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 K 2 SO 4(s) K 2 SO 4 is made of K + and SO 4 2- K 2 SO 4(s)  2 K + (aq) + SO 4 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 MgBr 2(s) Al(NO 3 ) 3(s) CuSO 4(s) (NH 4 ) 3 PO 4(s)

Remember from yesterday Soluable ionic compounds, separate into their respective ions when dissolved in water. Ex: CaCl 2(s)  Ca 2+ (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: AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(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: AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq)

Try these Mg(NO 3 ) 2(aq) + Na 2 CO 3(aq)  (NH 4 ) 3 PO 4(aq) + Ba 2 NO 3(aq)  Pb(NO 3 ) 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