Direct Current in Electrolysis
Electrolysis requires a steady, one-way flow of charge, known as direct current (DC), to ensure ions migrate to the correct, consistently charged electrodes for decomposition. Using alternating current (AC) would constantly reverse the process, preventing any stable products from being formed.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- In electrolysis, positive ions (cations) must travel to the negative electrode (cathode) and negative ions (anions) must travel to the positive electrode (anode).
- Direct Current (DC) provides a constant voltage polarity, meaning the cathode is always negative and the anode is always positive.
- This constant polarity allows for a continuous, one-way migration of ions to their respective electrodes where they are discharged to form products.
- Alternating Current (AC) rapidly and periodically reverses its polarity. If AC were used, the electrodes would constantly switch between being positive and negative.
- This switching would cause ions to oscillate back and forth between the electrodes, reversing any reaction that occurs and resulting in no net production of the desired substances.
Definitions
- Direct Current (DC)
- An electric current that flows in a single, constant direction. In electrolysis, this maintains a fixed negative cathode and positive anode.
- Alternating Current (AC)
- An electric current that periodically reverses its direction of flow, causing the polarity of the connected electrodes to switch continuously.
Worked example
A student attempts to electroplate a copper key with silver from a silver nitrate solution. They mistakenly use a mains AC supply instead of a DC battery. Explain why, after some time, the key is not coated in silver.
- 1
Identify the goal:
To coat the key with silver, it must be the cathode (negative electrode) to attract Ag+ ions from the solution.
- 2
The required reaction at the key is reduction:
Ag+(aq) + e- → Ag(s).
- 3
Recall the nature of AC:
The current direction and electrode polarity reverse many times per second.
- 4
In the first half of a cycle, the key is negative.
It acts as a cathode, attracting Ag+ ions and a thin layer of silver may deposit.
- 5
In the second half of the cycle, the key becomes positive.
It now acts as an anode, and any silver that just deposited is oxidised back into Ag+ ions:
Ag(s) → Ag+(aq) + e-.
- 6
Conclude the overall effect:
This rapid cycle of deposition and dissolution prevents any net accumulation of silver on the key's surface.
Answer: No silver coating is formed because the AC supply constantly reverses the electrode polarity. The key alternates between being a cathode (where silver deposits) and an anode (where silver dissolves back into the solution), resulting in no net change.
Common mistakes
- ×Thinking that AC means no reaction occurs at all. Reactions do occur, but they are immediately reversed, leading to zero net product.
- ×Forgetting that the core issue with AC is the *reversal of polarity*, not just a fluctuation in current strength.
- ×Confusing the roles of cathode and anode. With DC, these roles are fixed, which is essential for collecting a specific product at a specific electrode.
No-calculator tips
- ✓Visualise DC as a 'one-way street' for ions, leading them to their fixed destinations (the electrodes).
- ✓Think of AC as a 'revolving door' for ions – they move back and forth at the electrode surface but never permanently leave the solution as a product.
- ✓This topic is purely conceptual. Focus on the cause (polarity) and effect (ion movement and net reaction) rather than looking for numbers to calculate.