Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Argyroneta on April 08, 2017, 10:57:04 AM
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We set up oxidation-reduction daniel cell experiment.
ZnlZn2+(aq)llCu2+(aq)lCu
Anode half cell reaction : Zn(s) :rarrow: Zn2+(aq) + 2e- εA° : 0.760V
Cathode half cell reaction : Cu2+(aq) + 2e- :rarrow: Cu(s) εA° : 0.340V
Total cell reaction : Zn(s) + Cu2+(aq) :rarrow: Zn2+(aq) + Cu(s) ε°cell : 1.100V
We connected the litmus paper and filter paper to the ends of wires respectively. Litmus paper was moistened with NaCl and filter paper was moistened with phenolphthalein + NaCl . We have taken up 0.1M Cu and Zn. At second replication we have taken up 0.01M Zn. We used NaCl as salt bridge. I have 2 question I couldn't solve .
1- Explain the reason of the color changes in litmus paper and in filter paper moistened with phenolphthalein solution (I didn't understand why voltage changed the colors of papers)
2- Does changing in molarity of Zn effect the voltage?
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Not clear to me how the experimental setup looked like. How is the charge transferred from the wires to the NaCl solution?
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Not clear to me how the experimental setup looked like. How is the charge transferred from the wires to the NaCl solution?
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Answer my question.
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[...] I didn't understand why voltage changed the colors of papers [...]
An electrochemical reaction happened that changed the pH locally.
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Answer my question.
We fastened the litmus paper to wires end and dripped NaCl solution on litmus paper firstly . At second stage , we fastened filter paper instead of litmus paper and dripped NaCl and phenolphthalein solution on it . I don't know how transferred , I have already been searching the reason.
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[...] I didn't understand why voltage changed the colors of papers [...]
An electrochemical reaction happened that changed the pH locally.
Can you explain the reason more clearly please . I need the reason of change color .
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Charge transfer through the phase boundary (solid/liquid in this case) requires an electrochemical reaction. In other words, there is an electrolysis going on on the wire surface.
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At the beginning of this thread, you wrote out two half-reactions, you know that electrons are being moved in these reactions (that's the way we model it anyway) and you call these reactions reduction and oxidation. What does all that mean?
You know the NaCl in water didn't change color with phenolphthalein, yet when the wire connected the wet paper you got a color change. Why? Why does phenolphthalein ever change color?