Thermodynamics

Question 1

What is the equation for energy change?

(first law of thermodynamics)

Answer 1

• ΔE = q + w
  • E (or U) stands for internal energy (total energy, kinetic + potential)
  • First law of thermo: Internal energy can be transferred as heat or work, but not created or destroyed.
    
    • Work is organized energy. Heat is chaotic energy.
      
      • Within work, there is PV work (expansion of a gas), and non-PV work (electrical, free energy).
• When volume is constant: ΔE = q_v
  • _​​Heat at constant volume. (Bomb calorimetry gives this.)

Question 2

What is enthalpy and its equations?

Answer 2

• Just like energy change, but for reactions under constant pressure
• ΔH= q_p
  
  • _​heat at constant pressure. (Solution calorimetry gives this.)
• To calculate the difference between energy and enthalpy:
  
  • ΔH= ΔE + PΔV
  • can also be written as ΔH= ΔE + Δn_gasRT

Question 3

What is a spontaneous process, and where does it take us?

Answer 3

• A reaction that occurs by itself, and continues without outside assistance
• It takes us to equilibrium, Q=K
  
  • Ex: adding or removing a reactant or product, or changing volume or temperature is spontaneous. Le Chatlier’s principle.
• It takes us to the minimum free energy, ΔG=0
• Most exothermic reactions occur spontaneously
• Most reactions that increase entropy will occur spontaneously
  
  • But either the enthalpic or entropic term can take over, so if possible calculate ΔG = ΔH−TΔS

Question 4

What drives nonspontaneous reactions?

Answer 4

• Spontaneous reactions, some source of energy
  
  • Ex: photosynthesis needs sunlight

Question 5

What is entropy (S) and equations for it?

Answer 5

• A state function for the randomness/dispersal of energy, a measurement of disorder in a system
• positive ΔS = increase in entropy of a system
  • ΔS = S_final – S_initial
• Two ways to think about entropy:
  
  1. Statistically, the more ways energy can be distributed in a system, the higher the entropy
     
     • S = k * lnW
       
       • ​where k = Boltzman’s constant, and W = the number of microstates, the ways of distributing energy, Ω. Plug in.
  2. How much heat is transferred. (Heat is chaotic energy whereas work is disorganized energy)
     
     • ΔS = q_rev/T or ΔS = ΔH/T
• Sidenote: We can get an absolute value for S, not just its change (unlike ΔE and ΔH)
• Common units for S: J/mol*K

Question 6

What factors influence the magnitude of entropy, and how?

Answer 6

ΔS = S_final – S_initial

• Volume: For gases, entropy increases with volume.
  
  • When Δn_gas is positive, so is the entropy change
• Temperature: Entropy increases with temperature
• Heat: Entropy increases with heat.
  
  • At one temperature, the amount of heat transferred in a reversible process is your change in entropy. (Applies to phase change.)
• Physical state: the greater freedom of molecular movement, the higher the entropy
• # Particles: Entropy increases with the number of particles in a system, all else being equal
• Molecular complexity: Entropy increases with complexity
  • Especially without doube bonds
  • Ex: beta carotene, a long molecule, has more ways to distribute energy by wiggling around
• Forming mixtures

Question 7

What three factors can influence spontaneity?

Answer 7

​

ΔG = ΔH - TΔS

• Enthalpy change
  
  • Generally, enthalpy has the greater influence on spontaneity. (ΔH is in KJ/mol, ΔS in J/Kmol.)
• Entropy change
  
  • Entropy affects spontaneity. Entropy of the UNIVERSE predicts spontaneity.
• Temperature
  
  • The free energy of the system changes with temperature

Question 8

What is entropy change for the universe?

(2nd law of thermodynamics)

Answer 8

ΔS^o_universe = ΔS^o_system + ΔS^o_surroundings

• A spontaneous change increases the entropy of the universe
• A process is considered to be spontaneous under standard conditions if ΔS^o_universe > 0
• Calculate entropy change of universe:
  
  • ΔS^o_surroundings = -ΔH^o_system/T ADD TO
  • ΔS^o_system

Question 9

What is the Gibbs free energy function?

Answer 9

ΔG = ΔH - TΔS

Good Honey Tastes Sweet

• Combines enthalpy and entropy into a state function
  
  • ΔG = G_final - G_initial
• _​At constant temperature and pressure, a change can only be spontaneous if ΔG is negative
• G, the free energy, is the amount of electrical work a system can do
• Can be measured under any set of conditions. If the data are collected under standard-state conditions, the result is the standard-state free energy of reaction (ΔG^o).

Question 10

What is the difference between exergonic and endergonic?

Answer 10

• Exergonic: free energy decrease ΔG<0
• Endergonic: free energy increase ΔG>0

(extra vocab from book)

Question 11

• Determine spontaneity when:
  
  • • ΔH and + ΔS
  • +ΔH and - ΔS
  • ΔH and ΔS have same sign

Answer 11

ΔG = ΔH - TΔS

• When ΔH is negative (exo) and ΔS is positive, always spontaneous
• When ΔH is positive (endo) and ΔS is negative, never spontaneous
• When ΔH and ΔS are both positive, it will be spontaneous only at high temp
• When ΔH and ΔS are both negative, it will be spontaneous only at low temp
• When one is favorable and one isn’t, spontaneity will depend on temperature. When both are unfavorable, not spontaneous at any temperature.

Question 12

What is the third law of thermodynamics?

Answer 12

• The entropy of a perfect crystal at 0K, is 0.
• But that doesn’t exist. So all compounds in the world have positive molar entropy values

Question 13

What is standard entropy,

and how do you calculate entropy change in a reaction?

Answer 13

• The standard molar entropy, S°, of a substance is the entropy gained by converting 1 mol of it from a perfect crystal at 0 K to standard state conditions at the specified temperature.
• All substances have positive entropy values at temperatures above 0 K. Tabulated.
• Calculate standard molar entropy of a reaction by balancing chem equation, and plugging into S_products - S_reactants.
  
  • Don’t forget to multiply entropies by stoich coefficients, and look at the correct phase

Question 14

Give an example of a process that is:

• Exothermic and spontaneous
• Endothermic and spontaneous
• Exothermic and nonspontaneous
• Endothermic and nonspontaneous

Answer 14

• Exothermic and spontaneous
  
  • iron rusting
• Endothermic and spontaneous
  
  • cold pack - dissolving ammonium nitrate in water
• Exothermic and nonspontaneous
  
  • combustion in engine, requires a spark
• Endothermic and nonspontaneous
  
  • photsynthesis, requires sunlight
• So although exothermic processes tend to be favorable, they aren’t always spontaneous

Question 15

What is the entropy change at a phase change?

Graph and give equation.

Answer 15

• Entropy increases slightly with temperature
• At a phase change, the temperature is constant (isothermal process), but there is a big increase in entropy
• Calculate the entropy change for a phase change:
  
  • ΔS = q_rev/T or ΔS = ΔH/T
    
    • ΔH for the phase change is tabulated. Plug in the T at which it’s happening.

Question 16

If a reaction is spontaneous in the forward reaction,

it is _________ in the reverse.

Answer 16

nonspontaneous

(and vice versa)

Question 17

Kinetically favored vs.

thermodynamically favored

Answer 17

• The thermodynamically favored product is more stable, has lower energy
• The kinetically favored product has a lower activation energy, will form faster/more easily
  
  • Ex graph: the premium can opener is thermodynamically favored. The cheap one is kinetically favored.
• Bottom line: thermodynamics has nothing to do with the rate of a reaction
  
  • Even if something is spontaneous, it’s not necessarily going to be fast

Question 18

A reaction proceeds spontaneously towards what?

Answer 18

• Toward the minimum in free energy, which corresponds to equilibrium
  
  • Hint: slide down the curve

Question 19

Connect ΔG to equilibrium

Answer 19

• ΔG (not standard, any conditions) can be spontaneous in forward or reverse direction
• If it’s not spontaneous in either direction, you’re at equilibrium.
  
  • When Q=K, you’re at equilibrium, which means that you are at the minimum of free energy that your system can be at, ΔG = 0
• At equilibrium, you can solve for K using this equation -

Question 20

What is turnover temperature, and how do you find it?

How does this differ from phase change?

Answer 20

• Turnover is the temperature at which a reaction flips to spontaneous or nonspontaneous
• Using ΔG = ΔH - TΔS, set ΔG = 0
  
  • The answer is given as a range - above or below a certain threshold
• For a phase change, also set ΔG = 0
• Can do this because during a phase change, equilibrium exists between phases, so G = 0
• However, the answer is just that particular temperature (not a range)

Question 21

What does the sign of ∆G° indicate?

Answer 21

∆G° = – RT ln K

• The ratio of the amount of products to reactants at equilibrium
• The thermodynamic favorability of the reaction
• If ∆G° is negative at equilibrium, then we will have lots of products at equilibrium, K>1
• If ∆G° is positive at equilibrium, then we will have lots of reactants at equilibrium, K<1

Question 22

What does the sign of ∆G indicate?

Answer 22

• ∆G = 0 at equilibrium. (NOT ∆G°)
• Since with ∆G we are NOT necessarily at equilibrium, the sign of ∆G can be thought of as a predictor about which way the reaction will go.
  • If ΔG is negative, the reaction is moving forward towards product. (Slide down, forwards.)
  • If ∆G is positive, then to allow ∆G to reach zero, the reaction will need to from more reactants. (Slide down, backwards.)
• Look at the slope of the curve for the sign of ∆G, or use this equation to solve:
• ∆G = ∆G° + RT ln Q

​

Question 23

For a pure element, what is ΔG^o_formation, ΔH^o_formation, and ΔS^o_formation?

Answer 23

• ΔG^o and ΔH^o​ are 0
• ΔS^o​ has its own value

Question 24

Enthalpy driven vs. entropy driven

Answer 24

• Enthalpy driven if exothermic
  
  • Enthalpically favored
• Entropy driven if an endothermic reaction occurs because of a highly positive ΔS
  
  • Entropically favored
  • Ex: solution of ammonium nitrate
• Use the Gibbs equation to determine the relative importance of the enthalpy and entropy terms as driving forces behind a particular reaction

Question 25

Why does changing Temperature change K_eq?

Answer 25

• If you change the temperature past the turnover temperature, you change the free energy of all of the substances, and that changes the spontaneity of all of the system.
  
  • To get turnover temperature, set ΔG=0.

ΔG = ΔH - TΔS

* On the graph, ΔG<sup>o</sup> flips to positive or negative * Remember: If you raise the temperature for endothermic reaction, you make more products, so the K increases.

Question 26

Rewrite the ΔG^o equation as a line.

Answer 26

• ΔG^o = -ΔS^o(T) + ΔH^o​
• y = mx + b
  
  • The y intercept is the change in enthalpy. The slope is the negative change in entropy.

Question 27

STP vs. standard conditions

Answer 27

• In this chapter, we’re focusing on standard conditions/states. Indicated by superscript: Δ_rG^o
  
  • Standard if you have all your reactants and products at 1 molar concentrations for solutions, or 1 bar for gases
    
    • What you have in your beaker in your lab usually isn’t at standard state. So this is just useful as a reference point.
    • ΔG (no superscript) applies under any conditions, more useful
• STP is most commonly used when performing calculations on gases. 273 K (or 0° Celsius or 32° Fahrenheit) and 1 atm pressure.

Question 28

What is an equilibrium mixture?

Answer 28

• Composition of the reaction mixture at equilibrium
  
  • For most reactions, the minimum of free energy includes some reactants and some products
• If that minimum point occurs closer to the right (product side) then it’s a product favored reaction, vice versa

Question 29

Interpret this graph and label the points.

(Spontaneous in which direction? Q=K, Q>K, Q<k>
</k>

Answer 29

Graph of ΔG vs. rxn progress

• Spontaneous in the forward direction under standard conditions
• Q<k>
  </k><li><span>Q=K at the equilibrium point, and the reaction is product favored</span></li><li>Q&gt;K on the product side of the curve, spontaneous in the reverse direction</li><li>
  <p>ΔG^o is negative (the difference between all products and all reactants) so product favored</p>
  </li>

</k>

Question 30

What can thermodynamics be used to determine?

Answer 30

• The temperature at which a rxn is spontaneous
• The direction in which a rxn is spontaneous
• The extent to which a rxn occurs
• The entropy change of a rxn
• NOT the rate of a rxn (kinetics)

Question 31

What is the equation to relate free energy and equilibrium?

Answer 31

ΔG= ΔGº + RTln(Q)

ΔGº = -RTln(K)

• Hint: grtlnk
• K is called the thermodynamic equilibrium constant
  
  • Kc for solution or Kp for gases.
• The grtlnk equation is just for ΔG^o

Question 32

What are all four laws of thermodynamics?

(0 through 3)

Answer 32

Question 33

When does ΔG^o = ΔG?

Answer 33

• When the reactants and products are in standard state conditions
  
  • Then all are 1 M, and lnQ = 0
  • ΔG = ΔG^o + RTlnQ