Oxidation-reduction reactions Pgs 636 - 645 Oxidation-reduction reactions

Oxidation-Reduction Reactions Reactions that occur when electrons are transferred between atoms Also called Redox Reactions Oxidation = LOSS of electrons Reduction = GAIN of electrons LEO the lion goes GER

Oxidation – Reduction Reactions Reaction of sodium metal with chlorine gas: 2Na (s) + Cl2 (g)  2NaCl (s) What’s really happening in this reaction? Na Cl Na+Cl- Metals ALWAYS lose electrons to nonmetals  metals = ox, nonmetals = reduced Na+Cl- Each sodium gives-up an electron to the chlorine resulting in ions!

Practice!! In each of the following reactions, identify what is being oxidized, and what is being reduced? 2Mg (s) + O2 (g)  2MgO (s) 2Al (s) + 3I2 (s)  2AlI3 (s)

Can reactions between nonmetals be redox reactions? Yes, but it’s harder to see CH4 (g) + 2O2 (g)  CO2 (g) + H2O (g) + energy Use oxidation states to determine where the electrons are going!

Oxidation States Also called oxidation numbers Positive and negative numbers assigned to an INDIVIDUAL atom to help of keep track of electrons during redox reactions

Rules for Assigning Oxidation Numbers Oxidation number of a free element = zero Examples  HONClBrIF, solid metals (Na, Ag) Oxidation number of a monatomic ion is the same as it’s charge Examples  NaCl CaCl2 Oxygen’s oxidation number in compounds is -2, EXCEPT for peroxides where it is -1 Examples  O2 H2O H2O2

Rules for Assigning Oxidation Numbers Hydrogen’s oxidation number = +1 in covalent compounds Examples  H2O HI NH3 For a neutral compound, the sum of the oxidation states must be ZERO Examples  CuCl2 Fe2O3 For polyatomic ions, the sum of the oxidation numbers must equal the charge of the ion Examples  SO42-

Practice!! Assign oxidation numbers to each of the following atoms: SO3 N2O5 C2H6 SO32- PF3

What do we use oxidation numbers for? To determine what is being oxidized and what is being reduced in a redox reaction Example: Identify what is oxidized and reduced in the following reaction: HINT  Oxidation = INCREASE in oxidation # Reduction = DECREASE in oxidation # CH4 (g) + 2O2 (g)  CO2 (g) + H2O (g)

Practice!! Determine what is oxidized and reduced in the following reactions: Cu (s) + 2AgNO3 (aq)  Cu(NO3)2 + 2Ag Br2 (g) + 2NaF (aq)  2NaBr (aq) + F2 (g) CH3OH (g) + O2 (g)  CO2 (g) + H2O (g)

Two more terms to know… Reducing Agent electron donor What is oxidized in the reaction Oxidizing Agent  electron acceptor What is reduced in the reaction Identify the reducing agent and oxidizing agent in the previous 3 reactions: Cu (s) + 2AgNO3 (aq)  Cu(NO3)2 + 2Ag Br2 (g) + 2NaF (aq)  2NaBr (aq) + F2 (g) CH3OH (g) + O2 (g)  CO2 (g) + H2O (g)