Lecture 17- Thermodynamics 3

Question 1

Thermodynamically favored process

Answer 1

• spontaneous
• proceeds without any assistance from outside the system
• Ex. Water evaporates at 25C, iron rusts with O2 and H2O
• NaCl dissolves in water
• A process that is thermodynamically favored in one direction is non-thermodynamically favored in the other direction

Question 2

TFP exothermic

Answer 2

• exothermic reactions are often thermodynamically favored
• nature tends to favor processes that cause a reduction in energy
• In an exothermic reaction, the bonds in the products contain less energy than the bonds in the reactants. The excess energy is released as heat.

Question 3

TFP endothermic

Answer 3

• endothermic reactions can be thermodynamically favored
• evaporation is thermodynamically favored
• dissolving soluble compounds is thermodynamically favored

Question 4

entropy

Answer 4

• a measure of the disorder of a system
• a greater degree is favorable
• a positive value for ^S is favorable
  ^S=S(products)-S(reactants)

Question 5

laws of thermodynamics

Answer 5

1. The energy contained within the universe is constant

2. The entropy of the universe is constantly increasing

Question 6

S>0

Answer 6

• melting
• vaporization
• reactions where the products are in the same phase as the reactants but contain more particles than the reactants
• making most solutions
• adding heat
• increasing volume of gas
• when products have more particles
• volume increases

Question 7

calculating ^S

Answer 7

^S=sum n S(products) - sum n S(reactants)

Question 8

How to tell if a process is thermodynamically favored

Answer 8

• exothermic reactions are favored so ^H0

Question 9

Two methods for determining if a process is thermodynamically favored

Answer 9

1. ^S(universe)- theoretical

2. ^G (Gibbs free energy)

Question 10

^S universe

Answer 10

^S (universe)=^S (system) + ^S (surroundings)

- If ^S>0, the reaction is thermodynamically favored, its its negative then it won’t happen at all

Question 11

Gibbs free energy

Answer 11

^G=^H-T^S

• G= free energy change KJ
• H=enthalpy change KJ, calculate using calorimetry and Hess’s law or the enthalpy of formation values
• T- temperate of system K
• ^S= entropy change KJ/K, calculate using entropy values
• If G

Question 12

Temperature and Thermodynamic favorability

Answer 12

^H and ^S: 
- + =yes at all temps
\+ - = no at all temps
- - =yes at low, no at high 
\+ + = yes at high, no at low

Question 13

Free energy of formation

Answer 13

^Grxn=sum n G(products) - Sum n G (reactants)

Question 14

Find the ^S for the following reactions using the S values given

Answer 14

^S=sum n S(products) - sum n S(reactants)

Question 15

calculate ^G

Answer 15

^G=^H-T^S

Question 16

at what temperature does it become thermodynamically favored?

Answer 16

rearrange ^G=^H-T^S and solve where G=0.

Question 17

why can it still be thermodynamically favored when the entropy of the system decreases so dramatically?

Answer 17

still exothermic and contributes to the entropy of the universe

Question 18

Find the value of g using the Gf vales

Answer 18

^Grxn=sum n G(products) - Sum n G (reactants)

Question 19

Spontaneous signs for G, S, H

Answer 19

spontaneous:

• G -
• S+
• H -

Question 20

the G positive at low temps and negative at high temps. predict sign for H

Answer 20

negative

Question 21

under what conditions does the is process become thermodynamically favored?

Answer 21

when temp is low and G is negative

Question 22

Find S at this temp

Answer 22

^G=^H-T^S rearrange to solve for ^S