Lecture Notes on Chemical Thermodynamics PDF

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Chemistry lecture notes on thermodynamics, spontaneity, Gibbs free energy, and entropy with equations, meanings of values, and graphs to help explain how to use the equations from Dr. Learn's course ...

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Chemical Thermodynamics: Study of the energy changes that occur in a chemical reaction Spontaneous vs. Nonspontaneous Change: ● Spontaneous- Proceeds without the external input of energy ○ Occurs without outside assistance until finished ○ Some endothermic reactions occur spontaneously (ice melting, evaporation of water) ■ Delta H (change in heat) and the change in energy are positive ■ Heat is absorbed and energy enters the system ○ Many reactions which occur spontaneously are exothermic (iron rusting, fuel burning) ■ The delta H (change in heat) and the energy of the system are negative ■ The change in heat is one factor that influences spontaneity ○ Tend to favor: ■ Decrease in energy ■ Increase in disorder (entropy) ● Nonspontaneous- External input of energy needed to keep the process going ○ Occurs only with outside assistance ○ Never occurs by itself ● Any process that is spontaneous in one direction will be nonspontaneous in the other direction ● Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures ● Energetics and entropy determine the spontaneity of a reaction Entropy (symbol S): ● Thermodynamic quantity ● It is a statistical probability that determines the disorder of a system ● Less disorder, S is smaller ● More disorder, S is bigger ● S is NEVER negative ● 2nd law of thermodynamics- the entropy of the universe is increasing over time ● Entropy is a state function. It is independent of path. ● Factors that affect entropy: ○ Volume ■ For gases, entropy increases as volume increases ○ Temperature ■ Entropy decreases as temperature decreases and vice versa (3rd law of thermodynamics) ○ Physical state ■ Solid has the lowest entropy and gas has the highest entropy ○ Number of particles ■ The more particles present, the more entropy Entropy changes in chemical reactions (calculations): ● Reactions involving gases:

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○ Calculate the change in the number of moles of gas, or the delta n of the gas ○ If the delta n of the gas is positive, then the delta S is positive Reactions without gases: ○ Calculate the number of moles of molecules ○ Delta n is equal to the n of the products minus the n of the reactants ○ If delta n is positive, entropy increases ○ More molecules mean more disorder

= [(sum S° products) – (sum S° reactants)] 1. Write and balance the reaction 2. Use a table of thermodynamic data of S to calculate ● ●

Delta H only varies slightly with temperature Delta S is highly dependent on temperature

Gibbs Free energy equation: ● ΔGrxn = ΔHrxn  – T ΔSrxn ○ ΔHrxn is the heat of reaction ○ T is the temperature in Kelvin ○ ΔSrxn is the entropy of reaction ○ ΔGrxn is the Gibbs Free Energy  ● ΔGrxn or Gibbs Free Energy, is the ultimate, final deciding factor as to whether a reaction  will occur spontaneously, anywhere in the universe ○ When ΔGrxn = – #, the reaction is spontaneous. ○ When ΔGrxn = + #, the reaction is nonspontaneous. ○ When ΔGrxn =  0, the reaction is at equilibrium (has no tendency to go one way or the other)...