Chemistry 20
Aqueous
• Up until this point we have said that when a chemical is dissolved in water it is aqueous.
• In reality, something else is happening
Dissociation
• When ionic compounds dissolve in water, they separate away from each other in a process called dissociation.
• When you have these ionic compounds in water dissociating into its individual ions, it is aqueous.
Dissociation
• If ionic compounds separate when they are in water, why do I get salt crystals again if I let salt water evaporate?
• One of the unusual properties of water is that it is able to keep these ions separate. They come back together when the water evaporates.
Electrolytes
• When an ion is free to move around in water, it can be referred to as an electrolyte.
• These electrolytes can carry electric current, because they move around freely.
Why is this important?
• When more than one ionic compound is in a solution, the free ions are able to form new bonds!
• Sometimes these reactions form precipitates, or new compounds that decompose.
Double Displacement
• Dissociation is extremely important to double displacement reactions!
• It is what allows the ions to actually be free to switch.
How to Show dissociation
• Write your reactant as you normally would.
• Determine what ions it is made of.
• Write those ions on the products side, showing charge. Specify these as being aqueous.
• Balance the number of atoms.
Example
• KOH(s)
• KOH is made of K+ and OH-
• KOH(s) K+(aq) + OH-
(aq)
• Balanced.
Another Example
• K2SO4(s)
• K2SO4 is made of K+ and SO42-
• K2SO4(s) 2 K+(aq) + SO4
2-(aq)
• Balanced.
Note!
• Some covalent compounds will also dissolve in water, such as sugar, but they do not usually form ions.
Examples on your own
• MgBr2(s)
• Al(NO3)3(s)
• CuSO4(s)
• (NH4)3PO4(s)
Remember from yesterday
• Soluable ionic compounds, separate into their respective ions when dissolved in water.
• Ex: CaCl2(s) Ca2+(aq) + Cl-
(aq)
Complete Ionic Formulas
• The next thing that we need to do is apply dissociation to our double displacement reactions.
• When we have our double displacement reactions, we must do the following steps.
Complete Ionic Equations
1. Predict our products and balance your equation.
Write the next two steps on a separate line.2. Any chemicals that are (aq), will be written as
separated ions.3. Any chemicals that are (s), will be left as they are.
These steps will give us a complete ionic formua
Example:
AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
Complete ionic equations
• You may notice that we may have the same ions on both sides of the equation. These are called spectator ions. They don’t take part in the formation of a precipitate.
• The next step is to write the net ionic equation. The net ionic equation does not include any spectator ions.
Example:
AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
Try these
• Mg(NO3)2(aq) + Na2CO3(aq)
• (NH4)3PO4(aq) + Ba2NO3(aq)
• Pb(NO3)2(aq) + NaCl(aq)
Energy Changes in Chemical Reactions
• Energy is always involved in a chemical reaction
• Some Reactions are Exothermic, and some are Endothermic.
• Heat can be written in a chemical equation, as being on one side or the other.
Exothermic Reactions
• In an exothermic reaction, the chemical reaction is giving off energy in the form of heat and light.
• By giving off heat/light they increase in temperature.
• Exothermic reactions have a negative ΔH value.
• Heat can be written as a product in an exothermic reaction
Endothermic Reactions
• In an endothermic reaction, the reaction absorbs energy, making it colder.
• Endothermic reactions have a positive ΔH value.
• Heat can be written as a reactant in an endothermic reaction