Oxidation-Reduction Reactions

Transcription

1 Oxidation-Reduction Reactions Chapter 4 Section 9 & 10

2 Oxidation Reduction Reactions Reactions in which one or more electrons are transferred. That means elements change their charge from reactants to products. Most reactions that produce energy are redox Oxidation States Provide a way to keep track of electrons in oxidation reduction reaction. Imaginary charges in covalent compounds

3 Assigning Oxidation States Element is 0 Monatomic ion is the charge of ion Fluorine is -1; Oxygen is -2; Hydrogen is +1 Except peroxide, O 2 2-, where Oxygen is -1 Sum of oxidation states in a molecule must equal zero (electrons can be fractional) Charges are n+ or n Oxidation States are +n or -n

4 Assigning Oxidation States Examples What is the oxidation state of carbon? C O 2 = 0 (1 x C) + (2 x -2) = = 0 C = +4

5 Assigning Oxidation States Examples What is the oxidation state of nitrogen? N 1- O 3 = 1- (1 x N) + (3 x -2) = = 1- N = +5 Try p 174 #67

6 Assigning Oxidation States Examples What is the oxidation state of iron? Fe 3 O 4 = 0 (3 x Fe) + (4 x -2) = = 0 Fe = +8/3 Can oxidation states be fractional? Yep.

7 Practice Problems Try page 174 #70 d, g, k H 2 O 2 PbSO 4 (NH 4 ) 2 Ce(SO 4 ) 3

8 Redox Reactions In reduction the element goes from a higher charge to a lower charge. Cl 2 0 in reactant to Cl -1 in the product In oxidation the element goes from a lower charge to a higher charge. Na 0 in the reactant to Na +1 in the product

9 Redox Reactions Ionic Example Na (s) + Cl 2 (g) Na Cl (aq) OXIDATION Na +1 + Cl -1 Na 0 Na e Loss of Electrons is Oxidation 0 {

10 { Redox Reactions half reactions Na (s) + Cl 2 (g) Na Cl (aq) REDUCTION Na +1 + Cl -1 2e - + Cl 2 0 2Cl Gain of Electrons is Reduction

11 Half Reactions preview to balancing 22Na [ 2Na ] 2 + Cl 2 2Cl - 2Na (s) + Cl 2 (g) 2Na + (aq) + 2Cl - (aq) Balance the charge of the half reactions. Then put them back together.

12 Redox Reactions Molecular Example CH 4 (g) + O 2 (g) 2 CO 2 (g) + H 2 O (aq) 4H + + 4e - + O H 2 O -2 REDUCTION Since the reaction takes place in an acidic solution, H + can be added wherever needed. Gain of Electrons is Reduction

13 Redox Reactions half reactions CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (aq) 2H 2 O + C -4 H 4 C +4 O 2 + 8e - + 4H + Since the reaction takes place in solution, H 2 O can be added wherever needed. OXIDATION Loss of Electrons is Oxidation

14 Redox Reactions half reactions CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (aq) 2H 2 O + C -4 H 4 C +4 O 2 + 8e - + 8H + OXIDATION If the O was S then you would have to include the S 2 (O 2 ) in both half reactions. Loss of Electrons is Oxidation

15 Half Reactions preview to balancing 2 2 [ 8H 4H + + 8e 4e OO 2H ] 2 4H 2 O 2H 2 O + CH 4 CO 2 + 8e - + 8H + CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (aq) Balance the charge of the half reactions. Then put them back together.

16 Agents Oxidizing Agent Causes something else to oxidize The thing that gets reduced In the example O 2 is the agent of oxidation because oxygen is reduced from 0 to -2. Reducing Agent Causes something to reduce The thing that is oxidized In the example CH 4 is the reducing agent because carbon is oxidized from -4 to +4

17 Agents Use the complete formula as the agent even through only one element changes Agents are always reactants Let s look back at the examples.

18 Half Reactions finding agents 22Na [ 2Na ] 2 + Cl 2 2Cl - 2Na (s) + Cl 2 (g) 2Na + (aq) + 2Cl - (aq) Sodium is oxidized; which makes it the reducing agent. Chlorine is reduced; which makes it the oxidizing agent.

19 Half Reactions finding agents 2 2 [ 8H 4H + + 8e 4e OO 2H ] 2 4H 2 O H 2 O + CH 4 CO 2 + 8e - + 8H + CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (aq) -2 Oxygen is being reduced; which makes oxygen the oxidizing agent. Carbon is being oxidized; which make methane the reducing agent.

20 Agents diagram from page 159 M X e - M + X - loss of e - OXIDIZED REDUCED gain of e - electron donor Oxidation state increases Reducing agent Oxidation state decreases Oxidizing agent electron acceptor

21

22 LEO GER

23 OIL RIG

24 Practice Problems Try page 174 #72 b,c,d 2AgNO 3 + Cu Cu(NO 3 ) 2 + 2Ag HCl + NH 3 NH 4 Cl SiCl 4 + 2H 2 O 4HCl + SiO 2

25 Balancing Redox Reactions - Acidic 1. Write the oxidation half reaction reaction separate from the reduction half reaction. 2. For each half reaction 1. Balance all elements except H & O 2. Balance O using H 2 O 3. Balance H using H + 4. Balance charges with e - 3. If necessary multiply each reaction by an integer to make the number of e - equal. 4. Combine half reactions, cancel identical species. 5. Double check elements and charges.

26 Practice Problems Sample Exercise 4.19 on page 165 H + + Cr 2 O C 2 H 5 OH Cr 3+ + CO 2 + H 2 O

27 Balancing Redox Reactions - Basic 1. Work as if H + were present like an acidic example 2. On each side add OH - to equal H + 3. Form H 2 O from OH - and H +

28 Practice Problems Sample Exercise 4.20 on page 167 Ag + CN - + O 2 Ag(CN) 2 -

29 Practice Problems Try page #73c, 75b As 2 O 3 + NO 3 1- H 3 AsO 4 + NO Mn 2+ + NaBiO 3 Bi 3+ + MnO 4 1-