AP Chemistry/Thermodynamics

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Kinetic Energy[edit]

Energy of an object due to the motion of the object. It is represented: KE=1/2mv2.

Enthalpy versus Entropy[edit]

Enthalpy (H) - heat of a system. Negative enthalpy means that the temperature increases. Entropy (S) - randomness of a substance.

Trends in Entropy[edit]

Dfferent phases have different amount of entropy.

Generally, substances in the phases towards the beginning of the list below have lower entropy, while substances at the end of the list have high entropy.

Solid, Liquid, Aqueous, Gas

Another trend is that when there are more moles of a substance produced and the particles or the particles are more spread out, the entropy increases.

Since, S is the amount of disorder, The delta S (change in entropy) is positive if the disorder increases. The delta S is negative if the disorder decreases. The delta S can be predicted without thermodynamic values using the trends listed above. For example:

N2 (l) --> N2 (g)

This physical change would be predicted to have a positive delta S because gasses are more disordered than liquids.


N2 (g) --> N2 (l)

The reverse process would have a negatve because of amount of disorder in liquids are less than gases and Hess's law appplies to entropy as well as enthalpy.

Entropy also applies to chemical processes with or without a phase change. Chemical reactions without phase changes and changes in the number of moles would have an entropy change close to zero. The same trends can be used to predict the entropy change in a chemical process.

2 H2 (g) + O2(g) --> 2 H2O (l)

Since gasses are on the reactants side and liquids are on the products, the delta S of this reaction is negative. The reverse of this chemical reaction would have a positive delta S.

Other Stuff[edit]

Specific heat capacity - number of joules of heat needed to warm 1 gram of a substance by 1K. Water's specific heat, in case you didn't remember, is 4.184J/g.

First Law of Thermodynamics = Law of Conservation of energy

Second Law of Thermodynamics - spontaneous reactions increase entropy (randomness).

Gibb's Free Energy[edit]

ΔG = ΔH - T ΔS, where T is temperature in Kelvin. If ΔG is negative, a reaction is spontaneous. Note that some reactions are always spontaneous, some are never, and some depend on the temperature. Also, H is measured in kJ, and S in J.