Ch. 20 Oxidation-Reduction Reactions

Types of Reactions There are many different types of reactions: 1.Redox 2.Acid-Base 3.Precipitation

20.1 Oxidation and Reduction Oxidation- Reduction Reaction: reaction in which electrons are transferred from one atom to another AKA- Redox Reaction

Oxidation The roaring fire shown to the right is an example of the rapid oxidization of the hydrocarbons making up the wood and the reduction of the Oxygen gas from the air.

The, very rusty, Iron hammer is also being oxidized by the Oxygen in the air, but at a much slower rate than the burning wood.

How do Oxidation and Reduction Differ? Oxidation- loss of electrons OIL – Oxidation is Loss Ex: Na → Na + + e -

Reduction Reduction- gain of electrons RIG – Reduction is Gain Ex: Cl + e - → Cl -

OIL RIG Oxidation Is Loss Reduction Is Gain

Changes in Oxidation number Oxidation number – is the number of electrons lost or gained by the atom when it forms ions. When an atom or ion is reduced, the oxidation number is lowered. Oxidation numbers keep track of the movement of electrons in a redox reaction.

Oxidation numbers are written with the positive or negative sign BEFORE the number. (+ 3, - 2) Ionic charge is written with the sign AFTER the number. (3+, 2-)

Oxidation # in Redox Reactions When an atom is oxidized, its oxidation number increases When an atom is reduced, its oxidation number decreases

Oxidation # in Redox Reactions The Magnesium has been oxidized. The Hydrogen has been reduced. The Chlorine stayed the same.

Applications of redox chemistry Removes tarnish from metal objects such as silver.

Chlorine bleach whitens clothes by oxidizing dyes, stains, and other materials that discolor clothes..

Hydrogen peroxide is an antiseptic because it oxidizes biomolecules of germs. It also lightens hair because it oxidizes the dark pigment of the hair.

OxiClean contains Hydrogen peroxide which is a strong oxidizing substance which will "bleach" the stains away.

Assigning Oxidation Numbers The oxidation number of an uncombined element is zero. Includes diatomic molecules, subscripts don’t matter. Examples: Fe: oxidation number = 0 H 2 : oxidation number = 0 S 8 : oxidation number = 0

The oxidation number of a monatomic ion is equal to the charge on the ion. Examples: Oxidation number of S 2- is -2. Oxidation number of Al 3+ is +3. Oxidation number of Ca 2+ is+2.

The oxidation number of the more electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion. Ex: SiCl 4 ; Cl is -1

Hydrogen (H) has two possible oxidation numbers: +1 when bonded to a nonmetal -1 when bonded to a metal The oxidation number of fluorine (F) is always -1

The sum of the oxidation numbers of all atoms in a neutral compound = 0. Multiply subscripts by oxidation numbers, then add. Ex: NaCl (+1) + (-1) = 0 CaBr 2 (+2) + 2(-1) = 0

The sum of the oxidation numbers of all atoms in a polyatomic ion = the charge on the polyatomic ion (look on polyatomic sheet). Multiply subscripts by oxidation numbers, then add. Ex: NH 4 +1 (-3) + 4(+1) = +1

Assigning Oxidation Numbers Let's start with Carbon Dioxide, CO Neutral compound charge = 0 2. Oxygen gets its standard charge of -2 for each oxygen atom. 3. In order to tally to zero, we MUST assign Carbon the oxidation number of +4.

Neutral compound charge = 0 2. Chlorine is the most electronegative element charge of -1 for each Chlorine atom. 3. both Hydrogen atoms are given an oxidation number of In order to tally to zero, we MUST assign Carbon the oxidation number of 0.

Polyatomic 1. This ion has a charge = Oxygen gets its standard charge of -2 for each oxygen atom. 3. In order to tally to zero, we MUST assign Sulfur the oxidation number of +6.

Oxidation Number in Redox Reactions 1. When an atom is oxidized, its oxidation number increases. 2. When an atom is reduced, its oxidation number decreases.

Acid-Base Reactions In an acid-base reaction, an acid and a base combine to produce a salt and water. Recall: all acids start with H

Example of Acid-Base Reaction HCl(aq) + NaOH (aq)  NaCl (aq) + H 2 O (l) Acid + Base  salt + water How to identify acid-base? All acids start with H All acid-base produce water as a product

Precipitation reactions In a precipitation reaction, two aqueous solutions are combined to form a solid precipitate, which settles out on standing. 2FeCl 3(aq) + Cu(SO 4 ) (aq)  Fe 2 (SO 4 ) 3 (aq) + 3CuCl 2(s) How to identify precipitation reaction There is a small (s) on the product side of the reaction

How to identify a precipitate if not told Use your solubility chart!!!!!!