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Some Key Principles In Undergraduate Chemistry Notes

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Some Key Principles in Undergraduate Chemistry

1. Second Law of Thermodynamics

The Second Law of Thermodynamics determines whether a chemical reaction may take place or not. It is empirically derived.

Second Law: In a spontaneous process the entropy of the Universe increases.

Entropy is a property of matter. It is a measure of randomness or disorder.

The Universe consists of the SYSTEM and the SURROUNDINGS.

Heat (the transfer of energy by random molecular motion) leads to an increase in entropy, as it increases the motion of the molecules. The lower the temperature the greater the increase in entropy.

In an exothermic process, heat is given out to the surroundings, increasing the entropy of the surroundings.

In an endothermic process, heat is taken in from the surroundings, decreasing the entropy of the surroundings.

The entropy change of the system can be measured.

To calculate the entropy change of the Universe, simply sum together the entropy changes of the system and the surroundings.

Temperature is important since it influences how important the heat transfer is. The lower the temperature the greater the influence of the change in enthalpy of the reaction (under constant pressure).

The entropy change of the universe is given by:
ΔSuniv = ΔSsys + ΔSsurr

However, it is not convenient to consider the surroundings explicitly, so the equation is manipulated and expressed in terms of the system alone. This results in the GIBBS FREE ENERGY.

First, express ΔSsurr in terms of the enthalpy change of the reaction. Next multiply all terms by negative Tsys, and the result is the Gibbs energy change of the system:
ΔG = ΔH - TΔS

Daniel Day

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