Chapter 10: Problem 24
What is Hess's law? Why is it useful?
Short Answer
Expert verified
Hess's Law states that the total enthalpy change of a reaction is constant, irrespective of the path taken. It is useful for calculating the enthalpy change of reactions that cannot be measured directly by using intermediate steps with known enthalpy changes.
Step by step solution
01
Understanding Hess's Law
Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps the reaction is carried out in. This is because enthalpy is a state function, meaning that it depends only on the initial and final states of the system, not on the path taken.
02
Implications of Hess's Law
The law is useful because it allows the enthalpy change (heat change) for a reaction to be calculated even if the reaction does not occur directly. By using a series of intermediate steps with known enthalpy changes, the overall enthalpy change can be determined without directly measuring it.
03
Practical Utility of Hess's Law
Hess's Law is particularly useful in the calculation of enthalpy changes for reactions which are difficult to measure directly, such as combustion reactions. It also allows for the determination of enthalpy changes for hypothetical steps, which can aid in the understanding of reaction mechanisms and energy efficiency.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Enthalpy Change
Enthalpy change is a fundamental concept in thermochemistry, describing the heat absorbed or released during a chemical reaction at constant pressure. It's symbolized by ewline ewline \(ewline ewline \Delta Hewline ewline \), where a negative \(ewline ewline \Delta Hewline ewline \) indicates that the reaction releases heat, and a positive value means that the reaction absorbs heat. This energy change is an intrinsic characteristic of chemical processes and is crucial in determining whether a reaction is endothermic or exothermic.
For clarity, imagine melting ice. The ice absorbs heat from its surroundings, which causes a positive enthalpy change. Conversely, when water freezes, it releases heat to its surroundings, resulting in a negative enthalpy change. Both processes involve the same start and end substances, but the direction of heat flow changes, illustrating the relativity of enthalpy change.
For clarity, imagine melting ice. The ice absorbs heat from its surroundings, which causes a positive enthalpy change. Conversely, when water freezes, it releases heat to its surroundings, resulting in a negative enthalpy change. Both processes involve the same start and end substances, but the direction of heat flow changes, illustrating the relativity of enthalpy change.
State Function
A state function is a property of a system that depends only on its current state, not the path taken to reach that state. Common examples of state functions include enthalpy, entropy, and internal energy.
Consider altitude as an analogy: it doesn't matter whether you hike straight up a mountain or take a winding path, your elevation at the top is the same. Similarly, in chemistry, the enthalpy change between reactants and products is the same, no matter the route the reaction takes. This property is instrumental to Hess's Law, as it assures us that we can use known state functions of intermediate products to calculate the enthalpy changes for reactions that can't be measured directly.
Consider altitude as an analogy: it doesn't matter whether you hike straight up a mountain or take a winding path, your elevation at the top is the same. Similarly, in chemistry, the enthalpy change between reactants and products is the same, no matter the route the reaction takes. This property is instrumental to Hess's Law, as it assures us that we can use known state functions of intermediate products to calculate the enthalpy changes for reactions that can't be measured directly.
Thermochemistry
Thermochemistry is the study of energy and heat associated with chemical reactions and physical transformations. It focuses on concepts like enthalpy change and conservation of energy, providing the framework for understanding how energy is absorbed and released in chemical processes.
Key Principles of Thermochemistry
It is underpinned by the first law of thermodynamics, which asserts that energy cannot be created or destroyed. This principle forms the basis for Hess's Law, as it implies the total enthalpy change for a process - a system's heat content at constant pressure - must remain constant regardless of the path taken. Thermochemistry also helps us predict reaction spontaneity and the feasibility of industrial processes, making it an indispensable tool in both theoretical and applied chemistry.Reaction Mechanisms
A reaction mechanism is the microscopic pathway by which a chemical reaction occurs, detailing the step-by-step sequence of elementary reactions. Understanding a mechanism includes knowing the intermediates, transition states, and the overall kinetic and thermodynamic properties of a reaction.
Hess's Law can be particularly insightful when exploring reaction mechanisms. Though the overall enthalpy change is the primary focus, the law can deduce the enthalpy changes of individual steps within a mechanism, thus aiding in the reconstruction of the full process when direct measurements are impractical or impossible. This deep dive into the 'behind-the-scenes' of chemical reactions not only satisfies academic curiosity but also drives advancements in fields like drug discovery and materials science.
Hess's Law can be particularly insightful when exploring reaction mechanisms. Though the overall enthalpy change is the primary focus, the law can deduce the enthalpy changes of individual steps within a mechanism, thus aiding in the reconstruction of the full process when direct measurements are impractical or impossible. This deep dive into the 'behind-the-scenes' of chemical reactions not only satisfies academic curiosity but also drives advancements in fields like drug discovery and materials science.