Chapter 19:

Oxidation-Reduction(Redox)Reactions

A clear example of oxidation

Oxidation and reduction reactions:what the gosh darn heck are they?

Oxidation: the LOSS of electrons

Reduction: the GAIN of electrons

Redox reactions what they mean!!!!1!

LEO the lion says GER! OIL RIG

Loss of Electrons is Oxidation.Gain of Electrons is Reduction.

Oxidation Is Loss of electrons.Reduction Is Gain of electrons.

Connection to Electrochemistry

RED CAT AN OX

REDuction occurs at the CAThode.OXidation occurs at the ANode.

Half Reactions• Show what happens to electrons in the two parts of

a redox reaction: the reduction and the oxidation• Steel hulls in ships have zinc blocks attached

because they oxidize and release electrons. These electrons get consumed by the steel and prevent corrosion.

Rules for Writing Half-Reactions Equations

1. The number of electrons gained must equal

the number of electrons lost.

2. The equation must balance with respect to

both atoms and charge.

(Charge and mass are conserved in redox equations.)

Assigning Oxidation Numbers

Write the half-reaction equations for the redox reaction

Mg (s) + Br2 (l) → MgBr2 (s)

• Step 1: Use oxidation numbers to determine what is oxidized and what is reduced

0 0 2+ 1-

Mg (s) + Br2 (l) → MgBr2 (s)

What happened to Mg?

Mg gained a positive charge so it lost electrons.

It is oxidized.

0 0 2+ 1-

Mg (s) + Br2 (l) → MgBr2 (s)

What happened to Br2?

Br2 got a negative charge so it gained electrons.

It is reduced.

• Step 2 : Write two separate equations: one showing oxidation and another showing reduction. Use coefficients to balance the number of atoms.

Mg → Mg2+

Br2 → 2Br-

Step 3: Make the charge balance in each equation by adding electrons.

Mg → Mg2+ + 2e-

Br2 + 2e- → 2Br-

Check:1. The number of electrons gained must

equal the number of electrons lost.

2. The equation must balance with respect to both atoms and charge.

Redox Reactions and Covalent Bonds

• When hydrogen burns in chlorine, a covalent bond forms from the sharing of two electrons.

• The pair of electrons is more strongly attracted to the chlorine atom because of its higher electronegativity.

2 2

0 0 +1 1

H + Cl 2HCl–

• Neither atom has totally lost or totally gained any electrons.

• Hydrogen has donated a share of its bonding electron to the chlorine but has not completely transferred that electron.

Objectives

• Explain what must be conserved in redox equations.

• Balance redox equations by using the half-reaction method.

• Relate chemical activity to oxidizing and reducing strength.

• Explain the concept of disproportionation.