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CHEM 481 Lecture Notes _Engle 2nd - Chapter 2_

CHEM 481 Lecture Notes _Engle 2nd - Chapter 2_ - Chapter 2...

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Last printed 1/21/2010 9:40 AM 2-1 Chapter 2- The First Law G ETTING S TARTED As stated earlier (chapter 1), as far as thermodynamics is concerned, the universe can be separated into two parts: (1) the System (part we care about) and (2) the Surroundings (everything else). The First Law of Thermodynamics is concerned with the exchange of energy between the system and the surroundings Energy The capacity of a system to do work As far as thermodynamics is concerned energy can only exchanged between system and surroundings in the form of heat or work . H EAT AND W ORK Work (w): From 1 st year physics we know that work is the movement of an object against an opposing force () m N Units of Energy (J) Work Force Distance 1J 1Nm = ±²³ ²´ ±²³ ² ´± ² ²³ ²²´ For thermodynamics, we need to keep track of who does the work: Work is done ON the system o Compress a gas o Wind a spring Energy (of system) increases Work is done BY the system o Expand a gas o Unwind spring Energy (of system) decreases Molecular Interpretation of Work: Transfer of energy to (or from) the system that causes atoms in the surroundings to move in an organized way F h Example: Lifting a weight

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Last printed 1/21/2010 9:40 AM 2-2 When system does work on its surroundings atoms in surrounding move in organized way Expansion Work o Mass moves upward (organized motion) o System does work o Energy transferred TO surroundings o Mass move downward (organized motion) o Surroundings do work o Energy transferred TO system Heat (q) : Transfer of energy due to a temperature difference between the system and surroundings. When a heater is immersed in a beaker of water, the capacity of the system to do work is increased. This is because hot water can be used to do more work that cold water. When we consider heat transfer between the system and surroundings we need to consider the type of boundary and type of process Energy System Surroundings piston Burn a gas cylinder
Last printed 1/21/2010 9:40 AM 2-3 Process type Endothermic Process absorbs heat Exothermic Process gives off heat Boundary Type Adiabatic (does not conduct heat) System CAN’T get heat from surroundings T decreases System CAN’T give heat to surroundings T increases Diathermic (conducts heat) Heat flows from surroundings T is constant Heat flows to surroundings T is constant Molecular Interpretation of Heat : Heat is the transfer of energy that causes (or makes use of) chaotic molecular motion. Sign Convention: Work and heat refer to CHANGES in the amount of energy (We never say that a system has a certain amount of “work”, for example.) Therefore we must be consistent in the sign on those quantities. In thermodynamics, we adopt the following convention: w,q is NEGATIVE for energy flowing OUT of system w,q is POSITIVE for energy flowing INTO system F IRST L AW OF T HERMODYNAMICS The change in internal energy (U) of a system must be equal to the energy that flows into (or out of) the system in the form of heat and work , i.e.

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CHEM 481 Lecture Notes _Engle 2nd - Chapter 2_ - Chapter 2...

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