Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Heat required to raise the temperature of \(1 \mathrm{~mol}\) of a substance by \(1^{\circ}\) is called (a) specific heat (b) molar heat capacity (c) water equivalent (d) specific gravity

Step-by-Step Solution

Answer

(b) molar heat capacity.

Step 1: Understand the Problem Statement

The problem asks for the term that describes the heat required to raise the temperature of one mole of a substance by one degree Celsius. We need to identify the correct term from the given options.

Step 2: Define Relevant Terms

Let's define each term provided in the options: - **Specific Heat**: The amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. - **Molar Heat Capacity**: The amount of heat required to raise the temperature of one mole of a substance by one degree Celsius. - **Water Equivalent**: A measure used in calorimetry representing the mass of water that would absorb the same amount of heat as the calorimeter for the same temperature change. - **Specific Gravity**: The ratio of the density of a substance to the density of a reference substance, usually water.

Step 3: Match Terms with Definition

By comparing the problem statement with the definitions, we find that the term 'Molar Heat Capacity' specifically matches the requirement of heat needed to raise the temperature of one mole of a substance by one degree Celsius.

Step 4: Conclusion

Based on the definitions and the problem statement, the correct answer to the question is 'molar heat capacity,' as it directly corresponds to the heat needed for one mole of a substance.

Key Concepts

Specific Heat

The concept of specific heat is a fundamental idea in thermodynamics. It refers to the amount of heat per unit mass required to raise the temperature of a substance by one degree Celsius. Think of it as a measure of how much energy a specific substance can absorb before it experiences a noticeable change in temperature.

Specific heat is an intrinsic property, which means it doesn't rely on the amount of substance you have — just its temperature change and the heat applied. For example:

• Water has a high specific heat capacity, which means it can absorb a lot of heat without much change in temperature.
• Metals like iron have a lower specific heat, meaning they heat up and cool down more quickly.

This property is crucial when considering energy changes in physical and chemical processes, making it a key player in fields like meteorology and energy management.

Calorimetry

Calorimetry is an experimental technique used to measure the heat transferred to or from a substance. The main device used in calorimetry is a calorimeter, which isolates a sample from its environment to ensure accurate heat measurements.

In a typical calorimetry experiment:

• You measure the initial temperature of the sample.
• Allow the sample to interact with a heat source or sink, such as another substance or chemical reaction.
• Measure the final temperature.

The changes in temperature, combined with knowledge of the substance's specific heat or molar heat capacity, allow you to calculate the energy exchanged during the process. By using calorimetry, scientists can explore reaction enthalpies, phase changes, and other important thermodynamic processes. It is an indispensable tool for chemists and engineers.

Temperature Change

Temperature change is a straightforward but important concept in thermodynamics. It describes how much a substance's temperature increases or decreases when heat is added or removed.

When you apply heat to a substance, its molecules move faster, causing the temperature to rise. Conversely, when you remove heat, the molecules slow down, and the temperature decreases.

• A positive temperature change means the substance has absorbed heat and its temperature has increased.
• A negative temperature change indicates that the substance has released heat, leading to a decrease in temperature.

This concept is key in many scientific calculations, such as determining energy changes in chemical reactions or predicting how materials behave under thermal stress.

Mole Concept

The mole concept is a cornerstone of chemistry, allowing chemists to count atoms, ions, and molecules in a given substance by using a defined quantity. A mole is equal to Avogadro's number, which is approximately \(6.022 \times 10^{23}\) entities per mole.

This concept simplifies the study of chemical reactions and compounds:

• It lets you directly relate the mass of a substance to the number of molecules or atoms it contains, which is crucial for stoichiometric calculations.
• Molar heat capacity, for instance, is typically expressed in terms of "per mole" and represents the amount of heat needed to raise one mole of a substance by one degree Celsius.

Understanding the mole concept is pivotal as it bridges the gap between the atomic scale and the macroscopic world, making it easier to conceptualize chemical quantities and reactions.