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.

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