SCH4U Lab
Calorimetry: Testing Hess’s Law
Name ______
Through a series of steps, the molar enthalpy of combustion (∆H˚comb) for magnesium will be determined.
Mg(s) + ½ O2(g) à MgO(s)
Styrofoam cup calorimetry will be performed to calculate the molar enthalpy change for two reactions involving magnesium and magnesium oxide with excess hydrochloric acid.
Pre-Lab:
· Write the balanced chemical equations for the 3 reactions below (include states of matter!)
· Use Hess’ Law to figure out how you will manipulate the 3 equations to determine ∆H˚comb of Mg(s)
(1) Magnesium is reacted with hydrochloric acid
(2) Magnesium oxide is reacted with hydrochloric acid
(3) Formation of water. Look up the enthalpy of formation for water
Introduction:
· Purpose and hypothesis (show how the 3 reactions will be used to determine ∆H˚comb of Mg)
· Related theory (calorimetry, laws of thermodynamics, Hess’ Law)
Procedure:
Design a procedure to calculate the molar enthalpy of the following 2 reactions
(1) Magnesium (0.5 g) will be reacted with hydrochloric acid (100 mL of 1.0 M)
(2) Magnesium oxide (1.0 g) will be reacted with hydrochloric acid (100 mL of 1.0 M)
** What materials and equipment will you need? How will you measure quantities? Safety precautions?
Observations:
Record all relevant data in table format
Results and Analysis
(1) Magnesium is reacted with hydrochloric acid
· Calculate the limiting reagent.
· Calculate the enthalpy change.
· Calculate the molar enthalpy of neutralization.
· Write the thermochemical equation.
(2) Magnesium oxide is reacted with hydrochloric acid
· Calculate the limiting reagent.
· Calculate the enthalpy change.
· Calculate the molar enthalpy of neutralization.
· Write the thermochemical equation.
(3) Write the formation of water and look up the enthalpy of formation for water
· Using Hess’s Law, determine ∆H˚comb of magnesium.
· Research the accepted value for the molar enthalpy of combustion of magnesium and calculate the percent difference from your experimental value.
Discussion:
· Sources of error. Improvements to experimental design
· Why did we use Hess’ Law? Why couldn’t we experimentally calculate ∆H˚comb of magnesium?
· Hydrogen gas is an ideal fuel because the product of its combustion is water. However, it requires a great deal of energy to extract hydrogen from water by electrolysis. Research how this technology is being developed. How will this impact society and the environment?
· Concluding statement that reflects on your purpose