Lecture 9: Intro to Thermodynamics

Question 1

2 Basic questions that chemical reactions raise

Answer 1

1. What will happen (what direction will the reaction proceed)
2. How fast? –> dependent on enzymes

Question 2

What determined whether a reaction takes place at all?

Answer 2

• free energy difference btwn products and reactants

- determines whether a reaction will occur spontatneously

Question 3

What is the free energy required to initiate the conversion of reactions to products?

Answer 3

Activation energy

- determines the rate of a reaction

Question 4

What is the first law of thermodynamics?

Answer 4

• the total energy of a system is conserved (just converting one thing to another)
• E is the internal energy – a function that keeps track of heat transfer and work expenditure in the system

ΔE = q + w

q is heat absorbed BY the system
w is work done ON the system

Question 5

What is the second law of thermodynamics?

Answer 5

• system tends toward disorder and randomness
• will go from ordered to disordered states
• all processes proceed toward equilibrium (i.e minimum potential energy)
  ex) protein wants to be folding at the lowest potential energy state

Question 6

What do we mean by “is the process favorable”

Answer 6

• exergonic: energy is released
• favorable reactions increase entropy
• ex) ATP –> Pi + ADP

Question 7

Gibbs Free Energy (G)

Answer 7

• available energy to do work
• refers to the difference in chemical bond energy btwn products and reactants
• energy change as system moves from initial state to equilibrium

Question 8

What does the magnitude of change in G depend on?

Answer 8

• the chemical rxn itself
• how far the system is from equilibrium
• [reactants] and [products] and number of bonds broken and made

Question 9

Free Energy for a reaction (ΔG)

Answer 9

• defined in terms of its enthalpy (H), entropy (S) and temperature (T) at constant pressure
• ΔH refers to the overall change in bond energy when bonds are broken or formed in a reaction (talking about both covalent and non covalent)
• a measure of the distance from the equilibrium of a reaction

Question 10

Relationship to the Equilibrium constant

Answer 10

• Keq = tendency of a reaction to go toward completion
• a large value for Keq indicates that the reaction will proceed until conversion to reactants to products is nearly complete

Question 11

ΔGº

Answer 11

free energy change under standard states

Question 12

ΔG

Answer 12

overall free energy change of the reaction

Question 13

Standard states

Answer 13

for gases, the gas at a pressure of 1 atm, for solutions a concentration of 1 mol/L

Question 14

Standard State reaction

Answer 14

ΔG = ΔGº + RTln [C]^c [D]^d / [A]^a [B]^b

T = degrees Kelvin (273 + ºC)
R = 8.31 J mol-1 K-1

Question 15

ΔGº and the relationship to Keq

Answer 15

set ΔG to 0, so then

ΔGº = -RTlnKeq

Question 16

ΔGº’

Answer 16

• refers to standard states when pH = 7
• need this bc the standard free energy change, ΔGº assumes a concentration of 1 M –> if [H+] = 1M then pH = 0
• but pH in most cells is near neutral

Question 17

Standard state for the concentration of H+ for biochemical reactions

Answer 17

Standard state for [H+] = 10^-7M, pH = 7

this modified standard state is given the symbol ΔGº’

Question 18

What does ΔGº’ indicate?

Answer 18

• whether a reaction releases or requires energy
• the amt of energy involved
• ratio of products to substrates at equilibrium
• point of reference for a comparison of energetics of all cellular reactions (whats more energetic)

Question 19

If Keq > 1 what is the ΔG’º and which way does the reaction proceed?

Answer 19

• negative

- proceeds forwards (spontaneous)

Question 20

If Keq = 1 what is the ΔG’º and which way does the reaction proceed?

Answer 20

• zero

- is at equilibrium

Question 21

If Keq < 1 what is the ΔG’º and which way does the reaction proceed?

Answer 21

• positive

- proceeds in reverse

Question 22

ΔG vs ΔGº’

Answer 22

• whether ΔG is < , > , or = to ΔGº’ depends on the concentration of reactants and products in the cell
• ΔG is therefore the relevant term for living systems

Question 23

How can reactions be made spontaneous?

Answer 23

• not spontaneous based on ΔGº’ but based on changing concentrations of reactants and products