Solution E is aqueous copper(II) sulfate. In this experiment you pass a current through solution E using two carbon electrodes and find the mass of metal de...
Question 1 Report
Solution E is aqueous copper(II) sulfate. In this experiment you pass a current through solution E using two carbon electrodes and find the mass of metal deposited.
Clean both carbon electrodes with emery paper, wash them with distilled water, dry them and weigh the one you will use as the negative electrode. Pour 100 cm3 of solution E into a beaker. Clamp the two electrodes so that they dip about 4 cm into the solution without touching. Connect them through an ammeter to the power supply, close the switch and start the stopclock at once. Keep the current steady for fifteen minutes, then switch off. Lift out the negative electrode, rinse it gently with distilled water, dry it in a warm oven, let it cool and weigh it again.
Table 5.1
| measurement | value |
|---|---|
| mass of negative electrode before the experiment / g | |
| current / A | |
| time for which the current passed / s | |
| mass of negative electrode after washing and drying / g | |
| increase in mass / g |
(a) Record in Table 5.1 the mass of the clean dry negative electrode before the experiment. [1]
(b) Record in Table 5.1 the current shown on the ammeter and the time for which the current passed. [2]
(c) Record in Table 5.1 the mass of the negative electrode after it has been washed, dried and cooled. [1]
(d) Use your readings to work out the increase in mass of the negative electrode. [1]
(e) Record your observations at the positive electrode during the fifteen minutes. [2]
(f) Describe the test you would use on the gas collected from the positive electrode, and give the result you expect. [2]
(g) Record what happens to the blue colour of solution E during the experiment, and state which ion is being removed. [2]
(h) Suggest why the negative electrode is rinsed with distilled water and dried before the second weighing. [2]
(i) Plan how you would use the same apparatus to find out whether doubling the current doubles the mass of metal deposited in fifteen minutes. [3]
Answer Details
This is an electrolysis experiment: a current is passed through aqueous copper(II) sulfate using carbon electrodes, copper deposits on the negative electrode (cathode) and oxygen is released at the positive electrode (anode). The mass gained by the cathode is measured.
(a) Starting mass of the negative electrode [1]. Record it to 0.01 g, a sensible value of about 2 g to 10 g [1] (for example 5.42 g).
(b) Current and time [2]. Record the current with the unit A, about 0.2 A to 1.0 A [1]; record the time as 900 s (15 minutes) [1].
(c) Mass after washing and drying [1]. Record the second mass to the same precision and larger than the first [1] (for example 5.63 g), because copper has been added.
(d) Increase in mass [1]. Increase = final mass minus starting mass, correctly subtracted, a small positive value with unit g [1] (for example \(5.63 - 5.42 = 0.21\) g).
(e) Observations at the positive electrode [2]. Bubbles of a colourless gas form on the positive electrode [1]; the bubbling is steady, the gas has no smell and the carbon electrode may crumble slightly [1].
(f) Test for the gas [2]. Collect the gas in an inverted test tube held over the electrode and put a glowing splint into the mouth of the tube [1]; the splint relights, showing the gas is oxygen [1].
(g) The blue colour and the ion removed [2]. The blue colour becomes paler as the experiment goes on [1]; this is because the blue copper(II) ions, \(\text{Cu}^{2+}\), are being removed from the solution and deposited as copper [1].
(h) Why rinse and dry before reweighing [2]. Any solution left on the electrode would dry to leave solid copper(II) sulfate, which would be weighed along with the copper [1]; and water left on the electrode would itself add to the mass, so the increase would come out too large [1].
(i) Testing whether doubling the current doubles the mass [3]. Repeat the whole experiment with a fresh 100 cm3 portion of solution E and a freshly cleaned, dried and weighed electrode [1]; use the variable resistor to set the current at twice the first value while keeping the time at fifteen minutes and keeping the temperature, the depth of the electrodes and the concentration the same [1]; compare the two increases in mass, repeating each current twice to check the results agree [1].
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