Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

When petroleum is heated gradually, first batch of vapours evolved will be rich in (a) kerosene (b) petroleum ether (c) diesel (d) lubricating oil

Step-by-Step Solution

Answer

The first batch of vapours will be rich in petroleum ether (b).

Step 1: Understanding the Boiling Points

Petroleum is a complex mixture of hydrocarbons that can be separated by heating, taking advantage of the different boiling points of its components. Lighter fractions with lower boiling points evaporate first.

Step 2: Identify Key Fractions and Their Order

Petroleum fractions are distilled based on their boiling points. Common fractions include petroleum ether, kerosene, diesel, and lubricating oil, listed from lowest to highest boiling points.

Step 3: Determine the First Fraction to Evaporate

Since lighter fractions evaporate at lower temperatures, the first vapors evolved from heating petroleum will be from the fraction with the lowest boiling point—petroleum ether.

Step 4: Conclusion and Selection

Based on their boiling points, petroleum ether will vaporize first. Therefore, the first batch of vapors will be rich in petroleum ether.

Key Concepts

Boiling Points of Hydrocarbons

Petroleum is composed of a complex array of hydrocarbons, which are organic compounds made up of hydrogen and carbon atoms. The boiling point of a hydrocarbon is a key property that determines how these compounds behave when heated. Boiling points are important because they dictate when each component of a mixture will turn into a gas. The boiling point of a hydrocarbon is influenced by its molecular weight and structure.

• Lighter hydrocarbons, like methane, have lower boiling points and vaporize easily at lower temperatures. These are often found in gases and light liquids.
• Heavier hydrocarbons, like those found in oils and waxes, have higher boiling points.

Understanding these characteristics helps in the process of separating petroleum into useful products based on their volatility.

Petroleum Fractions

Petroleum is divided into fractions through distillation. Each fraction consists of hydrocarbons with similar boiling points and thus similar properties. These fractions range from light to heavy ones and include the following:

• Petroleum Ether: The lightest fraction. It's a volatile and flammable liquid, often used as a solvent.
• Kerosene: Often used as jet fuel and for heating. It has a moderate boiling point.
• Diesel: Used primarily as fuel in engines. It is heavier than kerosene and evaporates at a higher temperature.
• Lubricating Oil: This fraction is used to reduce friction in engines and other machinery, with one of the highest boiling points in petroleum products.

Each fraction has a unique role in industrial applications, emphasizing the importance of the distillation process.

Fractional Distillation

Fractional distillation is the process used to separate petroleum into its component fractions. This method relies on the distinct boiling points of the various hydrocarbons present in petroleum. During fractional distillation, petroleum is gradually heated, and fractions are collected at different temperatures as they vaporize.

• Initial Heating: The petroleum mixture is heated in a distillation column. The lightest fractions vaporize first since they have the lowest boiling points.
• Rising Vapors: As the vapors rise through the column, they cool and condense on trays or packing material.
• Collection of Fractions: Different fractions are collected at multiple levels of the distillation column based on their boiling ranges.

This technique is extremely efficient for separating complex mixtures and is fundamental to the petroleum refining industry.