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Lab manual_6blank - experiment 3 The reaction between Cu 2...

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UOIT Chem. 1800U, W10; Exp. 6-7.9 124 Experiment 6 - Electrochemistry Name: ____________________________ Partner ____________________________ Day: ____________________________ Time: ____________________________ TA Name: ____________________________
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UOIT Chem. 1800U, W10; Exp. 6-7.9 125 Results, Discussion and Questions Part I Complete the following table. Anode half-reaction Cathode half-reaction Exp. Voltage Theor. Voltage 1 2 3 4 5 6 Part II Voltage before addition of NH 3 ____________________ Voltage after addition of NH 3 ____________________
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UOIT Chem. 1800U, W10; Exp. 6-7.9 126 Questions 1. Why is no voltage observed before the porous cup is immersed in the glass electrode jar? 2. If a voltaic cell were constructed with two copper half cells and the concentrations of Cu 2+ were the same in each half-cell, what would the measured voltage be? If the Cu 2+ concentration in the anode half cell were 0.0020 mol L -1 and Cu 2+ concentration in the cathode half cell were 0.20 mol L -1 , what would the observed voltage be (assume T= 298.15 K)? Why is it not practical to perform this experiment with the voltmeters used in this
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Unformatted text preview: experiment? 3. The reaction between Cu 2+ and NH 3 is: Cu 2+ (aq) + 4NH 3 (aq) g Cu(NH 3 ) 4 2+ (aq) (6.14) For which an equilibrium constant can be written: (6.15) K Cu NH Cu NH f = + + [ ( ) ] [ ][ ] 3 4 2 2 3 4 From the observed voltage in part II (after adding NH 3 ) and the Nernst Equation: (6.16) E E RT nF Cu Cu cell cell cup jar =-+ + 2 2 ln [ ] [ ] calculate the concentration of “free” Cu 2+ (aq) ([Cu 2+ ] cup ) in the porous cup (assume T = 298.15 K). This quantity should be very small. Estimate the moles of Cu(NH 3 ) 4 2+ formed, assuming all the original Cu 2+ in the porous cup reacted to form the Cu(NH 3 ) 4 2+ . Because [Cu 2+ ] cup should be very small this is a reasonable approximation. Calculate the [Cu(NH 3 ) 4 2+ ] in the porous cup. UOIT Chem. 1800U, W10; Exp. 6-7.9 127 Assuming the concentration of ammonia in the porous cup is 1.1 mol L-1 , calculate the equilibrium constant, K f ....
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