SS2 Chemistry Lesson Note on Factors Affecting the Discharges of Ions

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FACTORS AFFECTING THE DISCHARGES OF IONS/ PRODUCT OF ELECTROLYSIS

During electrolysis the ions in the electrolyte are positively and negatively charged. The negatively charge ion will migrate to the negative charged cathode. In order to know which of the ions will be preferentially discharged at the electrodes, the following factors must be taken into consideration.

1. The position of ions in the electrochemical series

 

CATIONS		ANIONES
K+	If two cations are present in an electrolyte the cation below the other one in the electrochemical series will be preferentially discharged.	If two anions are present in electrolyte, the anions that is above the other one in the electrochemical series will be preferentially discharged	OH-
Na+			I-
Ca2+			Br-
Mg2+			Cl-
Al3+
Zn2+
Fe2+			NO3
Sn2+
			SO42-
Pb2+
H+
			F-
Cu2+
Hg2+
Ag+
AU+
Pt+

 

ELECTROLYSIS OF SODIUM CHLORIDE SOLUTION USING INERT ELECTRODES

Note: Inert electrodes are the electrodes that will tamper with the electrolysis. Examples are carbon (graphite) and platinum.

At the anode: Both OH^‑ and Cl^– will migrate to the anode where OH^‑ ions will be preferentially discharged because they are above Cl^– in the electrochemical.

Cathodic half reaction: 4OH^‑_(aq)        →     2H₂O_(l) + O_2(g) + 4e^–

At the cathode: Both Na⁺ ions and H⁺ ions migrate to the cathode where H+ ions are preferentially discharged because they are below Na+ ions in the electrochemical series.

Anodic half reaction: 4H+_(aq) + 4e^–      →       2H_2(g)

Overall reaction: 4OH^–_(aq) + 4H⁺_(aq)      →   2H_2(l) + O_2(g) + 2H_2(g).

Products of electrolysis: Hydrogen gas is produced at the cathode and oxygen gas is produced at the anode.

Effect of electrolyte on the electrolysis: At the end of the electrolysis, the electrolyte will more concentrated since OH^‑ and H⁺ have been removed from the electrolyte.

2. CONCENTRATION OF IONS IN THE ELECTROLYTE e.g. electrolysis of concentrated NaCl _(aq) using inert electrodes. The electrolytic cell is the same as in number factor above, but the electrolyte is concentrated NaCl_(aq)

Ions in the electrolyte:        

CATIONS                               ANIONS

Na⁺                                          OH^–

H⁺                                            Cl^–

At the anode: Both, Cl^– and OH^– will migrate to the anode where Cl^– ions will be preferentially discharged because they are more concentrated. (i.e they are more in concentration inside the electrolyte).

Anodic half reaction: 2Cl^–_{(aq)      →}      Cl_2(g) + 2e^–

At the cathode: Both the Na⁺ and H⁺ will migrate to the cathode where H⁺ will be preferentially discharged. This is because even though Na⁺ ions are more in concentration in the electrolyte, the distance between Na⁺ and H⁺ in the electrochemical series is very far.

Cathodic half reaction: 2H⁺_(aq) + 2e^–     →    H_2(g)

Overall reaction of electrolysis: Chlorine gas is produced at the anode and hydrogen gas at the cathode.

 

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