Chemistry Forum

Hi everyone
In my lab, observe Energy production by metals and solutions.
-Take a strip of filter paper, and place it in a plastic dish.
- Place one small drop of copper sulfate (CuSO₄) solution at the very end of the filter paper then place a small piece of copper metal (a penny) and place it on the spot of copper sulfate solution.
-Then add one small drop of zinc sulfate (ZnSO₄) solution at the other end of the filter paper.Take a small piece of zinc metal (a washer) and place it on the spot of zinc nitrate solution.
- Turn on the voltmeter, then put one voltmeter probe on the copper, the second on the zinc, and record the voltage (if it fluctuates, guess at the average value). Then remove the voltmeter.
- Take a dropper bottle of sodium nitrate (NaNO₃) solution and squeeze out a thin strip of solution that connects the drops of copper sulfate and zinc nitrate solutions.
- Repeat the voltage measurement. If the reading is negative, reverse the order of your two probes on the metal pieces. This reading should be bigger than the initial reading.
(This is a bit tricky, however one should observe a voltage only when a current can travel between the voltmeter probes. Dry paper is not conductive. An ionic solution, such as sodium nitrate is conductive.)

* My explain :
Personal, I observed the voltmeter of penny and zinc metal is 0.007 v. The voltmeter of compound after addition of NaNO₃ is 0.015v.
1/ I think that Cu + CuSO₄ (can’t reaction) , Zn+ZnSO₄ ( also can’t reaction). Therefore, the voltmeter can measure between Cu or CuSO₄, Zn or ZnSO₄.
2/ After add NaNO₃ into this compound (above), Cu(NO₃)₂ and Zn(NO₃)₂ are high conductivity. That is why the voltmeter of this compound is bigger than the initial compound.
2NaNO₃ + CuSO₄ -> Cu(NO₃)₂ + Na₂SO₄
2NaNO₃ + ZnSO₄ -> Zn(NO₃)₂ + Na₂SO₄

P/S : I don't really know about my explanation. If you guys know about how explain it , PLEASE help me !! Thanks a lot

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jessica wrote:* My explain :
Personal, I observed the voltmeter of penny and zinc metal is 0.007 v. The voltmeter of compound after addition of NaNO₃ is 0.015v.
1/ I think that Cu + CuSO₄ (can’t reaction) , Zn+ZnSO₄ ( also can’t reaction). Therefore, the voltmeter can measure between Cu or CuSO₄, Zn or ZnSO₄.

Not quite. It is a question of circuit completion. How do electric charges pass from one electrode to the other? Paper is not very conductive.

jessica wrote:2/ After add NaNO₃ into this compound (above), Cu(NO₃)₂ and Zn(NO₃)₂ are high conductivity. That is why the voltmeter of this compound is bigger than the initial compound.

Not exactly. You've completed the circuit with an ionic solution.


As the zinc electrode releases cations into the solution around it, electrons are released into the circuit, passing through the voltameter and to the copper electrode. At this surface electrons will attract copper cations to deposite copper metal.

This is the source of the smaller initial current.

Thus cation concentration will increase around the zinc electrode and copper cation concentration around the coper electrode will decrease (relative to the sulfate anions). This charge imballance in the solutions will inhibit the reactions, swiftly reducing the voltage, unless there is a path by which ions can flow to equalize ionic charge concentrations.

The sodium nitrate solution provides this passage.

HI ! Can I ask how you know about Voltmeter of Zn and Cu . Thanks a lot

Zn(s) = Zn²⁺ + 2 e^- + 0.76V
Cu²⁺(aq) + 2 e^- = Cu(s) + 0.34V
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Zn(s) + Cu²⁺(aq) = Cu(s) + Zn²⁺(aq) + 1.10V

jessica wrote:HI ! Can I ask how you know about Voltmeter of Zn and Cu

It's a textbook case. Quite literally: it is a classic example.

In a pinch, search wikipedia. The keywords are: Galvanic cell, cell voltage, or Standard electrode potentials.

What practical device uses this observed change? (Hint: Mobile phone, car, flashlights all use this.) this question also relates with the lab

My answer : Electromotive force means the force which makes current continuously flows through a conductor. This force can be generated from power generator, battery, flashlight battery and fuel cell, etc. Is it true ??

Yes. Batteries are comprised of one or more voltaic cells connected, usually, in series.