Chemistry 232 Properties of Solutions
Concentration Terms • Dilute - not a lot of solute. • Concentrated - a large amount of solute. • Concentration can be expressed quantitatively is many ways: • Molarity • Molality • Percentage • Mole fraction
Molarity and Molality • The molarity is the number of moles of solute in 1 litre of solution. • M = moles of solute / V sol’n (litres) • The molality is the number of moles of solute in 1 kg of solvent. • M = moles of solute / kg solvent • Conversion between the two requires the solutions density.
Partial Molar Thermodynamic Properties Define a partial molar thermodynamic property as Euler’s Theorem
The Chemical Potential We define the chemical potential of a substance as
The Wider Significance of Shows how all the extensive thermodynamic properties depend on system composition
Thermodynamics of Mixing Spontaneous mixing of two or more substances to form solutions Gibbs energy of the solution must be less than G(pure components)
The Ideal Solution TmixS/n kJ/mol TmixH/n 0 TmixG/n XA
Ideal Solution Def’n For an ideal solution
Raoult’s Law Consider the following system
Raoult’s Law #2 The chemical potential expressions
Raoult’s Law: Depression of Vapour pressure VP of solution relates to VP of pure solvent PA = XAP*A Solutions that obey Raoult’s law are called ideal solutions.
Raoult’s Law Example The total vapour pressure and partial vapour pressures of an ideal binary mixture Dependence of the vp on mole fractions of the components.
An Ideal Solution Benzene and toluene behave almost ideally Follow Raoult’s Law over the entire composition range.
Henry’s Law Henry’s law relates the vapour pressure of the solute above an ideally dilute solution to composition.
The Ideal Dilute Solution Ideal Dilute Solution Solvent obeys Raoult’s Law Solute obeys Henry’s Law
Henry’s Law #2 The chemical potential expressions • JO(H)is the Henry’s law standard state. • It is the chemical potential of J in the vapour when PJ = kJ.
Henry’s Law #3 The Standard State Chemical potential for Henry’s Law When the system is in equilibrium The chemical potential expressions reduce to Henry’s Law
Henry’s Law in terms of molalities The Standard State Chemical potential for Henry’s Law When the system is in equilibrium The chemical potential expressions reduce to Henry’s Law in terms of molalities
Chemical Potentials in terms of the Molality The chemical potential expressions oJ,m = chemical potential of the solute in an ideal 1 molal solution
The Gibbs-Duhem Equation The Gibbs-Duhem gives us an interrelationship amongst all partial molar quantities in a mixture
Colligative Properties • All colligative properties • Depend on the number and not the nature of the solute molecules • Due to reduction in chemical potential in solution vs. that of the pure solvent • Freezing point depression • Boiling Point Elevation • Osmotic Pressure
Boiling Point Elevation Examine the chemical potential expressions involved
Boiling Point Elevation #2 The boiling point elevation
Freezing Point Depression Examine the chemical potential expressions involved
Freezing Point Depression #2 Define the freezing point depression
Osmosis • The movement of water through a semi-permeable membrane from dilute side to concentrated side • the movement is such that the two sides might end up with the same concentration • Osmotic pressure: the pressure required to prevent this movement
Osmosis – The Thermodynamic Formulation - the osmotic pressure Equilibrium is established across membrane under isothermal conditions
The Final Equation The osmotic pressure is related to the solutions molarity as follows
Terminology Isotonic: having the same osmotic pressure Hypertonic: having a higher osmotic pressure Hypotonic: having a lower osmotic pressure
Terminology #2 Hemolysis: the process that ruptures a cell placed in a solution that is hypotonic to the cell’s fluid Crenation: the opposite effect
The Partial Molar Volume In a multicomponent system
Volume Vs. Composition The partial molar volume of a substance slope of the variation of the total sample volume plotted against composition. PMV’s vary with solution composition
The PMV-Composition Plot The partial molar volumes of water and ethanol at 25C. Note the position of the maxima and minima!!
Experimental Determination of PMV’s Obtain the densities of systems as a function of composition Inverse of density – specific volume of solution
Example with Methanol. Plot volumes vs. mole fraction of component A or B Draw a tangent line to the plot of volume vs. mole fraction. Where the tangent line intersects the axis – partial molar volume of the components at that composition
The Mean Molar Volume Define the mean mixing molar volume as V*J – the molar volume of the pure liquid Vm = V/nT
The Mean Molar Volume Plot VB-VB* VA-VA*
Infinite Dilution Partial Molar Properties The value of a partial molar thermodynamic property in the limit of zero volume is its infinite dilution value E.g., for the volumes
The Definition of the Activity For any real system, the chemical potential for the solute (or solvent) is given by
Activities of Pure Solids/Liquids The chemical potential is essentially invariant with pressure for condensed phases
Pure Solids and Pure Liquids or aJ = 1 For a pure solid or a pure liquid at standard to moderately high pressures
Activities in Gaseous Systems The chemical potential of a real gas is written in terms of its fugacity
Define the Activity Coefficient The activity coefficient (J) relates the activity to the concentration terms of interest. In gaseous systems, we relate the fugacity (or activity) to the ideal pressure of the gas via