Halides And Hydrides Of Alkali

Step 1: Understanding the reaction.
The reaction where acyl chloride is hydrogenated over palladium on barium sulfate to form an aldehyde is known as the Rosenmund reduction. This reaction is used to reduce acyl chlorides to aldehydes.

Step 2: Analyzing the options.
(A) Stephen reaction: This is used to reduce nitriles to aldehydes, not acyl chlorides.
(B) Rosenmund reduction: Correct — This is the reaction that reduces acyl chlorides to aldehydes.
(C) Etard reaction: This reaction is used to form aldehydes from aromatic compounds, not acyl chlorides.
(D) Wolff-Kishner reduction: This is a method used to reduce ketones and aldehydes to alkanes, not acyl chlorides.

Step 3: Conclusion.
The correct answer is (B) Rosenmund reduction, as it is the reaction that reduces acyl chlorides to aldehydes.

Step 1: Property of caesium.
Caesium has very low ionisation energy and easily emits electrons when exposed to light.
Step 2: Application based on property.
Due to its high photoelectric sensitivity, caesium is used in photoelectric cells.
Step 3: Eliminate other options.
Air conditioning uses refrigerants, boron extraction uses magnesium, and fast breeder reactors use liquid sodium.
Step 4: Conclusion.
Hence, caesium is used in devising photoelectric cells.