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| Chem1 Virtual Textbook - Chemical EnergeticsTruly excellent survey of Thermochemistry. Before going deeper into Chemical Thermodynamics and Equilibrium, first make sure you understand Internal Energy change and Enthalpy change, the heart of Thermochemistry. A good understanding of Thermochemistry and Chemical Thermodynamics will make Biochemistry much easier and much more interesting.
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Work, Energy, and Power Electricity Heat and Temperature The Ideal Gas and Kinetic Theory The First Law of Thermodynamics Thermochemistry Chemical Thermodynamics and the Equilibrium State | We need to keep coming at these important concepts, which are not the easiest in science, from different angles. The only way to understand is to keep at it.
Several modules ago in the Main Sequence of this MCAT course, we were discussing the electrical force. We developed our sense of the changes in energy matter undergoes at the atomic, molecular or intermolecular level.
In earlier modules we developed our understanding of Thermochemistry to account for the conservation of energy defining the interplay internal energy, heat flow, and work. Now we are working to develop an understanding of Chemical Thermodynamics to understand what drives spontaneous change.
To help us cross the bridge from Thermochemistry to Chemical Thermodynamics, let us review the path which has led us here from our understanding of the electric force.
Imagine the the thermodynamic system plus its surroundings. Energy moves between the system and its surroundings by either heat flow (quantity of energy transferred across the boundaries of the system by conduction or radiation) or thermodynamic work (a directed force acting through a distance). From the 1st law of thermodynamics, we know that whether a change is simple or complex, the sum of the heat transferred to the system and the work done on the system equals the change in internal energy. The energy added equals what went in. That is it.
What does it mean, in a chemical system for the internal energy to change? For a chemical system, there are two primary kinds of internal energy, really, the kinetic energy of the particles and the electrostatic potential energy associated with charge interactions at the particle level. Most chemical processes involve changing the relationship of charged particles with regard to each other. Ask yourself if in the new arrangement, are opposite charges closer together? Are like charges farther apart? In either of these two cases, internal energy will have decreased.
The internal energy change is determined exactly by the initial and final state of the system independently of the path through which the change has been effected. If the internal energy of the system has decreased, where did the energy go? The first law of thermodynamics, and by extension to thermochemistry, tells us that some combination of heat flow (enthalpy change) and work must have occurred.
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Work, Energy, and Power Electricity Intermolecular Forces Heat and Temperature The Ideal Gas and Kinetic Theory The First Law of Thermodynamics Thermochemistry The States of Matter Chemical Thermodynamics and the Equilibrium State | Let us continue our Thermochemistry review as we proceed into Chemical Thermodynamics with the familiar examples of melting and vaporization.
When heat is added to a solid below its melting point, the temperature begins to rise. Rising temperature means that the average kinetic energy of the particles is increasing (although with greater range of vibration along lines of intermolecular force, the electrostatic potential energy component is also increasing). So picture the molecules vibrating along lines of intermolecular force.
If the solid is heated at its melting point, at the melting point, the temperature remains constant until the solid has melted. The melting process requires the heat of fusion to flow into the matter to allow the particles to escape from the rigid intermolecular binding of the solid state into the less tight liquid arrangement.
When mutually attracting charged particles (the polarities which lead to intermolecular force) are moved apart from one another, the electrostatic potential energy of the system is increased. The heat flow into the matter at the melting point increases the electrostatic potential energy of the particles along lines of intermolecular force.
After all of the solid has melted, heating the liquid can now raise its temperature until the boiling point is reached. The heat of vaporization then causes the liquid to boil at constant temperature.
Similar to the heat of fusion, the internal energy increase brought about by the heat of vaporization is not increasing the average kinetic energy of the particles (the temperature is constant during vaporization) but the electrostatic potential energy increases as the particles are separated each from each other. The heat flow is not only increasing the internal energy. There is also a large change in the volume of the system, so the heat that flows in must also perform pressure-volume work. After all the liquid has been vaporized, the addition of more heat then raises the temperature of the gas. After vaporization, in which the particles escape from their mutual well of electrostatic binding energy, the internal energy increase caused by heat flow solely produces increased particle kinetic energy (and higher temperature).
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