Download ionic equation

Chemical Reaction Types
PS21 September 23, 2016
ACOS #5, p. 40…”observe and analyze
properties of substances before and
after chemical reaction – Chemical
Reaction types.”
Aqueous
Reactions
Aqueous Reaction Types
Will focus on reactions in aqueous solution and
their driving forces
• Precipitation Reactions
• Gas-forming Reactions
• Acid-base Reactions
• Redox Reactions
Aqueous
Reactions
Solutions
• Solutions are defined as homogeneous mixtures of
two or more pure substances.
• The solvent is the substance present in greatest
abundance.
• All other substances are solutes.
Aqueous
Reactions
Dissociation
• When an ionic
substance dissolves in
water, the solvent
pulls the individual
ions from the crystal
and solvates them.
• This process is called
dissociation.
Aqueous
Reactions
Electrolyte
• An electrolyte is a substances that dissociates
into ions when dissolved in water.
• A nonelectrolyte does not dissociate when
dissolved in water
Aqueous
Reactions
Electrolytes
• A strong electrolyte dissociates completely when
dissolved in water.
• A weak electrolyte only dissociates partially when
dissolved in water.
Aqueous
Reactions
Electrolytes
Strong Electrolyte Weak Electrolyte Nonelectrolyte
Ionic
All
None
None
Molecular
Strong acids
Weak acids &
All other
bases
compounds
.
Aqueous
Reactions
Electrolytes
Soluble ionic compounds are normally electrolytes
Nonelectrolytes
Molecular compounds tend to be
nonelectrolytes, except for acids and bases.
Aqueous
Reactions
Solution Chemistry
• It is helpful to pay attention to exactly what
species are present in a reaction mixture (i.e.,
solid, liquid, gas, aqueous solution).
• If we are to understand reactivity, we must be
aware of just what is changing during the
course of a reaction.
Aqueous
Reactions
Molecular Equation
The molecular equation lists the reactants and
products in their molecular form:
AgNO3(aq) + KCl(aq)  AgCl(s) + KNO3(aq)
Aqueous
Reactions
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:
Ag+(aq) + NO3−(aq) + K+(aq) + Cl−(aq) 
AgCl(s) + K+(aq) + NO3−(aq)
Aqueous
Reactions
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)
Aqueous
Reactions
Net Ionic Equation
• The only things left in the equation are those things that
change (i.e., react) during the course of the reaction:
Ag+(aq) + Cl−(aq)  AgCl(s)
• 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)
K+ and NO3- are spectator ions
Aqueous
Reactions
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.
Aqueous
Reactions
Reaction Types in Aqueous Solution
•
•
•
•
Precipitation Reactions
Gas-Forming Reactions
Acid-base Reactions
Redox Reactions
Can be identified from the net ionic equation
Aqueous
Reactions
Metathesis (Exchange) Reactions
• Metathesis comes from a Greek word that means
“to transpose.”
• It appears as though the ions in the reactant
compounds exchange, or transpose, ions:
AgNO3(aq) + KCl(aq)  AgCl(s) + KNO3(aq)
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Being Careful
Molecular equation:
2 KCl(aq) + Cu(NO3)2 (aq)  2 KNO3 (aq) + CuCl2(aq)
Ionic equation:
2 K+(aq) + 2 Cl-(aq) + Cu2+(aq) + 2 NO3-(aq) 
2 K+(aq) + 2 NO3-(aq) + Cu2+(aq) + 2 Cl-(aq)
Net ionic equation:
none – no reaction
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Precipitation Reactions
When one mixes ions
that form compounds
that are insoluble (as
could be predicted by
the solubility
guidelines), a precipitate
is formed.
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Example Precipitation Reaction
Molecular equation:
Pb(NO3)2(aq) + K2CrO4(aq)  2 KNO3(aq) + PbCrO4(s)
yellow
Ionic equation:
Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) + CrO42-(aq) 
2 K+(aq) + 2 NO3-(aq) + PbCrO4(s)
Net ionic equation:
Pb2+(aq) + CrO42-(aq)  PbCrO4(s)
Precipitation is the driving force
Aqueous
Reactions
Solubility Guidelines
• All Compounds of Na+, K+, NH4+ (most Li+)
• All Compounds of NO3-, NO2-, ClO3-, ClO4-,
{CH3COO- Slightly Soluble}
• All Compounds of Cl-, Br -, I-, SCN- (except Ag+,
Hg22+ ,Pb2+ {PbCl2 soluble in hot water})
• All compounds of SO42- (except Sr2+, Ba2+, Pb2+,
{Ag+, Ca2+, Hg2+ Slightly})
• Hydroxides: LiOH, NaOH, KOH, NH4OH, Ca(OH)2,
Sr(OH)2, Ba(OH)2
Aqueous
Reactions
Acids
• The Swedish physicist and
chemist S. A. Arrhenius defined
acids as substances that
increase the concentration of
H+ when dissolved in water.
Aqueous
Reactions
Strong Acids
There are only six strong acids (strong electrolytes):
• Hydrochloric (HCl)
• Hydrobromic (HBr)
• Hydroiodic (HI)
• Nitric (HNO3)
• Sulfuric (H2SO4)
• Perchloric (HClO4)
(although some books include chloric acid, HClO3)
Aqueous
Reactions
Bases
• Arrhenius defined bases as
substances that increase
the concentration of OH−
when dissolved in water.
Aqueous
Reactions
Bases
The strong bases (strong electrolytes) are the
soluble metal salts of hydroxide ion:
•
•
•
•
Alkali metals - LiOH, NaOH, KOH, CsOH, RbOH
Calcium – Ca(OH)2
Strontium – Sr(OH)2
Barium – Ba(OH)2
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Acid-Base/Neutralization Reactions
In an acid–base reaction, the acid donates a
proton (H+) to the base.
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Neutralization Reactions
Generally, when solutions of an acid and a base are combined, the
products are a salt and nonelectrolyte (often water):
CH3COOH(aq) + NaOH(aq)  CH3COONa(aq) + H2O(l)
salt
water
Net ionic equation:
CH3COOH(aq) + OH-(aq)  CH3COO-(aq) + H2O(l)
Driving force: formation of nonelectrolyte and salt
Aqueous
Reactions
Neutralization Reactions
When a strong acid reacts with a strong base, the net ionic
equation is H+(aq) + OH−(aq)  H2O(l)
E.g., Molecular equation:
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Ionic equation:
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)  Na+(aq) + Cl-(aq) +
H2O(l)
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Gas-Forming Reactions
• At least one of the products is a gas
• Driving force is production of the gas
Aqueous
Reactions
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):
CaCO3(s) + 2 HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
Ionic equation:
CaCO3(s) + 2 H+(aq) + 2 Cl-(aq)  Ca2+(aq) + 2 Cl-(aq) +
CO2(g) + H2O(l)
Net ionic equation:
Aqueous
CaCO3(s) + 2 H+(aq)  Ca2+(aq) + CO2(g) + H2O(l)
Reactions
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)
Aqueous
Reactions
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)
Aqueous
Reactions
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)
Aqueous
Reactions
Household Examples
• Baking Soda
NaHCO3 + H+ → Na+ + CO2 + H2O
• Baking Powder
14 NaHCO3 + 5 Ca(H2PO4)2 → 14 CO2 +
Ca5(PO4)3OH + 7 Na2HPO4 + 13 H2O
• Alka Seltzer
Citric acid + 3 HCO3-  citrate + 3 CO2 +
3 H2O
© 2012 Pearson Education, Inc.
Aqueous
Reactions
Types Are Not Exclusive
• By coincidence, all the gas forming reaction
above are also???
Aqueous
Reactions
Oxidation-Reduction (Redox) Reactions
•
•
•
•
An oxidation occurs when an atom or ion loses electrons.
A reduction occurs when an atom or ion gains electrons.
One cannot occur without the other.
Driving force is transfer of electrons
Aqueous
Reactions
Oxidation Numbers
To determine if an oxidation–reduction
reaction has occurred, we assign an oxidation
number to each element in a neutral
compound or charged entity.
Aqueous
Reactions
Oxidation Numbers
• Elements in their elemental form have an
oxidation number of 0.
• The oxidation number of a monatomic ion
is the same as its charge.
Aqueous
Reactions
Oxidation Numbers
• Nonmetals tend to have negative oxidation
numbers, although some are positive in
certain compounds or ions.
– Oxygen has an oxidation number of −2, except
in the peroxide ion, in which it has an oxidation
number of −1.
– Hydrogen is −1 when bonded to a metal, +1
when bonded to a nonmetal.
Aqueous
Reactions
Oxidation Numbers
• Nonmetals tend to have negative oxidation
numbers, although some are positive in
certain compounds or ions.
– Fluorine always has an oxidation number of −1.
– The other halogens have an oxidation number
of −1 when they are negative; they can have
positive oxidation numbers, however, most
notably in oxyanions.
Aqueous
Reactions
Oxidation Numbers
• The sum of the oxidation numbers in a
neutral compound is 0.
• The sum of the oxidation numbers in a
polyatomic ion is the charge on the ion.
Aqueous
Reactions
Identifying Redox Reactions
• Look for element(s) with different oxidation
number on each side of equation
• Often easy to find by looking for elemental
form of an element on only one side of
equation
2 Ca(s) + O2(g)  2 CaO(s)
Oxidation numbers:
Ca 0 O 0
Ca +2 O -2
Aqueous
Reactions
Displacement Reactions
• In displacement reactions, ions oxidize an element.
• The ions, then, are reduced.
Aqueous
Reactions
Displacement Reactions
In this reaction,
silver ions oxidize
copper metal:
Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s)
Aqueous
Reactions
Displacement Reactions
The reverse reaction,
however, does not
occur:
Cu2+(aq) + 2Ag(s) 
x Cu(s) + 2Ag+(aq)
Aqueous
Reactions
© 2012 Pearson Education, Inc.
Activity Series
Aqueous
Reactions