Gases and the Zeroth Law of Thermodynamics

Iuch of physical chemistry can be presented in a developmental manner One can grasp the easy ideas first and then progress to the more challenging ideas, which is similar to how these ideas were developed in the first place. Two of the major topics of physical chemistry—thermodynamics and quantum mechanics— lend themselves naturally to this approach. 

Chemistry understands that matter is composed of atoms and molecules, so we will also need to understand how physical chemical ideas relate to these particles that is, we can take a molecular approach to the topic. We will adopt this approach many times in the next few chapters. 

In chemistry, the study of large, or macroscopic, systems involves thermodynamics in small, or microscopic, systems, it can involve quantum mechanics. In systems that change their structures over time, the topic is kinetics. But they all have basic connections with thermodynamics. We will begin the study of physical chemistry with thermodynamics the study of heat and work in chemistry. 

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At equilibrium the two gases are at the same temperature (the zeroth law of thermodynamics), so we could, if we wanted, define the temperature to be T = (3/2). However, because the scale of temperature we use is degrees Kelvin, we need a constant of proportionality. This is k (Boltzmann s constant) when n is the number of molecules [JcT = (3 2)) and R, the gas constant, when n is the number of moles (RT - (3 2)]. If you stop and think for a moment, this result immediately gives a physical meaning to the absolute temperature as the point where molecular motion ceases. 

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