Introduction: The main purpose of this experiment was to determine the enthalpy of formation of Magnesium Oxide [MgO(s)] and to determine the specific heat of an unknown metal. This was accomplished by using a coffee cup calorimeter and measuring the heat given off in the reaction of the two components: Mg and hydrochloric acid (HCl). There is also a heavy use of Hess’s Law, which is pivotal in determining the enthalpy of formation of MgO because there are multiple reactions, which means that there are, in turn, a multitude of enthalpy changes throughout the experiment. Magnesium is used in many alloys and mixtures of two metals to form solid solution. At the same time, water has one of the highest specific heat capacities known, meaning that it takes a much larger amount of energy to change the overall temperature of water. The theory Hess’s law was used in this experiment, and it says that if a reaction is carried out in a series of steps, the enthalpy of formation for the reaction will equal the sum of the enthalpy changes for individual steps. The following equations are required in order to find the change in enthalpy formation (∆Hf) of MgO (s):
A). H 2 (g) + ½O 2 (g) → H 2 O(l) ∆Hf = -68 kCal/mol B). Mg(s) + 2HCl (aq) → MgCl 2 (aq) +H 2 (g) C). MgO(s) + 2HCl(aq) → MgCl 2 (aq) + H 2 O(l) D). Mg(s) + ½O 2 (g) → MgO(s)
Hess’s Law: ∆Hf(MgO) = ∆HB - ∆HC + ∆HA = ∆HD
Procedure: Determination of the Heat of Formation of MgO(s) 1. Set up coffee cup calorimeter 2. Prepare 250 mL of 1 HCl from the 3 M stock solution 3. Use a graduated cylinder to transfer exactly 100 mL of the dilute HCl solution into a coffee cup calorimeter 4. Clean a piece of Mg ribbon with steel wool and find the mass 5. Wind the Mg ribbon around the loop of the stirrer 6. Record the initial temperature of the HCl solution after inserting the thermometer 7. Record the highest temperature reached 8. Place 100 mL of fresh acid solution into the Styrofoam cup 9. Weigh about 0 g of MgO and record the exact weight 10. Repeat steps 1-7 using MgO in place of Mg
Specific Heat of an Unknown Metal 1. Get metal rod and record the number stamped on 2. Record the mass of the rod 3. Fill a beaker of water and place it on a hot plate bringing it to a simmer 4. Suspend the metal in the beaker of water 5. Transfer 50 mL of room temperature water to a Styrofoam cup
Data/Observations:
Heat Formation of MgO(s) Mg MgO Initial Temperature ( ̊C) 22 22. Final Temperature ( ̊C) 31 26 Temperature Change ( ̊C) ∆T 8 3. Mass (g) .213. Atomic mass (g/mole) 24 40. Number of Moles (mole) .0088. Heat given off (kCal) 110-3 -. ∆Hrxn (kCal/mole) 110-3 -.
Specific Heat of a Metal Mass of H 2 O (g) 50 Initial H 2 O Temperature ( ̊C) 25 Hot Metal Temperature ( ̊C) 100 Equilibrium Temperature ( ̊C) 25. ∆T of Metal ( ̊C) -74. ∆T of H 2 O ( ̊C). Specific Heat of H 2 O (Cal/g ̊C) 1. Specific Heat of Metal (Cal/g ̊C 7*10-
Calculations: Determination of Heat of Formation of MgO(s) Mg:
