Solutions in Chemistry

You are responsible for taking notes from this powerpoint!

• In class you may work with your group to do calculations and answer questions

• You are expected to read through the powerpoint and appropriate pages in the book and take notes

• If you are not prepared for class, you will have to work alone

Solutions are Mixtures• What is a mixture? • How are mixtures different than

compounds?• What is a homogeneous mixture?• Suspensions and Colloids

• If the particles in a mixture are too big, a suspension or colloid forms

• Suspension: particles will separate out over time (oil and water)

• Colloid: particles stay suspended but large enough to reflect light (milk)

Solutions: Made of particles so small the parts of the mixture cannot be differentiated

• Solute: substance in lesser amount of a solution - said to dissolve in the solvent

• Solvent: substance in greater amount of a solution - said to have solute dissolved in it

• Examples:• Solid: alloys (brass is copper and

zinc)• Liquid: alcohols, vinegar• Gas: air• Gas/liquid: soda• Solid/liquid: coffee• Solid/gas: pollution particles in air

Go to the following site

• Solutions

• Answer the following:• How do you know if a substance is a

solution?

• How would you separate a solution?

• The video also reviews molarity..you may want to watch before moving on!

Solvation• Solvation (solute dissolving in solution)

occurs when there is an attractive force between the solute and solvent

• “like dissolves like”

Polar and ionic substances attract to each other through dipole-dipole interactions, hydrogen bonding and ion-dipole attraction

Substances with dispersion forces attract to each other, but not to substances with stronger attractive forces

Solvation con’t

• Generally, the force of attraction between the solvent and solute must be stronger than the attraction between the solute particles themselves for a substance to dissolve If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

• salt dissolving in water

Electrolytes

• Many ionic substances dissociate when dissolved in water

• This solution is conductive due to the charged particles in solution

• NaCl (s) → Na+ (aq) + Cl- (aq)

• The attraction for the water molecule must be strong enough to overcome the electrostatic attraction between the ions

Types of solution

• Saturated solution: no more solute may be dissolved in solvent (temperature dependent)• How do you know when a solution is

saturated?• Unsaturated solution: more solute may be

dissolve in solution

• Supersaturated solution: more solute is dissolve in solution than expected at a given temperature

Factors affecting solubility• Temperature – why?

• For most solids, the solubility of a solid in water increases with temperature: more interaction between solvent and solute particles and more energy to separate solute particles

• For gases, solubility decreases with temperature: more KE, more volume and pressure of gas

• Pressure – does not affect solid dissolution, but does affect gases – why?• Gases are more soluble with increasing

pressure – need more pressure to come out of solution (soda)

Solubility curves1. Which solid

shows the greatest

increase of solubility with temperature?

2. Why is the NH3 line falling

with temperature?

Questions?

• Write down any questions that you have at this point and bring them to class

Concentration:

• Molarity: What is it??

What mass of NaCl is needed to make a 1.65 M NaCl solution?

What is the concentration of IONS in solution?

Concentration of ions

34.5g BaCl2 is dissolved in 500.ml water. What is the concentration of each ion in solution?

- BaCl2 (s) → Ba+2 (aq) + 2Cl- (aq)

-Calculate moles using molar ratio above

- Calculate molarity using given volume

Dilution

• The key to dilution problems is to remember that the amount of moles or grams does not change – just the volume changes

• Example: 250.ml of a .340M NaCl solution is diluted by adding 100.ml of water. What is the new concentration?• Determine the number of moles

• .250L x .340 mol/L = .0850 mol• Divide by the new volume

• .0850mol/.350L = .243M

Questions?

• Write down any questions that you have at this point and bring them to class

Colligative properties

• Properties of solutions that are determined by ratio of solute particles to solvent particles, not on the kind of particle present.• Vapor pressure• Boiling point and freezing point

• Molality: Concentration term : “m”

• Molality = moles solute/kg solvent

• Gives direct relationship between moles of solute to moles of solvent (kg may be converted to moles)

Calculating molality

• What is the molality of a solution made by mixing 45.0g sucrose in 500.ml H2O?

• 45.0g sucrose x 1mole/342 g = .132 moles

• 500.ml H2O x 1.00g/1ml x 1kg/1000ml = .500kg

• .132mole/.500kg = .263m

Freezing point depression and Boiling point elevation

• Adding a solute to solvent will change the physical properties of the solvent

• When freezing, solute particles interfere with crystallization process and lower temperature needed to crystallize

• Freezing temperature is lowered• When boiling, fewer solvent particles will be at surface

(interacting with solute particles) and vapor pressure is lowered.

• Boiling temperature is raised

Calculating changes in freezing and boiling points

• For any solution, the number of solute particles in solution will determine how much the freezing and boiling point is changed

• The temperature change from the normal freezing/boiling point will be in constant proportion to the molality of the solution:• ΔT =(Kf) (m) or (Kb) (m)

• For water: Kf = 1.86oC/m (freezing)

or Kb = 0.512oC/m (boiling)

Calculating freezing point depression and boiling point elevation

Example: What will be the freezing point of a .263m sucrose solution?

ΔT = (Kf) (m)

= (1.86oC/m) (.263m)= .489oC

ΔT =.489oC

Freezing point = 0oC - .489oC = -.489oC

What questions do you have??

• Write down any questions and bring them to class