Chemistry 104 Laboratory

 

 

This Week

Answer Keys

Gen Chem Lab

Periodic Table

 

 

 

á       Expt 1:  Brita, Scum, and Scale (Week 1, Part A)

o   Complete Part A individually

o   Plan Week 2 work

á       Report

o   Purpose, Procedure, Report Sheets due today

o   Week 1 Worksheet due in two days

 

Next Week

á       Expt 1:  Brita (Week 2, Parts B,C)

á       Quiz 2:  Expt 2 (Part C), Expt 3

 

Notes for Today

               

    

                    

 

  • Scale facts
    • Composition:  CaCO3 and/or MgCO3
    • Source: Hard water containing Ca2+ or Mg2+, HCO3-
    • Removal: Add reactant to remove either Ca2+ (Mg2+) or CO32- 

 

CaCO3 (s) <--> Ca2+ (aq) + CO32-  (aq)

 

    • Prevention: Remove Ca2+, Mg2+ from water by ion exchange (softening)

 

 

  • Controlling the Position of Equilibrium

 

Equation for equilibrium:

 

Cu(H2O)62+ + 4NH3 <--> Cu(NH3)42+  + 6H2O

                                                      

 

addition of aqueous NH3

observations

The pale blue copper solution turns deep violet-blue as ammonia is added. 

 

net ionic equation for the equilibrium shift

     Cu(H2O)62+ + 4NH3 --> Cu(NH3)42+  + 6H2O

 

 

explanation for equilibrium shift

Addition of NH3 increases the [NH3].  System consumes the excess NH3, shifting the equilibrium to the right.  Solution becomes violet-blue as more product, Cu(NH3)42+, is formed. Because NH3 is a direct participant in the equilibrium, addition of NH3 is a direct stress. The stress (increase in [NH3]) and the remedy (consumption of excess NH3) are clearly identified.

 

Inadequate explanation: The equilibrium shifts right because NH3 is added. This identifies neither the stress nor the remedy.

 

 

addition of HCl

 

observations

The violet-blue solution becomes pale blue again when HCl is added.

 

 

net ionic equation for the equilibrium shift

      Cu(NH3)42+  + 6H2O --> Cu(H2O)62+ + 4NH3

 

 

explanation for equilibrium shift

Adding acid lowers [NH3]:  H+ + NH3 --> NH4+

 

The original equilibrium must shift left to produce more NH3. Shifting left also produces more Cu(H2O)62+ (from Cu(NH3)42+) so the solution becomes pale blue. Because H+ (HCl) is not a participant in the original equilibrium, addition of H+ (HCl) is an indirect stress.

 

 

Inadequate explanation: The equilibrium shifts left because HCl is added.

This does not identify the stress on the original equilibrium (lowering of [NH3]) or the remedy (shifting of equilibrium to replenish the lost NH3).