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Chapter 6: Problem 43

State Hess's law. Explain, with one example, the usefulness of this law in thermochemistry.

Short Answer

Expert verified
Hess's Law, crucial in Thermochemistry, states that the total enthalpy change during the complete course of a reaction is the same whether the reaction is made in one step or in several steps. It allows the heat of reaction for challenging reactions to be calculated in stages instead of a total sum. For example, the calculation of the enthalpy change of burning methane can be done through intermediary reactions, making the process manageable and practical.

Step by step solution

01

Define Hess鈥檚 Law

Named after Germain Hess, a Russian-Swiss chemist, Hess鈥檚 Law, also Fondly referred to as Hess's Law of Constant Heat Summation states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several, provided that the final and initial conditions are the same for each case.
02

Highlight the importance of Hess鈥檚 Law in Thermochemistry

In thermochemistry, Hess's Law is fundamentally important as it allows the calculation of the heat of reaction for reactions which are challenging to perform experimentally. Hess's Law facilitates the measurement of the heat of reaction in stages instead of a total sum, providing a more manageable approach to calculate the heat absorption or release during the reaction.
03

Example Demonstrating the Usefulness of Hess's Law

Let's consider an example of burning methane. It's difficult to measure the enthalpy change of this reaction directly. However, this can be done in steps through reactions where enthalpy changes can be measured more easily.\[CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l) \quad \triangledown H_1\]\[C(s) + O_2(g) \rightarrow CO_2(g) \quad \triangledown H_2\]\[H_2(g) + \frac{1}{2}O_2(g) \rightarrow H_2O(l) \quad \triangledown H_3\]Here, \(\triangledown H_1\) is hard to measure, but \(\triangledown H_2\) and \(\triangledown H_3\) can be measured easily. Then applying Hess's Law, we can find \(\triangledown H_1 = \triangledown H_2 + 2\triangledown H_3\)This simplifies the process of finding the heat of reaction for complex reactions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermochemistry
Thermochemistry is the branch of chemistry that studies the relationships between chemical reactions and energy changes. It primarily focuses on understanding enthalpy and heat flow during reactions.

In thermochemistry, energies often transform from one form to another. Chemical reactions may release or absorb energy, driving various processes in the surroundings. Understanding energy changes helps predict reaction behaviors and their practicality.

Thermochemistry plays an integral role in developing and improving processes for industrial applications such as fuel combustion, energy storage, and material synthesis.
  • It provides insights into reaction spontaneity and feasibility.
  • Can determine if a reaction is exothermic (releases heat) or endothermic (absorbs heat).
Hess's Law, a pivotal thermochemical principle, allows us to calculate enthalpy changes in reactions that are difficult to measure directly, aiding in energetic studies of chemical processes.
Enthalpy Change
Enthalpy change, often denoted as \( \Delta H \), is the difference in the total heat content of a system before and after a reaction. It provides critical insight into the energy consumed or released during chemical reactions.

This concept is crucial in thermochemistry, as it helps quantify the heat flow between the system and its surroundings. An understanding of enthalpy change allows chemists to predict whether a reaction is energetically favorable.
  • \( \Delta H \) signifies an exothermic reaction if it is negative, indicating heat release.
  • A positive \( \Delta H \) implies an endothermic reaction, where heat is absorbed.
In practice, calculating \( \Delta H \) can be simplified using Hess's Law. By breaking down a complex reaction into simpler steps with known enthalpy changes, we can determine the overall enthalpy change without directly experimenting on potentially hazardous reactions.
Heat of Reaction
Heat of reaction, also known as enthalpy change of reaction, refers to the heat exchanged during a chemical reaction at constant pressure. It's the measure of the change in enthalpy, \( \Delta H \), associated with a particular chemical process.

Understanding the heat of reaction is essential in designing efficient chemical processes. It aids in energy management by predicting how much energy a reaction will release or require.
  • Helps in choosing suitable reaction conditions to optimize energy use.
  • Guides industrial applications to improve energy efficiency and safety.
Hess's Law is particularly helpful in calculating the heat of reaction for reactions that are impractical for direct measurement. It uses known intermediate reactions, adding their enthalpy changes to obtain the desired reaction's heat change. This approach enhances our ability to explore energetic patterns in complex chemical reactions.

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Most popular questions from this chapter

These are various forms of energy: chemical, heat, light, mechanical, and electrical. Suggest ways of interconverting these forms of energy.

A quantity of \(2.00 \times 10^{2} \mathrm{~mL}\) of \(0.862 \mathrm{MHCl}\) is mixed with an equal volume of \(0.431 M \mathrm{Ba}(\mathrm{OH})_{2}\) in a constant-pressure calorimeter of negligible heat capacity. The initial temperature of the \(\mathrm{HCl}\) and \(\mathrm{Ba}(\mathrm{OH})_{2}\) solutions is the same at \(20.48^{\circ} \mathrm{C}\), For the process $$\mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)$$ the heat of neutralization is \(-56.2 \mathrm{~kJ} / \mathrm{mol}\). What is the final temperature of the mixed solution?

A 46-kg person drinks 500 g of milk, which has a "caloric" value of approximately \(3.0 \mathrm{~kJ} / \mathrm{g}\). If only 17 percent of the energy in milk is converted to mechanical work, how high (in meters) can the person climb based on this energy intake? [Hint: The work done in ascending is given by \(m g h,\) where \(m\) is the mass (in kilograms), \(g\) the gravitational acceleration \(\left(9.8 \mathrm{~m} / \mathrm{s}^{2}\right),\) and \(h\) the height (in meters).]

Suggest ways (with appropriate equations) that would enable you to measure the \(\Delta H_{\mathrm{f}}^{\circ}\) values of \(\mathrm{Ag}_{2} \mathrm{O}(s)\) and \(\mathrm{CaCl}_{2}(s)\) from their elements. No calculations are necessary.

Define these terms: system, surroundings, open system, closed system, isolated system, thermal energy, chemical energy, potential energy, kinetic energy, law of conservation of energy.
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