Secs 4.1-4: Aqueous Reactions
Review: What is a solution?
Aqueous solutions: solutions in
which H2O is the dissolving medium
Denoted as (aq) , e.g., HCl(aq)
Two components of a solution:
Solvent
Solute
Electrolytes
solutes which exist as ions in
aqueous solution are called
electrolytes
solutions of electrolytes have the
property that they will conduct
electricity
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Note: most soluble ionic compounds are
electrolytes in aqueous solution
E.g., What dissolved species are present when NaCl is
added to H2O? What about potassium sulfate?
Most molecular substances do not form ions in solution these nonionizing substances are called nonelectrolytes
Solutions of nonelectrolytes do not conduct electricity
E.g., What species are present when CH3OH is added to
H2O?
Electrolytes may be further classified as being either strong or
weak:
Substances which exist almost completely as ions in
solution: strong electrolytes
Soluble ionic compounds are strong electrolytes –
they dissociate completely into ions in aq. solution
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Complete dissociation into ions is written with one arrow,
e.g.,
NaCl(aq)  Na+(aq) + Cl-(aq)
e.g, write equations to show how the following electrolytes
ionize in aqueous solution:
K2SO4(s)
CaCl2(s)
Compounds which do not ionize completely in H2O: weak
electrolytes
e.g., acetic acid CH3COOH : ionizes only partially in H2O:
CH3COOH(aq) = H+(aq) + CH3COO-(aq)
Note the double arrow: the reaction occurs to a significant
extent in both directions - this indicates a state of
chemical equilibrium
How to distinguish between strong/weak/nonelectrolytes?
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To do this properly, we need to examine acids and bases;
for now: water-soluble ionic compounds are strong
electrolytes
Precipitation Reactions
Reactions between two soluble solutes which produces an
insoluble solid, e.g.
AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)
How to predict whether or not a compound will be
insoluble in water?
The solubility of a compound is the amount (g or mol)
of the compound that can be dissolved in a given quantity
of solvent (mL or L) at a given temperature
e.g. suppose the solubility of NaCl in H2O is 35.2 g/100
mL at 25oC. Meaning?
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Solubility rules (table 4.1, p. 121)
Table 4.1 is organized according to the anion in the
compound…
Predict whether the following compounds are soluble or
insoluble in water
MgBr2
PbI2
Sr(OH)2
ZnSO4
Na2CO3
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Goal: we want to be able to predict whether a precipitate
forms when we mix aqueous solutions of two strong
electrolytes…
e.g. will a precipitate form when aq solutions of Mg(NO3)2
and NaOH are mixed?
Steps:
1. Look at the ions present in the reactants
2. Switch the anions and cations to form products – the
formulas of the products are determined by ionic
charges!!!!!!
3. Use Table 4.1 to determine if any of the products are
insoluble.
4. Write a balanced equation.
E.g. Will precipitation occur when solutions of AgNO3(aq)
and Na2CO3(aq)are mixed? If so, write a balanced
equation.
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Notice: in the above examples, we switched cations and
anions among the reactants to form products…
These reactions have the general form
AX + BY  AY + BX
These reactions are known as exchange or metathesis
reactions
Precipitation reactions conform to this pattern (as do
neutralization reactions between acids and bases)
E.g. Write a balanced equation for the reaction between
BaCl2(aq) and K2SO4.(aq).
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Ionic equations
molecular equation: shows complete chemical
formula of products/reactants, e.g.,
BaCl2 (aq) + K2SO4(aq)  BaSO4(s) + 2KCl(aq)
What if the dissolved substances (i.e., BaCl2, K2SO4, KCl)
are strong electrolytes?
Write complete ionic equation with strong electrolytes in
ionic form:
Ba2+(aq) + 2Cl-(aq) + 2K+(aq) + SO4-2(aq)  BaSO4(aq) + 2K+(aq) +
2Cl-(aq)
treat the arrow sign as an algebraic "=" sign: are there
identical ions on both sides of the = sign? what cancels?
K+ and Cl- are spectator ions
If complete ionic equation is written without spectator
ions, we have the net ionic equation:
Ba2+(aq) + SO4-2(aq)  BaSO4(aq)
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Net ionic equation includes only those ions & molecules
which are DIRECTLY involved in the reaction
Writing net ionic equations
questions:
Is the substance soluble? (i.e., aq)
If the substance is soluble, is it a strong
electrolyte?
If you answered yes to both of the above, write the
substance in ionic form
ONLY SOLUBLE STRONG ELECTROLYTES ARE
WRITTEN IN IONIC FORM!!!!
write other substances (weak/nonelectrolytes,
molecular, solid, liquid, gas) in 'molecular' form
cancel spectator ions
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E.g., write balanced net ionic equations for the
following:
Cr(OH)3(s) + HNO3(aq) H2O(l) + Cr(NO3)3(aq)
CuBr2(aq) + NaOH(aq)  Cu(OH)2(s) + NaBr(aq)
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Write a balanced net ionic equation for the following reaction:
Fe(NO3)2(aq) + KOH(aq) 
Acids, Bases, and Neutralization Reactions
Acids
Substances which donate a proton (H+) in aqueous
solution
e.g., HCl, HNO3, H2SO4, etc. (name these!)
Monoprotic acids: donate one proton, e.g.
HCl(aq)  H+(aq) + Cl-(aq)
HNO3(aq)  H+(aq) + NO3-(aq)
Diprotic acids: donate 2 protons, in a stepwise way:
H2SO4(aq)  H+(aq) + HSO4-(aq)
HSO4-(aq) = H+(aq) + SO42-(aq)
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Bases
Substances which accept H+ OR which produce OH- in
aqueous solutions
e.g., NaOH, KOH, Ca(OH)2, NH3
why is NH3 a base?
NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)
(It accepts protons and forms OH -)
How to tell acid/base strength?
Strong acids (Memorize!!)
HBr, HCl, HI, HNO3, H2SO4 (first proton!), HClO3, HClO4
These strong acids are strong electrolytes – exist 100
% ionized in aqueous solution
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Any acid that isn’t strong is referred to as a weak acid
– it is a weak electrolyte and exists in partially ionized
form in aqueous solution
E.g., CH3COOH; HF; HNO2
Strong bases (Memorize!!!)
All Group 1A metal hydroxides
Heavy group 2A (Ca2+, Sr2+, Ba2+) metal hydroxides
These bases exist in 100% ionized form in aqueous
solution – they are strong electrolytes
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Summary:
Strong electrolytes: 100% ionized
Soluble ionic compounds (salts)
Strong acids
Strong bases
Weak electrolytes: partially ionized
Weak acids
Weak bases (e.g. NH3)
Nonelectrolytes: not ionized
Molecular compounds that aren’t weak
acids/bases, e.g. CH3OH; C6H12O6
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Problems du Jour
Classify each of the following substances as a
nonelectrolyte or strong/weak electrolyte in H2O
HF
NH3
Cu(NO3)2
Ni(OH)2
C2H5OH
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