Thermodynamics and chemical equilibrium: thermodynamic functions and 'activity'

Question 1

What are solids?

Answer 1

characterised by having rigid structures in which the constituent particles are packed tightly together; solids have a fixed rigid state

Question 2

What does crystalline mean?

Answer 2

the constituent particles are arranged in regular, ordered patterns

Question 3

What defects can a crystal have?

Answer 3

• vacancies - an atom/molecule is missing from its place in the regular pattern
• interstitial defects - an extra atom/molecule in a position between the usual locations in the crystal
• line defects - e.g. where a plane of the regular pattern stops abruptly in the middle of the crystal
• planar defects - complete misalignment between the crystal structure on either side of a plane through the solid

Question 4

What is the term for solids that are not crystalline?

Answer 4

amorphous

Question 5

What are amorphous solids?

Answer 5

the constituent particles occupy well-defined positions but are not arranged in regular patterns and lack ‘long-range order’

Question 6

What are liquids?

Answer 6

the particles are free to move relative to one another; some regions are locally ordered, but this ‘short-range order’ is transient and constituents are in constant motions; no ‘long-range order’

Question 7

Which states of matter belong to the class ‘fluids’?

Answer 7

liquids and gases

Question 8

What is a gas?

Answer 8

the constituents are in constant random motion; very low density

Question 9

Define pressure of a gas

Answer 9

P= F / A

the pressure of a gas is related to the force exerted by the gas molecules every time they collide with the sides of the contained

Question 10

Define the force exerted by a molecule during a collision with the wall

Answer 10

F(i) = d(m(i)v(i))/dt = m(i) dv(i)/dt

Question 11

What is the implication of velocity on force?

Answer 11

the greater the component of the molecule’s velocity in the direction of the wall immediately before the collision, the greater the force exerted by the molecule during the collision

Question 12

Which parameters have a direct effect on gas pressure?

Answer 12

n, T, V

Question 13

What is the ideal gas equation?

Answer 13

pV = nRT

Question 14

What is a plasma?

Answer 14

a gas in which a portion of the atoms or molecules has been ionised, meaning that they have been charged by the removal of electrons; the electrons themselves tend to remain part of the plasma, which is therefore generally close to electrically neutral

Question 15

What interactions are found in plasmas?

Answer 15

although the constituents of a plasma are not directly bound to each other, they interact strongly via the electromagnetic fields that they generate as they move; as a result of these strong interactions, the behaviour of plasma is very different from that of a gas

Question 16

What is a liquid solution?

Answer 16

a homogeneous mixture of two or more distinct chemical species in the liquid phase

Question 17

Why isn’t milk a solution?

Answer 17

it consists of a mixture of fatty molecules in water, forming an emulsion, since the fats and water never produce a homogeneous single phase

Question 18

Why isn’t a pure liquid a solution?

Answer 18

more than one chemical species must be present

Question 19

What is relative permittivity?

Answer 19

also known as the dielectric constant, ε(r); a measure of how well the solvent is able to store electrical energy by concentrating lines of electrical flux; a measure of polarity

Question 20

What is viscosity?

Answer 20

η; a measure of the resistance to flow

Question 21

What are electrolytes?

Answer 21

ionic solutes

Question 22

What do the properties of he solute molecules in solution depend on?

Answer 22

the nature of the solvent and the amount of solute

Question 23

What is an ideal solution?

Answer 23

in an ideal solution, the solute molecules interact with the solvent molecules in exactly the same way as they interact with each other and so in solution behave as if they were in a pure liquid; the closer to this ideal one can get, the easier it is to dissolve solute in solvent

Question 24

Into which categories are van der Waals forces split?

Answer 24

• Keesom forces - between molecules with permanent dipole moments (asymmetric charge distributions)
• Debye forces - between one molecule with a permanent dipole and one with a transient (induced) dipole moment
• London forces - between two molecules, neither of which has a permanent dipole; the interaction is a result of mutual instantaneous dipoles

Question 25

When dissolving a highly polar or ionic substance, what should be taken into consideration?

Answer 25

a high value of ε(r)

Question 26

What is molarity?

Answer 26

C(B) in mol.dm^-3

defined as number of moles of B present in a given volume of solution

C(B) = n(B)/V

Question 27

What does molarity depend on?

Answer 27

molarity depends on T because V depends on T (as the solution is heated, it will expand)

Question 28

What is molality?

Answer 28

b(B) in mol.kg^-1

the amount of B dissolved in a known mass of solvent

b(B) = n(B) / m(A)

where m(A) is the mass of solvent

Question 29

What does molality depend on?

Answer 29

molality does not depend on T, since the mass does not depend on T

Question 30

What is the mole fraction?

Answer 30

x(B), dimensionless

the proportion of the molexules in the solution that are of species B

x(B) = n(B) / [ n(A) + n(B) ]

the sum of the mole fraction for all species in the solution (including the solvent) is equal to 1

Question 31

What does the mole fraction depend on?

Answer 31

the mole fraction does not depend on temperature

Question 32

What is vapour pressure?

Answer 32

the pressure of the gas inside a container at equilibrium

Question 33

What is Raoult’s Law?

Answer 33

P(A) = P(A)* x(A)

where P(A) is the partial pressure of A; P(A)* is the vapour pressure of a pure liquid containing only that species; and x(A) is the mole fraction of A

Question 34

What is the relationship between Raoult’s Law and ideality?

Answer 34

Raoult’s Law always holds true for an ideal solution; Raoult’s Law is considered a consequence of ideality rather than the definition of an ideal solution

Question 35

What is the appearance of a graph of partial pressure against mole fraction for an ideal solution?

Answer 35

straight lines

Question 36

What is the appearance of a graph of partial pressure against mole fraction for a non-ideal solution?

Answer 36

lines are not straight; because the interactions that exist between two molecules of A are quite different in character from those that exist between a molecule of A and of B

but when the mole fraction of A is small (ie. the solution is almost purely B), the partial pressure appears to follow Raoult’s Law

Question 37

Why is it the case that at small values of mole fraction, the partial pressure appears to follow Raoult’s Law?

Answer 37

the small amount of A in the solution means that the vast majority of B molecules only encounter other B molecules; the interactions are therefore almost the same as in pure liquid B and its behaviour is approximately ideal

Question 38

What is Henry’s Law?

Answer 38

P(B) = K(B) x(B)

where K(B) is some experimentally determined parameter

Question 39

What are ideal-dilute solutions?

Answer 39

very dilute solutions in which the solvent obeys Raoult’s Law and the solute obeys Henry’s Law

Question 40

Do macromolecules exhibit ideality?

Answer 40

although they are often present in only very dilute solutions, these solutions tend to exhibit strongly non-ideal characteristics as a result of their bulk, which is made even larger due to the extra solvent cage