Redox Reactions. CH102, Lab 11 Goals : Safety : Waste :

Transcription

1 Redox Reactions CH102, Lab 11 Goals : Safety : Waste : To explore the relative oxidizing and reducing strength of metals. To measure electrochemical potentials and use the reduction potentials to rank reduction half-reactions. KMnO 4 is acidic, corrosive and a strong oxidizing agent. 3M HCl is acidic, corrosive and gives off irritating vapors. If contact occurs with the eyes or skin, rinse with lots of water. Some of the metals produce flammable hydrogen gas in water or acid. DO NOT tightly cap the waste container. The oxidizing solutions from part A1 should be rinsed into the waste container provided for part A1. The metals from part A2 should be rinsed with water, blotted dry and placed in the waste container provided. DO NOT tighten the cap. The liquids can be flushed down the sink. The solutions from part B of the experiment should be rinsed into the waste container for part B redox solutions.

2 Energy Prelab Assistance Redox reactions : transfer of electrons from one chemical species to another. Zn Cu 2+ spontaneous Zn 2+ Cu Zn : Cu 2+ : Cu 2+ + Zn Cu + Zn 2+ oxidation state Reducing agent with high energy donor electrons. Loses electrons. Gets oxidized. Goes up in oxidation state (0 +2) during reaction. Oxidizing agent with low energy acceptor orbitals. Gains electrons. Gets reduced. Goes down in oxidation state (+2 0) during reaction.

3 Half Reactions (Mini Balanced Reactions) 1 O x i d a t i o n 2 r e a c t i o n : Z n Z n e - R e d u c t i o n 1 r e a c t i o n : 2 C u e - C u n e t r e d o x r e a c t i o n : C u Z n C u + Z n 2 + O x 1 R e d 2 R e d 1 O x 2

4 Part A1 Experimental Overview Students will determine the relative strength of oxidizing agents Cu 2+, Mg 2+ and acidic MnO 4 1- Add 3 drops of each oxidizing agent to each well. Add 2 drops of H 2 O 2 to each oxidizing agent. Note if a reaction occurs. H 2 O 2 C u 2 + M g 2 + M n O C u 2 + N O 1 - M g 2 + N O K H + 3 M n O 4

5 Part A1 Experimental Overview If no reaction is observed, prepare another well of the oxidizing agent. Add 3 drops of KI solution to each oxidizing agent. Note if a reaction occurs. Oxidizing agents get reduced: Cu e - Cu Mg e - Mg MnO H e - Mn H 2 O Hint on ranking : H 2 O 2 is a weaker reducing agent than I 1-. An oxidizing agent that reacts with H 2 O 2 will also react with I 1-.

6 Part A2 Experimental Overview Students will determine the relative strength of reducing agents Cu, Mg, and Zn. Add ml of fresh tap water and 3 drops of phenolphthalein indicator to three 30 ml beakers. Drop the Cu, Mg and Zn metal strips into each beaker. Cu Mg Zn If a reaction occurs, the generated OH 1- will turn the indicator pink near the reaction: 2H 2 O + 2 e - H OH 1-

7 Part A2 Experimental Overview If no reaction occurs, remove the metal and blot dry. Prepare new beakers containing 10 ml of water and 10 ml of 3M HCl for the metals that did not react. * Add the unreacted metals and note any evidence of reaction with H 1+ to produce H 2. Reducing Agents get oxidized : Cu Cu e - Mg Mg e - Zn Zn e - Hint on ranking : H 2 O is a weaker oxidizing agent than H 1+. An oxidizing agent that reacts with H 2 O will also react with H 1+. * Safety tip : Always add acid to a beaker of water. The opposite arrangement may result in dangerous splashing.

8 Part B Experimental Overview Students will measure the cell potential (E cell ) of different redox couples versus a Cu 2+ /Cu half cell. 6 7 Ag + /Ag 8 Anode (reference) Oxidation expected Pb 2+ /Pb KNO Cu 2+ /Cu Cu 2+ /Cu Zn 2+ /Zn Other Electrodes as Cathode (studied half cell) Reduction expected

9 Part B Experimental Overview E cell = E cathode - E anode From standard reduction table: Cu e - Cu E º cell = V E cell = E cathode 0.34 V E cathode = E cell V Calculated reduction potential of studied ½ cell measured Cu/Cu 2+ reference as anode Please note that Cu 2+ /Cu is assigned the anode position. This means the voltmeter expects Cu to be the stronger reducing agent and oxidation to occur at the anode. If a negative voltage is measured, the cathode has the stronger reducing agent.

10 Example : Postlab Assistance Writing Net Redox Reactions Write the spontaneous net redox reaction that occurs when Al metal is added to acidic solution (containing H 1+ ). 1) Write the more positive ½ reaction as a reduction : 2 H e - H V 2) Write the more negative ½ reaction as an oxidation : Al Al e V 3) Multiply the coefficients of the reaction such that the electrons will cancel : 3 x step 1 6 H e - 3 H 2 2 x step 2 2 Al 2 Al e - 6 H Al 3 H Al 3+

11 Postlab Assistance Writing Net Redox Reactions 6 H Al 3 H Al 3+ WebAssign Input Example : 6 H^ Al -> 3 H2 + 2Al^3+ Ox 1 + Red 2 Red 1 + Ox 2 Oxidizing agent Species 1 Reducing agent Species 2 Reduced form of Species 1 Oxidized form of Species 2