Monday, January 11th

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– oxidationCl2 + 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 +1

Oxidation: I− → IO3− + 6 e−

Reduction: Cl2 + 2 e− → 2 Cl−

Balancing Redox Reactions by the Half-Reaction Method

In this method, the reaction is broken down into two half-reactions, 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 Reactions1. 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) First balance elements other than H and O.

b) Add H2O where O is needed.

c) Add H+ where H is needed.

d) 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.

Balance the redox equation:

General ProcedureStep 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 18.1Balance the redox reaction in acidic solution:

Example 18.1 Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution

Balance the redox equation:

General ProcedureStep 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 18.2Balance the redox reaction in acidic solution:

Example 18.2 Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution

Balance the equation occurring in basic solution:

SolutionTo 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 18.3Balance the following redox reaction occurring in basic solution.

Example 18.3 Balancing Redox Reactions Occurring in Basic Solution

Homework

Review questions 1 – 8 due Wednesday Problem set 1 – due tomorrow Section 18.3 – due tomorrow