Electrolysis Flashcards
What is electrolysis?
Ionic compounds conduct electricity when molten or in solution as the ions are free to move. They move to the electrode of opposite charge where they either gain or lose electrons and the compound is broken down.
What are the negative and positive electrodes?
Positive: anode
Negative: cathode
What happens at the cathode in molten ionic compounds?
+ve ions move here and they gain e- (get reduced)
What happens at the anode in molten ionic compounds?
-ve ions move here and they lose electrons (get oxidised)
What happens in electrolysis of ionic compounds in aqueous solutions?
The ions are free to move. They move towards the electrodes, where electrons are lost or gained. But H2O is also present since they are in aqueous solutions, sometimes this reacts in place of one or both of the ions.
What happens at the cathode (-ve) in aqueous solutions?
Cations of more reactive metals (1,2 and Al3+) stay ions and H+ from water reacts in preference giving hydrogen gas.
If cations from less reactive ions are present they react plating the metal onto the electrode.
What happens at the anode (+ve) in aqueous solutions?
Halide ions react in preference to water giving Cl2, Br2 or I2. But OH- reacts in preference to other anions giving O2(g)
What are the two half equations for if water reacts when electrolysis occurs in an aqueous solution?
Hydrogen: 2H+ (aq) + 2e- => H2(g)
Hydroxide: 4OH- (aq) => O2 (g) + 2H2O (l) + 4e-
What is the typical material for electrodes and why?
Graphite and platinum, because they are unreactive
How can metals be purified using electrolysis?
By electrolysing a solution of their ions with electrodes made of that metal. The anode is the impure metal, and it gradually goes into the solution. The pure metal gradually builds up on the cathode.
What is the electrolysis of brine?
Brine is a concentrated solution of NaCl used in the industry to produce chlorine. When electrolysed it produces H2 at the cathode and Cl2 at the anode and leaves NaOH in solution.
What is the equation for the electrolysis of brine?
2NaCl (aq) + 2H2O(l) => Cl2 (g) + H2 (g) + NaOH (aq)
Why do the products of electrolysis of brine have to be kept separate?
Because they would react together.
