Unit 5

ELECTROCHEMISTRY

REDOX REACTIONS

OXIDATION: _____________________REDUCTION: ____________________REDOX: _____________________________

EXAMPLE

SHOW, WITH ARROWS, THE LOSS AND GAINS OF ELECTRONS

OXIDIZING AGENT: ___________________REDUCING AGENT: ____________________

PRACTICE PROBLEMS, PAGE 467, #1-4

HALF REACTIONS

• Oxidation and reduction can be represented in separate equations.

• HALF REACTION: BALACEND EQUATION THAT SHOWS THE NUMBER OF ELECTRONS INVOLVED IN EITHER OXIDATION OR REDUCTION.

Example...

Half-Reactions are balanced using the smallest possible whole-number coefficients

• Example:Is this a proper half reaction?

Disproportionation Reactions

A single element undergoes both oxidation and reduction in the same reaction.

Example:

Practice Problems, 5-8

Section Review

SR: 1-5

10.2 – Oxidation Numbers

We have seen net ionic equations with monatomic elements (Cu, Zn) and single-element ions.

We used ionic charges to describe the transfer of electrons.

Many reactions involve reactants or products with covalent bonds cannot use ionic charges to describe the transfer of electrons.

• Oxidation numbers: actual/hypothetical charges, assigned using a set of rules.– Describe redox reactions with covalent reactants

or products.

Oxidation Numbers from Lewis Structures

• To assign oxidation numbers:– Consider all bonding electrons be be ‘owned’ by the more

electronegative oxygen atom. • H+ has oxidation number +1• Oxygen has a filled octet, so -2.

• THESE ARE NOT IONIC CHARGES!– In ionic charges, (-) or (+) written AFTER the value

Molecules with Atoms of Equal Electronegativity• Ex// Cl2 : both atoms equally electronegative.

Oxidation numbers: 0 for both.

Molecules with Atoms of Different Electronegativity• Ex// for CN-,– Oxidation number for C: +2– Oxidation number for N: -3

Oxidation Number Rules

Oxidation # of:H: +1 except when

with metal hydrides: -1

O: -2 except peroxides and the OF2 compound.

Compounds w/o H or O: same as ionic charge.

“Average” Oxidation Numbers

• Oxidation numbers are not always integers.• Example: Fe3O4 – The sum of oxidation numbers for the oxygen

atoms will be -8.– In order for the compound to be neutral, each iron

atom would have to have an oxidation number of +3/8.

Hints:1) Use the oxidation number rules (obviously!)

- These take precedence.2) Use a variable (x) to figure out the unknown

oxidation number.

CHECK PAGE 477 FOR ANSWERS.CARRY OUT PPs 9-11, 12a&b ON PAGE 478.

Applying Oxidation Numbers to Redox Reactions

• Determine the oxidation numbers of the elements within the following reaction.

• TO DO SO...– Determine the ionic equation (dissociate)– Eliminate any spectator ions to get the net ionic equation– Look at the net ionic equation and figure out the oxidation

numbers for all of the reactants and products.

Redox reactions can also be described as...

Oxidation: a(n) ___________________ in oxidation number.

Reduction: a(n) ________________ in oxidation number.

Hints: • oxidizing/reducing agents are REACTANT MOLECULES!• If compounds are ionic and aqueous, dissociate!

COMPLETE: PPs #13-15, page 480.SR: 1-6.

10.3: The Half-Reaction Method for Balancing Equations

• We have learned how to divide balanced equations into separate reduction and oxidation reactions.

• We will now learn how to combine half-reactions into balanced equations.

REVERSE!

Formation Reaction of potassium chloride:

Half reactions of this reaction:

Are the atoms and charges balanced?

Balancing half-reactions in is more complicated when in acidic or basic solutions.

- H+ and OH- need to be taken into account.

Balancing Half-Reactions for Acidic Solutions

COMPLETE PPs 17-20.

Balancing Half-Reactions for Basic Solutions

COMPLETE PPs 21-24.

Half-Reaction Method for Balancing Redox Reactions

• Adding two half reactions results in an ionic equation.– No electrons are created or destroyed in a redox

reaction.– Add two half-reactions so that the number of

electrons lost in oxidation equals the number of electrons gained through reduction.

Example:Mg(s) + Al(NO3)3

Original Half-reactions:

Multiply half-reactions:

Add results:Eliminate electrons:Include spectator Ions:

Steps for Balancing by the Half-Reaction Method

Half-Reaction Method for Balancing Redox Equations

PPs, page 491. #25-28.SRs, #1-2, 5, 6.

10.4 – Oxidation Number Method for Balancing Equations

• When using the oxidation numbers to balance a chemical equation:– The total increase in the oxidation numbers of the oxidized element(s)

equals the total decrease in oxidation numbers of the reduced element(s).

– Oxidation number of nitrogen increases from -3 to +4, therefore increasing by 7.

– The oxidation number of oxygen decreases from 0 to -2, a decrease of 2.

– Two nitrogen atoms must react for every seven oxygen atoms so that the total increase and decrease in oxygen numbers are both 14.

PPs, page 496 #29-32 SR, #s 1-4

Chapter 10 ReviewPage 499#s 1-7, 9-10, 12