Electrolysis
- Electrolysis is a redox process where the cations undergo reduction and the anions undergo oxidation when an electric current is passed through a compound.
- Electrolysis results in the discharge of ions into atoms. Thus, electrolysis is simply the decomposition of compounds into elements by the passage of an electric current.
- The compound to be decomposed is either a molten ionic compound or an aqueous solution of ions. This is because the compounds need to have free ions to move under the influence of electricity. Such compounds are called electrolytes.
- Example I: Electrolysis of molten lead (II) bromide (PbBr₂).
  - The typical setup for electrolytic cells is as follows:
    - Two inert electrodes, which are carbon electrodes in this case,are connected to the source of direct current through the external circuit.
    - The electrode connected to the positive terminal will become a positive electrode and is called the anode. The electrode connected to the negative terminal will become a negative electrode and is called the cathode.
    - The two electrodes are dipped into a solid salt of lead (II) bromide (PbBr₂) which is the electrolyte.
    - Heat is applied to melt the solid lead (II) bromide (PbBr₂). The ionic bond between the lead and bromide ions is broken down due to high temperature. Hence, the ions become free to move around inside the crucible. Thus, the circuit becomes complete.
  - On applying direct current to the molten lead (II) bromide:
    - The anion that is the bromide ion will move towards the anode. It loses one electron to the anode and gets oxidised into liquid bromine. The electron gets transferred to the external circuit. The liquid bromine bubbles off as brown gas near the anode due to the high temperature.
    - The lead ion moves towards the cathode. The lead ion receives two electrons and gets reduced into lead atoms. Due to the high temperature of the electrolyte, the lead melts to the bottom of the crucible.
- Example-II: Electrolysis of molten zinc chloride (ZnCl₂)
  - When an electric current is passed through the electrolyte, zinc (II) ions move toward the cathode, while the chloride ion moves towards the anode.
  - The redox reaction occurring at the two electrodes:
    - At the anode: Each chloride ion loses one electron to get oxidised to form a neutral chlorine molecule. This bubbles off as pale green gas.
    - At the cathode: Each zinc ion gains two electrons to get reduced into a neutral zinc atom. It is converted to liquid zinc.

End products of electrolysis:

Electrolysis of aqueous solution of ionic compounds:
- Water is a polar covalent molecule. Therefore, dissolving salt or acid increases the conductivity of water.
- Once the ionic compounds are dissolved in water, each ion gets surrounded by the side of water molecules that have the opposite charge to that of the ion.
  - The water molecule that moves to the cathode, with the cation, gets reduced to form hydrogen gas and hydroxide ions.
  - The water molecule that moves to the anode, with the anion,gets oxidised to form oxygen gas and hydrogen ions.
- The hydrogen ions formed get discharged at the cathode into neutral hydrogen gas molecules. While the hydroxide ions formed get oxidised to produce oxygen gas at the anode.
  
  2 H⁺(aq) + 2e⁻ → H₂(g)
  4OH⁻(aq)→ O₂(g) + 2H₂O(l) + 4e⁻
- Either hydrogen ions or metal ions are reduced at the cathode to give hydrogen gas and metal respectively.
- On the other side, either hydroxide ions or non-metal ions are oxidised at the anode to give oxygen gas or non-metals.
Prediction of cation and anion that will be discharged at the two electrodes during the electrolysis of aqueous solution:
- Cations: Depends on the position of the ions in the electrochemical series.
  - The electrochemical series is also a series of elements arranged according to their ability to get oxidised. It isin decreasing order or their ability to get reduced in increasing order.
  - The position of elements in the electrochemical series is determined by their standard electrode potential values, or more precisely, the reduction potential of the ions. It is written as E^Ө.
    - The more positive the value, the easier it is for the ion of an element to get reduced.
    - The more negative the value, the easier it is for the ion of an element to get oxidised.
    - The lower the metal is in the series, the easier it is to reduce its ion.
  - There are slight differences in the arrangement of the metals in the reactivity series and the electrochemical series. However, one can still use the reactivity series to predict whether the hydrogen ion or metal ion will get reduced at the cathode.
- Anions: Depends on the discharge series for anions.
  - The ease of discharge of cations and anions increases down the series.
- In the electrolysis of an aqueous solution of sodium chloride:
  - The hydrogen ion will get discharged at the cathode into hydrogen gas, because the E^⦵ of hydrogen is more positive than that of sodium.
  - At the anode, the anion with a lower E^o value is more likely to get discharged. It can also be decided from the discharge series for anions. In this case, hydroxide ions will get oxidised and form oxygen gas at the anode, as it lies below the chloride ion in the series.
    - This is true only if the electrolyte is a dilute aqueous solution of sodium chloride.
    - If the aqueous sodium chloride solution is concentrated, chloride ions will be discharged. Because the E^⦵ values of ions of halogens, such as chlorine, bromine and iodine, don’t have that much difference with that of hydroxide ions. If the concentration of halide salt or hydrogen halide is high in the solution, more halogens are produced at the anode as compared to oxygen gas.
- The end products of the electrolysis of aqueous sulfuric acid at the two inert electrodes:
  - At the cathode, hydrogen gas would be produced.
  - Hydroxide ions are more likely to be discharged to produce oxygen gas at the anode, as sulphate ions lie way above the hydroxide ion in the discharge series for anions.
    - Sulphate ions and nitrate ions are very difficult to oxidise even in their concentrated solution.
Use of reactive electrodes during electroplating and electrorefining of metals:
- In electroplating and electrorefining, the metal to be plated or refined is used as anode and the metal over which the refined or plating metal is deposited is used as cathode.
- We have to usea suitable electrolyte, one that contains the ion of the same metal as the anode to make sure that other ions are not deposited at the cathode.
- Electrorefining copper:
  - Impure copper metal is used as an anode and a thin sheet of steel as the cathode.
  - Aqueous copper (II) sulphate solution is used as an electrolyte so that only copper ions are discharged at the cathode.
  - When the current is passed, instead of the hydroxide ions getting oxidised at the anode, the copper anode itself gets oxidised into copper(II) ions -the E^⦵ value for copper being higher than that for hydrogen -and is added to the copper(II) sulphate solution.
  - The copper ions then travel to the cathode and get reduced into copper. In this way, the copper from the anode gets deposited at the surface of the cathode.
  - During electrolysis, the size of the anode will decrease while the size of the cathode will increase.
  - In electrorefining, the impure copper at the anode gets separated from the impurities during its oxidation and gets deposited at the surface of the steel cathode as a pure solid copper layer.
- Electroplating of silver over a spoon made up of stainless steel:
  - The spoon is connected to the negative terminal of the power supply to turn it into a cathode. The silver rod is connected to the positive terminal of the power supply to turn it into an anode.
  - Aqueous silver nitrate solution is used as the electrolyte because silver nitrate is soluble in water and hence, contains silver ions.
  - Once the electric current is passed, a redox reaction occurs where the silver metal of the anode undergoes oxidation by losing one electron per silver atom to give silver ions.
  - The silver ions then go into the aqueous solution, which then moves towards the cathode – the spoon. At the surface of the spoon, each silver ion gets reduced to a silver atom by gaining an electron and it is then deposited on the surface.
    
    Over time, the whole surface of the spoon gets covered by silver metal.
    
    NOTE:
    Mostly those metals which lie lower than hydrogen in the series get to be electroplated or electrorefined using its aqueous solution like copper and silver which lie at the lower part of the electrochemical series.
    
    But, some metals with more negative E^⦵ value than hydrogen such as nickel, lead and zinc can also be electroplated or electrorefined using appropriate aqueous solution. Examples of such electrolytes are zinc sulphate, lead nitrate and nickel sulphate.

4.01 - Electrolysis (I)

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