Thermodynamics and chemical equilibrium: thermodynamic functions and 'activity' Flashcards
What are solids?
characterised by having rigid structures in which the constituent particles are packed tightly together; solids have a fixed rigid state
What does crystalline mean?
the constituent particles are arranged in regular, ordered patterns
What defects can a crystal have?
- 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
What is the term for solids that are not crystalline?
amorphous
What are amorphous solids?
the constituent particles occupy well-defined positions but are not arranged in regular patterns and lack ‘long-range order’
What are liquids?
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’
Which states of matter belong to the class ‘fluids’?
liquids and gases
What is a gas?
the constituents are in constant random motion; very low density
Define pressure of a gas
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
Define the force exerted by a molecule during a collision with the wall
F(i) = d(m(i)v(i))/dt = m(i) dv(i)/dt
What is the implication of velocity on force?
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
Which parameters have a direct effect on gas pressure?
n, T, V
What is the ideal gas equation?
pV = nRT
What is a plasma?
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
What interactions are found in plasmas?
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
What is a liquid solution?
a homogeneous mixture of two or more distinct chemical species in the liquid phase
Why isn’t milk a solution?
it consists of a mixture of fatty molecules in water, forming an emulsion, since the fats and water never produce a homogeneous single phase
Why isn’t a pure liquid a solution?
more than one chemical species must be present
What is relative permittivity?
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
What is viscosity?
η; a measure of the resistance to flow
What are electrolytes?
ionic solutes
What do the properties of he solute molecules in solution depend on?
the nature of the solvent and the amount of solute
What is an ideal solution?
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
Into which categories are van der Waals forces split?
- 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
When dissolving a highly polar or ionic substance, what should be taken into consideration?
a high value of ε(r)
What is molarity?
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
What does molarity depend on?
molarity depends on T because V depends on T (as the solution is heated, it will expand)
What is molality?
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
What does molality depend on?
molality does not depend on T, since the mass does not depend on T
What is the mole fraction?
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
What does the mole fraction depend on?
the mole fraction does not depend on temperature
What is vapour pressure?
the pressure of the gas inside a container at equilibrium
What is Raoult’s Law?
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
What is the relationship between Raoult’s Law and ideality?
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
What is the appearance of a graph of partial pressure against mole fraction for an ideal solution?
straight lines
What is the appearance of a graph of partial pressure against mole fraction for a non-ideal solution?
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
Why is it the case that at small values of mole fraction, the partial pressure appears to follow Raoult’s Law?
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
What is Henry’s Law?
P(B) = K(B) x(B)
where K(B) is some experimentally determined parameter
What are ideal-dilute solutions?
very dilute solutions in which the solvent obeys Raoult’s Law and the solute obeys Henry’s Law
Do macromolecules exhibit ideality?
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
