2 - Thermodynamic Concepts and Colligative Properties

Question 1

What is Enthalpy (H)?

Answer 1

The heat flow into the system while at constant pressure. Often referred to as heat content.

Question 2

What is a useful way to imagine the Change in Enthalpy?

Answer 2

The change in energy of the system due to the difference in the initial and final bond energies in a reaction.

Question 3

What is entropy?

Answer 3

Qualitatively a measure of evenness of energy distribution.

Quantitatively a measure of the unavailabilty of a system’s thermal energy to do mechanical work.

Question 4

What is the second law of thermodynamics?

Answer 4

Spontaneous change in an isolated system is always accompanies by an increase in entropy.
Alternatively; a system with constant volume and internal energy will adopt a conformation that maximises its entropy.

Question 5

What (qualitatively) is Gibbs free energy?

Answer 5

The maximum amount of energy in a system that is available to do non-mechanical work under constant temperature and pressure.

Question 6

What, technically, is the qualitative definition of temperature?

Answer 6

The measure of how much the internal energy of a system with constant volume changes when the entropy changes.

Question 7

What is the heat capacity of a system?

Answer 7

How much the energy changes with varying temperature.

Alternatively heat supplied to a system divided by change in temperature, at constant volume and pressure.

Question 8

What two ways can heat capacity be quantified?

Answer 8

Cp - Change in enthalpy per change in unit temperature, at constant pressure.

Cv - Change in internal energy per change in unit temperature, per unit volume.

Question 9

What assumptions are made when calculating heat capacity?

Answer 9

That the heat capacity itself does not change with temperature, which is not always true.
Also that the heat transfer is quasi-static.

Question 10

What are intensive and extensive properties.

Answer 10

Intensive properties are independent of the amount of the substance, extensive properties change with an increase or decrease in the amount of a substance.

Question 11

Is heat capacity an intensive or extensive property?

Answer 11

Extensive, as more energy is required to increase the temperature of more substance by the same degree.

Question 12

What two ways are heat capacity made into an intensive property of a substance?

Answer 12

Specific heat capacity - energy required to raise the temperature of 1kg of a substance by 1 degree.

Molar heat capacity (Cp,m) - energy required to raise the temperature of 1 mole of a substance by 1 degree.

Question 13

What equation defines the Standard Equilibrium Constant, K°, and also is notable for relating thermodynamic quantities to chemical ones?

Answer 13

ΔG° = -RT ln(K°)

Question 14

What are the chemical standard state conditions?

Answer 14

1 bar
298.15K (not technically a requirement, but assumed)
1 MolKg‾¹
Hence pH0 when H+ is involved
All assuming ideal-dilute conditions.

Question 15

What values are different for biological standard states?

Answer 15

pH7 used

1E-7 MolKg‾¹

Question 16

By finding equilibrium at constant temperature and pressure what is a system doing?

Answer 16

Adopting the conformation with the lowest Gibbs Free Energy.

Question 17

What values are Gibbs Free Energy dependent upon?

Answer 17

Temperature
Pressure
Moles of each component of a system

Question 18

What provides thermodynamic driving force for a reaction?

Answer 18

A reduction in free energy (-ΔG).

Question 19

Describe the phenomenon of equilibrium in terms of Free Energy.

Answer 19

G changes differently with the number of moles of each reaction participant. Whenever G is not at a minimum it can reach a lower value by changing the composition of the reaction participants until it is its lowest value at the given temperature and pressure.

Question 20

What is the chemical potential of a component?

Answer 20

How much the free energy of the system will change when the amount of the component changes.

i.e.; the slope of the free energy when plotted against changing number of moles.

Question 21

What are the relative chemical potentials of two components of a system at equilibrium?

Answer 21

Equal across all phases.

µA = µB

Question 22

What important factor is not technically part of the standard state definition?

Answer 22

Temperature.

Question 23

What are colligative properties?

Answer 23

Properties of a dilute solution that depend upon the nature of the solvent and the amount of solute but NOT on the nature of the solute.

Question 24

Why do colligative properties arise?

Answer 24

They arise solely due to the reduction in chemical potential of the solvent as a result. This is a purely entropic effect, being based on the number of solute particles.

Question 25

What assumptions do we make when discussing colligative properties?

Answer 25

That the solute, B, is non-volatile and that it will not dissolve in a solid form of the solvent, A. Also assumes a dilute solution.

Question 26

What does the word colligative mean?

Answer 26

Depending on the collection.

Question 27

What are the four colligative properties?

Answer 27

Vapour Pressure
Boiling Point
Freezing Point
Osmotic Pressure

Question 28

What is the cryoscopic effect?

Answer 28

The depression of a solution’s freezing point through colligative effect of increased solute. Various information about the solute can be gained from examination of this (cryoscopy).

Question 29

What information about the solute can be derived from cryoscopy (measuring the change in freezing point with changing [solute]?

Answer 29

The molar mass, with knowledge of the mass of the solvent and the solute dissolved.
This can be used to find an approximation of the extent of dissociation, α, of an acid (or other).
The extent of dissociation can then be used to find the dissociation constant, Ka.

Question 30

What equation relates osmotic pressure to molarity?

Answer 30

The Van’t Hoff equation.

Question 31

What is osmometry?

Answer 31

Finding the molecular mass of a molecule, often a macromolecule by measurement of osmotic pressure at the ‘mass concentration’ of the solute in gdm‾ᵌ.

Question 32

What is the energetic explanation of solute presence increasing boiling temperature?

Answer 32

It lowers the chemical potential (hence stabilising) the solution, but as it is non-volatile it has no effect on the chemical potential of the vapour. Hence G can be minimised by some extra molecules condensing.

Question 33

What is the entropic/molecular explanation of solute presence increasing boiling temperature?

Answer 33

The presence of the solute increases the entropy of the solution. As it is not volatile it does not decrease the entropy of the vapour, hence the solution has greater entropy. This means that entropy is maximised when there are more molecules in liquid phase.