1. Reactions in Aqueous Solutions
Double-Replacement Reactions in Water

2. Objectives: Describe aqueous solutions
Write complete ionic and net ionic equations for reactions in aqueous solutions
Predict whether reactions in aqueous solutions will produce a precipitate, water, or a gas

3. Reactions in Water are Vital…
70% of Earth covered by H2O
Your body is 60-70% H2O

4. Aqueous Solutions Aqueous from “Aqua” (Latin for water)
Aqueous Solution = Water with stuff dissolved in it
Solute = The stuff that is dissolved
Solvent = The most plentiful substance in the solution
Water = The solvent in an aqueous solution

5. Solutes
There are many possible solutes—sugar and alcohol are molecular compounds that exist as molecules in aqueous solutions.
Compounds that produce hydrogen ions in aqueous solutions are acids.
Ionic compounds can also be solutes in aqueous solutions.
When ionic compounds dissolve in water, their ions separate in a process called dissociation.

6. Types of Reactions in aqueous solutions
When two solutions that contain ions as solutes are combined, the ions might react.
If they react, it is always a double replacement reaction.
Three products can form: precipitates, water, or gases

7. Reactions that form solid precipitates
Aqueous solutions of sodium hydroxide and copper(II) chloride react to form the precipitate copper(II) hydroxide.
2NaOH(aq) + CuCl2(aq) →
2NaCl(aq) + Cu(OH)2(s)
Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations.
2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) →
2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)

8. Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations.
Formulas that include only the particles that participate in reactions are called net ionic equations:
2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)

9. Reactions that produce Water
Some reactions produce more water molecules.
No evidence of a chemical reaction is observable.
HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
Without spectator ions: H+(aq) + OH–(aq) → H2O(l)

10. Reactions that form gases
Gases that are commonly produced are carbon dioxide, hydrogen cyanide, and hydrogen sulfide.
2HI(aq) + Li2S(aq)→ H2S(g) + 2LiI(aq)

11. Reactions that form gases
Another example is mixing an acid and baking soda, which produces carbon dioxide gas:
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
H2CO3(aq) (carbonic acid) decomposes immediately:
H2CO3(aq) → H2O(l) + CO2(g)

12. Combining Reactions
Two reactions can be combined and represented by a single chemical reaction.

13. Combining Reactions Reaction 1
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
Reaction 2
H2CO3(aq) → H2O(l) + CO2(g)
Combined equation
HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)
Overall equation
HCl(aq) + NaHCO3(aq) → H2O(l) + CO2(g) + NaCl(aq)

14. This is what makes cakes rise!
Baking Soda + Acid
This is what makes cakes rise!

15. Review
Reactions between ionic compounds in water produce a gas, a solid precipitate, or a liquid (H2O, usually)
These reactions are double replacement reactions
Chemists write aqueous reactions as ionic equations – these show the ions dissolved in water and their interactions
A net ionic equation shows only those ions that interact to produce a solid, liquid or gaseous product
You can combine reactions and cancel out the ions and compounds that appear as both reactants and products.