Transcript AQUEOUS CHEMISRTY - University of the Witwatersrand

AQUEOUS CHEMISRTY
Chapter 4
General Properties of Aqueous
Solutions
Solution = a homogeneous mixture of two or
more substances.
Solvent = normally the component that is
present in greater
quantity
Solute = dissolves in the solvent.
Electrolytes and Nonelectrolytes
Electrolyte = a substance that dissolves in
water to give an electrically
conducting solution.
In general ionic solids that dissolves in water
are electrolytes.
Some molecular substances are also
electrolytes. eg. HCl (g)
Nonelectrolyte = a substance that dissolves in
water to give a nonconducting or
very poorly conducting solution.
Strong and Weak Electrolytes
Strong electrolyte  an electrolyte that exists
in solution almost entirely as ions.
Eg. Sodium Chloride
NaCl (s)
H2O
Na+(aq) + Cl-(aq)
Weak electrolyte  an electrolyte that
dissolves in water to give a relatively
small percentage of ions.
Eg. ammonia
NH3(aq) + H20(l)
NH4+(aq) + OH-(aq)
However, in the solution we will also find the
reverse reaction.
NH3(aq) + H20(l)
NH4+(aq) + OH-(aq)
For this situation we generally write a single
equation with a double arrow
NH3(aq) + H20(l)
NH4+(aq) + OH-(aq)
Types of Chemical Reactions
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
Precipitation reactions
 when two ionic solutions are mixed and a
solid ionic substance (precipitate) forms
Acid-base reactions
 involve the transfer of a proton
Oxidation-reduction reactions
 involve the transfer of electrons between
reactants
Other reactions
Precipitation Reactions
A precipitation reaction occurs in aqueous
solution because one product is insoluble.
Solubility Rules
Solubility of a substance  the amount of
substance that can be dissolved in a given
quantity of solvent.
The solubility of substances varies widely.
Precipitate  an insoluble compound formed
during a chemical reaction.
Reaction of Potassium iodide solution and
lead(II) nitrate solution
Shellfish form their shells from calcium
carbonate. The organism secretes calcium
ions from cells in contact with seawater,
which contains dissolved carbon dioxide,
some of which is present as carbonate ions.
The ions combine to give a precipitate of
calcium carbonate.
Kidney stones are generally also insoluble calcium
compounds such as calcium carbonate , calcium
phosphate or calcium oxalate.
Acid-Base Reactions
•Acids and bases are some of the most
important electrolytes
•Acids taste sour
•Bases taste bitter and feel soapy
Another property of acids and bases is their
ability to cause colour changes in certain dyes.
An acid-base indicator is a dye used to
distinguish between acidic and basic solutions
by means of the color changes it undergoes in
these solution.
Probably the most important characteristic of
acids and bases is how they react with each
other.
Some Common Acids and Bases
Name
Acids:
Acetic acid
Acetylsalicylic acid
Ascorbic acid
Citric acid
Hydrochloric acid
Sulfuric acid
Bases:
Ammonia
Calcium hydroxide
Formula
Remark
HC2H3O2
HC9H7O4
H 2 C 6 H 6 O6
H 3 C 6 H 5 O7
HCl
H2SO4
Found in vinegar
Aspirin
Vitamin C
Found in lemon juice
Found in gastric juice
Battery acid
NH3
Ca(OH)2
Often found in household cleaners
Slaked Lime (used in building
industry)
Milk of magnesia
Found in drain and oven cleaners
Magnesium hydroxide Mg(OH)2
Sodium hydroxide
NaOH
Acid and base definitions
An acid is the species that donates a proton
to another species in a proton-transfer
reaction.
A base is the species that accepts a proton in
a proton-transfer reaction.
NH3 (aq) + H2O (l)
NH4+ + OH- (aq)
Reaction of nitric acid with water
HNO3(aq) + H2O(l)
NO3-(aq) + H3O+(aq)
Note: the reaction simply involves the
Strong and Weak Acids and Bases
A strong acid is an acid that ionizes completely
in water, and is a strong electrolyte.
HCl (aq) + H2O (l)
H3O+ (aq) + Cl- (aq)
A weak acid is an acid that only partially ionizes
in water, and is a weak electrolyte.
HCN (aq) + H2O (l)
H3O+ (aq) + CN- (aq)
A strong base is a base that is present in
aqueous solution entirely as ions (one of
which is OH-) and is a strong electrolyte.
NaOH (s)
H2O
Na+ (aq) + OH- (aq)
A weak base is a base that is only partially
ionized in water.
NH3 (aq) + H2O (l)
NH4+ (aq) + OH- (aq)
Neutralization Reactions
One of the main properties of acid and bases
is that they neutralize one another.
A neutralization reaction is a reaction of an
acid and a base that results in an ionic
compound and possibly water.
The ionic compound that is a product of a
neutralization reaction is called a salt.
Acid + Base
2HCl (aq) + Ca(OH)2 (aq)
Salt + Water
CaCl2 (aq) + 2H2O (l)
2HCl (aq) + Ca(OH)2 (aq)
CaCl2 (aq) + 2H2O (l)
To discuss the essential reactions occurring we
have to write the reactions as ionic equations.
Writing the strong electrolytes in the form of
ions gives the following complete ionic
equation:
H+(aq) + 2Cl-(aq) + Ca2+(aq) + 2OH-(aq)
Ca2+(aq) + 2Cl-(aq) + 2H2O(l)
Canceling spectator ions and dividing by 2
gives the net ionic equation:
H+(aq) + OH-(aq)
H2O(l)
Example
Writing an equation for a neutralization rxn
Write the molecular equation and the net ionic
equation for the neutralization of sulfurous
acid by potassium hydroxide.
sulfurous acid =
potassium hydroxide =
Molecular equation:
Ionic equation:
Net ionic equation:
Monoprotic acids  acids that have only one
acidic hydrogen atom per acid molecule
eg. HCl and HNO3
Polyprotic acid  an acid that yields two or
more acidic hydrogens per molecule.
Phosphoric acid is an example of a triprotic
acid. By reacting it with different amounts of
base, a series of salts can be obtained.
H3PO4(aq) + NaOH(aq)
NaH2PO4(aq) + H2O(l)
H3PO4(aq) + 2NaOH(aq)
Na2HPO4(aq) + 2H2O(l)
H3PO4(aq) + 3NaOH(aq)
Na3PO4(aq) + 3H2O(l)
Acid-Base reactions with Gas Formation
Certain salts, notably carbonates, sulfites and
sulfides, react with acids to form a gaseous
product.
Na2CO3(aq) +2HCl(aq)
2NaCl(aq) + H2O(l) + CO2(g)
If we consider this reaction as an exchange, or
metathesis reaction, then we obtain the
following:
Na2CO3(aq) +2HCl(aq)
2NaCl(aq) + H2CO3(aq)
The last product in the above equation is
carbonic acid, which is unstable and
decomposes to water and carbon dioxide
Example
Writing an equation for a reaction with gas
formation.
Write the molecular equation and the net ionic
equation for the reaction of copper(II)carbonate
with hydrochloric acid.
Molecular equation:
Ionic equation:
Net ionic equation:
Working with Solutions
Most chemistry you have encountered to date
takes place in solution!
In general, reactions involving solids proceed
very slowly, if at all.
When carrying out reactions in solutions it is
very convenient to measure out the reactant by
volume.
Molar Concentration
Concentration = the quantity or amount of
solute in a standard quantity of solution.
Eg. Units: mol/L or g/ml
We also use the relative terms : dilute and
concentrated
A solution is dilute, when the solute
concentration is low
A solution is concentrated when the solute
concentration is high.
Molar concentration, or Molarity, M
 the moles of solute dissolved in one
liter of solution.
Molarity =
moles of solute
liters of solution
Units: mol.L-1 or M
n
c
v
Example
Calculating molarity from mass and volume.
A student prepared a solution by dissolving
1.345 g of potassium nitrate (KNO3) in enough
water to prepare 25.00 mL of solution. What is
the molarity (concentration) of the solution?
Example
Or the other way around!!
Calculate the mass of potassium permanganate
needed to prepare 250 mL of 0.0380 M KMnO4
(aq) solution.
Dilution
It is often necessary to reduce the
concentration of a solution - that is to dilute it.
This can be accomplished by taking a certain
volume of the more concentrated solution and
then diluting it with additional solvent.
NB - during dilution the number of moles of
solute remains constant.
The Dilution Equation
For initial solution :
C1 x V1 = n1
For final solution :
C2 x V2 = n2
However n1 = n2
therefore : C1 x V1 = C2 x V2
or :
C1V1 = C2V2
Example
Diluting a solution.
Calculate the volume of 0.0155 M HCl that we
should use to prepare 100 mL of a 5.23x10-4 M
HCl solution.
Quantitative Analysis
The determination of the amount of a
substance or species present in a material.
Volumetric Analysis
A titration is a procedure for determining the amount of
substance A by adding a carefully measured volume of
a solution with a known concentration of B until the
reaction of A and B is just complete.
A
B
A+B
Example
Calculating the Quantity of a Substance in a
Titrated Solution
A dilute solution of hydrogen peroxide is sold in
pharmacies as a mild antiseptic. A typical
solution was analyzed for the percentage of
hydrogen peroxide by titrating it with potassium
permanganate.
5H2O2 + 2KMnO4 + 6H+
8H2O + 5O2 + 2K+ + Mn2+
What is the mass percentage of H2O2 in a
solution if 57.5 g of peroxide solution required
38.9 mL of 0.534 M KMnO4?
Answer : 3.08%