Chapter 16: Electrochemistry

Electrolysis

Electrolysis is defined as the conduction of electricity by an electrolyte, leading to the decomposition of the electrolyte.

Components in an Electrolytic Cell:

There are five different scenarios of how electrolysis may be carried out:

⦁ Electrolysis using inert electrodes (Platinum and Carbon) with
(a) Molten electrolytes
(b) Dilute aqueous electrolytes
(c) Concentrated aqueous electrolytes

⦁ Electrolysis using reactive electrodes with
(a) Dilute aqueous electrolytes
(b) Concentrated aqueous electrolyte

Relative Ease of Discharge:

In molten electrolytes, typically only one cation and one anion exist, resulting in both ions to be discharged due to the absence of alternative options.

However, aqueous electrolytes contain multiple cations, and one of them is selectively discharged over others, as well as the case with anions.

The electrochemical series is useful in predicting which ion will be selectively discharged. The series indicates that as you move downwards, discharging the ions becomes easier.

1st Scenario:
Electrolysis of MOLTEN Sodium Chloride Using Inert Electrodes

Question 1

The electrolysis of molten lithium hydride, LiH, was carried out using graphite electrodes. Which correctly shows the likely observations made at the two electrodes?

Explanation:

Since molten LiH is used, there are only Li⁺ and H^– ions present. The Li⁺ ions will be reduced at the cathode to form Li metal in molten state due to the heat to keep electrolyte in molten state.

On the other hand, the H^– ions will be attracted and oxidised at the anode to form H₂ gas. H₂ gas will extinguish a lighted splint with a ‘pop’ sound. Hence, the answer is B.

2nd Scenario:
Electrolysis of AQUEOUS Sodium Chloride Using Inert Electrodes

Question 2

The cell below was used to electrolyse dilute sulfuric acid. If gas 1 was to react with gas
2, what would the product be?

A Carbon dioxide
B Dilute sulfuric acid
C Sulfur dioxide
D Water

Explanation:

Cathode half equation: 4OH^– (aq) → O₂ (g) + 2H₂O (𝑙) + 4e^–

Anode half equation: 2H⁺ (aq) + 2e^– →H₂ (g)

The cation present is H⁺ while the anions present are OH^– and SO₄^2-. Thus, OH^– ions will be oxidised at the anode to form O₂ gas while the H⁺ will be reduced to form H₂ gas at the cathode. The 2 gases will form water when reacted. Hence, the answer is D.

3rd Scenario:
Electrolysis of CONCENTRATED Sodium Chloride (Brine) Using Inert Electrodes

Two electrodes, X and Y were connected to a battery and placed on a piece of red litmus paper soaked with concentrated sodium chloride solution.

Explanation:

Cathode half equation: 2H⁺ (aq) + 2e^– → H₂ (g)

Anode half equation: 2C𝑙^– (aq) → C𝑙₂ (g) + 2e^–

The electrode P (anode) will attract the anions, C𝑙^– and OH^– to itself. As the electrolyte is concentrated NaC𝑙, C𝑙^– is selectively oxidised to form C𝑙₂ gas. This will turn red litmus paper white as C𝑙₂ gas has bleaching properties.

The electrode Q (cathode) will attract the cations Na⁺ and H⁺, to itself. The H⁺ ion is selectively reduced to form H₂ gas. Since the concentration of H⁺ ion is lower than the concentration of OH⁺ ions at the cathode, the overall solution at the cathode becomes alkaline in nature. This causes the red litmus paper to turn blue. Hence, the best answer is C.

4th Scenario:
Electrolysis of AQUEOUS copper(II) chloride using Inert Electrodes vs using Reactive Electrodes

Inert electrodes:

Reactive electrodes (any other electrodes except Platinum and Carbon):

Question 4

A sample of concentrated copper(II) chloride solution is electrolysed using copper electrodes.

Explanation:

Anode half equation: Cu (s) → Cu²⁺ (aq) + 2e^–

Since electrode A is an anode, it will attract the negative ions, C𝑙^– and OH^– towards itself. However, as Cu is a reactive electrode, it undergoes oxidation more readily compared to anions. Thus, the answer is B.

Simple Electric Cells

A simple electric cell consists of two different metal electrodes placed in an electrolyte, as shown in the example above.

Metals higher up in the reactivity series (more reactive) lose electrons more readily and become the negative electrode (anode) in the cell. Metals lower in the reactivity series will be the positive electrode (cathode) in the cell.

The naming of anode and cathode follows the type of ions they attract. The anode attracts anions while the cathode attracts cations. But in a simple cell, anodes become negatively charged while cathodes are positively charged (opposite from electrolytic cells).

Zinc being more reactive than copper, is the negative electrode. Copper is the positive electrode.

Voltage of electric cell depends on the relative positions of the two metals in the reactivity series; the further apart they are, the higher the voltage.

Question 5

A student set up three different electrochemical cells each containing two of the metals, W, Y or Z, immersed in an electrolyte. The table shows the voltage and the positive terminal of each electrochemical cell.

Explanation:

The more reactive electrode will be the negative electrode while the less reactive electrode will be the positive electrode. Thus, it can be concluded that Z is more reactive than Y in electrochemical cell 1. In cell 2, W is less reactive than Y as W is the positive electrode. Hence, the answer is C.