Oxidation-Reduction Reactions

1. Oxidation-Reduction Reactions

2. Oxidation • Reactions in which the atoms or ions of an element attain a morepositive (or less negative) oxidation state are oxidation processes. • A species that undergoes a change in oxidation state in a positive direction is said to be oxidized. • Loss of electrons = oxidation

3. Reduction • Reactions in which the atoms or ions of an element attain a morenegative (or less positive) oxidation state are reduction processes. • A species that undergoes a change in oxidation state in a negative direction is said to be reduced. • Gain of electrons = reduction

4. Mnemonic Devices • Leo the Lion goes “ger” Lose electrons = oxidation Gain electrons = reduction • Oil Rig Oxidation is loss (of electrons) Reduction is gain (of electrons)

5. Oxidation-Reduction Reactions • A chemical process in which elements undergo a changeintheiroxidation number. • Called “redox” reactions for short. • Combines the two processes of reduction and oxidation into one equation.

6. Oxidation Numbers • We assign oxidation numbers or oxidation states to atoms and ions within compounds to help us “see” the movement ofelectrons when we look at the species within a redox reaction. • We have rules for assigning these numbers.

7. Assigning Oxidation Statesp. 655 • The oxidation state of an atom in an uncombined element is 0. • The oxidation state of a monatomic ion is the same as its charge. • Oxygen is assigned an oxidation state of -2 in most of its covalent compounds. (Important exception: peroxides; in these oxygen has an oxidation state of -1.) • In its covalent compounds with nonmetals, hydrogen is assigned an oxidation state of +1. (When hydrogen combines with metals in a hydride, the hydrogen has an oxidation state of -1.) • In binary covalent compounds (2 nonmetals), the element with the greater electronegativity is assigned a negative oxidation state equal to its charge as an anion in its ionic compounds. • For an electrically neutral compound, the sum of the oxidation states must be zero. • For an ionic species, the sum of the oxidation states must equal the overall charge.

8. Redox Reactions 2 Na (s) + Cl2 (g)  2 NaCl (aq) 0 0 +1 -1 Na goes from 0 to +1 (hence gives away 1 electron) so it is oxidized Cl goes from 0 to -1 (hence gets 1 electron) so it is reduced

9. Half Reactions • Half Reactions divide a redox reaction into two parts: one that shows oxidation and one that shows reduction • Half Reactions also show the transfer of electrons. • Oxidation Half Reaction shows loss of electrons so they are shown as products. • Reduction Half Reaction shows gain of electrons so they are shown as reactants.

10. Half Reactions for synthesis of table salt 2 Na (s) + Cl2 (g)  2 NaCl (aq) 0 0 +1 -1 Ox. H.R. : 2 Na  2 Na+1 + 2 e- Red. H.R.:Cl2 + 2 e-  2 Cl-

11. Terminology • The substance that loses electrons is said to be oxidized; we can also call it the reducing agent (because it causes the other substance in the equation to be reduced). • The substance that gains electrons is said to be reduced; we can also call it the oxidizing agent (because it causes the other substance to be oxidized).

12. 2 Na (s) + Cl2 (g)  2 NaCl (aq) 0 0 +1 -1 Ox. H.R. : 2 Na  2 Na+1 + 2 e- Red. H.R.:Cl2 + 2 e-  2 Cl- So, in our example above, sodium is oxidized and is the reducing agent; chlorine is reduced and is the oxidizing agent.

13. Practice 3 Mg (s) + Al2(SO4)3(aq) 2 Al (s) + 3 MgSO4(aq) 0 3+ 6+ 2- 0 2+ 6+ 2- Mg: 0 to 2+ Al: 3+ to 0 S and O: no change loses e-, so oxidized gains e- , so reduced Ox. H.R. : 3 Mg0 3 Mg2+ + 6 e- Red. H.R.: 2 Al3+ + 6 e-  2 Al0 So, Mgis oxidized and is the reducingagent; Alis reduced and is the oxidizingagent.