Ch. 19: Free Energy and Thermodynamics

Question 1

spontaneous process

Answer 1

One that occurs without ongoing outside intervention (such as the performance of work by some external force).

_{ex.: book dropped in a gravitational field}

Question 2

Entropy

Answer 2

It is a thermodynamic function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state. Unit is in J/K. It is an extensive property – it depends on the amount of the substance.

S = klnW

k is the Boltzmann constant (the gas constant divided by Avogadro’s number, R/N_A = 1.38 x 10^-23 J/K)

W is the energetically equivalent ways to arrange the components of a system. Unitless

• _{Exothermic process increases the entropy of the surroundings.}
• _{An endothermic process decreases the entropy of the surrounds.}

Question 3

macrostate

Answer 3

The overall state of a system as defined by a given set of conditions (such as P, V, and T).

Question 4

microstate

Answer 4

The exact distribution of internal energy at any one instant among the particles that compose a system.

Question 5

Second law of thermodynamics

Answer 5

For any spontaneious process, the entropy of the universe increases (ΔS_UNIV > 0)

Question 6

Standard entropy change for a reaction (ΔS°_rxn)

Answer 6

Change in entropy for a process in which all reactants and products are in their standard states. Function of state.

ΔSº_rxn = Σn_pS°(products) - Σn_rSº(reactants)

_{n represents the stoichiometric coefficients}

Question 7

standard molar entropies (S°)

Answer 7

A measure of the energy dispersed into one mole of a substance at a particular temperature.

_{Increases with increase in molar mass. (Holds for elements in same state)}

_{Increases with increasing molecular complexity. (translational, rotational, vibrational motion)}

Increases when there is a greater energy dispersal.

Question 8

third law of thermodynamics

Answer 8

The entropy of a perfect crystal at absolute zero (0 K) is zero.

Question 9

allotropes

Answer 9

One of two or more forms of the same element; each form has different structure.

Question 10

Change in entropy for the surroundings

Answer 10

ΔS_surr = -ΔH_sys/T (constant P, T)

ΔS_surr = -q_sys/T

q_sys = ΔH_sys

Question 11

Gibbs free energy (G)

Answer 11

ΔG = ΔH - TΔS

ΔH and ΔG represents the system. T is in kelvins. At constant temperature.

ΔG = -TΔS_univ

ΔG is proportional to the negative of ΔS_univ.

_{ΔS is criterion of spontaneity.}

_{Also called chemical potential becuase it is analogous to mechanical potential energy (tend toward lower chemical potential).}

A decrease (ΔG < 0) corresponds to a spontaneous process.

A increase (ΔG > 0) corresponds to a nonspontaneous process.

Question 12

Effect of ΔH, ΔS, T on Spontaneity

Answer 12

1. ΔH Negative, ΔS positive: ΔG is negative at all temperatures
2. ΔH positive, ΔS negative: ΔG is positive at all temperatures.
3. ΔH negative, ΔS negative: ΔG negative at low temperatures and positive at high temperatures.
4. ΔH positive, ΔS positive: ΔG positive at low temperatures and negative at high temperatures.

Question 13

standard change in free energy (ΔGº_rxn)

Answer 13

The change in free energy for a process when all reactants and products are in their standard states. The more negative it is, the more spontaneious the process – the further it will go toward products to reach equilibrium.

Question 14

free energy of formation (ΔGº_f)

Answer 14

The change in free energy when 1 mol of a compound in its standard state forms from its constituent elements in their standard states. Pure elements in their standard states is zero.

ΔG°_rxn = Σn_pΔG°_f(products) - Σn_rΔG°_f​(reactants)

This method of calculating works only at the temperature for which the free energies of formation are tabulated, namely, 25°C.

Standard free energy of formation of a pure element in its standard state is zero.

Question 15

Calculating ΔG°_rxn for a stepwise reaction

Answer 15

1. If a chemical equation is multiplied by some factor, ΔG_rxn is also multiplied by the same factor.
2. If a chemical equation is reversed, ΔG_rxn changes sign.
3. If a chemical equation can be expressed as the sum of a series of steps, ΔG_rxn for the overall equation is the sum of the free energies of reactions for each step.

Question 16

reversible reaction

Answer 16

A reaction that achieves the theoretical limit with respect to free energy. It occurs infinitesimally slowly, and the free energy can only be drawn out in infinitesimally small increments that exactly match the amount of energy that the reaction produces during that increment.

Question 17

irreversible reactions

Answer 17

A reaction that does not achieve the theoretical limit of available free energy.

Question 18

free energy change of a reaction under nonstandard conditions (ΔG°_rxn)

Answer 18

ΔG°_rxn = ΔG°_rxn + RTlnQ

R = 8.314 J/mol·K

Under standard conditions, Q is always equal to 1.

Under equilibrium conditions, ΔG_rxn is zero. (Not spontaneous at either direction.

Use law of mass action to calculate Q.

i.e.: 2 NO(g) + O₂(g) → 2 NO₂

Q = [P²_NO2]/[P²_NO][P_O2]

Question 19

Relationship of (K) equilibrium constant and ΔG°_rxn

Answer 19

At equilibrium, Q = K and ΔG°_rxn = 0.

So, ΔG_rxn = ΔG°_rxn + RT ln Q will turn into

ΔG°_rxn = -RT ln K

• When K < 1, ln K is negative and ΔG°_rxn is positive. Under standard conditions (when Q = 1), the reaction is spontaneous in the reverse direction.
• When K > 1, the reaction is spontaneous in the forward direction.
• When K = 1, the reaction happens to be at equilibrium under standard conditions.

Small changes in ΔG°_rxn have a large effect on K.

-ln K = [(ΔH°_rxn/R)(1/T)] + [ΔS°_rxn/R]
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