Thermodynamics and Thermochemistry

Question 1

Different thermodynamic systems

Answer 1

• Isolated system: no exchange of heat, work, or matter with the surroundings.
• Closed system: exchange of heat and work, but not matter with the surroundings.
• Open system: exchange of heat, work and matter with the surroundings.

Question 2

State function

Answer 2

• A state function is path-independent and depends only on the initial and final states.
• State functions include: ΔH (enthalpy), ΔS (entropy), ΔG (free energy change), ΔU (internal energy change).

Question 3

Conservation of energy

Answer 3

• The total energy of an isolated system remains constant.
• The total energy of a closed or open system plus the total energy of its surroundings is constant.
• Total energy is neither gained nor lost, it is merely transferred between the system and its surroundings.

Question 4

Endothermic/exothermic reactions

Answer 4

• Endothermic = energy is taken up by the reaction in the form of heat. ΔH is positive.
• Exothermic = energy is released by the reaction in the form of heat. ΔH is negative.

Question 5

Formation Reaction

Answer 5

A formation reaction is where a compound or molecule in its standard state is formed from its elemental components in their standard states. The standard state is where things are in their natural, lowest energy, state. For example, oxygen is O₂(diatomic gas) and carbon is C (solid graphite).

Question 6

Hess’ law of heat summation

Answer 6

ΔH_rxn = sum of ΔHf (products) - sum of ΔHf (reactants)

Question 7

Bond dissociation energy as related to heats of formation

Answer 7

• Bond dissociation is the energy required to break bonds.
• ΔHrxn = Bond dissociation energy of all the bonds in reactants - bond dissociation energy of all the bonds in products
• ΔHrxn = Enthalpy of formation of all the bonds in products - Enthalpy of formation of all the bonds in reactants.
• Bond dissociation energy is positive because energy input is required to break bonds.
• The enthalpy of formation of bonds is negative because energy is released when bonds form.

Question 8

Heat Capacity

Answer 8

Heat capacity = the amount of heat required to raise the temperature of something by 1 °C.

Question 9

Entropy

Answer 9

Entropy = measure of disorder = energy / temperature = J / K

Question 10

Relative entropy for gas, liquid, and crystal states

Answer 10

• Entropy of gas > liquid > crystal states.
• At room temperature, the gas molecules are flying around, but the table in front of you is just sitting there. So, gases have more disorder.
• Reactions that produces more mols of gas have a greater increase in entropy.

Question 11

Free energy G

Answer 11

• Free energy is the energy available that can be converted to do work.
• ΔG = ΔH - TΔS
• T is temperature in Kelvin.

Question 12

Spontaneous reactions

Answer 12

• Spontaneous reactions are reactions that can occur all by itself.
• Spontaneous reactions have negative ΔG.

Question 13

When can an exothermic reaction be non-spontaneous?

Answer 13

large, negative ΔS can cause it to become nonspontaneous

Question 14

When can an endothermic reaction be spontaneous?

Answer 14

large, positive ΔS can make it spontaneous.

Question 15

Zeroth Law of Thermodynamics

Answer 15

• 0th law of thermodynamics basically says that heat flows from hot objects to cold objects to achieve thermal equilibrium.
• Mathematically, if T_A = T_B, and T_B = T_C, then T_A = T_C. Where T is temperature.

Question 16

First Law of Thermodynamics

Answer 16

1st law of thermodynamics is based on the principle of conservation of energy, and it basically says that the change in total internal energy of a system is equal to the contributions from heat and work.

Question 17

ΔE = q + w

Answer 17

• ΔE is the same thing as ΔU, which is the change in internal energy.
• Q is the contribution from heat
  
  • Q is positive when heat is absorbed into the system (ie. heating it).

Q is negative when heat leaks out of the system (ie. cooling it).

• W is the contribution from work.
  
  • W is positive when work is done on the system (ie. compression).

W is negative when work is done by the system (ie. expansion).

Question 18

Equivalence of mechanical, chemical, electrical and thermal energy units

Answer 18

• If it’s energy, it’s Joules. It doesn’t matter if it’s potential energy, kinetic energy, or any energy - as long as it’s energy, it has the unit Joules.
• Energy is equivalent even if they are in different forms. For example, 1 Joule of mechanical energy can be converted into 1 Joule of electrical energy (ignoring heat loss) - no more, no less.

Question 19

Second Law of Thermodynamics

Answer 19

• The 2nd law states that the things like to be in a state of higher entropy and disorder.
• An isolated system will increase in entropy over time.
• An open system can decrease in entropy, but only at the expense of a greater increase in entropy of its surroundings.
• The universe as a whole is increasing in entropy.

Question 20

Reversible and irreversible processes as related to entropy and temperature

Answer 20

For reversible processes ΔS = q / T.

For irreversible processes ΔS > q / T.

Question 21

Conduction

Answer 21

heat transfer by direct contact. Requires things to touch.

Question 22

Convection

Answer 22

heat transfer by flowing current. Need the physical flow of matter.

Question 23

Radiation

Answer 23

heat transfer by electromagnetic radiation (commonly in the infra-red frequency range). Does not need the physical flow of matter, can occur through a vacuum.

Question 24

Heat of fusion

Answer 24

the energy input needed to melt something from the solid to the liquid at constant temperature.

Question 25

Heat of vaporization

Answer 25

the energy input needed to vaporize something from the liquid to the gas at constant temperature.

Question 26

The energy it takes to melt a solid

Answer 26

ΔH_fus x #mols of that solid

Question 27

The energy it takes to vaporize a liquid

Answer 27

ΔH_vap x #mols of that liquid

Question 28

PV diagram

Answer 28

work done = area under or enclosed by curve

Question 29

Adiabatic process

Answer 29

no heat exchange, q = 0. ΔE = W

Question 30

Isothermal process

Answer 30

no change in temperature ΔT = 0.

Question 31

Isobaric process

Answer 31

pressure is constant, W = PΔV.

Question 32

Isovolumetric (isochoric) process

Answer 32

volume is constant, W = 0. ΔE = q

Question 33

Calorimetry

Question 34

How much energy does it take to heat ice from -20 °C to water at 37 °C (Theoretical Approach)

Answer 34

For the ice phase from -20 °C to 0 °C, use q = mc_iceΔT, where ΔT is 20.

For the phase transition, use heat of fusion: q = ΔHfus x #mols of ice/water, where ΔHfus is in energy per mol. (note: if the heat of fusion is given in energy per mass, then you should multiply it by the mass to get energy)

For the water phase from 0 °C to 37 °C, use q = mc_waterΔT, where ΔT is 37.