Ch. 21 - Chem 1036 Chapter 21 1 of 32 Chem 1036 Chapter 21...

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Chem 1036 Chapter 21 1 of 32 Chapter 21 Electrochemistry: Chemical Change and Electrical Work In this chapter, we will look at electron transfer reactions See Chapter 4 pages 150-157 for introduction to electron transfer reactions, oxidation-reduction reactions or redox reactions. Example: Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Write as two half-reactions : chemical equations describing the individual oxidation and reduction processes. Chem 1036 Chapter 21 2 of 32 Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Reduction Oxidation Ag + (aq) + e - Ag(s) Cu(s) Cu 2+ (aq) + 2e - Oxidizing agent: Reducing agent: Chem 1036 Chapter 21 3 of 32 Oxidation Numbers To keep track of electrons lost and gained, oxidation numbers are assigned to every atom in the reaction. Rules for Assigning Oxidation Numbers The oxidation number of an atom in the free (uncombined) element is 0 H 2 , O 2 , Cl 2 , Mg(s), P 4 In their compounds, Group 1 metals always have an oxidation number of +1. In their compounds, Group 2 metals always have an oxidation number of +2 In its compounds, fluorine is always -1 In its compounds, hydrogen is usually +1 (H is –1 in some compounds with metals) In its compounds, oxygen is usually –2 (can also be –1 in O 2 2- or –0.5 in O 2 - ) In their binary (two-element) compounds with metals, Group 15 is –3, Group 16 is –2 and Group 17 is –1. The total of the oxidation numbers of all the atoms in a molecule is 0. For an ion this total is equal to the charge on the ion. Chem 1036 Chapter 21 4 of 32 KMnO 4 C 2 O 4 2- Steps to Balancing Redox Reactions Oxidation and reduction half reactions combine so the (1) atoms balance, (2) charge balances, and (3) electrons cancel. 1. Assign all oxidation numbers and determine what elements are being oxidized and reduced. 2. Write separate half-reactions for oxidation and reduction 3. Insure that the element being oxidized or reduced is balanced. 4. Multiply reaction(s) by constant(s) so reduction electrons cancel oxidation electrons 5. Add the reactions together. 6. Balance the remaining elements: Acidic solution : a. Balance oxygen by adding H 2 O molecules b. Balance hydrogen by adding H + Basic solution : a. Balance the charge by adding OH - b. Balance hydrogen and oxygen by adding H 2 O rinted with FinePrint - purchase at www.fineprint.com
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Chem 1036 Chapter 21 5 of 32 Example Problem : Balance the following redox reaction, first in acidic solution and then in basic solution. Identify the reducing and oxidizing agents. Cr 2 O 7 2- + SO 2 Cr 3+ + SO 4 2- First, balance in acidic solution: 1. Assign all oxidation numbers: Cr 2 O 7 2- + SO 2 Cr 3+ + SO 4 2- 2. and 3. Write separate half-reactions and balance the element being oxidized or reduced: Oxidation: Reduction: Oxidizing agent: Reducing agent: Chem 1036 Chapter 21 6 of 32 4. Multiply reaction(s) by constant(s) so reduction electrons cancel oxidation electrons Oxidation: Reduction:
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Ch. 21 - Chem 1036 Chapter 21 1 of 32 Chem 1036 Chapter 21...

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