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Lecture 49 – Lecture 50
Reactions in Aqueous Solutions
Ozgur Unal
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What changes take place at the
atomic level, when salt is dissolved
in water?
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An aqueous solution contains one or more substances called
solutes dissolved in the water.
In this case water is the solvent – the most plentiful
substance in the solution. Salt is the solute.
Molecular compounds can be dissolved in water.
Some of them exist as molecules in the solution  sugar
solution.
Some molecular compounds form ions when dissolved in
water  HCl
HCl (aq)  H+ (aq) + Cl- (aq)
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Ionic compounds (such as NaCl) can also be dissolved in
water.
When ionic compounds dissolve, their ions separate.
NaCl (aq)  Na+ (aq) + Cl- (aq)
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When two aqueous solutions that contain ions as solutes are
combined, the ions might react with one another.
These reactions are always double-replacement reactions.
There are 3 types of products that can form from double
reactions in aqueous solutions: a precipitate, water or a gas.
Reactions that form precipitate:
Some reactions that occur in aqueous solutions produce
precipitates.
Example:
2NaOH (aq) + CuCl2 (aq)  2NaCl (aq) + Cu(OH)2 (s)
Na+ and Cl- ions after the reaction remain dissolved in the
new solution.
Cu(OH)2 precipitates at the bottom of the solution
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To show the details of reactions that involve ions in aqueous
solutions, chemists use ionic equations  different than
chemical equations
In ionic equations, the ions in the solution are explicitly
written:
2Na+ (aq) + 2OH- (aq) + Cu+2 (aq) + 2Cl- (aq)  2Na+ (aq) + 2Cl- (aq) + Cu(OH)2 (s)
An ionic equation that shows all of the particles in a
solution as they exist is called a complete ionic equation.
Note that the sodium and chlorine ions do not participate in
the reaction  spectator ions
Net ionic equations are ionic equations that include only the
particles that participate in the reaction.
2OH- (aq) + Cu+2 (aq)  Cu(OH)2 (s)
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Consider the reaction between hydrobromic acid and
sodium hydroxide:
HBr (aq) + NaOH (aq)  ???
What are the products of this reaction?
Does this reaction produce a precipitate?
The reaction above produces NaBr (aq) and water.
The complete ionic equation for this reaction is:
H+ (aq) + Br- (aq) + Na+ (aq) + OH- (aq)  H2O (l) + Na+ (aq) + Br- (aq)
The net ionic equation is:
H+ (aq) + OH- (aq)  H2O (l)
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Example: Write the chemical, complete ionic and net ionic
equations for the reaction between hydrochloric acid and
aqueous lithium hydroxide. The reaction produces water and
aqueous lithium choride.
Example: Write the chemical, complete ionic and net ionic
equations for the reaction between the following substances.
Mixing sulfuric acid and aqueous potassium hydroxide
produces water and aqueous potassium sulfate.
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Some double replacement reactions produce gases, such as
CO2, HCN and H2S
Example:
2HI (aq) + Li2S (aq)  H2S (g) + 2LiI (aq)
The complete ionic equation for this reaction is:
2H+ (aq) + 2I- (aq) + 2Li+ (aq) + S+2 (aq)  H2S (g) + 2Li+ (aq) + 2I- (aq)
What are the spectator ions here?
The net ionic equation is:
2H+ (aq) + S-2 (aq)  H2S (g)
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Write down the complete ionic, net
ionic and chemical equations for the
reaction between baking soda and
vinegar.
Example: Write chemical, complete ionic, and net ionic
equations for the reactions between perchloric acid (HClO4)
and aqueous potassium carbonate forming CO2 and water.
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Consider the reaction between HCl and NaHCO3.
HCl (aq) + NaHCO3 (aq)  H2CO3 (aq) + NaCl (aq)
When H2CO3 is formed, it decomposes immediately:
H2CO3 (aq)  H2O (l) + CO2 (g)
Therefore, there are two reactions taking place: double
replacement and decomposition reactions.
When these two reactions are combined, we get the overall
reaction:
H+ (aq) + HCO3- (aq)  H2O (l) + CO2 (g)
Follow the steps on page 307.
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