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Energy Changes in Chemical Reactions

   

   

Thermochemistry: Energy Changes in Chemical Reactions

Thermochemistry deals with transfers of energy between reacting chemicals and the world around them.
Systems, Surroundings, and Boundaries
The word system refers to that particular part of the universe we wish to study. The system might be the chemicals reacting in a beaker or the chemicals in a battery cell reacting to give electricity, or the system of a living cell.

The word surroundings refers to whatever is entirely outside the defined system, everything in the universe except the system itself.

A boundary, real or imaginary, separates the system from its surroundings. When the system is in a beaker, the boundary exists wherever the solution contacts the beaker or the air above it. If the boundary can prevent any transfer of heat between the system and the surroundings, we say that the system is insulated from its surroundings. Styrofoam makes a good insulating boundary for keeping a cup of coffee hot, but no material is a perfect insulator.

Another term that is used frequently is the state of a system. Each system has a state defined by listing its temperature, pressure, volume, and composition (including concentration terms). We say that a system undergoes a change of state whenever any change occurs in one or more of the variables that define the system.
 

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