HalfReactionBalancing

Add electrons to balance these half reactions a b no3

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Unformatted text preview: practice? Do more. 1. Add electrons to balance these half-reactions. a. NO3─ + 4 H+ b. H2O2 N O + 2 H2 O 2 H+ + O2 2. Balance these half-reactions. a. SO42─ + H+ b. H2S S8 + c. MnO2 + H+ Mn2+ + H2O d. NH4+ + H2O NO3─ + H+ SO2 + H2O H+ Balancing Half-Reactions By the CA-WHe Method In textbook and test problems, for redox reactions run in aqueous solutions, the H+, H2O, and electrons needed to balance a half-reaction often are not supplied. In those cases, you will need to add those terms to balance the half-reactions. You can construct half-reactions by using The CA-WHe! Method To balance redox half-reactions, 1. first balance the “central atom” (CA), usually one that is not O or H. Then, 2. add Water if needed to balance the oxygens. 3. Add H+ if needed to balance the hydrogens. 4. Add electrons to balance the charge. 5. Check that atoms and charge are the same on both sides. It helps to memorize: “To balance half-reactions, balance the central atom, then WHe!” Use the CA-WHe! method on the following example. ©2009 ChemReview.net v. k1 Page 382 Module 16: Half-Reaction Balancing Q. Balance this half-reaction. Cr2O72─ ***** Answer Cr3+ 1. First, balance the central atom (usually an atom that is not O or H). Cr2O72─ 2 Cr3+ 2. Add Water to balance oxygen atoms. Cr2O72─ 2 Cr3+ + 7 H2O 3. Add H+ to balance hydrogen. 14 H+ + Cr2O72─ 4. Add electrons to balance charge. 2 Cr3+ + 7 H2O 6 e─ + 14 H+ + Cr2O72─ 2 Cr3+ + 7 H2O 5. After balancing, check to see that the count for each kind of atom is the same on both sides and the total charge is the same on both sides. Check: 14 H, 2 Cr, 7 O on both sides; +6 charge on both sides. Balanced. Practice B Complete the balancing of these half-reactions, adding H+ and H2O if needed. Assume a particle is neutral if no charge is shown. Do the odd problems, checking your answers as you go. Do the evens during your next practice session. 1. ClO3─ Cl2 5. Br2 BrO4─ 2. PbSO4 Pb + SO42─ 6. Au Au3+ 3. SO2 7. H2O 4. MnO4─ SO42─ O2 MnO2 Balancing Half-Reactions That Include Hydroxide Ion (OH─) Some redox reactions require basic conditions for the desired reaction to take place. In these cases, the redox half-reactions may include OH─ ions instead H+ ions. The steps to balance these half-reactions are 1. First balance by the CA-WHe! method using H+ and H2O. 2. Then neutralize the H+: using the H+ coefficient, add that number of OH─ ions to both sides. This “neutralization of H+” will replace the number of H+ ions in the reaction ©2009 ChemReview.net v. k1 Page 383 Module 16: Half-Reaction Balancing with an equal number of H2O molecules, and add the same number of OH─ ions to the side opposite the original H+. 3. Adjust the H2O coefficients on both sides if needed. Use those steps to do the following problem. If you get stuck, read part of the answer below, then complete the problem. Q. Balance this half-reaction using OH─ ions instead H+. Cr2O72─ ***** 2 Cr3+ Answer 1. Balance the half-reaction using the CA-WHe! method. 6 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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