Lecture 4: Thermodynamics

Question 1

Types of systems

Answer 1

1. Open: exchange mass and energy
2. Closed: exchange only energy
3. Isolated: don’t exchange anything

Question 2

State functions

Answer 2

1. Independent of the pathway used to achieve the state of eqm of the system
2. 2 types
   2a. Extensive property: sum of all values from every part of the system
   2b. Intensive property: independant of the amount of matter in a system

Question 3

Path functions

Answer 3

1. Dependant on the path followed by a system when moving between 2 states
2. Work and heat

Question 4

2 ways to transfer energy

Answer 4

1. Heat
2. Work

Question 5

3 forms of heat (q)

Answer 5

1. Conduction: direct physical contact
   1a. Objects ability to conduct heat is thermal conductivity (k)
2. Convection: hot air (less dense) rises and cool air (more dense) lowers causing currents
3. Radiation: transfer via electromagnetic waves

Question 6

2 types of work a chemical system at rest can do

Answer 6

1. Non Pressure-Volume work: common in living organisms and batteries as a form of electricity
2. Pressure-Volume work: transferring energy between system and surroundings
   2a. Work done on gas: W=-PV
   2b. Work done on surroundings: W=PV

Question 7

First law of thermodynamics

Answer 7

1. Energy of a system and surroundings is conserved (Energy can’t be created or destroyed)
   1a. E= Q+W

Question 8

Second law of thermodynamics: Carnot cycle

Answer 8

1. Entropy (disorder) always decreases (S) and thermal energy from hot reservoir is converted to work
2. Steps
   2a. Reversible isothermal expansion: P decrease & V increase
   2b. Reversible adiabatic expansion: P decrease
   2c. Reversible isothermal compression: P increase & V decrease
   2D. Reversible adiabatic compression: P increase

Question 9

Second law of thermodynamics: theory

Answer 9

1. Thermal energy can’t be changed completely into work in a cyclical process
2. Work is required to transfer energy from cold to hot reservoir

Question 10

Molecular energies

Answer 10

1. Translational: kinetic energy due to motion of center mass of molecule
2. Rotational: kinetic energy due to rotation around centres of mass
3. Vibrational: when atoms vibrate relative to eachother
4. Intermolecular potential: potential energy via intermolecular molecules
5. Electronic: potential energy via nucleus-electron attraction and electron-electron repulsion
6. Rest mass: E=mc^2

Question 11

Zeroth law of thermodynamics

Answer 11

1. 2 systems in thermal equilibrium with a third system are in thermal equilibrium with eachother : temperature

Question 12

Equipartition theorum

Answer 12

1. States that for a normal system each mode of motion will have the same average energy and each energy will be equal to (!/2)kT

Question 13

Pressure of an Ideal gas

Answer 13

1. PV=(2/3)K

Question 14

Enthalpy (H)

Answer 14

1. H= U +PV
   1a. U=internal energy
   1b. P=pressure
   1c. V=volume

Question 15

Enthalpy (H): Hess’ law

Answer 15

1. Sum of enthalpy changes for each step is equal to total enthalpy

Question 16

Endothermic vs exothermic

Answer 16

1. If enthalpy change is positive=endothermic (put in heat/unfavourable)
2. If enthalpy change is negative=exothermic (release heat/favourable)

Question 17

Entropy (S)

Answer 17

1. Measure of disorder: +ve disorder is favourable
2. Entropy increases with number, volume and temperature
3. S=q/T

Question 18

Gibbs free energy change

Answer 18

1. G<0: spontaneous and G>0=nonspontaneous

Question 19

Gibbs free energy with respect to enthalpy and entropy

Answer 19

1. G=H-TS
   1a. If both H and S are -ve: spontaneous at low T and nonspontaneous at high T (opposite if both positive)
   1b. If H&G=-ve and S=+ve: spontaneous (if opposite: not spontaneous)

Question 20

Reversibility

Answer 20

1. Microscopic perspective: everything is reversible
2. Macroscopic perspective: no reaction is reversible