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Part 1 of 1
An oxidation state (or oxidation number) is a formal charge assigned to an atom in a molecule or polyatomic ion, reflecting the hypothetical charge it would carry if all bonds to different elements were 100% ionic.
Standard rules for determining oxidation states: 1. Uncombined free elements carry an oxidation number of 0 (e.g. O2, N2, H2, Zn, Fe, S8 all have oxidation state 0). 2. Monatomic ions have an oxidation state equal to their ionic charge (e.g. Na^+ is +1, Cl^- is -1, Fe^3+ is +3). 3. Group 1 alkali metals always have oxidation state +1 in compounds; Group 2 alkaline earth metals always have +2. 4. Fluorine is the most electronegative element and always has oxidation state -1 in compounds. 5. Hydrogen is typically +1 when bonded to non-metals (e.g. HCl, H2O, CH4), but is -1 in metal hydrides (e.g. NaH, CaH2). 6. Oxygen is almost universally -2 in compounds (e.g. H2O, CO2, SO4^2-), with exceptions in peroxides such as H2O2 where oxygen is -1, and in OF2 where oxygen is +2. 7. In a neutral molecule, the algebraic sum of all oxidation numbers must equal 0. In a polyatomic ion, the sum must equal the net charge of the ion.
Calculating unknown oxidation states: - In potassium permanganate (KMnO4): K is +1, each oxygen is -2 (-8 total). To sum to 0: (+1) + Mn + 4(-2) = 0, which yields Mn = +7. - In sulfuric acid (H2SO4): Each H is +1 (+2 total), each oxygen is -2 (-8 total). To sum to 0: 2(+1) + S + 4(-2) = 0, which yields S = +6.
Worked example
Step 1: Calculate the oxidation state of sulfur in sulfurous acid (H2SO3) and sulfate ion (SO4^2-). Step 2: For H2SO3 (neutral compound): 2(H) + S + 3(O) = 0. Substitute H = +1 and O = -2: 2(+1) + S + 3(-2) = 0 implies 2 + S - 6 = 0, yielding S = +4. Step 3: For SO4^2- (polyatomic ion with net charge -2): S + 4(O) = -2. Substitute O = -2: S + 4(-2) = -2 implies S - 8 = -2, yielding S = +6. Step 4: Confirm: Sulfur has oxidation state +4 in H2SO3 and +6 in SO4^2-.
For the exam
1.The oxidation number of manganese in potassium permanganate (KMnO4) is +7.
2.In pure molecular nitrogen (N2), each nitrogen atom carries an oxidation state of -3.
3.The oxidation state of sulfur in sulfuric acid (H2SO4) is equal to +6.
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