Units 15 & 16 Solutions & Acids and Bases. Solutions All solutions are composed of two parts: The...
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Transcript of Units 15 & 16 Solutions & Acids and Bases. Solutions All solutions are composed of two parts: The...
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
Liquid in liquid
Example – VinegarSolute: Acetic acid
Solvent: Water
Solid in liquid
Example – Ocean WaterSolute: Sodium Chloride
(solid)
Solvent: Water (liquid)
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
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
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.
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 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?
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.
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.
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
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