Chem Final: Entropy and Gibbs Free Energy

Process for finding entropy of reaction

"n' means moles, meaning coefficients 1) Calculate enthalpy (how much heat it absorbed) with hess's law- ndelta H products - ndelta H reactants 2) Calculate entropy of reaction (nS products - nS reactants)

3 ways to calculate ∆Grxn

(1) from ΔH° andΔS∘ (2) from free energies of formation (only at 25 °C) (3) from the ΔG°'s of reactions that sum to the reaction of interest.

Methods for Calculating the Free Energy of Formation (ΔG∘rxn)

(1) ΔG∘rxn=ΔH∘rxn−TΔS∘rxn (2) ΔG∘rxn=∑npΔG∘f(products)−∑nrΔG∘f(reactants) (3) ΔG∘rxn (overall)=ΔG∘rxn (step 1)+ΔG∘rxn (step 2)+ΔG∘rxn (step 3)+...

Third Law of Thermodynamics

No system can reach absolute zero

second law of thermodynamics

in every energy transaction, some energy is lost to the surroundings

negative ΔG∘rxn means.. positive ΔG∘rxn means...

negative- theoretical amount of energy available positive- minimum amount of energy to make a nonspontaneous process occur

Predict the sign of delta S: H2(g) + F2(g) -> 2HF(g)

neutral delta S or delta S=0 because equal moles of gas on either side

if ∆G is positive If ∆G is negative

positive- nonspontaneous negative- spontaneous

If ∆S is positive If ∆S is negative

positive- spontaneous negative- spontaneous

If ∆H is positive if ∆H is negative

positive- spontaneous negative- nonspontaneous

∆S for reversible process

qrev/T

Why is delta S useful to know

so we can calculate entropy of reactants/products and figure out why it proceeds

Thermodynamics

study of the spontaneity of reactions

If a system releases heat (exothermic), then delta S (system) and delta S (surrounding) are...

system- negative surrounding- positive

Entropy (s)

the number of energetically equivalent ways in which the components of a system can be arranged and is a measure of energy dispersal per unit temperature

∆Ssurr formula

∆S= -∆H/T

positive delta S means... negative delta S means...

+ = disorder increases - = disorder decreases

Predict the sign of delta S: HgO(s) -> 2HG(l) + O2(g)

+delta S because solid to liquid and gas

Predict the sign of delta S: 2H2S(g) + SO2(g)-> 3S(s) + 2H2)(g)

-delta S because gas to sold AND 2 moles of gas on reactants with 3 moles of solid on products

If ∆G is negative, 1) K will be what 2) Will the products or reactants be favored 3) Spontaneous or nonspontaneous

1) Large (K>1) 2) products favored 3) Spontaneous

4 Rules for Prediction of positive delta S

1) When solid -> liquid, delta S=0 2) When solid -> gas, delta S>0 3) When the number of molecules of a gas increase, delta S>0 4) When temperature increases, delta S>0

Entropy of reaction

a calculation of the change in entropy that occurs when the reactants rearrange into products

spontinaity

a reaction that is thermodynamically capable of happening without outside energy

first law of thermodynamics

energy can be neither created nor destroyed.

Gibbs free energy

function that is proportional to the negative of the change in the entropy of the universe