SNS - General Chemistry - Phases and Phase Changes Flashcards
Colligative Properties
Lowering the Vapor Pressure
If a solute A is added to a pure solvent B, the vapor pressure above the solvent decreases. The more particles dissolved in a solvent, the lower the vapor presure
Colligative Properties Lowering the Vapor Pressure Raoult’s Law
Change in vapor pressure = (mole fraction) x vapor pressure of pure solvent Mole fraction = moles of solute / (moles of solute + moles of solvent)
Colligative Properties Lowering the Freezing Point
The more particles of solute added to a solution, the greater the reduction of temperature needed ot allow the molecules to orient themselves in a crystallline structure Change in freezing point depression = Kf x molarity where Kf is the constant for the solvent (1.86⁰cm⁻¹ for water)
Colligative Properties Raising the Boiling Point
Boiling occurs when the vapor pressure is equal to the external pressure on the surface. Since adding solutes lowers the vapor pressure, a higher temperature is needed to raise the vapor pressure and cause the solution to boil Change in boiling point = Kb x Molality Where Kb is a constant depending on solvent
Colligative Properties Osmotic Pressure
Solvent moves across a semipermeable membrane is the solute concentrations on either side differ Osmotic pressure = molarity of solution x R x T Where R is a constant and T is the temperature in K
Phase Equilibria
Gas-Liquid
- The tempertaure of a liquid is related to the average kinetic energy of the liquid molecules. A few near the surface may have sufficient energy to leave the liquid phase and enter the gas phase - evaporation
- Each time the liquid loses a high-energy the temperature of the remaining liquid decreases
- If a cover is placed above, the escaping molecules are trapped above the solution and exert a countering pressure which forces some of the gas back into the liquid phase - condensation. Atmospheric pressure acts in a similar fashion as a solid lid
Phase Equilibria
Gas-Liquid
Vapor Pressure
As evaporation and condensation proceed, an equilibrium is reached in which the rates of the two processes become equal. Once this is reached, the pressure that the gas exerts over the liquid is called the vapor pressure of the liquid.
Increases as temperature increases since more molecules have sufficient kinetic energy to escape into the gas phase
Phase Equilibria
Gas-Liquid
Boiling Point
Temperature at which the vapor pressure of a liquid is equal to the external pressure
Phase Equilibria
Solid-Liquid
- Vibrations of molecules within a solid increase as heat is applied
- If atoms and molecules in the solid absorb sufficient energy, the 3D structure breaks down and the liquid phase begins - melting point
Phase Equilibria
Gas-Solid
When a solid goes directly to the gas phase is called sublimation, eg dry ice
Transition of gas into solid is called deposition
The Gibbs Function
For each equilibrium (eg gas-liquid, liquid-solid, solid-gas) the change in Gibbs free energy must be equal to zero: ∆G = 0. For an equilibrium between a gas and a solid
∆G = G (g) - G (s)
so G (g) = G (s) at equilibrium
Colligative Properties
Physical properties derived solely from the number of particles present, not the nature of those particles
Liquids and Solids Homogenous Reactions
Occur in one phase, for example solid only
Liquids and Solids Energy Changes Accompanying Changes of State
The Calorie is a unit of thermal energy. One calorie is equal to 4.184 J
Characteristics of States of Matter Expandability
- Gases - most expandable 2. Liquids 3. Solids - least expandable
