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Hess's Law and Conservation of Energy

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A series of free High School Chemistry Video Lessons.

In this lesson, we will learn

  • Conservation of Energy
  • Second Law of Thermodynamics
  • Entropy
  • Hess’s Law

Conservation of Energy
Conservation of energy, sometimes called the first law of thermodynamics, states that energy can neither be created nor destroyed. Energy, however, can be transferred from one form to another.
How energy is neither created nor destroyed.
Second Law of Thermodynamics
The Second Law of Thermodynamics can be rephrased in several ways. Fundamentally, it says that heat always flows from hot objects to cold objects (unless work is exerted to make it flow the other direction). It can also be expressed using the concept of entropy as saying that the system's entropy will always naturally increase if no work is exerted to decrease it. These rephrasings mean fundamentally the same thing because heat deals with kinetic energy and increasing a system's kinetic energy will increase the system's entropy.
Understanding and applying the second law of thermodynamics.

Entropy measures the amount of disorder in a system. Nature tends towards disorder, so as time elapses, entropy naturally increases. Energy is required in order to decrease entropy.
Understanding the concept of entropy.
A discussion of what entropy is and what it isn't.

Hess's Law
Hess's Law states that the energy of a chemical reaction is the same regardless of the number of steps needed or the reaction mechanism. Hess's law is directly related to the law of conservation of energy.
Understanding Hess' Law.
This chemistry tutorial covers how to solve for the enthalpy of reaction for an given reaction by using Hess's Law and the delta H values for other known chemical reactions. This tutorial involves several examples demonstrating the use of Hess's Law, which allows for the calculation of an unknown enthalpy of reaction from other reactions due to the fact that enthalpy is a state function.


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