Electrolysis: Selective discharge of ions

In MOLTEN compounds, the only ions present are the ions from the ionic compound.

What are the ions present in MOLTEN NaCl ?

• Na⁺ only

• Cl^- only

• Na⁺ and Cl^- only

• Na⁺ , H+ , Cl^- and OH^-

In AQUEOUS compounds, the ions present include both the ions from the ionic compound and the ions from water (aqueous).

What are the ions present in AQUEOUS NaCl ?

• Na⁺ only

• Cl^- only

• Na⁺ and Cl^- only

• Na⁺ , H⁺ , Cl^- and OH^-

Selective discharge of ions:

Rule #1, Ions are prefarably discharged according to the position of ions in the reactivity series

(refer to notes pg10)

Ions present in aqueous NaCl are Na⁺ , Cl^- , H⁺ and OH^- .

Which are most likely to be discharged?

• Negative electrode: Na⁺ ; Positive electrode: Cl^-

• Negative electrode: Na⁺ ; Positive electrode: OH^-

• Negative electrode: H⁺ ; Positive electrode: Cl^-

• Negative electrode: H⁺ ; Positive electrode: OH^-

Selective discharge of ions:

Rule #1, Ions are prefarably discharged according to the position of ions in the reactivity series

(refer to notes pg10)

Ions present in aqueous Pb(NO₃)₂ are Pb²⁺ , NO₃^- , H⁺ and OH^- .

Which are most likely to be discharged?

• Negative electrode: H⁺; Positive electrode: OH^-

• Negative electrode: Pb²⁺; Positive electrode: OH^-

• Negative electrode: H⁺; Positive electrode: NO₃^-

• Negative electrode: Pb²⁺; Positive electrode: NO₃^-

Ions present in aqueous Pb(NO₃)₂ are Pb²⁺ , NO₃^- , H⁺ and OH^- . Ions discharged are H⁺ and OH^- as they are less reactive, hence easier to be discharged.

H⁺ ions are discharged at the cathode (negative electrode).

OH^- ions are discharged at the anode (positive electrode).

Which of the following is the correct ionic equation? (AN OILRIG CAT)

• Cathode: 2H⁺ + 2e^- → H₂

  Anode: 4OH^- → 2H₂O + O₂ + 2e^-

• Cathode: H⁺ + 2e^- → H₂

  Anode: 4OH^- → 2H₂O + O₂ + 2e^-

• Cathode: 2H⁺ + 2e^- → H₂

  Anode: 4OH^- → 2H₂O + O₂ + 4e^-

• Cathode: H⁺ + 2e^- → H₂

  Anode: 4OH^- → 2H₂O + O₂ + 4e^-

Selective discharge of ions: (pg 10)

Rule #2, Concentration of ions. The MORE concentrated the ions is, the EASIER it will be discharged. (rule #1 may be ignored)

Eg. AQUEOUS PbCl₂: Pb²⁺ , Cl^- , H⁺ and OH^- ion present.

Aqueous PbCl₂: H⁺ and OH^- ions discharged.

Concentrated PbCl₂: Pb²⁺ and Cl^- ions discharged.

What are the ions discharged in concentrated aqueous CuBr₂ ?

• H⁺ and OH^- ions discharged

• Cu²⁺ and Br^- ions discharged.

• H⁺ and Br^- ions discharged

• Cu²⁺ and OH^- ions discharged.

Selective discharge of ions: (pg 10)

Rule #2, Concentration of ions. The MORE concentrated the ions is, the EASIER it will be discharged. (rule #1 may be ignored)

Eg. AQUEOUS PbCl₂: Pb²⁺ , Cl^- , H⁺ and OH^- ion present.

Aqueous PbCl₂: H⁺ and OH^- ions discharged.

Concentrated PbCl₂: Pb²⁺ and Cl^- ions discharged.

What are the ions discharged in concentrated aqueous HCl ?

• Ions present in concentrated aqueous HCl: H⁺ , Cl^- and OH^-

  
  

  Ions discharged are: H⁺ and Cl^- ions

• Ions present in concentrated aqueous HCl: H⁺ , Cl^- and OH^-

  
  

  Ions discharged are: H⁺ and OH^- ions

Rule #2, Concentration of ions. The MORE concentrated the ions is, the EASIER it will be discharged. (rule #1 may be ignored)

HOWEVER, THERE ARE EXCEPTIONS. The following ions cannot be discharged eventhough they are in high concentration: F^- , SO₄^2- , NO₃^-, CO₃^2-, K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺ (SHOULD BE COPIED IN YOUR NOTES ALREADY)

Eg. AQUEOUS NaCl: Na⁺ , Cl^- , H⁺ and OH^- ion present.

Aqueous NaCl: H⁺ and OH^- ions discharged.

Concentrated NaCl: H⁺ and Cl^- ions discharged.

What are the ions discharged in concentrated aqueous MgCl₂ ?

• H⁺ and OH^- ions discharged.

• Mg²⁺ and OH^- ions discharged.

• H⁺ and Cl^- ions discharged.

• Mg²⁺ and Cl^- ions discharged.