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HyperPhysics - First Law of Thermodynamics

HyperPhysics - Internal Energy

PY105 Notes - The first law of thermodynamics
Excellent, clear presentation of the 1st Law of Thermodynamics including discussion of model thermodynamic processes.



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Special points of emphasis

Work, Energy, and Power

The Ideal Gas and Kinetic Theory

The First Law of Thermodynamics

Let us take a kinetic theory perspective on why a gas loses energy during an expansion. The gas particles within a piston cylinder are in constant collision with the surface of the piston. If the piston is expanding, the piston head is moving away as the gas particles collide with its surface. During the collision, the particles exert force. A force upon an object moving over a distance performs mechanical work and expends energy. A gas contained within an expanding volume performs work and a gas contained in a contracting volume receives work. The particles, having performed work on the piston head as it expands, rebound with less energy. Likewise, if the piston head moves inward, they particles rebound with more energy. Think about what happens to the energy of a tennis ball if it collides with a racquet moving along with its direction of motion versus one that collides moving directly into the motion of the ball. If no other energy transfers are taking place (no heat flow; i.e. the process is adiabatic) the temperature of the gas will still be changing. In other words, an adiabatic expansion results in a decreased temperature while an adiabatic compression results in temperature increase.



Work, Energy, and Power

Electricity

Atomic Theory

Periodic Properties

The Chemical Bond

The First Law of Thermodynamics

Thermochemistry

Thermochemistry in General Chemistry is conceptually analogous to the discussion of the First Law of Thermodynamics in Physics. In both topics, work means the same thing, i.e. the product of the pressure and the change in volume of the system. Furthermore, heat flow, is the identical concept in both physics and chemistry, although chemistry usually references heat flow as a change in a state function enthalpy. The concept of enthalpy, as a state function, is very useful in chemistry because, among other uses, it allows heat flows to be summed over alternative pathways.

However, there is a big difference though between chemistry and physics in the concept of internal energy, though, which is a much richer idea in chemistry. We begin the discussion with the ideal gas whose internal energy is only the kinetic energy of the particles. In chemistry, though, the internal energy is much more complicated. It helps to understand that there really are two primary kinds of internal energy relevant to chemistry. These are the kinetic energy, which will have rotational and vibrational modes in addition to translational modes in most chemical substances, and the electrostatic potential energy associated with the arrangement of charged particles (the protons and electrons in a particular atomic, molecular, or intermolecular arrangement). Crucial to being able to conceptualize chemical change in the context of the First Law of Thermodynamics is a clear sense of the electrostatic potential energy changes in chemical substances as they impact internal energy change. The internal energy changes associated with a chemical process are often directly determined by the rearrangement of charge as bonding or intermolecular arrangements have undergone transformation.




Work, Energy, and Power

Electricity

Intermolecular Forces

The First Law of Thermodynamics

Thermochemistry

Let's use the First Law of Thermodynamics to interpret something we are all familiar with: boiling a liquid. According to the First Law of Thermodynamics, there are two ways a thermodynamic system exchanges energy with its surrounds, heat flow and pressure-volume work.

The first stage of boiling a liquid is to heat the liquid to the boiling point. This is practically an isovolumetric process (although there can be interesting problems with the slight volume changes of liquids with changing temperature). If the volume isn't changing, no macroscopic work is being performed, so, as a consequence of the First Law of Thermodynamics, the heat flow into the system must only be devoted to increasing the internal energy. The temperature is increasing. The internal energy change is being reflected in an increase in the average kinetic energy of the particles.

At the boiling point, however, the temperature is constant. The input of heat is still increasing internal energy, but this internal energy increase is in the form of increasing electrostatic potential energy between the molecules. The mutually attracting particles are overcoming the intermolecular forces that bind them as they escape from one other into the gas phase.

It is important to remember that increasing electrostatic potential energy is not all which is being accomplished by the heat flow into the system during vaporization. As the liquid is transformed into vapor, the system must also expand macroscopically. This means that work is being performed against the surroundings, pressure-volume work. In summary, in terms of the First Law of Thermodynamics, a portion of the heat of vaporization is going into the increase in electrostatic potential energy associated with intermolecular force as particles escape from each other, and a portion of the heat of vaporization performs the pressure-volume work necessary to expand the volume of the system.








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