GCSE · Chemistry · Edexcel · Spec 1CH0

Core Practical: electrolysis of copper sulfate solution with inert and copper electrodes

Same solution. Swap the electrodes and the result changes. By the end you'll know exactly why.

Copper electrodes: where does the copper go?

d.c. supplye⁻Copper sulfate solution−Cathode (−)Pure copper+Anode (+)Impure copper

Electrolyte: Copper sulfate solution (aqueous). Cathode (−), made of Pure copper, half-equation Cu²⁺ + 2e⁻ → Cu, product Pure copper, deposited on the cathode, observation Mass and size increase.. Anode (+), made of Impure copper, half-equation Copper atoms form Cu²⁺ ions that go into the solution, product Cu²⁺ ions in solution, observation Mass and size decrease.. Cu²⁺ (positive) migrates to the cathode. e⁻ in the external circuit: anode to cathode. static

Follow a Cu²⁺ from the anode side across to the cathode

Cathode (−)

Cu²⁺ + 2e⁻ → Cu

Product: Pure copper, deposited on the cathode

Observation: Mass and size increase.

Anode (+)

Copper atoms form Cu²⁺ ions that go into the solution

Product: Cu²⁺ ions in solution

Observation: Mass and size decrease.

An impure copper anode and a pure copper cathode in copper sulfate solution. The electrolyte has to contain the same Cu²⁺ ions as the metal being purified.

Exam line: With copper electrodes the anode gets smaller and the cathode gets bigger. With inert carbon electrodes in the same solution, copper forms on the cathode and oxygen forms at the anode.
Watch out: The electrolyte is the same in both cells, so don't assume the result is. You'll run the graphite cell next.

Chemistry · The apparatus

Build the cell
ElectrolytePower pack−+

Showing 4 layers: Beaker and electrolyte, Petri dish lid, Graphite electrodes, Leads and power pack

Layers

Explore

Tap a part to see its job

Red goes to positive, black goes to negative.

Tap a part to see what it is for. Peel the layers to see what sits under what.

Chemistry · Method

Run the practical in the right order

Put the inert-electrode method in the order you would actually do it

1 · First step7 · Last step

  1. Switch on and record any changes at each electrode in a table

  2. Measure 50 cm³ of electrolyte with a measuring cylinder and pour it into the beaker

  3. Once enough gas has collected, lift the tube, let the electrolyte out, then test the gas

  4. Put on safety glasses

  5. Fill a test tube with electrolyte, keep a finger over the end, invert it under the electrolyte and place it over an electrode

  6. Connect the electrodes to the power supply: red lead to positive (anode), black lead to negative (cathode)

  7. Place the lid on the beaker and push the electrodes through its holes

Predict, then check

At the graphite anode, gas collects in the test tube. You hold a glowing splint in it, and you test it with damp blue litmus paper.

The gas relights the glowing splint, and the damp blue litmus paper stays the same. Which gas is it?

Chemistry · Interpreting the result

?

Reason it through

Why does graphite in copper sulfate solution give copper at the cathode and oxygen at the anode?

Link 1 of 3

First link · your turn

Which ions are in the solution, and which electrode does each group head for?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Chemistry · Weighing the electrodes

Weigh before, run for 10 minutes, weigh after

In one 10-minute run with copper electrodes, the anode lost 0.91 g. How much mass did the cathode gain?

Your estimate

0.9g

0.7g1.1g

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Electrolysis breaks a substance down by passing a current through an electrolyte: a liquid or solution with freely moving ions.
  • The simple cell is two graphite (inert) electrodes pushed through a Petri dish lid on a beaker of electrolyte.
  • Red lead to the positive terminal makes that electrode the anode. Black lead to negative makes it the cathode.
  • Have a goA classmate announces, 'The red lead always means negative, obviously.' Is that right? Where should the red lead go?

    No. The red lead goes to the positive terminal, and the electrode it joins is the anode.

    Red joins the positive terminal, so that electrode is the anode. The cathode is the one on the black lead.

  • To catch a gas, invert a test tube of electrolyte over the electrode. When enough has gathered, lift it, drain it, test it.
  • Gas tests: hydrogen gives a squeaky pop with a lit splint, oxygen relights a glowing splint, chlorine turns damp blue litmus red then white.
  • Have a goA gas turns damp blue litmus paper red and then bleaches it white. Which of the three gases is it?

    Chlorine.

    Only chlorine bleaches damp blue litmus. Hydrogen gives a squeaky pop with a lit splint, and oxygen relights a glowing splint.

  • Graphite electrodes in copper sulfate solution give orange-brown copper at the cathode and oxygen gas at the anode.
  • Copper forms at the cathode because copper is less reactive than hydrogen. Oxygen forms at the anode because no halide ion is present.
  • Have a goGraphite electrodes, copper sulfate solution. You hold a test tube over the cathode. Do you expect to collect any gas there?

    No. Copper forms on the cathode instead.

    Copper is less reactive than hydrogen, so copper forms at the cathode as an orange-brown deposit rather than hydrogen gas.

  • Copper electrodes change things: an impure copper anode, a pure copper cathode, and an electrolyte containing Cu²⁺ ions.
  • With copper electrodes the anode shrinks as its copper atoms become Cu²⁺ ions, and the cathode grows through Cu²⁺ + 2e⁻ → Cu.
  • Have a goCopper electrodes this time. The cathode gets a layer of new copper. Where did that copper come from?

    From the anode. Its copper atoms form Cu²⁺ ions that go into the solution, then gain electrons at the cathode.

    The anode shrinks while the cathode grows. The copper is not coming from the electrolyte alone.

  • Weigh the anode and cathode at the start, run the electrolysis (for example 10 minutes), then weigh both again.

The big picture

In copper sulfate solution with graphite (inert) electrodes, copper forms at the cathode and oxygen forms at the anode. With copper electrodes the anode shrinks and the cathode grows, because copper atoms at the anode become Cu²⁺ ions and Cu²⁺ ions gain electrons at the cathode. The electrolyte is the same, but the electrodes decide the result.

Key points

1Electrolysis uses an electric current to decompose a substance in an electrolyte containing freely moving ions.
2Graphite electrodes in copper sulfate solution: copper at the cathode, oxygen at the anode.
3Copper electrodes in the same solution: the anode loses mass and size, the cathode gains mass and size.
4At the cathode with copper electrodes: Cu²⁺ + 2e⁻ → Cu.
5The electrolyte is the same in both cells, but the results are not.

Worked example

Problem

In a copper-electrode run, the anode went from 12.40 g to 11.76 g and the cathode went from 10.15 g to 10.75 g. Work out the change in mass of each electrode and say what the difference suggests.

⚠ Watch out

Thinking the solution alone decides the result, so both cells should match. They don't. Graphite electrodes give copper at the cathode and oxygen at the anode. Copper electrodes give a shrinking anode and a growing cathode.

🧠

Memory hook

Ask it every time: where did the copper come from, and where did it go? In both cells copper ends up on the cathode. With copper electrodes, the anode is where it came from.

✓

Check yourself

Without looking back: say what forms or changes at the cathode and at the anode when you use graphite electrodes, and then when you use copper electrodes.

Flashcards

(14)
What is electrolysis?
The decomposition (break-down) of a substance by passing an electric current through an electrolyte.
What is an electrolyte, in electrolysis?
A liquid or aqueous salt solution that contains freely moving ions.
Which lead goes to which terminal, and what is the electrode called?
Red lead to the positive terminal, and that electrode is the anode. Black lead to the negative terminal, and that electrode is the cathode.
Why measure the electrolyte with a measuring cylinder?
It is the most accurate of the equipment listed. Measure 50 cm³ and pour it into the beaker.
How do you collect the gas at an electrode?
Fill a test tube with electrolyte, keep a finger over the end, invert it under the electrolyte and place it over the electrode. Once a good amount has collected, lift it, let the electrolyte out, then test the gas.
What does oxygen do to a glowing splint?
It relights it.
What do lit splints and damp blue litmus show for hydrogen and chlorine?
Hydrogen burns with a squeaky pop at a lit splint. Chlorine turns damp blue litmus red and then bleaches it white.
Graphite electrodes, copper sulfate solution: what forms at the cathode?
An orange-brown deposit of copper. No gas forms there.
Graphite electrodes, copper sulfate solution: what forms at the anode, and why?
Oxygen gas, because no halide ion is present.
Why does copper, not hydrogen, form at the cathode?
Copper is less reactive than hydrogen.
What are the electrodes and electrolyte in the purifying-copper cell?
An anode of impure copper, a cathode of pure copper, and an electrolyte that contains Cu²⁺, the same ions as the metal being purified.
What happens to the copper anode?
Its mass and size decrease, because copper atoms form Cu²⁺ ions that go into the solution.
What is the cathode half-equation with copper electrodes?
Cu²⁺ + 2e⁻ → Cu
How do you show the copper electrodes have changed?
Weigh the anode and the cathode at the start, run the electrolysis (for example 10 minutes), then weigh both again at the end.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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