Units 15 & 16

Solutions & Acids and Bases

Solutions

All solutions are composed of two parts:The solute and the solvent.

The substance that gets dissolved is the solute

The substance that does the dissolving is the solvent (Usually present in the larger amount)

***A solution may exist as a solid, liquid or gas depending on the state of the solvent.

Types of Solutions

Gas in liquid Example – soda water

Solute: carbon dioxide (gas) Solvent: water (liquid)

Solid in solidExample – SteelSolute: carbon

Solvent: iron

Types of Solutions

Gas in gas

Example - AirSolute: Oxygen

Solvent: Nitrogen

Types of Solutions

Liquid in liquid

Example – VinegarSolute: Acetic acid

Solvent: Water

Solid in liquid

Example – Ocean WaterSolute: Sodium Chloride

(solid)

Solvent: Water (liquid)

Aqueous Solution

Any mixture where water is the solvent. Something is dissolved in water

Soluble - a substance that dissolves in another substance.

Insoluble - a substance that does not dissolve in another substance.

Solubility

Solubility

Immiscible - two liquids that are insoluble in each other.

Miscible - two liquids that are soluble in each other.

Solvation – a process that occurs when an ionic solute dissolves in a solvent

Solvation (Hydration)

Solvation Video

Solvents of a specific polarity or type will dissolve solute of similar polarities or types!

Aqueous solutions of ionic compounds:The charged ends of the water molecules attract the positive and negative ions making up an ionic solid, forcing them to separate.

Aqueous solutions of molecular compounds:Molecular compounds that have polar sections easily form aqueous solutions with water.

“Like Dissolves Like”

Solubility – the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature and pressure to produce a saturated solution

•Units for solubility: grams of solute per 100 g solvent

• Example: At 20˚C, NaNO3 has a solubility of 74 g/100 g H2O

Solubility

SolubilitySaturated Solution - contains the maximum

amount of dissolved solute

Unsaturated Solution - contains less than the maximum amount of dissolved solute

Supersaturated Solution – contains more solute than can theoretically be dissolved at a given temperature

Supersaturated Solutions

How can you dissolve more solute than possible??

Solubility Curves

Solubility of a solid generally increases with increasing temperatureThe higher the temperature, the greater

amount of solute that will be dissolved in the solvent

Solubility can be represented in a chart called a solubility curve

Solubility Curves

Factors that affect solubility

Temperature-

Generally, as temperature increases, more solid solute will dissolve in the same amount of liquid solvent. The opposite is true for gaseous solutes.

Concentration –the measure of the amount of solute dissolved in a given quantity of solvent

Solution Concentration

Expressing concentration:

Concentration Description Ratio

Percent by mass

Percent by volume

Molarity

solution of liters

solute of moles (M)Molarity

100 * solution of volume

solute of volume by volumepercent

100 * solution of mass

solute of mass massby percent

Percent By Mass

Percent by mass -ratio of the solute’s mass to the solution’s mass expressed as a percent.

Example: An aquarium contains 3.6 g NaCl per 100.0 g of water. What is the percent by mass of NaCl in the solution?

Percent By Volume

Percent by volume-ratio of the volume of the solute to the volume of the solution expressed as a percent.

Example: What is the percent by volume of ethanol in a solution that contains 35 mL of ethanol dissolved in 115 mL of water?

Molarity (M) = moles of solute

liters of solution

Moles of solute dissolved in 1 liter of solutionExample: 0.23 M solution = 0.23 moles of solute dissolved in 1 L of solution

M is read as “molar” when next to a number

4 M HCl = 4 molar hydrochloric acid

Keep in mind that the liters of solution takes into account the volume of the solute AND the volume of the solvent

Example: What is the molarity of a solution that contains 0.65 mol of CuCl2 in 500 mL of water?

Example: What is the molarity of a solution that contains 5.10 g of glucose (C6H12O6) in 100.5 mL of solution?

Preparing Molar Solutions

How is a solution of known molarity made?

Convert moles of solute to grams and measure the amount out.

Add solvent so that the total volume of the solution is 1 L.

For any volume other than 1 L we must adjust the amount of solute needed by multiplying it by the fraction of a liter of solution we need.

Example

Example: How many grams of CaCl2 would be dissolved in 1.0 L of water to make a 0.10 M solution of CaCl2?

Example

Example: How many grams of NaOH are in 250 mL of a 3.0 M NaOH solution?

Dilutions are used to decrease the concentration (or molarity) of a solution

M1V1=M2V2

Steps to Performing a Dilution

1.Calculate how many mL of the original (stock) solution to start with

2.Measure out the volume of stock solution (using a graduated cylinder or a pipet) and place in appropriately sized volumetric flask

3.Add water to the mark on flask

Example:

What volume, in milliliters, of 2.00 M CaCl2 is needed to make 0.50 L of 0.300 M CaCl2 solution?

Colligative Properties

Colligative means “depending on the collection.”

Depends only on the number of dissolved particles, not on the identity of dissolved particles.

Includes vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure

Electrolytes and Colligative Properties

Electrolyte: Soluble ionic compounds. When they dissolve in solution, they dissociate into their component ions and conduct electricity.

Ex: NaCl (s) Na+ (aq) + Cl- (aq)

Covalent molecules in aqueous solution: Covalent particles do not dissociate when in

solution, so the # of molecules = the # of particles.

Boiling Point Elevation

Boiling occurs when vapor pressure equals atmospheric pressure.

Boiling point of a solution is higher than the boiling point of the pure solvent.

Dissolving substances increases the boiling point of a solvent.

Examples of Colligative Properties

Freezing Point Depression

Freezing point of a solution is lower than the freezing point of the pure solvent.

Dissolving substances lowers the freezing point of a solvent.

Ex: Icy pavement - throw down CaCl2 or NaCl, and the water will then freeze at a lower temperature

Ex: Antifreeze: a solution of ethylene glycol in water

1. Prevents car’s radiator from freezing in the winter.

2. Prevents car’s radiator from boiling over in the summer

The more ethylene glycol in the water, the lower the freezing point, and the higher the boiling point.

Acids & Bases

Properties of Acids

Physical: Taste sour

Chemical: React with metals to produce H2 gas Neutralized when reacted with a base

Litmus Indicator: Turns blue litmus paper red

Ions in Solution: H+, H3O+ (hydronium ion)

Properties of Bases

Physical: Taste Bitter Slippery

Chemical: Neutralized when reacted with an acid Do not react with metals Why are bases used as drain cleaners?

Litmus Indicator: Turns red litmus paper blue

Ions in Solution: OH-1

Arrhenius Acids & Bases

Arrhenius Model:

ACIDS: Acids contain the H+ ion

Ex.) HCl, HBr, HNO3

BASES: Bases contain the OH-1 ion

Ex.) NaOH, KOH, Ca(OH)2

Bronsted-Lowry Acids & Bases

Bronsted-Lowry Model: For every acid, there must be a base Acid = proton donor Base = proton acceptor

HCl (aq) + NH3 (aq) NH4+ (aq) + Cl-1 (aq)

Conjugate Pairs

NHNH33 / NH / NH44++ is a is a conjugate pairconjugate pair — — related by the related by the

gain or loss of Hgain or loss of H++

Every acid has a conjugate base, Every acid has a conjugate base, formed when H+ formed when H+ is removed from the acid.is removed from the acid.

Every base has a conjugate acid, Every base has a conjugate acid, formed when H+ formed when H+ is added to the base.is added to the base.

Conjugate Pairs

Identify the conjugate acid-base pairs: HCl in water

NaOH in water

NH3 in water

Types of Acids

Monoprotic and Polyprotic Acids Acids can contain 1 or more hydrogens that are

acidic **Not ALL hydrogens are acidic (Ex. Vinegar)

Identify the following as monoprotic or polyprotic:

HNO3, H2SO4, HClO, HClO4, H3PO4, HC2H3O2

Strength of Acids/Bases

Strengths of Acids Strong Acid Give off LOTS of H+

100% Dissociation

Strong Acids: HCl, HI, HBr, HNO3, H2SO4, HClO4

That’s it! Everything else is “weak”

Weak Acid Give off smaller amounts of H+

Equilibrium occurs (breaks apart and then recombines) Not all H+ ions separate (not 100% dissociation)

Strength of Acids/Bases

Strengths of Bases Strong Base Give off LOTS of OH-1

100% Dissociation Generally, Group I, II Hydroxides (except H, Be, Mg)

Ex.) Ca(OH)2, NaOH Everything else is “weak”

Weak Base Give off smaller amounts of OH-1

Equilibrium occurs (breaks apart and then recombines)

Not 100% dissociation

Strength of Acids/Bases

Strong or weak vs. concentrated and dilute

Strong/weak tells you how much it dissociates

Concentrated/dilute indicates the concentration (amount of solute in the solvent)

pH

pH & pOH pH tells us the acidity or basicity

of a solution Based on measuring the [H+]

(a.k.a. [H3O+])

pH Scale Ranges 0 to 14 Acid ~ 0 to 7 Bases ~ 7 to 14

Definition: Hydronium Ion

In aqueous solution, H+ does NOT exist!

Note: In problems, [H+] = [H3O+]

H+ + H2O H3O+

(hydronium ion)

pH

pH = - log [H3O+] pOH = - log [OH-] Make sure you have the negative sign! Find the “log” function on your calculator!

pH + pOH = 14 [H+][OH-] = 1.0 x 10-14

pH Calculations

What is the pH and pOH for a solution with a H+ concentration of 3.0 x 10-6 M H+?

pH Calculations

What is the H+ and OH- concentration of blood with a pH of 7.40?

Neutralization Reactions

Neutralization reaction: Reaction in which acid and base react to

neutralize one another

Acid + Base Water + Salt

***Salt = Any ionic compound formed as a

by-product of an acid-base reaction

Neutralization

Acid-base Titration: Definition:

Lab technique which allows you to get moles of acid and base EXACTLY equal to another

Complete neutralization Allows you to calculate the concentration of an

unknown acid or base

Definitions

The titrant is the substance of known concentration used to determine the unknown concentration of the other substance.

An indicator- substance that changes color at a certain pH—is added to tell us when the neutralization is complete.

Example: Phenolphthalein undergoes a color change between pH 8 and 10

clear in acid Light pink in neutralDark pink in base

Neutralization

Procedure: Add known volume of acid or base to

Erlenmeyer flask Add a known concentration of the

other to a buret Add an indicator to the flask Slowly dispense titrant (what you’re

adding with a buret) into the flask Stop when 1 drop of titrant causes

the indicator to switch from one color to another

Neutralization

Equivalence point: pH at which amount of acid = base

Indicator: Compound that changes color due to a change in pH Common Indicators and pH Range

Litmus: 5.5 to 8.0 (red= acid, blue = base) Phenolphthalein: 8.2 to 10.6 (colorless to magenta)

End point: Point at which the volume of titrant added makes the amount of acid and base are equal and the indicator changes color