RecapPrecipitation Reactions:

ions combine to form insoluble products

Neutralization Reactions:H+ ions and OH- ions combine to form H2O

Next:

Oxidation-Reduction (Redox) Reactions:Atoms or ions donate or accept electrons

Redox Reaction

• Corrosion of a metal

battery terminal

iron rusting

metal

Oxygen,acid

Metal Corrosion

• metal loses electron

forms a cation

= metal is oxidized

AND another reactant gains electrons

= is reduced

Oxidation-Reduction Reactions

• An oxidation occurs when an atom or ion loses electrons.

• A reduction occurs when an atom or ion gains electrons.

• One cannot occur without the other.

Redox Reactions

• Oxidation-Reduction Reactions (Redox Reactions)– reactions that involve the transfer of electrons

between two reactants– an element in one reactant is oxidized while an

element in another reactant is reduced

Mg (s) + 2 H+ (aq) Mg2+ (aq) + H2 (g)

oxidized reduced

Redox Reactions

• Oxidation:– the loss of electrons

• chemical species becomes more positively charged

– the gain of oxygen– the loss of hydrogen

Redox Reactions

• Reduction:– the gain of electrons

• the chemical species becomes more negatively charged

– the gain of hydrogen– the loss of oxygen

Redox Reactions

• LEO:– Lose Electrons

Oxidation

• GER:– Gain Electrons

Reduction

GER

LEO

LEO says GER

Redox Reactions

Oil Rig

Oil : Oxidation Involves

Loss of e-

Rig : Reduction Involves

Gain of e-

Redox Reactions• Electrons are not explicitly shown in chemical equations.

• Oxidation Numbers are used to keep track of electrons gained and lost during redox reactions.

• Oxidation number – a hypothetical number assigned to an individual atom

present in a compound using a set of rules.

• May be positive, negative, or zero

Rules for Oxidation Numbers

• Oxidation numbers are always reported for individual atoms or ions not groups of atoms or ions!!!!!!!!!!!

• For an atom in its elemental form, the oxidation number is always zero.

– H2: oxidation # = 0 for each H atom

– Cu: oxidation number = 0

– Cl2: oxidation # = 0 for each Cl atom

Rules for Oxidation Numbers

• For any monoatomic ion:

oxidation # = charge on ion• K+ oxidation # = +1• Cl- oxidation # = -1• S2- oxidation # = -2

Rules for Oxidation Numbers• Group 1A Metal Cations:

Always +1

• Group 2A Metal Cations:

Always +2

• Hydrogen (H)

+1 when bonded to nonmetals

-1 when bonded to metals

Rules for Oxidation Numbers

• Oxygen (O)-1 in peroxides (O2

2-)

-2 in all other compounds

• Fluorine (F) always -1

Rules for Oxidation Numbers

• The sum of the oxidation numbers of all atoms in any chemical species (ion or neutral compound) is equal to the charge on that chemical species

H2O:

MgCl2:

MnO4-:

1 + 1 + (-2) = 0

2 + (-1) + (-1) = 0

7 + (-2) + (-2) + (-2) + (-2) = -1

Oxidation Numbers

• For many compounds, you will be able to directly apply the rules to determine the oxidation number of all atoms except for one.

– Use the last two rules to determine the oxidation number of that last element.

Practice determining oxidation numbers

Example: Determine the oxidation state of all elements in SO3.

Is it elemental?

Are any monoatomic ions present?

Which elements have rules?

Set up an equation to find the remaining oxidation number.

No

No

O = -2

S + 3(-2) = 0 S = +6

Oxidation Numbers

Example: Determine the oxidation number of Mn and O in MnO4

-.

Mn + 4(-2) = -1 so Mn = +7

Is it elemental?

Are any monoatomic ions present?

Which elements have rules?

Set up an equation to find the remaining oxidation number.

No

No

O = -2

Oxidation Numbers

Example: Determine the oxidation state of all elements in NaNO3

Is it elemental?

Are any monoatomic ions present?

Which elements have rules?

Set up an equation to find the remaining oxidation number.

No

Na+

Na = +1, O = -2

1 + N + 3(-2) = 0 N = +5

Practice

1) Determine the oxidation number of P in HPO42-.

2) Determine the oxidation state of all elements in Cr2O72-.

3) Determine the oxidation state of Sn in SnBr4.

Redox Reactions

• There are many different kinds of redox reactions.– Combustion

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

– Oxidation of Metals by Acids or Metal Salts

Cu (s) + H2SO4 (aq) CuSO4 (aq) + H2 (g)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• The reaction between a metal and an acid or between a metal and a metal salt is called a displacement reaction.– a reaction in which an element reacts with a

compound and displaces an element from that compound

A + BX AX + B

Redox ReactionsOxidation of Metals by Acids or Metal Salts

Examples:

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

Zn (s) + 2 HBr (aq) ZnBr2 (aq) + H2 (g)

Mn (s) + Pb(NO3)2 (aq) Mn(NO3)2 (aq) + Pb (s)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• How do you know if a redox reaction has occurred?

• You must examine the oxidation number of each of the elements present in the reactants and products.– If the oxidation number changes, then a redox

reaction has occurred.

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• When oxidation occurs:– Electrons are lost– Oxidation number increases

• When reduction occurs:– Electrons are gained– Oxidation number is reduced (decreases)

e-

e-

Redox ReactionsOxidation of Metals by Acids or Metal Salts

Mg(s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

0

-1

+2

-1

0+1

Oxidation # of Mg increased oxidation

Oxidation # of H+ ion reduced reduction

Redox ReactionsOxidation of Metals by Acids or Metal Salts

Example: Identify the element that has been oxidized and the one that has been reduced.

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

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• Find the oxidation numbers:

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

0 +1

-1 -1+2

0

oxidized

reducedZn is the reducing agent.HCl is the oxidizing agent.

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• Oxidizing agent:– The reactant that causes another reactant to be

oxidized– The reactant that contains the element that was

reduced

• Reducing agent:– The reactant that causes another reactant to be

reduced– The reactant that contains the element that was

oxidized.

Redox ReactionsOxidation of Metals by Acids or Metal Salts

Example: Identify the oxidizing and reducing agents in the following reaction.

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• You can also write a net ionic equation to describe a redox reaction:– Write all soluble strong electrolytes as ions– Eliminate the spectator ions

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

Zn (s) + 2 H+ (aq) + 2 Cl- (aq) Zn2+ (aq) + 2 Cl- (aq) + H2 (g)

Zn (s) + 2H+ (aq) Zn2+ (aq) + H2 (g)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

Example: Write the complete ionic and net ionic equations for the reaction. Which element is oxidized? What is the oxidizing agent?

2 Al (s) + 3 Ni(NO3)2 (aq) 2 Al(NO3)3 (aq) + 3 Ni (s)

2 Al (s) + 3 Ni2+(aq) + 6 NO3- (aq) 2 Al3+ (aq) +

6 NO3- (aq) + 3 Ni (s)

2 Al (s) + 3 Ni 2+(aq) 2 Al3+ (aq) + 3 Ni (s)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

2 Al (s) + 3 Ni(NO3)2 (aq) 2 Al(NO3)3 (aq) + 3 Ni (s)

0 +2-2

+3

+5 +5

-2 0

oxidized

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• Based on the previous equation, we wouldn’t want to store a solution of Ni(NO3)3 in an aluminum container.

– The aluminum container would react and dissolve!!!

• Metals differ in the ease with which they are oxidized.

– Al(s) is oxidized by Ni(NO3)3 (aq)

– Ag(s) is NOT oxidized by Ni(NO3)3 (aq)

Redox ReactionsOxidation of Metals by Acids or Metal Salts

• Activity series:– A list of metals arranged in order of

decreasing ease of oxidation– Used to predict whether a metal will react with

an acid or with a metal salt– See Table 4.5– Any metal on the list can be oxidized by, i.e.

will lose electrons to, the ions of a metal below it.

Activity Series

Activity series:Any metal on the list can be oxidized by, i.e. will lose electrons to, the ions of a metal below it.

Cu can lose electrons to Ag+