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CHEM463 Lecture Notes - Thermal Energy, Isolated System, Heat Capacity

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Darren De Lorey

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System: part of the universe chosen for study. It can be as large as all the oceans on Earth or
as small as the contents of a beaker.
Surroundings: part of the universe outside the system with which the system interacts.
Open system: free exchanges energy and matter with its surroundings.
Closed system: can exchange energy with its surroundings, but not matter.
Isolated system: does not interact with its surroundings.
Energy: the capacity to do work.
Work: done when a force acts through a distance. Moving objects do work when they slow
down or are stopped.
Kinetic Energy: the energy of a moving object.
Potential energy: due to condition, position, or composition; it is an energy associated with
forces of attraction or repulsion between objects.
Thermal energy: kinetic energy associated with random molecular motion. When the
potential energy in the ball is converted to kinetic energy found in the ball, the surface and the
surrounding air. In general, thermal energy is proportional to the temperature of a system. The
more vigorous the motion of the molecules in the system, the hotter the sample and the greater is
its thermal energy. However, the thermal energy of a system also depends on the number of
particles present. A small sample at a high temperature such as 75ºC may have less thermal
energy than a pool at 30ºC.
Heat: energy transferred between a system and its surroundings as a result of temperature
difference. At the molecular level, molecules of the warmer body, through collisions, lose kinetic
energy to those of the colder body. Thermal energy is transferred until the average molecular
kinetic energies of the two bodies are equal, thus the temperatures are equal. Change in
temperature can also change a state of matter. Energy is required to overcome the attractive
forces in a given state of matter. During the transition phases from one state to the next, the
temperature remains constant, while the thermal energy is raised and used to overcome the forces
holding the matter together. This is said to be Isothermal. The energy of a system is called
internal energy. Heat is simply a form in which a quantity of energy may be transferred across a
boundary between a system and its surroundings. Therefore, the quantity of heat, q, required to
change the temperature of a substance depends on:
How much the temperature is to be changed.
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