Chapter 4: Reactions in Aqueous Solutions
Biomineralization: The formation of insoluble compounds by living organisms
http://oceanworld.tamu.edu/resources/oceanography-book/Images/emiliana-huxleyi.jpg
Animations on solutions forming:
http://www.deciencias.net/proyectos/0cientificos/Tiger/Tiger.htm
http://www.earth.northwestern.edu/people/seth/107/Ridges/smoker.JPG
General Properties of Aqueous Solutions
Let us investigate the ability of aqueous solutions to conduct electricity. If there are ions in solution, these ions can
carry charge and will conduct electricity.
Predict whether each would be: non-conductor (no ions in solution) , weak conductor (some ions in solution), or a
strong conductor of electric current (lots of ions in solution)
Solution
Tap water
Distilled water
NaCl (salt water)
C12H22O11 (sugar water)
Vinegar (a weak acid)
HCl (a strong acid)
NaOH (a strong base)
CH3OH (methanol)
BaSO4 (barium sulfate)
What general observations can we make?
Prediction
Observation
Classification of Compounds in Our Experiment
Which solutions did not conduct electricity and why did this happen?
Which solutions conducted electricity weakly and why did this happen?
Which solutions conducted electricity strongly and why did this happen?
Electrolytes: Strong
Electrolytes: any substance that produces ions in an aqueous solution
• Strong electrolytes are either molecules that are strong acids or soluble ionic compounds
All soluble ionic compounds are strong electrolytes why? By dissolving they are breaking up into ions.
Below are the only solubility rules we want you to know, a compound only need one cation or anion below to
be soluble
• Cations (+): any ammonium ion NH4+ or any group I (Li+, Na+, K+, Rb+, Cs+) ion containing compound is
soluble
• Anions (-): any nitrate NO3- or chlorate ClO3- or acetate CH3COO- ion containing compound is soluble
If the compound does not contain any of the ions above it is insoluble.
All strong acids are strong electrolytes why?
What are the common strong acids? HCl, HI, HBr, HNO3, HClO4 and H2SO4
Electrolytes: Weak
Electrolytes: any substance that produces ions in an aqueous solution
• Weak electrolyte:
Examples:
• Weak acids:
• Weak bases:
A non-electrolyte is a substance which does not release ions when dissolved in water.
Most molecular compounds like sugar, alcohol, or acetone are non-electrolytes. There are some exceptions:
strong acids are molecular compounds but they are strong electrolytes. Some molecular species react with water
to produce ions, like the weak acids and bases above.
Anything that is insoluble cannot be an electrolyte.
Precipitation Reactions
Some pairs of ions form insoluble compounds. This is a property of the pair, not of the
individual ions.
Examples:
Pb(NO3)2 lead nitrate
is soluble
KI
potasium iodide
is soluble
KNO3
potassium nitrate
is soluble
PbI2
lead iodide
is insoluble
Draw a microscopic
picture of Pb(NO3)2
Draw a microscopic
picture of KI
What happens when we mix together? Draw a microscopic picture?
Exchange Reactions Writing Complete Balanced Equations
Exchange Reactions (Double Displacement, Metathesis) are a type of chemical reaction where the
reactants are composted of two anion-cation pairs which react by changing partners to form two
new anion-cation pairs
• How would you write the balanced molecular equation for the following exchange reaction:
A solution of lead nitrate is mixed with a solutions of potassium iodide to form the yellow
precipitate, lead iodide.
Step 1: Exchange anion-cation reacting pairs
Step 2: Write chemical formulas of products as neutral species
Step 3: Balance the overall reaction, both reactants and products
Step 4: Add phase labels. Exchange Reactions: More Examples
Pb(NO3)2 (aq) + 2KI (aq)
PbI2(s) + 2K(NO3)(aq)
• Write complete balanced equation for the reaction that occurs when a solution of lead
chlorate is mixed with a solution of sodium iodide and a yellow precipitate of lead iodide is formed.
Pb(ClO3)2 (aq) + 2NaI (aq)
PbI2(s) + 2Na(ClO3)(aq)
• Write the equation for the reaction that occurs when a solution of lead acetate is mixed with a
solution of ammonium iodide and a yellow precipitate of lead iodide is formed.
Pb(CH3COO)2 (aq) + 2NH4I (aq)
What do all of these reactions have in common?
PbI2(s) + 2NH4(CH3COO)(aq)
Writing Ionic and Net Ionic Equations
Net ionic equations: This type of equation shows only those ions and molecules participating
in the reaction.
How do you write an net ionic equation using a balanced molecular equation?
1. write the ionic equation by dissociating (breaking apart) all strong electrolytes (all strong
acids, all soluble ionic compounds), keeping all weak acids and weak bases as molecules.
2. identify and cancel ions that do not participate in the reaction (these are
called spectator ions)
What is the ionic and net ionic equation for:
Pb(NO3)2 (aq) + 2KI (aq)
PbI2(s) + 2K(NO3)(aq)
Writing Ionic and Molecular Equations
You find a beaker in a lab and determine it contains the following ions and solid material. Write the net ionic and balanced molecular reaction would produce these products.
Concentrations of Solutions
Let us define an expression for concentration: Molarity (M) =
Consider a 1.0 M solution of CuSO4. How many moles of Cu2+ are in 1.0 liter?
Making Solutions and Dilutions
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Stoichiometry of Precipitation Reactions:
Given that silver chloride (AgCl) is insoluble, predict will happen when we mix 200 ml of 0.10 molar AgNO3 and 150 ml 0.10 molar CaCl2. What did you observe?
Write the balanced molecular equation and the net ionic equation for this reaction.
Add: 200.0 ml 0.1 M AgNO3 + 150 ml of 0.10 M CaCl2 Consider the reaction above. Determine the concentration of all the resulting species in solution after the reaction has come to completion. Acids and Bases
Definition of an Acid:
Examples:
Strong -
Weak -
Compounds that don’t look like acids but actually are.
Compounds that look like acids but actually are not.
Acids and Bases
Definition of a Base:
Examples:
Strong -
Compounds that don’t look like Bases but actually are (weak bases)
Compounds that look like bases but actually are not.
Neutralization Reactions: Write the net ionic equation for each
of the balanced molecular equations below
HCl + NaOH à NaCl + H2O
HF + NaOH à NaF + H2O
H2SO4 + 2 NaOH à Na2SO4 + 2 H2O
Titrations
A titration is a technique for determining the amount of one chemical present in a sample. It involves a reaction between two substances, this reaction can be any type of
reaction like a neutralization reaction, a precipitation reaction or an oxidation-reduction
reaction. Titration experiments proceed until the chemical reaction between the two
substances is complete, this is the end of the titration (called the Endpoint) and is signaled by
either a change in color or conductivity of the solution. Because one substance has a known
concentration and the other is at an unknown concentration we can determine the amount of
unknown material by knowing the amount of known material that has reacted.
Titration
Titrations
Example
• Consider the following neutralization reaction
2HNO3(aq) + Sr(OH)2(aq)
Sr(NO3)2(aq) + 2H2O(l) Suppose it required 10 ml of 1.0 M Sr(OH)2 to neutralize a 10 ml sample of HNO3 in a titration experiment. What is the concentration of the nitric acid in original sample? Titration of a Mixture
Benzoic Acid and gallic acid are both colorless white solids; they are also both monoprotic acids.
• Let ‘HBen’ represent benzoic acid. Write a complete, balanced equation for the neutralization
of HBen with NaOH
• Given the following information:
Benzoic acid (“HBen”)
molar mass = 122g/mol
Gallic acid (“HGal”)
molar mass = 170 g/mol
You have 1.00 gram of a mixture of these two acids. You titrate the mixture against 0.500 molar
NaOH, and find that it takes a total of 14.7 ml of base to completely neutralize both acids. Determine
The mass of each acid present in the original mixture.
Oxidation-Reduction Reactions
• Acid-Base reactions involve a transfer of a proton (H+), Oxidation and Reduction reactions
involve a transfer of electrons.
• Consider the reaction of Zn with H+. Is this an acid-base reaction?
Oxidation-Reduction Reactions and Oxidation Numbers
Oxidation Numbers are used to keep track of electron changes in chemical reactions. Each
element or ion has an oxidation number, if that number changes during a reaction then an oxidationreduction reaction has occurred. (also called redox reactions) To assign oxidation numbers follow the
following rules in order.
1. Pure elements: For any atom in elemental form the oxidation number is zero. 2. Monatomic ions: For monatomic ions in ionic compounds the oxidation number equals the charge
on the ion.
3. Fluorine: always -1
4. Hydrogen: +1 unless violated because rules 2,3. 5. Oxygen: -2 unless when violated by rules 2,3,4
“oxidation state rule”: For neutral species oxidation numbers add to zero, for ions they add to charge on ion.
Oxidation-Reduction Reactions and Oxidation Numbers
HS2O3-
NH2F
Na2FeO4