Chemistry Exam Revision - Redox & Electrochemical Series
- Deﬁne oxidation and reduction in term of loss and gain of oxygen; loss and gain of electrons and change in oxidation state (number).
• Oxidation is the loss of electrons; an increase in oxidation state or a gain in oxygen by a molecule, atom, or ion. • Reduction is the gain of electrons; a decrease in oxidation state or a loss of oxygen by a molecule, atom, or ion. • OIL RIG (Oxidation is Loss, Reduction is Gain) - Rules for assigning the oxidation numbers of an atom.
• • • • • • • • • • •
The cation (+) is written ﬁrst, followed by the anion (-). Eg. NaH (H is H-) HCl (H is H+) A free element is always 0. Eg. He, N2 A monatomic ions equals the charge of the ion. Eg. N3- is -3 Hydrogen is usually +1. Oxygen in compounds are usually -2. Group IA (ﬁrst column) in a compound is +1. Eg. H, Li, Na Group IIA (second column) in a compound is +2. Eg. Be, Mg, Ca Group VIIA (second last column) in a compound is -1. Eg. F, Cl, Br, I Sum of the oxidation numbers in a neutral compound is 0. Sum of the oxidation numbers in a polyatomic ion equals its charge. Eg. SO42- is -2 || NO3 = -1 || SO4 = -2 ||
- Balance equations using oxidation numbers.
1. Assign oxidation numbers to all the atoms in the reactants and products. 2. Identify which atoms have changed their oxidation number. Note what has been oxidised and to what and what has been reduced and to what. 3. Make the number of atoms that change oxidation number the same on both sides of the equation by inserting temporary coefﬁcients. 4. Calculate the total change in oxidation number for the oxidation and the reduction. Make the total increase in oxidation number equal the total decrease in the oxidation for these two processes. Multiply through by appropriate factors. 5. Balance the remainder by inspection.
+5 ! " ! Zn: ∆ of +2 - Oxidation N: ∆ of -1 - Reduction
Zn + HNO3 -> Zn(NO3) + NO2 + H2O 0 +2
1Zn : 2N !
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