Chapter 4: Chemical Quantities & Aqueous Reactions CHE 123: General Chemistry I Dr. Jerome Williams, Ph.D. Saint Leo University

Overview Oxidation & Reduction Principles Oxidation Numbers Activity Series of Elements

Oxidation & Reduction Principles Oxidation is the loss of electrons. Reduction is the gain of electrons. Oxidation and reduction must occur together. They cannot exist alone. Recall definition using pneumonic “OIL RIG”.

Oxidation & Reduction Principles Oxidizing Agent is the species that undergoes reduction. Reducing Agent is the species that undergoes oxidation.

Oxidation Numbers Assigning Oxidation Numbers: All atoms have an “oxidation number” regardless of whether it carries an ionic charge. 1.An atom in its elemental state has an oxidation number of zero. 2. An atom in a monatomic ion has an oxidation number identical to its charge.

Oxidation Numbers 3. An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. – A. Hydrogen can be either +1 or –1. – B. Oxygen usually has an oxidation number of –2. In peroxides, oxygen is –1. – C. Halogens usually have an oxidation number of –1. When bonded to oxygen, chlorine, bromine, and iodine have positive oxidation numbers.

Oxidation Numbers 4.The sum of the oxidation numbers must be zero for a neutral compound and must be equal to the net charge for a polyatomic ion. – A. H 2 SO 4 2(+1) + (?) + 4(–2) = 0 net charge ? = 0 – 2(+1) – 4(–2) = +6 – B. ClO 4 – (?) + 4(–2) = –1 net charge ? = –1 – 4(–2) = +7

“Number Line” Analogy 5. Whenever one atom loses electrons (is oxidized), another atom must gain those electrons (be reduced). – A substance which loses electrons (oxidized) is called a reducing agent. Its oxidation number increases. – A substance which gains electrons (reduced) is called the oxidizing agent. Its oxidation number decreases.

Oxidation Numbers Assign oxidation numbers to each atom in the following substances: – A. CdSB. AlH 3 C. Na 2 Cr 2 O 7 D. SnCl 4 – E. CrO 3 F. VOCl 3 G. V 2 O 3 H. HNO 3 – I. FeSO 4 J. Fe 2 O 3 K. H 2 PO 4 L. MnO 4 – – M. Cr 2 O 7 2–

Oxidation Numbers For each of the following, identify which species is the reducing agent and which is the oxidizing agent. Ca(s) + 2 H + (aq)  Ca 2+ (aq) + H 2 (g) 2 Fe 2+ (aq) + Cl 2 (aq)  2 Fe 3+ (aq) + 2 Cl – (aq) SnO 2 (s) + 2 C(s)  Sn(s) + 2 CO(g) Sn 2+ (aq) + 2 Fe 3+ (aq)  Sn 4+ (aq) + 2 Fe 2+ (aq)