Chapter 4Chapter 4Chapter 4Chapter 4

11

Reactions in Aqueous Solution

Observing and Predicting Reactions

22

How do we know when a reaction is taking place?

Precipitate

Fire!

Smoke

Bubbles

Explosion!!

Change in Temperature

Properties of Aqueous Solutions

– Solution: homogeneous mixture of two or more substances

– Solute: substance present in smaller amount

– Solvent: substance present in greater amount

– Aqueous solution: solvent is water

Most reactions in general chemistry take place in an aqueous (aq) environment. What does that

mean?

NaCl

Electrolytes in Aqueous Solutions

Electrolytesconduct

electricity in aqueous

solutions by moving toward

the oppositely charged

electrode

Non-electrolytes,

however, do not have

charges in aqueous

solutions and are not

attracted to the electrodesElectrolytes Non-electrolytes

Properties of Aqueous Solutions

Substances behave differently when they are placed in water; specifically

ionic versus covalent compounds.

NaCl CH3OH

Na+

Na+Cl-

Cl- CH3OH

CH3OH

NaCl “breaks apart” or ionizes

CH3OH does not“break apart”. It does not ionize

Na+Cl-

CH3OH

Electrolyte Non-

Electrolyte

Electrolytic Properties

Strong electrolyte:

substance that, when dissolved

in water, results in a solution that can conduct electricity

Weak electrolyte:

substance that is a poor conductor of electricity

when dissolved in water

Non-electrolyte:substance that doesn’t conduct electricity when

dissolved in water (CH3OH – methanol)

Ionize

<100%

Ionize

100%

Ionize 0%

Six General Types of Reactions

• Double Replacement (Precipitation)

• Single Replacement

• Acid – Base Neutralization

• Combustion

• Combination

• Decomposition

Most of the time, products of these

reactions can be

easily predicted

Sometimes products of these

reactions cannot

be easily predicted

Double Replacement / Precipitation Reactions

Soluble ionic compound AB dissolved in H2O

Insoluble precipitate (BC) formed from mixing AB + CD

Will not dissolve in H2O

Soluble ionic compound CD dissolved in H2O

Pb(NO3)2 (aq) + 2KI (aq) → ?

AB(aq) + CD(aq) → AD(aq) + BC(s)In general:

Example:

Predicting Products of Double Replacement

MgCl2 (aq) + AgNO3 (aq) → ???

MgCl2 (aq) + 2AgNO3 (aq) → Mg(NO3)2 (aq) + 2AgCl (s)

To predict products, switch partners with cations

To determine states of mater, check the solubility rules of

the products

If the ionic compound is soluble → (aq)

If the ionic compound is insoluble → (s)

If both products are soluble (aq), there is NO REACTION

How do we know what is soluble?We have rules…

Pb(NO3)2 (aq) + 2KI (aq) → 2KNO3 (aq) + PbI2 (s)

Chemistry

Humor…tell all your friends!

We’re a clever

Lot!

Solubility Rules

• Determine if the following ionic compounds will be soluble (aq) or insoluble (s) in water:

• K2CO3

• BaSO4

• PbCl2• NaClO4

• Ag2S

• (NH4)3PO4

• Cu(OH)2

Solubility Rules Answers

• Determine if the following ionic compounds will be soluble (aq) or insoluble (s) in water:

• K2CO3 soluble (aq)

• BaSO4 insoluble (s)

• PbCl2 insoluble (s)

• NaClO4 soluble (aq)

• Ag2S insoluble (s)

• (NH4)3PO4 soluble (aq)

• Cu(OH)2 insoluble (s)

Molecular, Ionic, and Net Ionic Equations

1414

Mg2+(aq) + 2Cl-(aq) + 2Ag+(aq) + 2NO3-(aq) → Mg2+(aq) + 2NO3

-(aq) + 2AgCl(s)

Molecular Equation

MgCl2 (aq) + 2AgNO3 (aq) → Mg(NO3)2 (aq) + 2AgCl (s)

Net Ionic Equation

2Cl- (aq) + 2Ag+(aq) → 2AgCl (s)

Ionic Equation

Any ionic compound that is aqueous (aq) is separated into ions

Any ion found on both sides of the equation is removed from the

net ionic equation. Solids liquids and gases are not removed

Complete/Balance These Equations

• Identify any precipitate(s) that form(s) and identify the spectator ions.

• Na2S(aq) + CuCl2(aq) �

• MgSO4(aq) + BaCl2(aq) �

• KNO3(aq) + CaCl2(aq) �

• CuSO4(aq) + NaOH(aq) �

1515

Complete/Balance These Equations

• Na2S(aq) + CuCl2(aq) � 2 NaCl(aq) + CuS(s)

– Cu2+(aq) + S2-(aq) � CuS(s)

• MgSO4(aq) + BaCl2(aq) � MgCl2(aq) + BaSO4(s)

– Ba2+(aq) + SO42-(aq) � BaSO4(s)

• KNO3(aq) + CaCl2(aq) � KCl(aq) + Ca(NO3)2(aq)

– No reaction

• CuSO4(aq) + 2 NaOH(aq)� Cu(OH)2(s) + Na2SO4(aq)

– Cu2+(aq) + 2 OH-(aq) � Cu(OH)2(s)1616

Give the net ionic equation for each

Acid-Base Reactions

Acid: a substance, when dissolved in water, increases the H3O

+ concentration.

Base: a substance, when dissolved in water, increases the OH- concentration.

HCl(aq) + H2O(l) → H3O+

(aq) + Cl-(aq)

NaOH(s) + H2O(l) → H2O(l) + OH-(aq) + Na

+(aq)

Sometimes represented

as H+

NaOH(s) → OH-(aq) + Na+(aq)

H2O(l)

-or-

Acid-Base Reaction

Acid + Base → H2O + Salt

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

Water is created from H+ and OH-

The salt is created from spectator ions

HCl Mg(OH)2

Acid-Base Neutralization

• What does the word “neutralization”mean to you?

• How might these two beakers react?

+OH-

Na+

Na+

OH-

H+

H+ Cl-

Cl-

Single-Replacement (Displacement)A single-replacement is somewhat similar to a double replacement in

that one chemical “replaces” another.

Except, instead of two ionic compounds reacting, one of the reactants is

a pure metal (not an ion). And there is no precipitate.

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)Molecular

EquationTo predict products, “switch places”

Solid Cu metal (no charge) becomes a

Cu2+ ion The Ag+ ion becomes solid metal (no charge)

Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)

Net IonicEquation

A(s) + BC(aq) → AC(aq) + B(s or g)In general:

Single-Displacement Reactions

2121

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

What’s actually happening?

Cu(s)

Ag+(aq)

NO3-(aq)

NO3-(aq)

Cu2+(aq)

Cu(s)

Ag(s)→

Notice that the NO3-’s aren’t really doing anything.

Single Displacement: Li + H2O

2222

Some metals can also displace hydrogen from water and

acids. In this case, the solid metal and hydrogen switch

places. Products are predicted by “switching places” also.

2Li(s) + 2H2O(l) → H2(g) + 2LiOH(aq)

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

In both reactions, hydrogen is displaced to

give hydrogen gas, H2.

Will this reaction take place?

If we mix any metal with any aqueous ionic solution (or acid), will a reaction take

place? Ummm….no. Once again, we have rules.

The Activity Series Li K

Ba Ca

NaMg

Al Mn

Zn Cr

Fe Co

Ni Sn

H2Cu

Ag Hg

Pt Au

For a single replacement reaction to occur, the solid metal must be higher on the

list than the metal it is displacing

Represents

Acids

The first five will

react with H2O

Anything

above H2

will react

with an acid

Predict whether a reaction occurs

• Na + H2O

• Fe + HCl

• Ni + Pb(NO3)2

• Ag + Mg(NO3)2

• Zn + Co(NO3)2

2424

Combustion Reactions

What does your local (non-nuclear) power plant, your running car, and wildfires all have in common?

In each case, a combustion reaction is taking place. During

the reaction, carbon dioxide and water are produced (in a perfect world). In reality, side-reactions also take place. We’ll

ignore these for now.

CO2(g) + H2O(g)

Recognizing a Combustion Reaction

In each case, a carbon containing compound is being burned in oxygen.

Coal Gasoline Plant Matter

CxHyOz + O2(g) → CO2(g) + H2O(g)

A reactant

containing C and H

or C, H, and O

A combustion reaction usually will contain:

O2 as the other reactant

CO2 and H2O as products

+ →

Combination and Decomposition Reactions

Simpler compounds and/or elements → More complex compound(s)

More complex compound(s) → Simpler compounds and/or elements

Combination

Decomposition

A + B → C

C → A + B

2H2(g) + O2(g) → 2H2O(g)

2H2O(g) → 2H2(g) + O2(g)

Combination Reactions

Combination reactions that produce ionic compounds can be easily predicted

Predict: Cl2(g) + Na(s) → ??

Cl2(g) + 2Na(s) → 2NaCl(s)

However, when covalent compounds are formed, more than one compound is possible

Predict: C(s) + O(g) → ??CO2(g)

CO(g)

Even if all the products of combination reactions cannot be

predicted, you should be able to recognize this type of reaction

Decomposition Reactions

Explosions are caused by decomposition of unstable compounds. In the reaction, gases are rapidly produced and expand from heat formed in

the reaction. Think about how much more space gases occupy than

solids and why this might be destructive!!

2C7H5(NO2)3(s) → 7CO(g) + 7C(s) + 5H2O(g) + 3N2(g)

TNT Gases produced

Decomposition Reactions

• Oxides, peroxides (O22-) → O2

• Nitrates (NO3-) → NO2

• Carbonates (CO32-) → CO2

• Ammonium (NH4+) salts → NH3

3232

Many decomposition reactions produce gases

Types of Reactions Summary

• Precipitation: use Solubility Rules

• (AB + CD � AD + CB)

• Acid-Base Neutralization: acid + base �salt + water

• (AB + CD � AD + H2O)

• Combination: start with elements

• (A + B � AB)

• Decomposition: often produces gas

• (AB � A + B)

• Single Displacement: use Activity Series to predict if a reaction occurs

• (A + BC � B + AC)

• Combustion:

– Hydrocarbon + O2(g)� CO2(g) + H2O(g)

Predict Products and Types of Reactions

+

Ca Ca Ca Ca

Ca Ca Ca

H+

Cl-

Cl-

Cl-

Cl- H+

H+

H+

Predict Products and Types of Reactions

+

Na

Cl Cl

Na Na

NaNa Na

Cl Cl

Cl Cl

Predict Products and Types of Reactions

+Ag+

NO3-

Ag+

Ag+ NO3-

NO3- Cl-

Cl-

Cl-Na+

Na+

Na+

Predict Products and Types of Reactions

+Cl-

H+

K+ OH-

OH-

K+

H+

Cl-

Classify (and balance) these rxns

1. AlF3(aq) + H2O(l) ���� Al(OH)3(s) + HF(aq)

2. Fe(s) + HCl(aq) ����

3. Ca(OH)2(s) ���� CaO(s) + H2O(g)

4. Ca(s) + H2O(l) ����

5. CaO(s) + CO2(g) ����

6. Cu(s) + AgNO3(aq) ����

7. BaCl2(aq) + Na2SO4(aq) ����

4040

Classify these reactions by type

8. H2SO3(aq) ���� H2O(l) + SO2(g)

9. HgO(s) ���� Hg(l) + O2(g)

10. KOH(aq) + HNO3(aq) ����

11. Li(s) + O2(g) ����

12. Mg(OH)2(aq) + 2HCl(aq)����

13. NH3(g) + HCl(g) ���� NH4Cl(g)

14. NiCO3(s) ����

15. P4(s) + F2(g) ���� PF5(g)

1. AlF3(aq) + 3H2O(l) � Al(OH)3(s) + 3HF(aq)

2. Fe(s) + 2HCl(aq) � FeCl2(aq) + H2(g)

3. Ca(OH)2(s) � CaO(s) + H2O(g)4. Ca(s) + 2H2O(l) � Ca(OH)2(aq) + H2(g)

5. CaO(s) + CO2(g) � CaCO3(s)

6. Cu(s) + 2AgNO3(aq) � Cu(NO3)2(aq) + 2Ag(s)7. BaCl2(aq) + Na2SO4(aq) � BaSO4(s) + 2NaCl(aq)

8. H2SO3(aq) � H2O(l) + SO2(g)9. 2HgO(s) � 2Hg(l) + O2(g)

10. KOH(aq) + HNO3(aq) � KNO3(aq) + H2O(l)

11. 4Li(s) + O2(g) � 2Li2O(s)12. Mg(OH)2(aq) + 2HCl(aq) � 2MgCl2(aq) + H2O(l)

13. NH3(g) + HCl(g) � NH4Cl(s)

14. NiCO3(s) � NiO(s) + CO2(g)15. P4(s) + 10F2(g) � 4PF5(g)

precipitation

single disp./H2

decomposition

single disp./H2

combination

single disp./metal

precipitationdecomposition

decomposition

neutralizationcombination

neutralizationcombination

decomposition

combination

Classify these reactions by type

Oxidation-Reduction Reactions

Oxidation-reduction reactions are reactions in which electrons are transferred to/from atoms. While seemingly a

different “type” of reaction than the six previously mentioned, it another way of describing those reactions.

Double-replacement

Acid-Base

Single-Replacement

Combination

Decomposition

Combustion

Usually redox reactions

Assigning Oxidation Numbers

In order to determine if a redox reaction takes place, we first have to assign oxidation numbers to all of

the atoms in a reaction

Assigning Oxidation Numbers

Assigning Oxidation Numbers

4545

#1) The oxidation number of atoms of an uncombined element (in elemental state) is 0.

Fe(s) O2(g) N2(l)

#2) Hydrogen has an oxidation number of +1 (unless it

is combined with alkali metals, in which case it has an oxidation number of –1).

H2O(l) CH4(g) NaH

+1 -1

Exception to the rule

Assigning Oxidation Numbers

#3) Oxygen usually has an oxidation number of –2 in its compounds.

Exceptions:

Oxygen has an oxidation number of –1 in peroxides, which contain the O2

2- ion (e.g., K2O2 and H2O2).

KNO3(aq) H2O(g) H2O2(aq)

-2 -1

Exception to the rule

#4) For binary ionic compounds, the oxidation number is the same

as the charge of the ion.

NaCl(aq) MgS(s) AlP(s)

+1 -1 +2 -2 +3 -3

#5) For covalent compounds, the oxidation number of the halogens

is -1.

Assigning Oxidation Numbers

Some elements can only be determined using the last rule

#6) The sum of the oxidation numbers of all atoms in a substance must equal the total charge

on the species

NaCl

+1 + -1 = 0Since NaCl is not a charged species, the

oxidation numbers must add up to 0.

H3O+

Since H3O+ is a charged species, the oxidation

numbers must add up to the charge.3(+1) + -2 = +1

CH4Hydrogen is +1. But what about carbon?

Carbon has to be -4? + +1(4) = 0

Assigning Oxidation Numbers

For many covalent compounds, you will need to use rule #6 to determine oxidation numbers for elements not covered by the other rules

CO2

CO

CCl4

? + 2(-2) = 0

? + 1(-2) = 0

? + 4(-1) = 0

Carbon is +4

Carbon is +2

Carbon is +4

Example

• H2SO4

– H = +1

– O = –2

– S unknown, so leave this for last.

• The overall charge on this compound is 0.

• Use algebra to solve for S:

• 2(+1) + 1(x) + 4(-2) = 0

• Solve for each element: MgCr2O7

4949

Assigning Oxidation Numbers

• Determine values of the oxidation number of each element in these compounds or ions:

H2O SO2CCl4 H2O2Fe3(PO4)2 MnO4

-

NaNO3 KClO4

5050

Assigning Oxidation Numbers

• Determine values of the oxidation number of each element in these compounds or ions:

H2O

SO2

CCl4H2O2

Fe3(PO4)2MnO4

-

NaNO3

KClO4

5151

H: +1, O: -2S: +4, O: -2C: +4, Cl: -1H: +1, O: -1Fe: +2, P: +5, O: -2Mn: +7, O: -2Na:+1, N:+5, O: -2K: +1, Cl: +7, O: -2

Oxidation-Reduction Reactions

Now that you have assigned oxidation numbers, let’s determine if something is oxidized or reduced.

2H2(g) + O2(g) → 2H2O(g)

0 0 +1 -2

H2 is oxidized because the oxidation number increased.

It lost electrons.

O2 is reduced because the oxidation number decreased.

It gains electrons.

Oxidation-Reduction Reactions

• The substance which is oxidized is called the reducing agent.

• The substance which is reduced is called the oxidizing agent.

• 4 Fe(s) + 3O2(g) � 2Fe2O3(s)

Oxidation-Reduction Reactions

• Identify the element oxidized/reduced. Also identify the oxidizing agent and the reducing agent.

• Cu(s) + 2AgNO3(aq) � 2Ag(s) + Cu(NO3)2(aq)

• Zn(s) + 2HCl(aq) � ZnCl2(aq) + H2(g)

• H2O(l) + Na(s) � H2(g) + NaOH(aq)