

insoluble products:

NaOH(aq) + FeCl3(aq)

Pb(NO3)2(aq) + FeSO4(aq)

FeCl2(aq) + H2S(g)

FeCl3(aq) + (NH4)3PO4(aq)

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insoluble products:

2NaOH(aq) + FeCl3(aq) 3NaCl(aq) + Fe(OH)3( )

Pb(NO3)2(aq) + FeSO4(aq) PbSO4( ) + Fe(NO3)2(aq)

FeCl2(aq) + H2S(g)


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insoluble products:



FeCl2(aq) + H2S(g) FeS( ) + 2HCl(aq)


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insoluble products:



FeCl2(aq) + H2S(g) FeS( ) + 2HCl(aq)

FeCl3(aq) + (NH4)3PO4(aq) FePO4( ) + 3NH4Cl(aq)

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4.6 DESCRIBING REACTIONS IN SOLUTION

There are many different ways of representing reactions insolution, each revealing different types of information:

Molecular:

Here, the overall reaction stoichiometry is shown but not theactual chemical forms of the reactants and products:

KCl(aq) + AgNO3(aq) AgCl( ) + KNO3(aq)

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Complete Ionic:

Here, all strong electrolytes are represented as ions:

K+(aq) + Cl(aq) + Ag+(aq) + NO3(aq)

AgCl( ) + K+(aq) + NO3(aq)

In the above example, the K+(aq) and NO3(aq) remain

unchanged and are therefore known as Spectator Ions

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Net Ionic:

Here, only those species that undergo change are shown, notthe spectator ions:

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

Here, the spectator ions that appear on each side of thechemical equation are cancelled out to focus on the mostimportant information

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4.7 SELECTIVE PRECIPITATION

Precipitation can be used to separate mixtures of ions, forexample in:

analytical measurement, eggravimetric analysis

sample cleanup, eg removal of iron form solution

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Starting with a mixture of silver, barium, iron (III) and nitrateions in aqueous solution, suggest a scheme for the

separation of the mixture by Selective Precipitation of silver,barium, iron (III) ions in that order:

1: add aqueous sodium chloride solution:

Ag+

(aq) + Cl

(aq) AgCl( )Barium and iron (III) ions remain unchanged, so the silverions can be removed as AgCl(s) by filtration

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2: add aqueous sodium sulphate solution to the filtrate:

Ba2+(aq) + SO42(aq) BaSO4( )

Iron (III) ions remain unchanged, so the barium ions can beremoved as BaSO4(s) by filtration

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3: add aqueous sodium hydroxide solution to the filtrate:

Fe3+(aq) + 3OH(aq) Fe(OH)3( )

What is left in the filtrate after step three?

Aqueous solution of sodium and nitrate ions, plus traces ofexcess chloride, sulphate and hydroxide ions

Chem 101 Lecture 13

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