Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

12,716 Students Helped

Homework Questions

Summary

Learning Objectives

Key Concepts

Example Problems

Explanations

Common Mistakes

Summary

Thermochemistry is essential in understanding the energy dynamics of chemical reactions through the study of enthalpy changes, whether they involve the absorption or release of heat. Techniques such as Hess's Law and calorimetry provide precise methods for calculating these energy changes. Mastery of these concepts is vital for both academic research and practical industrial applications, ensuring efficient and safe chemical processes.

Learning Objectives

Explain the fundamental concepts of thermochemistry and its importance in understanding energy changes in chemical reactions.

Describe how to calculate enthalpy changes and determine if a reaction is endothermic or exothermic.

Apply Hess's Law to compute overall reaction enthalpy from component reactions.

Understand the principles of calorimetry and its role in measuring energy changes in laboratory and industrial settings.

Key Concepts

CONCEPT

DEFINITION

Thermochemistry

The branch of chemistry that studies the energy changes accompanying chemical reactions, particularly the heat absorbed or released.

Enthalpy (ΔH)

A measure of the heat content in a system at constant pressure; used to indicate the energy change in chemical reactions.

Exothermic Reaction

A chemical reaction that releases heat to its surroundings, resulting in a negative enthalpy change (ΔH < 0).

Endothermic Reaction

A chemical reaction that absorbs heat from its surroundings, resulting in a positive enthalpy change (ΔH > 0).

Hess's Law

A principle stating that the total enthalpy change for a reaction is the same, regardless of the pathway taken, so long as initial and final conditions are identical.

Calorimetry

An experimental technique used to measure the amount of heat absorbed or released during a chemical reaction.

Example Problems

Example 1

Which of the following statement is false? (1) Thermochemistry is the study of relationship between the heat energy and chemical energy. (2) An exothermic reaction is one which is accompanied by evolution of heat. (3) An endothermic reaction is one in whieh heat is absorbed. (4) The unit that represents largest amount of energy is electron volt.

Example 2

The wrong statement among the following is (1) An exothermic reaction is that in which the reacting substances have more energy than the products. (2) Electrolysis of water is accompanied with absorption of encrgy. (3) Evaporation of water is an endothermic change. (4) The law of conservation of cnergy states that the internal cnergy of a system is constant.

Example 3

Which is not characteristic of a thermochemical equation? (1) It indicates physical state of reactants and products. (2) It indicates whether the reaction is exothermic or endothermic. (3) It indicates allotrope of the reactants if present. (4) It indicates whether a reaction would oecur or not.

Example 4

The correct thermochemical cquation is (1) $\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} ; \Delta H=94 \mathrm{kcal}$ (2) $\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} ; \Delta H=+94 \mathrm{kcal}$ (3) $\mathrm{C}_{(n)}+\mathrm{O}_{(\mathbb{B})} \longrightarrow \mathrm{CO}_{2 \mathrm{~g}} ; \Delta H=94 \mathrm{kcal}$ (4) $\mathrm{C}_{(8)}+\mathrm{O}_{\mathrm{cg}} \longrightarrow \mathrm{CO}_{2 \mathrm{~m}} ; \Delta H=+94 \mathrm{kcal}$

Example 5

For the reactions $\mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{HCl}(\mathrm{g})+X_{1} \mathrm{~kJ}$ $2 \mathrm{IICl}(\mathrm{g}) \longrightarrow 1 \mathrm{I}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \quad X_{2} \mathrm{~kJ}$ (1) $X_{1}$ and $X_{2}$ are numerically cqual. (2) $X_{1}$ and $X_{2}$ are numerically different. (3) $X_{1} \quad X_{2}>\mathrm{O}$ (4) $X_{1} \quad X_{2}<0$

Step-by-Step Explanations

QUESTION

How can you determine the overall enthalpy change for a reaction using Hess's Law?

STEP-BY-STEP ANSWER:

Step 1: Identify the overall chemical reaction for which you need to find the enthalpy change.
Step 2: Break the overall reaction into a series of steps or intermediate reactions for which the enthalpy changes are known.
Step 3: Adjust the known reactions if needed (reversing or multiplying) to ensure that when they are added, they produce the overall reaction.
Step 4: Sum the enthalpy changes of the individual steps to obtain the total enthalpy change for the overall reaction.
Final Answer: The overall enthalpy change is the algebraic sum of the enthalpy changes of the individual steps based on Hess's Law.

Hess's Law

QUESTION

How do you use calorimetry to determine the heat change during a chemical reaction?

STEP-BY-STEP ANSWER:

Step 1: Prepare the calorimeter and measure the initial temperature of the surroundings (usually water) in the device.
Step 2: Conduct the chemical reaction in the calorimeter and record the final temperature.
Step 3: Calculate the change in temperature (ΔT) by subtracting the initial temperature from the final temperature.
Step 4: Use the specific heat capacity formula (q = m × c × ΔT) to calculate the heat change, where m is the mass of the substance, c is its specific heat capacity, and ΔT is the temperature change.
Final Answer: The calculated q value represents the heat energy absorbed or released during the reaction, indicating if the reaction is endothermic or exothermic.

Calorimetry

Common Mistakes

Confusing endothermic and exothermic reactions due to sign errors in enthalpy changes.
Forgetting to adjust reaction equations properly when applying Hess's Law.
Neglecting the calibration and proper insulation of a calorimeter, which can lead to inaccurate measurements.
Assuming that energy calculations in thermochemistry are only important in academic contexts rather than practical industrial applications.