Cambridge O Level Chemistry · Syllabus 5070 · Electrochemistry
Anode
What is Anode?
The positive electrode of a simple electrolytic cell, joined to the positive terminal of the power supply. Negative ions move through the electrolyte towards it and lose electrons there, so oxidation happens at the anode and the product is a non-metal other than hydrogen.
This definition is part of the Electrochemistry chapter in Cambridge O Level Chemistry.
Anode in context
At the cathode, either a metal or hydrogen is formed. At the anode, a non-metal other than hydrogen is formed.
Common mistakes with Anode
- 2. "Positive ions go to the positive anode." Repair Opposite charges attract, so positive cations move to the negative cathode and negative anions move to the positive anode. Taught in Lesson A.
- 3. "The anode is positive, in every cell, everywhere." Repair In a simple electrolytic cell the anode is positive and the cathode is negative because an external power supply makes them so; that sign pattern belongs to this context and should not be carried unexamined into other kinds of cell. Taught in Lesson A.
- 4. "Oxidation is gain of electrons." Repair Oxidation is loss of electrons and happens at the anode; reduction is gain of electrons and happens at the cathode — write the full words, then check the side your electrons are on. Taught in Lesson B.
- 6. "Chlorine is a colourless gas" — or — "concentrated sodium chloride solution gives oxygen at the anode." Repair Chlorine is a pale yellow-green gas with a sharp choking smell, and it is the anode product from a concentrated aqueous chloride; oxygen is what a dilute one gives. Taught in Lesson E.
- 7. "A copper electrode behaves like a carbon one." Repair Carbon and platinum electrodes take no chemical part, but a copper anode is itself oxidised and passes into the solution as copper ions, so the anode product is completely different. Taught in Lesson G.
- 8. "Connect the spoon you want to plate to the positive terminal." Repair Metal ions are reduced onto the object, and reduction happens at the cathode, so the object being plated is the negative cathode and the coating metal is the positive anode. Taught in Lesson I.
Examiner tips on Anode
- The arrow that costs marks. A single electron arrow drawn straight from the anode across to the cathode, ignoring the power supply, describes a cell that does not exist. Electrons in an electrolytic cell are pushed by the supply; draw them leaving the negative terminal on one side and arriving at the positive terminal on the other.
- Where the oxygen atoms come from. Count them in the anode equation: four hydroxide ions bring four oxygen atoms, and those four end up as two in the \(\mathrm{O_2}\) molecule and two in the two water molecules. Every oxygen atom is accounted for, and none of them came from a sulfate ion.
- A careful way to say it. Write "the anode loses mass and the cathode gains mass" — that is what is observed. Do not claim the two changes are exactly equal unless a question gives you data or a calculation that establishes it.
Questions students ask about Anode
What is electrolysis?
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. The electrolyte is the molten or aqueous substance being decomposed; its mobile ions carry charge inside the cell. Cations move to the negative cathode and gain electrons, anions move to the positive anode and lose electrons, and the products are released at the electrode surfaces. At the cathode either a metal or hydrogen is formed; at the anode a non-metal other than hydrogen is formed.
What is the difference between the anode and the cathode?
In a simple electrolytic cell the anode is the positive electrode, joined to the positive terminal of the power supply, and the cathode is the negative electrode, joined to the negative terminal. Oxidation is loss of electrons and happens at the anode, where anions arrive and give up electrons. Reduction is gain of electrons and happens at the cathode, where cations arrive and take electrons. Write the full words before any mnemonic, because the exam marks the words, and remember that these signs belong to this powered cell.
Do electrons flow through the electrolyte during electrolysis?
No. Electrons move only in the external metallic circuit: they leave the negative terminal of the power supply, travel along the wire to the cathode, and electrons released at the anode travel back to the positive terminal. Inside the electrolyte charge is carried by mobile ions and by nothing else: positive cations move to the negative cathode and negative anions move to the positive anode, because opposite charges attract. The two carriers meet only at the electrode surfaces, where an ion gains or loses electrons.
Why does concentrated sodium chloride solution give hydrogen at the cathode instead of sodium?
Because the solution contains water as well as sodium chloride, more than one species is available at each electrode, so an aqueous cell does not behave like a molten one. At the cathode the product is hydrogen, not sodium, even though sodium ions are the majority cation in the solution. At the anode, chlorine is released because the chloride solution is concentrated; a dilute solution gives oxygen instead. Chlorine is a pale yellow-green gas with a sharp choking smell, not a colourless gas.
What happens when aqueous copper(II) sulfate is electrolysed with copper electrodes instead of carbon?
Carbon and platinum electrodes are inert: they conduct but take no chemical part, so with carbon electrodes copper is deposited at the cathode and oxygen gas is released at the anode. A copper anode is different: it is an active electrode, so the copper of the anode itself is oxidised and passes into the solution as copper ions, while copper is deposited on the cathode. No gas forms at the anode. Write what is observed: the anode loses mass and the cathode gains mass.
How do you write an ionic half-equation for an electrode?
Write the ion on the left and the product on the right, then add the electrons on the correct side: on the left when the ion gains them (reduction at the cathode, for example \(\mathrm{Pb^{2+}} + 2e^- \rightarrow \mathrm{Pb}\)) and on the right when the ion loses them (oxidation at the anode, for example \(2\mathrm{Br^-} \rightarrow \mathrm{Br_2} + 2e^-\)). Then make two checks: count the atoms of each element on both sides, and add up the total charge on each side counting each \(e^-\) as \(-1\). Both must match.
Which electrode do you connect the object to when electroplating?
The object being plated is made the negative cathode. Metal ions from the electrolyte are reduced onto the object, and reduction happens at the cathode, so connecting the object to the positive terminal would be wrong. The coating metal is made the positive anode, and the electrolyte is a solution containing ions of the coating metal. Electroplating is done for two reasons, and a question asking why wants both: to improve the appearance of a metal object and to improve its resistance to corrosion.