Aqueous Solutions
We have talked numerous times about substances which dissolve in water (soluble). When a substance
dissolves in water, a solution forms (homogenous mixtures). A 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. An aqueous solution is a solution in which the solvent is water.
**It is important to know the difference between a solute and a solvent.
Let’s begin our discussion with a description of water, since it is considered the universal solvent (or, in
laymen’s terms –the most common dissolving agent).
In general, polar molecules (molecules that have partial charges, such as water) will arrange themselves
to satisfy the attractive forces that occur between the partially negative end and the partially positive
end.
The hydrogen atoms are white and the oxygen atoms are red in the diagram above. The dashes represent
the attractive force that exists between the partially positive hydrogen of one water molecule and the
partially negative oxygen of a different, nearby water molecule.
Now, when an ionic compound or another polar covalent compound is added to water (both of which are
made up of charged/partial charged particles), the partial charges of the water molecule can pull the
compounds apart – see the below diagram of NaCl(aq)
This process is what we call dissolving or dissociation!!!
**When ionic compounds dissolve in water, their ions separate. The equation below shows an
aqueous solution of the ionic compound sodium chloride.
NaCl (aq)  Na+(aq) + Cl-(aq)
When two aqueous solutions are combined, the ions may react with one another. These reactions are
always double-replacement reactions. The solvent molecules, which are all water molecules do not react.
Three types of products can form the double-replacement reaction: precipitate, water, or gas.
Question: When salt water evaporates we can still observe salt crystals in the container. Why is this?
Answer: The properties of water allows the ions of sodium chloride to be separate. However, when the
water evaporates the ions come back together.
What are electrolytes?
Electrolytes are ions that are able to move freely in water. Electrolytes can carry an electric charge when
they move around freely. This is why ionic molecules often carry charges whereas covalent molecules
do not. Covalent molecules do not separate into ions.
Not all ionic compounds are water soluble. Some are too strong to be pulled apart in water!!!
Most polar covalent compounds will dissolve in water,
BUT most non-polar covalent molecules will not.
MEMORY TRICK: Like dissolves Like!!!
Complete Ionic Equations:
Example:
2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s)
When aqueous solutions of sodium hydroxide and copper (II) chloride are mixed, a double –
replacement reaction occurs in which the precipitate cooper (II) hydroxide forms.
This chemical equation does not show the details of this reaction. Sodium hydroxide and copper (II)
chloride are ionic compounds. Therefore, in aqueous solution they exist as Na+, OH-, Cu2+, and Cl- ions.
When their solutions are combined, Cu2+ ions in one solution and OH- ions in the other solution react to
form the precipitate copper (II) hydroxide, Cu(OH)2(s). The Na+ and Cl- ions remain dissolved in the
new solution. An ionic equation is an equation which shows all the ions in the equation. An ionic
equation that shows all of the particles in a solution as they realistically exist is called a complete ionic
equation.
2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq)  2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s)
Note that the sodium ions and the chloride ions are both reactants and products. Because they are both
reactants and products, they do not participate in the reaction. Ions that do not participate in a reaction
are called spectator ions and are not usually shown in ionic equation. Ionic equations that include only
the particles that are written from complete ionic equations by crossing out all spectator ions is called
the net ionic equation. For example, a net ionic equation is what remains after the sodium and chloride
ions are crossed out of the complete ionic equation.
2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq)  2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s)
Net Ionic Equation:
2OH-(aq) + Cu2+(aq) +  Cu(OH)2(s)
Example:
Write the chemical equation, complete ionic equation, and the net ionic equation for the following
reaction:
MgSO4 (aq) + 2NaOH(aq)
Chemical Equation Step 1: Predict the products and state of the products. Balance this equation.
MgSO4 (aq) + 2NaOH(aq)  Mg(OH)2(s) + Na2SO4(aq)
Complete Ionic Equation Step 2: Write any aqueous compounds as there ions. DO NOT SEPARATE
SOLIDS, GASES, OR LIQUIDS INTO THEIR IONS.
Mg2+(aq) + SO42-(aq) + 2Na+(aq) + 2OH-(aq)  Mg(OH)2(s) + 2Na+(aq) + SO42-(aq)
Net Ionic Equation Step 3: Indicate the spectator ions and eliminate them from the complete ionic
equation.
Spectator ions: SO42- and 2Na+
Mg2+(aq) + 2OH-(aq)  Mg(OH)2(s)
Practice:
1. HCl(aq) + Ca(OH)2(aq) 
2HCl(aq) + Ca(OH)2(aq)  2H2O(l) + CaCl2(aq)
Complete ionic: 2H+ (aq) + 2Cl- (aq) + Ca2+ (aq) + 2OH-(aq)  2H2O(l) + Ca2+(aq) + 2Cl- (aq)
Net ionic: H+ (aq) + OH- (aq)  H2O(l)
**Reduce each coefficient by 2.
2. 2HI (aq) + Li2S(aq)  H2S(g) + 2LI(aq)
Complete Ionic: 2H+(aq) + 2I- (aq) + 2Li+(aq) + S2-(aq)  H2S(g) + 2Li+(aq) + 2I-(aq)
Net Ionic: 2H+(aq) + S2-(aq)  H2S(g)
Endothermic and Exothermic Reactions
It is important to note that all reactions give off or absorb heat. Heat is a form of energy and energy is
measured in joules. A joule is a rather small unit so we often express the energy lost or gained in a
chemical reaction in unit of kilojoules.
Exothermic Reactions:
- System loses heat to surroundings
- Heat and light is given off
- Increase in temperature
Endothermic Reactions:
- System absorbs energy from surroundings
- Decrease in temperature
In an equation, heat of a reaction, written as ∆H, can by written in two ways:
1. As a term in the balanced chemical
2. As a ∆H separate from the balanced
equation:
chemical equation:
Example:
Example:
(a) Exothermic:
(a) Exothermic:
Energy is produced in a chemical process (energy Energy is produced in a chemical process (energy
is a product)!!!
is a product)!!! ∆H is negative!!!
2H2(g) + O2(g)  2H2O(l) + 484KJ
2H2(g) + O2(g)  2H2O(l) ∆H= - 484KJ
(b) Endothermic:
(b) Endothermic:
Energy is required in a chemical process (energy
is a reactant)!!!
Energy is required in a chemical process (energy
is a reactant)!!! ∆H is positive!!!
2H2O(l) + 968KJ  4H2(g) + 2O2(g)
2H2O(l)  4H2(g) + 2O2(g) ∆H= + 968KJ
Consider 2N2O5(g)  4NO2(g) + O2(g) + 230kJ
(a) Is this an exothermic reaction or an endothermic reaction? Explain.
The reaction is exothermic – we determine this by looking at the heat term in the balanced chemical
equation. The heat is a product, meaning that excess heat is produced.
(b) Write this equation with the heat as a ∆H separate from the balanced chemical equation.
2N2O5(g)  4NO2(g) + 1O2(g) ∆H=- 230kJ
(c) Write a word equation for this reaction.
230kJ of energy is released when Dinitrogen pentaoxide gas breaks up into oxygen gas and
nitrogen dioxide gas.
OR
Dinitrogen pentaoxide gas  oxygen gas + nitrogen dioxide gas + 230kJ of heat