Bell Work: Molarity Moles of solute divided by liters of solution is called _________. Moles of solute divided by mass of solution is called _________. The ratio to convert grams to moles is 1 ____ divided by the ____ ____. The ratio to convert moles to grams is the ____ ____ divided by 1 ____. Molarity measure ( diluteness / temperature / concentration ) of a solution.

Colligative Properties

Colligative Properties Colligatiave properties are: physical properties influenced by the number or amount of solute particles but NOT by the identity of those particles. Analogy: A gold-digger is a person who only cares about the amount of money a person has not who the person is. This is the same with colligative properties.

There are three colligative properties that change when the amount of solute is changed. vapor pressure lowering boiling point elevation freezing point depression

Vapor Pressure Lowering Pressure exerted in a closed container (like a soda bottle with the top on) by the solvent particles as they evaporate to form a gas. The vapor exerts pressure on the container and the surface of the liquid. More solute, less vapor pressure. They are inversely related.

Boiling Point Elevation When the vapor pressure (pushing up) equals atmospheric pressure (pushing down), the solvent boils. If the vapor pressure is lowered by the solute, it has to get hotter to make enough vapor to boil. So the more solute, the more the boiling point is elevated. They are directly related.

Freezing Point Depression Temperature is a measure of the average kinetic energy of particles. Freezing is the point when solvent particles get very close together because they do not have enough kinetic energy to stay far apart so they are pulled together to solidify. kinetic energy of solids

The solute particles get in the way of the formation of the solid so the more solute, the lower the freezing point. They are inversely related.

How do the number of solute particles in a solution change and affect these 3 physical properties? Solute particles INTERFERE with the solvent particles! They act like a “third wheel” in the way of everything. When a solute is added to a solvent, solvation and dissociation begin to take place.

Only ionic compounds are able to break apart as they dissociate These compounds are called electrolytes because their aqueous solutions can conduct an electric current. strong electrolyte

Ionic solutes break apart and always produce more than one ion which makes the number of solute particles increase. NaCl electrolyte

Remember: The more. solute particles that are in Remember: The more solute particles that are in the solution, the more the colligative properties are affected!

Let’s Summarize Ionic Compounds will dissociate and produce electrolytes that are capable of conducting an electric current The following solutes are placed into water. What happens to these solutes as they begin to dissolve?

They dissociate into: NaCl  1 Na+1 + 1 Cl-1 MgCl2  Na3PO4  strong electrolyte They dissociate into: 2 moles of ions NaCl  MgCl2  Na3PO4  1 Na+1 + 1 Cl-1 1 Mg+2 + 2 Cl-1 3 moles of ions 3 Na+1 + 1 PO4-3 4 moles of ions

Q: Which substance above produces more solute particles in solution? A: Na3PO4 ; therefore, it will have the most influence on colligative properties ***Remember: A colligative property depends on the number of solute particles in solution, not the identity of the particles

Covalent solutes dissolve, but they do NOT dissociate into ions. Why? They do NOT have regions of opposite charge to attract to the nonpolar solvent!

Instead, covalent substances dissolve as whole molecules. The whole molecules form nonelectrolytes and they DO NOT conduct an electric current! nonelectrolyte

nonelectrolyte

The solutes below are placed into water. What happens to these solutes as they begin to dissolve? C6H12O6  C12H22O11  **They dissociate one molecule at a time! 1 mole of particles 1 C6H12O6 1 C12H22O11 1 mole of particles

Q: Which covalent substance above would have the most influence on colligative properties? A: Neither! They would have the same effect b/c each covalent compound produces only ONE mole of solute particle and colligative properties depend on the number of particles!

Let’s Summarize Covalent Compounds will dissociate into only one mole of particles and produce nonelectrolytes that are NOT able to conduct an electric current!

Now it’s your turn! Why do ionic compounds more strongly affect colligative properties than covalent compounds? Ionic compounds always dissociate into two or more particles while covalent compounds always dissociate into only one! Since colligative properties depend on the NUMBER of particles, and ionic compounds produce more particles, they affect colligative properties more!

How does the addition of a solute affect each of the 3 colligative properties? Vapor pressure decreases with addition of solute to solvent! There are solute particles interfering with the solvent particles at the surface of the solution. Less of the solvent can evaporate and push upwards b/c the solute is at the surface, too. Since less solvent is evaporating, vapor pressure drops b/c there is not gas pushing upwards. vapor pressure

Vapor Pressure of a Solution: A Colligative Property Which flask has a lower vapor pressure? Explain. Flask A Flask B Pure Solvent Solution (no solute present) Solute + Solvent

Solute particles interfere with the solvent particles at the surface. How does the addition of a solute affect each of the 3 colligative properties? Boiling point elevates (increases) with addition of a solute to a solvent. For a substance to boil, atmospheric pressure pushing down must equal vapor pressure pushing up. Solute particles interfere with the solvent particles at the surface.

It will take more energy to get the solvent particles to “push” their way to the surface to evaporate. This means you have to heat the solution up to a higher temperature to give the solvent particles more energy to increase vapor pressure.

How does the addition of a solute affect each of the 3 colligative properties? Freezing point depresses (decreases) with addition of solute to solvent. Solute particles are mixed in with the solvent particles interfering with the ability of solvent particles to “get close” together.

The solute is “in the way.” This means temperature has to drop A LOT for the solvent to push the solute out of the way in order to get close together and solidify.

ions are called electrolytes Ionic Compounds Covalent Compounds polar (have + and – regions) nonpolar (no oppositely charged poles) since no ions, they dissociate one particle at a time dissociate into two or more particles at a time ions are called electrolytes no ions so they are called non-electrolytes conduct electricity non-conductors of electricity strongly affect colligative properties more weakly affects colligative properties