Questions & Answers

Which acid is used in a car battery?
(A)Dilute sulphuric acid
(B) Dilute hydrochloric acid
(C) Dilute nitric acid
(D) None

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Hint: In a car battery electrochemical reactions are involved in converting chemical energy into electrical energy. An oxidation and reduction reaction takes place as an electrochemical reaction.

Complete step by step solution:
Diluted sulphuric acid is used in a car battery.
1. In the battery, electrochemical reactions are involved which is an oxidation-reduction reaction.
2. Car acid batteries contain at least 30%-50% diluted sulphuric acid in which only 29% concentrated sulphuric acid is present.
3. As the batteries discharge the sulphuric acid reacts with lead to form lead sulfate and water. To change the battery the reaction is reversed. Thus dilute sulfuric acid is used in the car battery.
Hence, option (A) is the correct option.

Additional information:
A hydrometer is a device to check the state of change of the battery by measuring the strength of the acid. During battery discharge, the electrons released at the cathode travel through an external circuit and reach the anode. This establishes charge neutrality inside the battery. Working with a lead-acid battery involves the formation of lead sulfate.
Here the product of battery discharge is lead sulfate the total cell reaction is sometimes referred to as double sulfate reaction.

The formation of lead sulfate involved sulphuric acid, which is the electrolyte used in lead-acid batteries. The sulphuric acid is consumed during the discharge of the battery and released during the charge of the battery, which means the specific gravity of the electrolyte changes during battery use and charge.

Hence, the specific gravity of the electrolyte is used as an indicator of the state of charge of the lead-acid battery.

Note: The basic reaction in the battery involves uptake and release of sulphuric acid. Hence, in these batteries electrolyte of sulphuric acid is used.
Car batteries consist of about 30 to 50% of sulfuric acid in water with 29% of a mole fraction of it in. The density of sulfuric acid is 1.25 kg/liter and the pH is acidic as 0.8.
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