solutions suroviec spring 2014 chapter 12. i. types of solution most chemical reaction take place...

Solutionssuroviec

Spring 2014

Chapter 12

I. Types of Solution

Most chemical reaction take place between ions/molecules dissolved in water or a solvent.

Solution: homogenous mixture of 2 or more substances in a single phase

II. Molecular view of solution process

Liquids and solids held together by intermolecular attractions

Looking at specifically Solvent-solvent Solvent - solute Solute – solute

3 steps Separate solvent molecules

Separate solute molecules

Mix together

Ex. Dissolve CaCl2 (s) in H2O

What happens to the concentration of Ca2+ and Cl-?

Saturated: stable solution in which the maximum amount of the solute has been dissolved

Equilibrium in solution

Solubility: concentration of solute in equilibrium with undissolved solute in a saturated solution

Examples:

CCl4 and CH4NaCl and CCl4

III. Concentration

A. Molarity: (M) Moles of solute / liter of solution

Does not tell us the EXACT amount of volume to use

B. Molality: (m)Mole of solute / mass of solvent (kg)

Cannot be the same measure of a solution

Example

The concentration of a K2CrO4 solution is undetermined, but it was prepared from 38.3 g of K2CrO4 (MW 19.4 g/mol) and 1.00 kg of water. What is the molality?

Calculate the molality of 1 L of 1.22M sucrose (MW 342.3 g/mol) solution. The density of the solution is 1.12 g/mL.

C. Mole Fraction

Amount of the component we are interested divided by the total amount of all the components of the mixture

Mole fraction of A (XA) =

example

Consider a solution that has 1.00 mole ethanol (46.1 g) dissolved in 9.00 moles of water (162 g). Find the mole fraction of each.

D. Mass Percent

Common unit of measurement for consumer products

Mass of one component divided by the total mass of the mixture multiplied by 100%

Example

Consider an alcohol-water mixture that is prepared by dissolving 46.1 g of ethanol in 116.2 g of water. What is the mass percent of ethanol in the mixture?

IV. Pressure, Temperature and Solubility

A. Solid solubility and Temperature When a solid is dissolved an

equilibrium is established An increase in temp will shift the

equilibrium

B. Gas Solubility

Solubility of gasses decreases with increasing temperature

Why do fish go to bottom of lake in the summer?

Gases that dissolve to an appreciable extent in water do so in an exothermic process

So what do you need to do to get the gas out of the water?

V. Pressure and Solubility

A. Henry’s Law• Solubility of a gas in a liquid is

directly proportional to the gas pressure• Henry’s Law• cg = kHPg

cg = molar concentration (mol/L)

kH = Henry’s law constant (mol/L · atm)

Pg =pressure of a gaseous solute (atm)

Example Deep-sea diving

Solubility of gases is the concentration of gases in equilibrium with substances in gaseous state

What would happen if we increased the pressure?

VI. Colligative Properties

Properties of a solution that depend only in the number of solute particles per solvent molecule and not the nature of the solute or solvent

A. Raoult’s Law

Equilibrium vapor pressure: pressure of the vapor when the liquid and the vapor are in equilibrium

Example

Solution contains 82.0 g of ethylene glycol in 100g of water at 22oC. (where is vapor pressure of water is 19.88 mm Hg) what is the new vapor pressure?

B. Boiling Point Elevation

You have a solution of 0.200 mole of sugar in 100.0 g of benzene

The normal vapor pressure of pure benzene is 400 mmHg, what is the new vapor pressure of this solution?

This vapor pressure lowering causes a BP elevation!

Normal BP is the temp at which vapor pressure =760 mm Hg

BP elevation

B. BP elevation

Leads to general equation:

The melting point of water is 0oC at 1atm. If 12.15 g of ZnSO4 (MW 161.5 g/mol) are dissolved into 161.2 g of water: What is the molality of the solution What is the new melting point of the solution?

C. Freezing Point Depression

Freezing point of a solution will be lower than of a pure solvent

General equation

Why add antifreeze to your car?

D. Osmosis

Movement of solvent molecules through a semi-permeable membrane from a region of lower concentration to a region of higher concentration

Movement continues until equilibrium is reached

D. Osmosis Over time water flows from region of low solute concentration to region of high solute concentration.

Flow continues until pressure exerted by column of solution in tube above water level is great enough to equal flow in and out of bag

This is measure of osmotic pressure