Lecture 1: Thermodynamics Flashcards
Thermodynamics
The relationship between energy, work, and heat.
Energy
Capacity to do work
Work
Transfer of energy from the system to surroundings that can raise a weight
Heat
Transfer of energy as a result of a difference in temperature
First law of thermodynamics
Any change in the internal energy (U) of a system must equal the transfer of energy as heat or work
Enthalpy
The thermodynamic potential of a system. At constant pressure, enthalpy is equivalent to the amount of heat in the system.
Spontaneous process
Has a tendency to occur without us or anyone else having a part in it.
Spontaneous vs non-spontaneous process
Spontaneous: tends to occur without the input of energy (breaking an egg on the floor)
Non-spontaneous: requires input of energy to occur (reforming of the broken egg)
Second law of thermodynamics
Entropy of a system tends to increase
Entropy (S)
Number of energetically equivalent arrangements of the system (J K-1). Entropy is a function of temperature
Change in entropy > 0
When a spontaneous process has no overall change in energy or entropy
Driving forces of spontaneous processes
- Negative change in enthalpy, meaning heat is transferred from the system to the surroundings
- Increase in entropy from the initial state to the final state
Gibbs free energy
The true indicator of spontaneity. Gibbs relates changes in entropy to enthalpy via temperature.
Mechanical example of Gibbs free energy
We have a weight that can either go up a hill or down a hill. The free energy of the weight at the bottom of the hill is lower than the free energy at the top of the hill.
Positive change in free energy: required to move the weight from the bottom to the top of the hill (endergonic, non-spontaneous process).
Negative change in free energy: when the weight is moving from the top to the bottom of the hill (exergonic, spontaneous process).
Chemical example of Gibbs free energy
If the free energy of the reactants is above the free energy of the products, free energy will be negative and the reaction will be spontaneous.
If the free energy of the reactants is lower than the free energy of the products, free energy will be positive and the reaction will be non-spontaneous.
ΔG ≤ 0 indicates an exergonic rxn.
ΔG > 0 indicates an endergonic rxn
Negative enthalpy and positive entropy
Enthalpically favored and entropically favored. Spontaneous at all temperatures
Negative enthalpy and negative entropy
Enthalpically favored and entropically unfavored. Spontaneous at temperatures below T = change in H/change in S
Positive enthalpy and positive entropy
Enthalpically unfavored and entropically favored. Spontaneous at temperatures above T = change in H/change in S.
Positive enthalpy and negative entropy
Enthalpically unfavored and entropically unfavored. Nonspontaneous at all temperatures
Chemical coupling
In coupled biological reactions, the usual source of free energy comes from ATP. Chemical coupling allows otherwise unfavorable reactions. The high energy ATP molecule reacts directly with the metabolite that needs activation.
Factors affecting free energy change
- Concentrations of reactants and products
- Standard conditions (25 C, 1 atm, pH 7)
Reaction at equilibrium
The net change in free energy for a reaction at equilibrium is 0. At equilibrium, the free energy change of the forward reaction exactly balances that of the reverse reaction
Equilibrium constant vs. temperature
The equilibrium constant is dependent on temperature.
Van’t Hoff equation
Shows the relationship between free energy change and the equilibrium constant. It resembles y = mx +b.
Plotting data using Van’t Hoff
Data can be plotted on a Van’t Hoff graph, where the y-axis is the natural log of the equilibrium constant and the x-axis is 1/T. The slope of the graph is equal to (-) the standard change in enthalpy over the gas constant R. The Y intercept is equal to the standard change in entropy over the gas constant R.
