Chemistry ppt

Adiabatic process

• An adiabatic process is one that occurs without transfer of heat or matter between a system and its surroundings. A key concept in thermodynamics, the adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics. For some practical and theoretical purposes, some chemical and physical processes occur so rapidly that they can be conveniently described as an "adiabatic approximation", meaning that there is hardly time for transfer of energy as heat. Such processes are often followed or preceded by processes that are not adiabatic.

• A process that does not involve the transfer of heat into or out of a system Q = 0, is called an adiabatic process, and such a system is said to be adiabatically isolated. The assumption of an adiabatic process or isolation is frequently made when analyzing a system from the stand point of thermodynamics. For example, the compression of the gas within a cylinder of a diesel engine is assumed to occur so rapidly such that on the time scale of the compression process, little of the system's energy can be transferred out as heat. Even though the cylinders are not insulated and are quite conductive, that process is idealized to be adiabatic.

• The assumption of adiabatic isolation is a useful one, and is often combined with other assumptions about a system so as to make the calculation of the system's behavior possible. Such assumptions are idealizations. The behavior of actual machines deviates from these idealizations, but the assumption of such "perfect" behavior are useful first approximations about how the real world works.

First Law of Thermodynamics

• The change in a system's internal energy is equal to the difference between heat added to the system from its surroundings and work done by the system on its surroundings.

• Mathematical Representation of the First Law
• Physicists typically use uniform conventions for representing the quantities in the first law of thermodynamics. They are:
• U1 (or Ui) = initial internal energy at the start of the process
• U2 (or Uf) = final internal energy at the end of the process
• delta-U = U2 - U1 = Change in internal energy (used in cases where the specifics of beginning and ending internal energies are irrelevant)
• Q = heat transferred into (Q > 0) or out of (Q < 0) the system
• W = work performed by the system (W > 0) or on the system (W < 0).
• This yields a mathematical representation of the first law which proves very useful and can be rewritten in a couple of useful ways:
• U 2 - U 1 = delta- U = Q - W
• Q = delta-U + W

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