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Chemical Effects of Electric Current - Chemical Changes during Electrolysis

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Electrolysis is the process of decomposing a chemical compound (electrolyte) by passing an electric current through it in its dissolved or molten state.

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When electric current passes through an electrolyte, it dissociates into ions. Positive ions (cations) move towards the negative electrode (cathode), and negative ions (anions) move towards the positive electrode (anode).

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Chemical changes during electrolysis can manifest as: 1. The evolution of gas bubbles at the electrodes, 2. The deposition of metals on the electrodes, 3. Changes in the color of the solution.

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In the electrolysis of water, oxygen (O2O_2) gas bubbles are formed at the positive electrode (anode) and hydrogen (H2H_2) gas bubbles are formed at the negative electrode (cathode).

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When current is passed through a copper sulphate (CuSO4CuSO_4) solution using copper electrodes, free copper ions (Cu2+Cu^{2+}) get drawn to the negative electrode and are deposited there as solid copper (CuCu).

📐Formulae

2H2O(l)→Electric Current2H2(g)↑+O2(g)↑2H_2O(l) \xrightarrow{\text{Electric Current}} 2H_2(g) \uparrow + O_2(g) \uparrow

CuSO4(aq)⇌Cu2+(aq)+SO42−(aq)CuSO_4(aq) \rightleftharpoons Cu^{2+}(aq) + SO_4^{2-}(aq)

Cu2++2e−→Cu (At Cathode)Cu^{2+} + 2e^- \rightarrow Cu \text{ (At Cathode)}

Zn→Zn2++2e− (At Anode during electroplating)Zn \rightarrow Zn^{2+} + 2e^- \text{ (At Anode during electroplating)}

💡Examples

Problem 1:

During the electrolysis of acidified water, if 10 mL10\text{ mL} of gas is collected at the cathode, how much gas is approximately collected at the anode, and identify the gases.

Solution:

The gas at the cathode is Hydrogen (H2H_2) and the gas at the anode is Oxygen (O2O_2). According to the chemical equation 2H2O→2H2+O22H_2O \rightarrow 2H_2 + O_2, the ratio of Hydrogen to Oxygen is 2:12:1. Therefore, at the anode, 5 mL5\text{ mL} of Oxygen is collected.

Explanation:

The decomposition of water molecules releases two atoms of hydrogen for every one atom of oxygen, leading to a 2:12:1 volume ratio in their gaseous forms.

Problem 2:

What happens when a tester is used to check the conduction of a copper sulphate (CuSO4CuSO_4) solution using two copper plates?

Solution:

When current passes, CuSO4CuSO_4 dissociates into Cu2+Cu^{2+} and SO42−SO_4^{2-}. The Cu2+Cu^{2+} ions move to the cathode and deposit as CuCu. Simultaneously, the copper anode dissolves to release Cu2+Cu^{2+} ions into the solution.

Explanation:

This process maintains the concentration of the solution. The copper lost from the solution is restored by the copper anode, effectively transferring copper from the anode to the cathode.