LAB #2:  HESS'S LAW




SAFETY PRECAUTIONS:
Handle concentrated HCI in the fume hood only, wear gloves and safety goggles (as always!). Rinse any spills immediately.


Background Information

Many ionic compounds naturally contain water as part of the crystal lattice structure. A hydrate is a compound that has one or more water molecules bound to each formula unit. Ionic compounds that contain a transition metal are often highly colored. Interestingly, it is common for the hydrated form of a compound to be of a different color than the anhydrous form, which has no water in its structure. A hydrate can usually be converted to the anhydrous compound by heating. For example, the anhydrous compound cobalt(II) chloride is blue, while the hydrate is a distinctive magenta color.




The hydrated form of cobalt(II) chloride contains six water molecules in each formula unit.  The name of the compound is cobalt(II) chloride hexahydrate and its formula is CoCl2•6H2O.  The formula for water is set apart at the end of the formula with a dot, followed by a coefficient that represents the number of water molecules per formula unit.


Experiment Summary 

The purpose of this investigation is using Hess's Law to calculate the enthalpy of reaction of:



The equations that will be use to "add up" to get the above equation include:


The lab component is to determine experimentally the enthalpy of reaction for A and B, and then with that information mathematically calculate the enthalpy of formation for MgO. 


Experimental Set-up


The following video provides the concept of the lab that you are going to do.  Please make sure that you follow our written procedure provided to you, which is also posted below.  Please take note of the following that we will NOT do which was described/demonstrated in the video:
  • try NOT to poke around with a glass stirring rod. 





Procedure 

View the procedure handout attached at the bottom of this page.  Please note that we will not look at the kinetics of crystal violet, but of another, very similar molecule. The principles operate similarly to the discussion above. Apply what you learned about crystal violet to the molecules and reactions under investigation.