Colligative Properties Flashcards
What is an ideal solution?
it can be defined as one in which there is no change in the properties of the components, other than dilution, when they are mixed to form a solution
no heat is evolved or absorbed during the mixing process
no shrinking or expansion occurs when the substances are mixed = no transformation
What is escaping tendency?
two bodies are in thermal equilibrium when their T are the same
if T1>T2 heat will flow from the hotter (T1) to the colder (T2)
the heat in body 1 has a greater escaping tendency
What is the definition of vapour pressure of a solution?
quantitative expression of escaping tendency
How to reduce the escaping tendency of a solution?
if an external force was applied that could keep the molecules down then escaping tendency would decrease/be diminished
the lower the pressure on the surface, the easier it is for molecules to escape
example - CFCs propellants
What are adhesion and cohesion?
adhesion - forces between different compounds
cohesion - forces between the same compounds
How can the forces in an ideal solution be described?
there is complete uniformity of attractive forces
How can the forces in a real solution be described?
there are 2 possibilities
positive deviation
- adhesion between A and B is weaker than cohesion between A and A/B and B
- above the ideal graph
negative deviation
- adhesion between A and B is stronger than cohesion between A and A/B and B
- below the ideal graph
What are colligative properties?
collected properties = property of the solution
- depends on the number of solute particles present rather than on the nature of the constituents
What are 4 factors affecting colligative properties?
Vapour pressure lowering
Boiling point elevation
Freezing point elevation
Osmotic pressure
What is vapour pressure lowering?
vapour pressure - pressure exerted by molecules on the container and liquid surface while in vapour phase
adding solute decreases vapour pressure
- solvent become less volatile
- solute-solvent interactions contribute to this effect
vapour pressure of a solution < vapour pressure of a pure solvent
What is the equation for vapour pressure lowering?
change in vapour pressure = original vapour pressure x mole fraction
What is normal boiling point?
the T at which the vapour pressure of the liquid equals the external pressure
760mmHg
What is the effect of lowering vapour pressure on boiling point and freezing point?
decreases freezing point
increases boiling point
- adding solute lowers vapour pressure
- a solution boils at a higher temperature than pure solvents
- T must be increased to achieve boiling point
How does osmosis occur? What is osmotic pressure?
a semi-permeable membrane separates a solution from a solvent
the membrane only allows the solvent molecules to pass through not the solute
the solvent passes through into the solution and dilutes it
osmotic pressure is the pressure applied to stop osmosis
Why are there different formulas for electrolytes and non-electrolytes in osmotic pressure?
electrolytes - dissociate into ions
non-electrolytes - do not dissociate into ions
if a solute dissociates into ions then each ion contributes to the osmotic pressure
