Monday, January

th
11
Grab a beige HW check sheet from the projector

Change the due date problem set 3 to 1/15

Take out your 18.1-18.2 notes – we will start by
supplementing them

Warm-up: For reactions #21-22 on the back of
Friday’s worksheet

Split the reaction into its half-reactions

Identify each reaction as oxidation or reduction
(should be easy with your answers from Friday!)
Oxidation–Reduction

Reactions where electrons are transferred from one
atom to another are called oxidation–reduction
reactions.
 Redox reactions for short

Atoms that lose electrons are being oxidized; atoms
that gain electrons are being reduced.
2 Na(s) + Cl2(g) → 2 Na+Cl–(s)
Na → Na+ + 1 e– oxidation
Cl2 + 2 e– → 2 Cl– reduction
Oxidation and Reduction

Oxidation is the process that occurs when
 the oxidation number of an element increases,
 an element loses electrons,
 a compound adds oxygen,
 a compound loses hydrogen, or
 a half-reaction has electrons as products.

Reduction is the process that occurs when
 the oxidation number of an element decreases,
 an element gains electrons,
 a compound loses oxygen,
 a compound gains hydrogen, or
 a half-reaction has electrons as reactants.
Half-Reactions

We generally split the redox reaction into two separate
half-reactions—a reaction just involving oxidation or
reduction.
 The oxidation half-reaction has electrons as products.
 The reduction half-reaction has electrons as reactants.
3 Cl2 + I− + 3H2O → 6 Cl− + IO3− + 6 H+
0
−1
+1 −2
−1
+5 −2
Oxidation: I− → IO3− + 6 e−
Reduction: Cl2 + 2 e− → 2 Cl−
+1
Balancing Redox Reactions by the
Half-Reaction Method


In this method, the reaction is broken down into two halfreactions, one for oxidation and another for reduction.
Each half-reaction includes electrons.
 Electrons go on the product side of the oxidation
half-reaction—loss of electrons.
 Electrons go on the reactant side of the reduction
half-reaction—gain of electrons.


Each half-reaction is balanced for its atoms.
Then the two half-reactions are adjusted so that the electrons
lost and gained will be equal when combined.
Balancing Redox Reactions
1. Assign oxidation states.
a)
Determine the element oxidized and the element reduced.
2. Write oxidation and reduction half-reactions, including
electrons.
a)
Oxidation electrons on right, and reduction electrons on left of
the arrow.
3. Balance half-reactions by mass.
a)
b)
c)
d)
First balance elements other than H and O.
Add H2O where O is needed.
Add H+ where H is needed.
If the reaction is done in a base, neutralize H+ with OH−.
4. Balance half-reactions by charge.
a)
Balance charge by adjusting electrons.
5. Balance electrons between half-reactions.
6. Add half-reactions
7. Check by counting atoms and total charge.
Example 18.1 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Balance the redox equation:
General Procedure
Step 1
Assign oxidation states to all atoms and identify the substances being oxidized
and reduced.
Step 2
Separate the overall reaction into two half-reactions: one for oxidation and
one for reduction.
Step 3
Balance each half-reaction with respect to mass in the following order:
• Balance all elements other than H and O.
• Balance O by adding H2O.
• Balance H by adding H+.
All elements are balanced, so proceed to the next step.
Example 18.1 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Continued
Step 4
Balance each half-reaction with respect to charge by adding electrons. (Make the sum
of the charges on both sides of the equation equal by adding as many electrons as
necessary.)
Step 5
Make the number of electrons in both half-reactions equal by multiplying one or both
half-reactions by a small whole number.
Step 6
Add the two half-reactions together, canceling electrons and other species as necessary.
Example 18.1 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Continued
Step 7 Verify that the reaction is balanced both with respect to mass and with respect
to charge.
For Practice 18.1
Balance the redox reaction in acidic solution:
Example 18.2 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Balance the redox equation:
General Procedure
Step 1
Assign oxidation states to all atoms and identify the substances being oxidized
and reduced.
Step 2
Separate the overall reaction into two half-reactions: one for oxidation and
one for reduction.
Step 3
Balance each half-reaction with respect to mass in the following order:
• Balance all elements other than H and O.
• Balance O by adding H2O.
• Balance H by adding H+.
Example 18.2 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Continued
All elements other than H and O are balanced so proceed to balance H and O.
Step 4
Balance each half-reaction with respect to charge by adding electrons. (Make
the sum of the charges on both sides of the equation equal by adding as many
electrons as necessary.)
Example 18.2 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Continued
Step 5 Make the number of electrons in both half-reactions equal by
multiplying one or both half-reactions by a small whole number.
Step 6 Add the two half-reactions together, canceling electrons and other species as
necessary.
Example 18.2 Half-Reaction Method of Balancing Aqueous Redox
Equations in Acidic Solution
Continued
Step 7 Verify that the reaction is balanced both with respect to mass and with
respect to charge.
For Practice 18.2
Balance the redox reaction in acidic solution:
Example 18.3 Balancing Redox Reactions Occurring in Basic
Solution
Balance the equation occurring in basic solution:
Solution
To balance redox reactions occurring in basic solution, follow the
half-reaction method outlined in Examples 18.1 and 18.2, but add
an extra step to neutralize the acid with OH– as shown in step 3.
1.Assign oxidation states.
2.Separate the overall reaction into two half-reactions.
Example 18.3 Balancing Redox Reactions Occurring in Basic
Solution
Continued
3. Balance each half-reaction with respect to mass.
• Balance all elements
other than H and O.
• Balance O by
adding H2O.
• Balance H by
adding H+.
• Neutralize H+ by
adding enough OH– to
neutralize each H+. Add
the same number of
OH– ions to each side
of the equation.
4. Balance each half-reaction with respect to charge.
Example 18.3 Balancing Redox Reactions Occurring in Basic
Solution
Continued
5. Make the number of electrons in both half-reactions equal.
6. Add the half-reactions together.
Example 18.3 Balancing Redox Reactions Occurring in Basic
Solution
Continued
7. Verify that the reaction is balanced.
For Practice 18.3
Balance the following redox reaction occurring in basic solution.
Homework

Review questions 1 – 8 due Wednesday

Problem set 1 – due tomorrow

Section 18.3 – due tomorrow