Why is lead sulphate not directly obtained by the reaction between sulphuric acid and lead oxide?
Lead oxide is poorly soluble in water. The solubility of lead oxide in dilute sulfuric acid is also very low. When lead oxide is added to dilute sulfuric acid, a thin layer of lead sulfate is formed on the surface of the solid lead oxide. This layer protects the rest of the lead oxide from getting converted to lead sulfate in what is known as passivation.
To increase the amount of lead sulfate formed, the lead oxide particles being added to the acid should be ground to a very fine powder. If the concentration of sulfuric acid is very high, a hydrated crystal of lead sulfate is formed that is soluble in the acid. This allows the formation of lead sulfate to continue without interruption.
An easier method of creating lead sulfate is with the reaction of lead nitrate and sulfuric acid. This works more efficiently as lead nitrate is soluble in water.