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Experimental Applications of Hess Law

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					                   Experimental Applications of Hess’s Law
There are many reactions for which the heat of reaction cannot be easily measured
experimentally. Some of these are dangerous to perform in the lab. Others generate so
much heat that simple calorimeters cannot be used. In such cases, it is practical to
examine a series of reactions whose net effect is the desired reaction, but whose heats of
reaction are more easily measured. Once that has been done, the required heat of reaction
can be calculated using Hess’s Law.

In this lab, you will conduct two such investigations. In the first one we will examine the
combustion of magnesium. Then burning of magnesium can be represented by the
equation
                                 2Mg + O2  2 MgO
This equation can be obtained by manipulating these three equations:
                           MgO + 2HCl  MgCl2 + H2O
                            Mg _ 2HCl  MgCl2 + H2
                               2H2 + O2  2H2O

In the second investigation, we will examine the heat of formation of solid calcium
hydroxide. The equation for the reaction is
                              Ca + O2 + H2  Ca(OH)2
It can be obtained by combining the following two equations:
                            Ca + 2H2O Ca(OH)2 + H2
                                2H2 + O2  2H2O


Apparatus and Material:
     Thermometer              100 mL Grad. Cylinder          Stir Rod
     Mg ribbon                Ca metal                       balance
     2 Styrofoam cups         Magnesium oxide                1M HCl
     water



Safety:
        Hydrochloric acid is corrosive. The calcium hydroxide solutions produced is very
caustic. Avoid spilling either of these on yourself, your clothes, or your friends. Wash
immediately if the solutions contact your skin. Safety goggles are required. Calcium
metal reacts vigorously with water. Be extremely careful that pieces of calcium metal do
not become wet. It is advisable to wear a lab coat or an apron throughout this
experiment.
Procedure~
Part One: The Combustion of Magnesium
    Mass an empty, dry Styrofoam cup.

      Put 100 mL of the hydrochloric acid solution into the Styrofoam cup. Carefully
       mass the cup with acid, and record in the data table.
      Record the initial temperature of this solution.
      Accurately find and record the mass of 1 g of solid magnesium oxide.
      Add the solid magnesium oxide to the acid solution already in the cup. Stir the
       mixture and record the highest temperature possible.
      Discard the solution and rinse the cup thoroughly with water.
      Put 100 mL of HCl solution in the Styrofoam cup. Record the initial temperature
       of this solution.
      Accurately find and record the mass of about 0.25 g of magnesium ribbon.
      Add the magnesium ribbon to the acid solution. Stir the mixture and record the
       highest temperature reached.
      Empty and rinse the cup.


   Part Two: The heat of formation of solid calcium hydroxide
      Punch two holes in the bottom of a Styrofoam cup. Put a thermometer through
       one hole, and leave the other free to vent hydrogen gas that is produced in the
       reaction. This cup will act as a lid to your original Styrofoam cup.
      Record the mass of the Styrofoam cup.
      Put 100 mL of water into the original Styrofoam cup (the one with no holes).
       Record the mass of the cup and the water.
      Put the lid on the cup. Tape the cups securely together.
      Insert a thermometer through one hole in the lid. Record the initial temperature of
       the water.
      Record the mass of about 1.0 g of calcium metal. Caution: do not allow any
       pieces of calcium metal to become wet while on the balance or the lab table. Use
       only the amount needed, and place the rest of the calcium back in its container.
      Add the calcium to the water through the pre-punched hole.
      Stir the mixture and record the highest temperature reached.
Calculations:
1. From your data in Part 1a and Part 1b (Magnesium), calculate the following:
      a. The T for each experiment.
      b. The mass of each solution used.
      c. The heat (q) absorbed by the solution (use Cp= 4.184 J/g*K)
      d. The number of moles of (1) magnesium oxide and (2) magnesium used.
      e. The heat of the reaction when hydrochloric acid reacted with (1)
          magnesium oxide and (2) magnesium.

2. From your data in Part Two: Calcium, calculate the following for the reaction.
      a. The T for each experiment.
      b. The mass of each solution used.
      c. The heat (q) absorbed by the solution (use Cp= 4.184 J/g*K)
      d. The number of moles of calcium metal used.
      e. The heat of the reaction when water reacted with calcium metal

3. Show how the equations for the magnesium reactions can be added together using
   Hess’s Law. Show also your calculations of the heat of the overall reaction
   using the heats found in calculation # 1.

4. Show how the equations for the actions found in the introduction can be added
   together using Hess’s Law to form the overall reaction for the calcium hydroxide.

5. Assuming that the heat of reaction for H2 + O2  H2O is –285 kJ, use Hess’s Law
   to calculate the overall heat of reaction for the formation of solid calcium
   hydroxide.
                                    Data Tables
Part 1 (a)
Volume of 1.0 M HCl solution used                         mL
Mass of magnesium oxide used (used only for mole calc)        g
                                                          o
Initial temperature of 1.0 M HCl solution before mixing       C
                                                          o
Final temperature of solution after MgO added                 C



Part 1 (b)
Volume of 1.0 M HCl solution used                         mL
Mass of magnesium ribbon used (use only for mole calc.)       g
                                                          o
Initial temperature of 1.0 M HCl solution before mixing       C
                                                          o
Final temperature of solution after Mg added                  C


Part 2
Volume of H2O used =                                      mL
Mass of calcium used =                                        g
                                                          o
Initial temperature of H2O before mixing =                    C
                                                          o
Final temperature of Ca(OH)2 solution =                       C
Calculations: Part 1a
                             o
ΔT                               C
Heat Energy produced, q      kJ
Moles of MgO used           mol
Hrxn                      kJ/mol

Calculations: Part 1b
                             o
ΔT                               C
Heat Energy produced, q      kJ
Moles of Mg used            mol
Hrxn                      kJ/mol
Calculations: Part 2
                             o
ΔT =                             C
Energy produced, E =         kJ
Moles of Ca used =          mol
Hrxn =                    kJ/mol

				
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