1

Lecture 6

Professor Hicks

Inorganic Chemistry (CHE151)

Metathesis reactions

• Precipitation reactions

• Acid-base (neutralization) reactions

• Reactions that form a gas

metathesis reactions

ions change partners

• How to write complete balanced reactions

2

Solubility

• Max amount of a substance that will dissolve

• Not increased with increased stirring, shaking, etc.

• For molecular compounds educated guesses of

solubility can be made based on the “like dissolves

like” rule

• Solubility of ionic compounds seemingly

unpredictable

• Detailed rules exist to predict solubility for ionic

compounds based on the identity of the ions

I will give you the solubility rules for ionic compounds for the exam

3

Precipitation reactions • Only some ionic compounds dissolve in water

• If solutions are mixed that contain ions that can form an

insoluble compound, the compound will form and

precipitate as a solid

AgCl

insoluble AgNO3

soluble

Ag+ (aq)

NO3- (aq)

NaCl soluble

Na+ (aq)

Cl- (aq)

mix

Na+ (aq)

NO3- (aq)

AgCl (s)

NaNO3 is

soluble

AgCl solid

precipitates

initially

cloudy

4

Precipitation of an ionic compound

is a metathesis reaction

AgNO3 + NaCl → AgCl + NaNO3

white solid forms

a precipitate

metathesis reaction = ions change partners

Acids • H+ + anion

H+ -

anion

• Ionic compounds must separate into ions to dissolve

• Acids are molecular compound because they can

dissolve without dissociating into ions

• Weak acids have a small percentage of molecules

separated into H+ and an anion, the rest stay together

as one particle

Strong acids

HCl HNO3 H2SO4

H+ Cl- H+ NO3- H+ SO4

2-

hydrochloric acid nitric acid sulfuric acid

Weak acids

HC2H3O2 HF

H+ C2H3O2

- H+ F-

acetic acid hydrofluoric acid

HF ~ 95% H+ and F- 5%

Strong acids separate 100% into H+ and anion in water HCl ~ 0 % H+ and Cl- ~ 100%

5

Molecular

compounds

All other

molecular

compounds dissolve

Dissolved

molecules

Ionic

compounds

dissolve

cations

(+ ions)

anions

(- ions)

separated ions H+ + F- ~5%

(separated ions)

HCl → H+ + Cl-

~ 100%

HF ~ 95%

(molecules)

Acids

dissolve

Acids are

Molecular compounds

Electrolytes

- anion

+ cation

• Molecular compounds = Non-Electrolytes

• Soluble ionic compounds and strong

acids are called Strong Electrolytes

- Form ions in water

- Mobile Ions = conduct electricity

• Weak acids = Weak Electrolytes

- Small % molecules → ions

- Conduct electricity poorly

water is a molecular compound and a non-electrolyte

(remember H is an exception it acts like a non-metal)

tap water only conducts b/c

ions are dissolved in it

6

Strong Bases

+

cation OH-

any

cation

hydroxide

ion

- Ionic compounds with hydroxide ion (OH-)

Examples: NaOH, LiOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2,

Acid-base neutralization reactions

acid + strong base → H2O (l) + salt

H+

- anion

+

cation

OH-

strong

base

acid

HCl (aq) + Ba(OH)2 (aq) → H2O (l) + BaCl2 (aq)

metathesis

reactions

ions change

partners

acid-base reactions sometimes require balancing

2 2

7

Weak Bases (molecular bases)

• Molecules that react with water to produce

OH-

Base + H2O → Base-H+ + OH-

NH3 (aq) + H2O (l) → NH4+ (aq) + OH- (aq)

Molecules that contain nitrogen are weak bases

Methylamine (CH3NH3), amphetamine, cocaine, or heroin

Free-base form is the form that has not accepted an H+ yet

Metathesis reactions that form a gas

acid + CO32- → CO2 (g) + H2O (l) + salt

acid + CN- → HCN (g) + salt

acid + S2- → H2S (g) + salt

strong base + NH4+ → NH3 (g) + H2O (l) + salt

Alka-seltzer

gas chamber

rotten eggs, swamps

cleaning solutions

You must memorize these reactions for the exam

acid + HSO3- or SO3

2- → SO2 (g) + H2O (l) + salt

preservative

8

If potassium cyanide is dissolved in water and hydrochloric acid is added a gas is formed. Write the molecular equation for this reaction.

How to write a complete molecular reaction Example: a metathesis reaction to form a gas

KCN (aq) + HCl (aq) → HCN (g) + salt

salt is the combination of the other ions from the metathesis reaction

salt = KCl check solubility rules = KCl is soluble written (aq)

+ KCl (aq) KCN (aq) + HCl (aq) → HCN (g)

HCl HNO3 HClO4 H2SO4

Hydrochloric Nitric Perchloric Sulfuric

Acid Acid Acid Acid

strong acids all soluble written (aq)

check solubility rules = KCN is soluble written as (aq)

memorize these

Strong acids

+ NaClO4 (aq) HClO4 (aq) + NaOH (aq) →

How to write a net ionic equation

Example: a neutralization reaction

Why is water described as liquid (l) not aqueous?

Pure substances - solids that precipitate, also substances

that would form separate liquid layer (oils), or are the

solvent itself are written as their physical state under

those conditions (solid, liquid, gas)

Write the molecular equation, total ionic equation, and net ionic equation

for the neutralization reaction of perchloric acid and sodium hydroxide.

Strong Electrolytes (strong acids and soluble

ionic compounds ) written as separate ions

(aq) in ionic equation

Molecular compounds including weak acids

are written as molecules – not as ions

H+(aq) + ClO4-(aq) + Na+(aq) + OH-(aq) → H2O(l) + Na+(aq) + ClO4

-(aq)

Complete Ionic Equation

H2O (l) + salt Molecular Equation

9

Net Ionic Equation from

Complete Ionic Equation

H+(aq) + ClO4-(aq) + Na+(aq) + OH-(aq) → H2O(l) + Na+(aq) + ClO4

-(aq)

H+ (aq) + OH- (aq) → H2O (l)

Complete Ionic Equation

Net Ionic Equation

cross out spectator ions

Spectator ions are ions are dissolved

(aq) on the reactants and products side

10

Formic acid (HCO2H) and sodium hydrogen sulfite

11

Sulfuric acid and barium cyanide

Chloric acid and strontium hydroxide

12

Oxidation-Reduction Reactions

• Oxidation is the loss of electrons

• Reduction is the gain of electrons

• They must always happen together

• Referred to as redox reactions

Oxidation-Reduction Reactions

4Na (s) + O2 (g) 2Na2O (s)

Na is oxidized O2 is reduced

Na is the reducing agent

O2 is the oxidizing agent

How can we tell O has gained / Na lost electrons?

Dissolve Na2O in water and the ions that form are 2Na+ (aq) and O2- (aq)

13

Oxidation-Reduction Reactions

2Ag+ (aq) + Cu (s) 2Ag (s) + Cu2+ (aq)

• Cu donates electrons to Ag+

• Cu is oxidized (loses electrons)

• Ag+ is reduced (gains electrons

Oxidation-Reduction Reactions with

molecular compounds

2C (s) + O2 (g) 2CO (g)

CO does not break into ions when dissolved in

water - it is a molecular not ionic compound

This type of reaction is considered an oxidation-

reduction reaction, based on the similarity of the

balanced reaction to a large number of reactions

where the product can be dissolved to yield O2-

ions

14

Concept of oxidation numbers

• Used to keeps track of electrons

• Allows fictional “charges” to be assigned

oxidation-reduction reactions identified

Electronegativity is a scale of

metallic/non-metallic character of elements

• Electronegativity scale runs 0-4

• Used to determine which of the elements

in a molecular compound will be thought of

as the metal and which the non-metal

• F has the highest EN

• Fr/Cs are the lowest

• H/B have the lowest EN of any non-metals

15

16

Oxidation numbers in

CO • O in CO is negative because it is more

electronegative than C

• if O is assigned a -2 charge as it forms in

ionic compounds the C must be +2

• these are the Oxidation Numbers aka

Oxidation States for O and C

Rules for the Assignment of Oxidation Numbers with additions from Professor Hicks

1. Oxidation numbers must add up to the charge on the molecule, formula unit, or ion.

2. The atoms of free elements have oxidation numbers of zero.

3. Metals in Groups 1A, 2A, and Al have +1, +2, and +3 oxidation numbers,

respectively. 4. H and F in compounds have +1 and -1 oxidation numbers,

respectively.

5. Oxygen has a -2 oxidation number. Except in peroxides where it is -1

6. Group 7A elements have a -1 oxidation number.

7. Group 6A elements have a -2 oxidation number.

8. Group 5A elements have a -3 oxidation number.

9. When there is a conflict between two of these rules or an ambiguity in assigning

an oxidation number, apply the rule with the lower number and ignore the

conflicting rule.

10. H is +1 with any other non-metal except Boron (H+ in most molecular compounds)

11. H is -1 with any metal or Boron (H- in all ionic compounds and B/H compounds)

NaH, LiH, NaBH4, etc.

The negative ion of hydrogen ,H-, is called the hydride ion.

17

18

19

a) O22- b)O2

- c) NH4+

a)BH3 b) LiH d) LiMgH3

20

21

O2 + 2 F2 2OF2

22

Combustion Reactions are Redox Reactions

Assign all the oxidation states below

C8H18 + 12½ O2 8CO2 + 9H2O

What was oxidized?

What was reduced?

C8H18 + 9 F2 C8F18 + 9H2

ement(s) are being are being

What was oxidized?

What was reduced?

23

Combustion Reactions are Redox Reactions

Assign all the oxidation states below

C2H4 + H2 C2H6

What was oxidized?

What was reduced?

Combustion Reactions are Redox Reactions

Assign all the average oxidation states below

C6H12O6 + 6O2 6CO2 + 6H2O

What was oxidized?

What was reduced?

The reaction above is the combustion of glucose.

It is the same reaction referred to as respiration

performed in a living organism. The reverse of this

reaction is photosynthesis.

24

Combustion Reactions are Redox Reactions

Assign all the average oxidation states below

C6H12O6 + ½ F2 C6H11O6F + ½ H2

What was oxidized?

What was reduced?

S based respiration/photosynthesis!

Assign all the oxidation states below

S + H2 H2S

What was oxidized?

What was reduced?

Instead of reducing CO2 to C6H12O6

(photosynthesis) purple bacteria reduce S

to H2S to get energy.