Lecture 9: Intro to Thermodynamics Flashcards
2 Basic questions that chemical reactions raise
- What will happen (what direction will the reaction proceed)
- How fast? –> dependent on enzymes
What determined whether a reaction takes place at all?
- free energy difference btwn products and reactants
- determines whether a reaction will occur spontatneously
What is the free energy required to initiate the conversion of reactions to products?
Activation energy
- determines the rate of a reaction
What is the first law of thermodynamics?
- 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
What is the second law of thermodynamics?
- 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
What do we mean by “is the process favorable”
- exergonic: energy is released
- favorable reactions increase entropy
- ex) ATP –> Pi + ADP
Gibbs Free Energy (G)
- 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
What does the magnitude of change in G depend on?
- the chemical rxn itself
- how far the system is from equilibrium
- [reactants] and [products] and number of bonds broken and made
Free Energy for a reaction (ΔG)
- 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
Relationship to the Equilibrium constant
- 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
ΔGº
free energy change under standard states
ΔG
overall free energy change of the reaction
Standard states
for gases, the gas at a pressure of 1 atm, for solutions a concentration of 1 mol/L
Standard State reaction
ΔG = ΔGº + RTln [C]^c [D]^d / [A]^a [B]^b
T = degrees Kelvin (273 + ºC) R = 8.31 J mol-1 K-1
ΔGº and the relationship to Keq
set ΔG to 0, so then
ΔGº = -RTlnKeq
ΔGº’
- 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
Standard state for the concentration of H+ for biochemical reactions
Standard state for [H+] = 10^-7M, pH = 7
this modified standard state is given the symbol ΔGº’
What does ΔGº’ indicate?
- 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)
If Keq > 1 what is the ΔG’º and which way does the reaction proceed?
- negative
- proceeds forwards (spontaneous)
If Keq = 1 what is the ΔG’º and which way does the reaction proceed?
- zero
- is at equilibrium
If Keq < 1 what is the ΔG’º and which way does the reaction proceed?
- positive
- proceeds in reverse
ΔG vs ΔGº’
- 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
How can reactions be made spontaneous?
- not spontaneous based on ΔGº’ but based on changing concentrations of reactants and products
