Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 17: 4 March

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Thermochemistry

Energy

Law of Conservation of Energy

Energy can be transformed, but is neither created nor destroyed in chemical processes. Thus radiant energy can be transformed into heat which may to used to expand the volume of a gas and vice versa, bond energy can be converted into heat and/or light, etc.

Problem here of course is- What is energy? Energy is the capacity to do work. So what's work? Work occurs when an object (mass) is moved against a force. Some common forms of energy important to our study include:

State Functions

Kinetic energy, potential energy, pressure, and volume are all examples of State Functions. They are all properties that depend only on the current state - they are all independent of the path used to reach this state.

The First Law of Thermodynamics

The First Law of Thermodynamics says that the energy of the Universe is constant. Thus it is another name for the law of conservation of energy. Symbolically it is written:

E = q + w

where E is energy, q is heat, and w is work.

Note that according to this law we can still do things with energy, its just that they are always compensated. (Thus as the Universe expands, work is done against gravity and the heat in the Universe decreases as manifested by a decreasing average temperature.)

Generally in thermodynamics we refer to systems. A system is simply a portion of the universe we wish to work with. For the expression

DeltaE = q + w

where E is the internal energy (the total KE and PE) of the system.

q = the quantity of heat exchanged by the system:

Notice in each case endo- and exo- are in respect to the system, not the surroundings. For example, a fire is exothermic, because heat comes out of the fire - the fuel loses heat, even though you (part of the surroundings) may gain some of it.

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© R A Paselk

Last modified 4 March 2013