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Chapter 5. Thermochemistry
5.1 The Nature of Energy
• Thermodynamics is the study of energy and its transformations.
• Thermochemistry is the study of the relationships between chemical reactions and energy changes involving heat.
- • Energy is the capacity to do work or to transfer heat.
- • Work is energy used to cause an object with mass to move.
- w = F × d
- • Heat is the energy used to cause the temperature of an object to increase.
- • A force is any kind of push or pull exerted on an object.
- • The most familiar force is the pull of gravity.
Kinetic Energy and Potential Energy
• Kinetic energy is the energy of motion:
• Potential energy is the energy an object possesses by virtue of its position or composition.
• Electrostatic energy is an example.
• It arises from interactions between charged particles.
• Potential energy can be converted into kinetic energy.
System and Surroundings
- • A system is the part of the universe we are interested in studying.
- • Surroundings are the rest of the universe (i.e., the surroundings are the portions of the universe that are not involved in the system).
- • Example: If we are interested in the interaction between hydrogen and oxygen in a cylinder, then the H2 and O2 in the cylinder form a system.
Transferring Energy: Work and Heat
• From physics:
• Force is a push or pull exerted on an object.
• Work is the energy used to move an object against a force.
5.2 The First Law of Thermodynamics
- • The first law of thermodynamics states that energy cannot be created or destroyed.
- • The first law of thermodynamics is the law of conservation of energy.
- • That is, the energy of system + surroundings is constant.
- • Thus, any energy transferred from a system must be transferred to the surroundings (and vice