HalfReactionBalancing

Adjust trial coefficients by trial and error if

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Unformatted text preview: n both sides. Balanced. ***** Lesson 16C: Separating Redox Into Half-Reactions The fastest way to balance a redox reaction is often to separate the equation into two halfreactions. The steps are 1. Find the 2 atoms that change their oxidation numbers. 2. Write the 4 particles that contain those two atoms in two separate half-reactions, one above the other, with arrows lined up. 3. Balance and add the half-reactions. 4. Put the resulting trial coefficients back into the original equation. Adjust trial coefficients by trial and error if needed to balance atoms and charge. Use those steps on the following problem. Q. Add two half-reactions, adding H+ and H2O if needed, to balance this reaction. Br ─ + SO42 ─ SO2 + Br2 Try Steps 1 and 2 above, and then check your answer below. ***** 1. Find the two atoms that change their oxidation numbers. Each -1 Br─ + + 6 -2 SO42 ─ +4 -2 0 SO2 + Br2 Bromine and sulfur change their oxidation number. 2. Write the 4 particles containing those two atoms in two separate half-reactions. Br─ SO42 ─ Br2 SO 2 Now try Step 3 above, and then check your answer below. ***** ©2009 ChemReview.net v. k1 Page 390 Module 16: Half-Reaction Balancing 3. Balance each half-reaction. 2 Br─ 2 e─ + 4 H+ + SO42 ─ Br2 + 2 e─ SO2 + 2 H2O Since 2 electrons are in each, they can be added without multiplying either equation. 2 Br─ Br2 2 e─ + 4 H+ + SO42 ─ + 2 e─ SO2 + 2 H2O 2 Br─ + 4 H+ + SO42 ─ SO2 + Br2 + 2 H2O Try Step 4. ***** The unbalanced equation was Br ─ + SO42 ─ With H+, H2O, and trial coefficients: 2 B r ─ + 4 H+ + SO42 ─ SO2 + Br2 SO2 + Br2 + 2 H2O Check the balancing. 4 H, 2 Br, 1 S, 4 O and zero net charge on each side. Practice 1. Balance these by adding half-reactions. Add H+ ions and H2O if needed. a. Br─ + ClO4─ Br2 + Cl─ b. Ni + NO3─ Ni2+ + NH4+ c. S2─ + NO3─ S + NO 2. In each reaction above, identify the reactant that is the reducing agent. 3. In each reaction above, identify the reactant being reduced. ANSWERS 1a. Br─ + ClO4─ Br2 + Cl─ Steps 1 and 2: Break into half-reactions. Br─ ClO4─ Step 3: Balance both using CA-WHe. Br2 Cl─ 2 Br─ 8 e─ + 8H+ + ClO4─ Br2 + 2 e─ Cl─ + 4 H2O 8 2 Br─ 4 Br + 8 2 e─ 2 (4x) 8 e─ + 8H+ + ClO4─ Then add. Cl─ + 4 H2O (1x) 8 Br─ + 8H+ + ClO4─ ©2009 ChemReview.net v. k1 4 Br2 + Cl─ + 4 H2O Page 391 Module 16: Half-Reaction Balancing Step 4: Check: 8 Br, 8 H, 1 Cl, and 4 O atoms on each side; net ─1 charge on both sides. 1b. Ni + NO3─ Ni2+ + NH4+ Ni2+ NH4+ Steps 1 and 2: Break into half-reactions: Ni NO3─ Step 3: Balance each half-reaction: Ni ─ + 10H+ + NO ─ 8e 3 4 Ni Then add: 8 e─ + 10H+ + NO3─ 4 Ni2+ + 8 2 e NH ++ 3 H O 4 2 Ni2+ + 2 e─ NH4+ + 3 H2O (4x) (1x) 4 Ni2+ + NH4+ + 3 H2O 4 Ni + 10 H+ + NO3─ Step 4: Check: 4 Ni, 10 H, 1 N, 3 O atoms on both sides; +9 charge on both sides. S + NO 1c. S2─ + NO3─ Steps 1 and 2: Break into two half-reactions. Step 3: Use CA-WHe! to balance each. S2─ NO3─ S NO S2─ 3 e...
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This note was uploaded on 10/28/2010 for the course CHAP 12 taught by Professor Lebec during the Spring '10 term at Marlboro.

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