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Predicting Chemical Reactions
Exchange Reactions
(Metathesis Reaction)
1
Precipitation Reactions
Predicting Products
Consider the reaction between AgNO3 and NaCl in aqueous
solution:
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Notice that the anions and cations appear to have
exchanged partners.
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Precipitation Reactions
• Metathesis Reactions
– reactions in which the positive ions and negative ions
present in the reactants appear to exchange partners
– also called exchange reactions
AX + BY  AY + BX
AgNO3 + NaCl  AgCl + NaNO3
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Precipitation Reactions
• How can you predict the products of a reaction
between ionic compounds?
– Identify the ions (including charge) present in the
reactants.
– Exchange the anions between the two cations
and write the correct formulas (electrically neutral)
for the possible products.
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Precipitation Reactions
• How can you predict the products of a reaction between
ionic compounds?
-Determine if any of the products are insoluble in water.
• If any are insoluble, the reaction will procede!
• Also, if possible products are in gas form (CO2) or are in
the liquid (not aq) form, the reaction will take place.
• If none of the products are a solid, a gas, or a liquid (i.e.
different physical state, physical state other than aq,)
there is no reaction….nothing happens.
-Write a balanced equation including the physical
states of reactants and products.
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Predicting Reactions
Example: Write the chemical equation for
the aqueous reaction between Ba(NO3)2 and
Na2SO4. Predict which product(s) will be
insoluble.
Ions: Ba2+ , NO3 -,
Na+,
At this point, you don’t
care how many of each
ion you started with.
SO426
Predicting Reactions
Possible Products:
Ba2+ and SO42-
BaSO4
Na+ and NO3 -
NaNO3
Make sure that you combine the ions so that
total positive charge = total negative charge
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Predicting Reactions
Determine solubility of possible products:
BaSO4 (s)
Insoluble = solid
NaNO3 (aq)
Soluble = aqueous
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Predicting Reactions
Write a balanced equation:
Ba(NO3)2(aq) + Na2SO4(aq)  BaSO4 (s) + NaNO3 (aq)
unbalanced
You must now make sure that you balance the
equation at this point.
Ba(NO3)2(aq) + Na2SO4(aq)  BaSO4 (s) + 2 NaNO3 (aq)
balanced
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Practice
• Predict the identity of the precipitate that
forms when solutions of Fe2(SO4)3 and
LiOH are mixed.
• Ions present in solution:
Fe3+
SO42Li+
OH-
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Practice (contd)
• Write a balanced equation for the reaction:
Fe2(SO4)3(aq)+ LiOH(aq)
Fe(OH)3 (s) + Li2SO4(aq)
Unbalanced!!
Fe2(SO4)3(aq)+ 6 LiOH(aq)
2 Fe(OH)3 (s) + 3 Li2SO4(aq)
Balanced
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Molecular Equation
The molecular equation lists the reactants and
products in their molecular form.
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
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Ionic Equation
• In the ionic equation all strong electrolytes (strong
acids, strong bases, and soluble ionic salts) are
dissociated into their ions.
• This more accurately reflects the species that are
found in the reaction mixture.
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Ag+ (aq) + NO3- (aq)+ K+(aq) + Cl-(aq)  AgCl (s) + K+ (aq) + NO3- (aq)
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Net Ionic Equation
• To form the net ionic equation, cross out anything
that does not change from the left side of the
equation to the right.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq)
Ag+(aq)
+ Cl-(aq)
AgCl (s)
The only things left in the equation are those
things that change (i.e., react) during the course of
the reaction.
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Net Ionic Equation
• To form the net ionic equation, cross out anything that does not change
from the left side of the equation to the right.
• The only things left in the equation are those things that change (i.e.,
react) during the course of the reaction.
• Those things that didn’t change (and were deleted
from the net ionic equation) are called spectator ions.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq)
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Writing Net Ionic Equations
1. Write a balanced molecular equation.
2. Dissociate all strong electrolytes.
3. Cross out anything that remains
unchanged from the left side to the
right side of the equation.
4. Write the net ionic equation with the
species that remain.
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Practice
• Write a net ionic equation for the
precipitation reaction that occurs when
aqueous solutions of silver nitrate and
potassium phosphate are mixed.
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More practice
• Lab 8:
• Write molecular, as well as ionic and net
ionic reactions (13 of them) if there is a
reaction taking place.
• Reaction #1:
Sodium carbonate plus hydrochloric acid
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Acids and Bases
and Their Reactions
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Acids
• Arrhenius defined acids
as substances that
increase the
concentration of H+
when dissolved in water.
• Brønsted and Lowry
defined them as proton
donors.
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Acids
Increase the H+ concentration in water:
Example: nitric acid
HNO3 (aq)
 H+ (aq) + NO3- (aq)
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Acids
• Examples of Acids:
– HCl
– HNO3
– CH3COOH
– H2SO4
– H3PO4
hydrochloric acid
nitric acid
acetic acid
sulfuric acid
phosphoric acid
Note: Acids can form different
numbers of H+ ions!
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Acids
• Monoprotic acids
– have one H in the formula
– form a single H+ ion when they ionize
HNO3 (aq)  H+ (aq) + NO3- (aq)
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Acids
• Diprotic acids
– have two H’s in the formula
– can form two H+ ion when they ionize completely
H2SO4 (aq)  2H+ (aq) + SO42- (aq)
• Polyprotic acids:
– Have two or more H’s in the formula
– Form two or more H+ ions when they ionize
completely
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Bases
• Arrhenius defined
bases as substances
that increase the
concentration of OH−
when dissolved in water.
• Brønsted and Lowry
defined them as proton
acceptors.
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Bases
• Bases:
– substances that accept (react with) H+ ions.
– any substance that increases the OHconcentration when added to water
Examples:
– Hydroxide ion (OH-)
OH- (aq) + H+ (aq)  H2O (l)
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Bases
• Examples (cont)
– Common hydroxide containing bases:
• NaOH, KOH, Ca(OH)2
• Note: These are strong electrolytes!
• NaOH (aq)  Na+ (aq) + OH- (aq)
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Bases
• Examples (cont):
– Ammonia (NH3)
• Does not contain OH• Accepts H+ ion from water and increases the OHconcentration in the water
NH3(aq) + H2O (l)
NH4+ (aq) + OH- (aq)
NH3 is a weak electrolyte!! (double arrow!)
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Acids & Bases
• Strong Acid:
– an acid that is a strong electrolyte
• ionizes completely in solution
• Weak Acid:
– an acid that is a weak electrolyte
• an acid that does not ionize completely
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Acids & Bases
• Strong acids:
– Know the names and formulas of the 7 common
strong acids:
•
•
•
•
•
•
•
HCl (aq)
HBr (aq)
HI (aq)
HClO3
HClO4
HNO3
H2SO4
hydrochloric acid
hydrobromic acid
hydroiodic acid
chloric acid
perchloric acid
nitric acid
sulfuric acid
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Acids & Bases
• Examples of Weak Acids
HF (aq)
H3PO4
CH3COOH
hydrofluoric acid
phosphoric acid
acetic acid
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Acids & Bases
• Strong Base:
– a base that is a strong electrolyte
– ionizes completely in solution
• Weak Base:
– a base that is a weak electrolyte
– does not ionize completely in solution
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Acids & Bases
• Strong Bases:
Know the names and formulas of the strong bases
– Alkali metal (1A) hydroxides
•
•
•
•
•
LiOH
NaOH
KOH
RbOH
CsOH
lithium hydroxide
sodium hydroxide
potassium hydroxide
rubidium hydroxide
cesium hydroxide
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Acids & Bases
• Strong bases to know (con’t):
– Heavy alkaline earth metal (2A) hydroxides
• Ca(OH)2
• Sr(OH)2
• Ba(OH)2
calcium hydroxide
strontium hydroxide
barium hydroxide
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Acids & Bases
• Examples of Weak Bases:
ammonia (NH3)
sodium bicarbonate (NaHCO3)
• baking soda
• a component of Alka-Seltzer
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Acids + Bases
Generally, when solutions of an acid and a base are
combined, the products are a salt and water.
CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)
Neutralization Reaction
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Neutralization Reactions
When a strong acid reacts with a strong
base, the molecular equation is…
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
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Neutralization Reactions
When a strong acid reacts with a strong
base, the ionic equation is…
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) 
Na+ (aq) + Cl- (aq) + H2O (l)
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Neutralization Reactions
When a strong acid reacts with a strong
base, the net ionic equation is…
HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) 
Na+ (aq) + Cl- (aq) + H2O (l)
H+ (aq) + OH-(aq)  H2O (l)
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Neutralization Reactions
The products of these reactions have very
different properties than the reactants.
HCl (aq) + NaOH (aq)  H2O (l) + NaCl (aq)
Sharp
sour
bitter
slippery
salt
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Acid-Base Reactions
• Salt:
– any ionic compound whose cation comes from a base
and whose anion comes from an acid
– an ionic compound that is neither an acid nor a base
• In general,
acid + metal hydroxide  a salt + water
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Acid-Base Reactions
• Neutralization reactions are a type of metathesis
reaction.
• To predict the products:
– identify the ions present
– exchange anions
– write the correct formulas for the products including
physical states
– write a balanced equation
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Acid-Base Reactions
Example: Write the balanced equation for the reaction
between HBr (aq) and Ca(OH)2 (aq).
Ions:
H+
Br-
Possible Products:
H-OH = H2O
Ca2+
OH-
CaBr2
2HBr (aq) + Ca(OH)2 (aq)  CaBr2 (aq) + 2H2O (l)
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Gas-Forming Reactions
• Some metathesis reactions do not give the
product expected.
• In this reaction, the expected product (H2CO3)
decomposes to give a gaseous product (CO2)
and H2O.
CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
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Gas-Forming Reactions
When a carbonate or bicarbonate reacts with an
acid, the products are a salt, carbon dioxide, and
water.
CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l)
“H2CO3”
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Gas-Forming Reactions
Similarly, when a sulfite reacts with an acid, the
products are a salt, sulfur dioxide, and water.
SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l)
“H2SO3”
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Gas-Forming Reactions
• This reaction gives the predicted product, but
you had better carry it out in the hood, or you will
be very unpopular!
• But just as in the previous examples, a gas is
formed as a product of this reaction.
Na2S (aq) + H2SO4 (aq)  Na2SO4 (aq) + H2S (g)
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