Question

Hess’s law is really just another statement of the first law of thermodynamics. Explain.

Short answer

In conclusion, Hess's law is indeed another statement of the first law of thermodynamics, as both laws are concerned with the conservation of energy. Hess's law focuses on the enthalpy change in a reaction, which is a state function and depends only on the initial and final states of the reactants and products. The first law of thermodynamics states that the total energy in a closed system remains constant, and it deals with the overall energy change as a sum of heat and work. Therefore, these laws are connected through the concept of conservation of energy, with Hess's law highlighting the enthalpy aspect of energy conservation in chemical reactions.

Step-by-step solution

Define Hess's law

Hess's law states that the total enthalpy change (ΔH) during a chemical reaction is the same whether the reaction occurs in one step or multiple steps. It means that the change in enthalpy of a reaction solely depends on the initial and final states of the reactants and products (state function).

Define the First law of thermodynamics

The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed, but it can only be transformed between different forms (such as chemical, kinetic and potential energy). In a closed system, the total energy remains constant.

Relating Hess's law to the First law of thermodynamics

Both Hess's law and the first law of thermodynamics involve the concept of conservation of energy. The first law of thermodynamics implies that the total energy change in a system is equal to the sum of the energy transferred as heat (q) and the work done (w) by or on the system. ΔE = q + w Enthalpy (H) is a state function and is related to energy (E), pressure (P), and volume (V) by H = E + PV. So, a change in enthalpy (ΔH) in a reaction can be connected to the change in energy (ΔE) of the system. Hess's law can now be related to the first law of thermodynamics by observing that both laws deal with conservation of energy: Hess's law focuses on enthalpy change, while the first law focuses on energy change. Since enthalpy change is related to energy change, Hess's law can be seen as another statement of the first law of thermodynamics.

Conclusion

In conclusion, Hess's law is related to the first law of thermodynamics as both laws revolve around the conservation of energy in a system. Hess's law specifically relates to enthalpy change in a reaction, while the first law of thermodynamics is more general and deals with the overall energy change in a closed system. Thus, Hess's law can be seen as just another statement of the first law of thermodynamics.

Key concepts

First Law of Thermodynamics

The first law of thermodynamics is a foundational concept in physical science. It states that energy cannot be created or destroyed. Instead, energy can only change forms, moving from one type to another. For example, in a chemical reaction, potential energy may change into kinetic energy or heat. This principle is also known as the conservation of energy.
In any closed system, the total amount of energy remains constant, even if it undergoes different transformations. This idea is crucial in understanding thermal dynamics, as it provides a framework for tracking energy flow and transformations.
In mathematical terms, the first law can be expressed as the equation \( \Delta E = q + w \), where \( \Delta E \) is the change in energy, \( q \) is the heat added or removed, and \( w \) represents the work done by or on the system. This equation shows that the internal energy change is accounted for by the heat and work added to the system.

Enthalpy Change

During a chemical reaction, enthalpy change ( abla H) represents the heat absorbed or released under constant pressure. It serves as a measure of energy transferred and is depicted as the difference between the enthalpies of the products and reactants.
Reactions can either absorb energy, making them endothermic, or release energy, making them exothermic. abla H provides useful information about these energy changes.
Since enthalpy is a state function, its value depends only on the current state of the system, not the path taken to reach that state. This simplification means chemists and physicists can easily calculate energy changes without examining every intermediate step, aligning with the principles of Hess's Law.

Conservation of Energy

The conservation of energy is a principle that acts as a backbone for various scientific laws, including the first law of thermodynamics and Hess's Law. By asserting that energy resources are constant in a closed system, it reinforces the idea that energy can change forms but is never lost.
In chemical reactions, features like enthalpy changes, heat exchange, and work all showcase energy conservation. This principle guarantees that all transformations and interactions within a system can be accounted for, providing a clearer understanding of energy dynamics.
The essence of conservation is not only pivotal in thermodynamics but also extends to disciplines such as physics and engineering.

State Function

State functions are special properties or quantities that determine the condition or "state" of a system, such as energy, temperature, and pressure. The unique attribute of state functions is that their values remain independent of how the system arrived at its present state.
This means that whether the system underwent a complex path of transformation or a direct change, the function's value solely depends on the system's current condition. This greatly simplifies analyses because intermediate variables or pathways need not be considered.
For example, the enthalpy change ( abla H) is a state function. Its value depends solely on the initial and final states of the chemical reaction, rather than any intermediary processes. This independence makes calculations more straightforward and aligns with the principles of both Hess’s Law and the first law of thermodynamics.