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The First Law of Thermodynamics Thermochemistry Chemical Thermodynamics and the Equilibrium State | What does it mean that a chemical system is in a state of equilibrium? When we look at the chemical equation, you have reagents on the left and products on the right. At equilibrium, the probability of the system moving a little bit to the right and forming a bit of product equals to the probability of the system moving a little bit to the left and forming a bit of reagent. This is the meaning of the term 'microscopic reversibility'. Small changes from the equilibrium state are reversible, i.e. nonspontaneous.
H = U + PV. That is the enthalpy of the system. If you picture a small change in a system at equilibrium, you are imagining a change in the enthalpy of the system, a bit of heat flows in or out. At equilibrium, these heat flows are reversible.
When we talk about heat flow into or out of a system at equilibrium, what we are describing is a heat flow that does not change the total entropy of the universe. If the reaction goes a bit in the exothermic direction and heat flows out into the environment, the entropy increase in the surroundings is exactly balanced by a decrease in disorder in the system.
When small changes occur at equilibrium, the entropy change of the environment due to heat flow and the entropy change of the system must be equal, [qrev = T(SB - SA) ] then wrev = HB - HA - T(SB - SA) or wrev = GB - GA [GB = HB - TSB]. What develops from this little bit of math is the idea of the Gibbs free energy. Don't worry about solving quantitative problems for the MCAT so much. What is important is the concept, one of the most important in physical science.
If change in the forward or reverse direction would increase the entropy of the universe, either by letting heat flow out into the environment or by increasing disorder in the system, or some combination, in which entropy change due to heat flow is not compensated by entropy change do to the system's order, then the system is not at equilibrium. Spontaneous changes are occurring. Heat flows are not reversible. Free energy is available for either the forward or reverse direction. Things will spontaneously change until the reagents and products have equal free energy. Then we are at the equilibrium state.
These are difficult concepts. Give yourself some time to think about it.
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Work, Energy, and Power Electricity Intermolecular Forces Thermochemistry The States of Matter Chemical Thermodynamics and the Equilibrium State | Here are some perspectives on entropy differences among states of matter to help improve your understanding of the concept of entropy. Comparing a solid to a liquid, generally, the distance between molecules (or atoms in an elemental crystal) is about the same but the average ligancy is less is the liquid. In other words, particles in a liquid have fewer bonding partners, so in terms of entropy, the increase of entropy in the fluid can be computed similarly to the computation of the entropy of mixing where there has been the addition and mixture of holes.
A second interesting perspective: with the transition from a liquid to a gas, the entropy of vaporization (which equals the ratio of the heat of vaporization to the temperature) is nearly constant across most substances. Higher temperature boiling substances require the addition of more heat to boil, because the boiling point is a measure of the amount of molecular agitation necessary to overcome intermolecular force; the boiling point is an indication of the magnitude of these forces. At vaporization, the kinetic energy of the particles, measured by the temperature, has grown sufficient for the random interplay of energy among its possible forms within the substance to increase the electrostatic potential energy along lines of intermolecular force sufficiently enough to lead to molecular separation. This relative balance between the electrostatic potential energy increase (the heat flow required from the surroundings) and the amount of kinetic energy present in the particles (gauged by the temperature), equals the entropy change (δH/T)).
A third interesting fact is that symmetry of the molecules has a pronounced effect on the melting point, but not the boiling point (the greater the symmetry, the higher the melting point). This is an advanced concept. If molecules are symetrical, then there will not be as great an increase in disorder when they move from solid to liquid. Flipping over does not change anything, so there is not as much order bound up in the crystal, so there is not so great a loss of the order of crystalization in the melting process, which makes melting somewhat less favorable. Think about how much more easily a molecule can go in the reverse direction (back to crystal) if it does not have to hit it just right.
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