C - Thermodynamics

Question 1

What is the First Law of Thermodynamics?

Answer 1

Energy cannot be created or destroyed.

This is known as the law of conservation of energy

Question 2

What is specific heat capacity?

Answer 2

The gradient of a plot of energy against temperature at constant pressure

Cp = dH / dT

Question 3

What is 1 calorie?

Answer 3

The energy change needed to increase the temperature of 1g of water by 1°C

1 calorie = 4.2 J

Question 4

What is the equation for work done?

Answer 4

E = Fx

where F = force constant

and x = distance

Question 5

What is the equation for gravitational potential energy?

Answer 5

PE = mgh

Question 6

What is the equation for kinetic energy?

Answer 6

E = ½ mv²

Question 7

What is the equation for the energy of an individual photon?

Answer 7

E = hν

Question 8

What is a calorimeter?

Answer 8

An insulated device in which the temperature change associated with a specific chemical or physical change can be measured. It is normally calibrated electrically.

It has a stirrer, a thermometer, and a lid to keep constant pressure

eg. school calorimeter, ‘bomb’ calorimeter

The latter has the material in excess oxygen, and when given a spark it combusts very quickly

Heat evolved: qp = n x Cp x ΔT = ΔH

Question 9

What is the equation for power dispersed?

Answer 9

P = dE / dt

P measured in Watts = J/s

Question 10

What is Kleiber’s law?

Answer 10

Power output, P ∝ M^3/4

Question 11

What do the terms universe, surroundings and adiabatic mean?

Answer 11

Universe = both system and surroundings

Surroundings = everything around the system that has physical contact with it; may or may not exchange energy and matter

Adiabatic = Energy is transformed only in the form of work

Question 12

What do the terms system, and this being open, closed or isolated, mean?

Answer 12

System = the chemical reaction or mechanism under observation

Open = system can exchange energy and matter with the surroundings

Closed = system can exchange energy but not matter with the surroundings

Isolated = no exchange can occur

Question 13

Describe the Maxwell-Boltzmann distribution.

Answer 13

It is the distribution of energy of atoms in a gas. It is weighted towards lower speeds, and tends towards zero.

As temperature increases, the curve flattens and moves towards the right, given that with increasing temperature the probability of finding particles at higher energy increases.

The highest point on the curve, the stationary point, is the modal energy of the atoms.

Question 14

How can the distribution within energy levels be described?

Answer 14

There is an exponential relationship.

As temperature increases, the higher energy states become more populated.

Question 15

What is the equation for predicting the occupation of a particular energy level (Maxwell-Boltzmann distribution)?

Answer 15

(population in state En / population in state Eo ) = e^{-(En - Eo)/kT}

if considering individual molecules, k = Boltzmann constant, 1.38 x 10^-23 J/K

if considering moles, k = R (gas constant) = 8.314 J

Question 16

What is the Zeroth Law of Thermodynamics?

Answer 16

Energy flows between objects so as to equalise their temperatures.

Temperature is therefore to a liquid as pressure is to a gas.

Alternatively, if two systems A and B are individually in equilibrium with a third system, C, then A and B are also in equilibrium with each other.

Question 17

What is the difference between intensive properties and extensive properties?

Answer 17

Intensive properties do not depend on the amount of material, whereas extensive properties do

eg. temperature = intensive, volume = extensive

Question 18

What is temperature?

Answer 18

Temperature determines the distribution of thermal energy in a system.

Higher temperatures imply faster molecules or more populated higher energy levels.

Question 19

What do the terms U, q and w mean?

Answer 19

U = internal energy ie the sum total of all possible energies in the system

q = heat transferred to the system

w = work done on the system

Question 20

What is a state function? Give an example of one.

Answer 20

A state function is path independent; it does not matter how the change has come about, but the value of the change does matter.

U is a state function as they are denoted with capital letters.

Question 21

What is heat?

Answer 21

The transfer of energy due to a difference in temperature between the system and the surroundings.

This results in a change in the random motion at the molecular level in the system.

Question 22

What is work?

Answer 22

The transfer of energy to or from the system that results in a coordinated motion of molecules of a system.

Question 23

What does it mean if q > 0 or q < 0 ?

Answer 23

q > 0 = energy is moving from the surroundings to the system = endothermic

q < 0 = energy moving from the systems to the surroundings = exothermic

Question 24

What does it mean if w > 0 or w < 0 ?

Answer 24

w > 0 = surroundings do work on the system = system gains energy

w < 0 = system does work on the surroundings = system loses energy

Question 25

What happens to ΔU, q and w if pressure says constant?

Answer 25

At constant pressure q = q_p w = -pΔV q_p = ΔU + pΔV = ΔH

Question 26

What happens to ΔU, q and w if both pressure and volume stay constant?

Answer 26

ΔU ≈ ΔH

Question 27

What is enthalpy?

Answer 27

Enthalpy is the thermal energy transferred in a process at constant pressure. It is a state function and extensive.

Question 28

What does it mean when ΔH \> 0 or ΔH \< 0 ?

Answer 28

ΔH \> 0 = energy is moving from the surroundings to the system = endothermic ΔH \< 0 = energy is moving from the system to the surroundings = exothermic

Question 29

What is Kirchoff's Law? What does it allow us to do? What is its limitation?

Answer 29

ΔH_T2 = ΔH_T1 + Cp_{T2 − T1} This allows us to correct enthalpies under standard conditions to physiological temperatures. It can only be applied to small temperature changes,

Question 30

What is isothermal calorimetry and what is it used for?

Answer 30

Two chambers are linked by feedback mechanisms. A constant power is applied to the sample cell in order to keep its temperature the same as the standard cell. Acid or salt is injected into the sample cell to unfold the protein. A ligand could be added instead to measure binding enthalpy. The added substance causes a change in temperature; power decreases if sample heats up, power increases if sample cools down. Power input is then measured over time

Question 31

What is the Second Law of Thermodynamics?

Answer 31

The entropy of any isolated system not in thermal equilibrium almost always increases

Question 32

What is entropy the ratio of?

Answer 32

The ratio of heat transferred to temperature: ΔS = q/T

Question 33

What does entropy establish?

Answer 33

Entropy establishes the direction of spontaneous change, but it does not tell its speed

Question 34

What is the equation for maximum efficiency of work?

Answer 34

w_max = q_transferred ( 1 - [T_cold / T_hot] )

Question 35

What does entropy measure?

Answer 35

Entropy is the measure of the "quality of energy", and how narrowly it is distributed. In other words, it is the measure of disorder.

Question 36

What is the equation for entropy in terms of microstates?

Answer 36

S = k_B ln W where k_B = constant of proportionality (Boltzmann constant) W = microstates

Question 37

What is the order of entropy for solids, liquids and gases?

Answer 37

Gases → liquids → solids Most entropy → least entropy

Question 38

What is the equation for the mole fraction, x?

Answer 38

x_A = moles of A / total moles of everything in sample

Question 39

What is the equation for molality?

Answer 39

molality = moles of solute (moles) / mass of solvent (kg)

Question 40

What does isothermal mean?

Answer 40

Isothermal is a system that doesn't rely on temperature differences to do work

Question 41

What is the equation for ΔG under constant pressure?

Answer 41

ΔG = ΔH - T ΔS only under constant temperature and pressure

Question 42

What happens when ΔG \> 0 , ΔG \< 0 and ΔG = 0 ?

Answer 42

ΔG \> 0 → endergonic ΔG \< 0 → exergonic ΔG = 0 → equilbrium

Question 43

What is the chemical potential, µ?

Answer 43

µ measures the driving force for a process. It is a measure of how G changes per molecule.

Question 44

What is the equation for ΔG°?

Answer 44

ΔG° = - RT ln K_eq

Question 45

What is Gibbs free energy?

Answer 45

The energy from a change in the system that will be available to do non-pV work

Question 46

Will the reaction take place if both ΔH and ΔS are positive?

Answer 46

The reaction will not be spontaneous at low temperatures. Above a critical temperature, it will switch over to spontaneous behaviour.

Question 47

Will the reaction take place if ΔH is negative and ΔS is positive?

Answer 47

The reaction will be spontaneous at all temperatures.

Question 48

Will the reaction take place if ΔH is positive and ΔS is negative?

Answer 48

The reverse reaction will be spontaneous at all temperatures.

Question 49

Will the reaction take place if both ΔH and ΔS are negative?

Answer 49

The reaction will be spontaneous at low temperature. Above a critical temperature, the reverse reaction will be favoured.

Question 50

What are osmosis and diffusion driven by?

Answer 50

Chemical potential

Question 51

What are mixing and unmixing driven by?

Answer 51

Entropy

Question 52

What causes protein folding and membrane formation?

Answer 52

They are driven by entropy. Clustering into micelles or folded proteins reduces the contact of non-polar molecules with the solvent. When proteins form membranes they release water, thereby increasing the entropy.

Question 53

What is a spontaneous process?

Answer 53

A process that occurs in a system without any input of energy from the surroundings. It is favoured by a decrease in H and an increase in S.

Question 54

What is the relationship between k_B (Boltzmann constant) and R (gas constant)?

Answer 54

In the equation for the Maxwell-Boltzmann distribution, using k_B gives the answer in J/molecule, where R gives the answer in J/mol. This is because k_B = R / Na where Na = Avogadro's number

Question 55

What is the enthalpy of combustion?

Answer 55

The enthalpy change when one mole of a compound burns in oxygen to produce H₂O and CO₂ under standard conditions.

Question 56

What is the enthalpy of formation?

Answer 56

The enthalpy change when one mole of a compound is formed from its constituent elements, with all substances in their standard states at standard conditions.

Question 57

How can the enthalpy of combustion be determined?

Answer 57

Calorimetry

Question 58

What is the Third Law of Thermodynamics?

Answer 58

For any perfect crystal at 0K, the entropy is zero.

Question 59

What is specific heat?

Answer 59

The energy needed to increase the temperature of 1g of a substance by 1°C C = q / ΔT

Question 60

Does an increase in gravitational potential energy cause an increase in U?

Answer 60

Yes because it is positive work (+w)

Question 61

What is the formula for expansion work?

Answer 61

w = -p x ΔV

Question 62

What is the thermodynamic standard state?

Answer 62

The most stable state of a substance at 1 atm, 25°C and 1M of all substances in solution

Question 63

What is the standard enthalpy of reaction?

Answer 63

The enthalpy change for a reaction measured under standard conditions

Question 64

What is the molar heat capacity of water?

Answer 64

4.186

Question 65

What is Raoult's Law?

Answer 65

The partial vapour pressure of a component in a mixture is equal to the vapour pressure of the pure component at that temperature multiplied by its mole fraction in the mixture. The freezing and boiling points of an ideal solution are respectively depressed and elevated relative to that of the pure solvent by an amount proportional to the mole fraction of solute.

Question 66

BASF’s Insulator Micronal PCM is a polystyrene foam filled with specialised wax droplets. How does it work to heat buildings?

Answer 66

During the heat of the day (≈23°C), the wax melts (endothermic) and so stores potential chemical energy, which is then released at cooler times as thermal energy via the wax solidifying (exothermic). This keeps the temperature of the building constant; the wax absorbs energy when heat is excessive and releases energy when the building cools below comfortable temperatures.