Toni L3 Flashcards
(21 cards)
Define chemical potential
μi = (δG/δni)nj,p,T
What is the meaning of chemical potential
The chemical potential of substance i shows how the free energy of the system changes as more of substance i is added, at constant composition of everything else (j), constant pressure (p) and constant temperature (T)
What does it mean if chemical potential is <0 (μ<0)
If μ<0 then there is a tendency for more of substance in to be produced
What does it mean if chemical potential is >0 (μ > 0)
If μ > 0 there is a tendency for i to be consumed by reactions in the system or to be transferred away (open systems)
Describe how we can calculate changes in chemical potential during a reaction
Chemical potential is a state function, therefor the changes in chemical potential can be calculated if the balances chemical equation is known
aA + bB -> cC + μ dD
ΔG = dμD + cμC + bμB - aμA
Define phase equilibria
Phase equilibria refers to the balance between different phases of matter (solid, liquid, gas, or more) coexisting in a system, where there is no net change in the amount of each phase over time.
If two phases phase 1 and 2 are in equilibrium then there chemical potentials are the same
Describe concentration gradients in terms of chemical potentials
When solute concentration varies across a solution, the chemical potential differs between regions. To lower the system’s free energy, solute molecules diffuse from areas of high to low chemical potential.
Equilibrium is reached when the solute is uniformly distributed and the chemical potential μi is constant throughout.
What is the equation for the molar entropy of mixing
ΔSmix = -nR{xAlnxa + xblnxb}
Describe the entropy of mixing value
The entropy of mixing is positive as the mixture is always more disordered than the pure components
Describe the mathematical reason that the entropy of mixing is positive
The mole fractions are positive numbers less than 1 by definition 0 < xA < 1. Therefore ln xA < 0, but the whole term in the bracket is
multiplied by −R.
What is the definition of ideal solution
A solution where the enthalpy of mixing is zero and the entropy of mixing is given by ΔSmix = -nR{xAlnxa + xblnxb}
Describe the free energy of mixing for an ideal solution
ΔG = ΔH - TΔS but for an ideal solution ΔH =0 therefore the molar free energy of mixing is ΔG = -TΔS
ΔGmix = RT{xa lnxa + xb lnxb}
Describe what we mean by free energy of mixing for ideal solutions
If the interaction energies A-A, B-B and A-B are the same, then no heat will be released when pure A is mixed with pure B. The sole driving force for the formation of the solution of A & B is the entropy of mixing
Give examples of nearly ideal solutions
- any mixture of unreactive gases at room temperature and ambient pressure
- molecules of similar structure interacting solely by weak forces like benzene and toluene
- very dilute solutions of uncharged solutes e.g dilute solutions of sucrose in water
Give examples of far from ideal solutions
- most strong electrolytes
- mixtures of liquids where strong intermolecular forces e.g hydrogen bonding are involved
Describe the concentration of water molecules in liquid water and how other solutes usually compare
The concentration of water molecules in liquid water is about 56 mol dm−3. The concentration of solute is usually less than this (not many compounds are so soluble) then xwater ≃ 1 and ln xwater ≃ 0.
What can the free energy of mixing be simplified to if the concentration of solutes is low
ΔGmix = Na/N RT{lnxA}
What is the mole fraction of the solute proportional to and what does this mean the molar free energy of mixing will depend on
The mole fraction of the solute will be proportional to its concentration, therefore the molar free energy of mixing will depend on ln c
Describe how we can calculate chemical potential from a graph
Plot a graph of gibbs free energy over moles, the gradient of this graph is chemical potential
Describe how we obtain a positive chemical potential
If you increase the number of mole you will increase the gibbs energy so positive chemical potential
