An Ice Calorimeter Determination of Reaction Enthalpy
Thermochemistry: An Ice Calorimeter Determination of Reaction
Enthalpy
D. F. Nachman
6/23/2010
Abstract: An ice calorimeter was used to study the reaction of magnesium metal and 1.00M sulfuric acid solution: Mg(s) + H2SO4(aq) →MgSO4(aq) + H2(g). We found the experimental molar enthalpy of reaction to be ΔH = –355 ± 17 kJ/mol at 0°C, 24% lower than the textbook value of ΔH° = –466.9 kJ/mol, reported at 25°C.
Introduction
Whether a chemical reaction occurs spontaneously or is driven by an outside force, it almost always exchanges energy with the surroundings. Energy exchange can occur as work or as heat flow. When a reaction occurs under constant-pressure conditions, we call the heat portion of the energy exchanged the enthalpy change of the reaction. Because the work can be hard to measure accurately, and because the work often represents only a minor fraction of the total energy change, we often focus only on the heat form of energy exchange. Measuring the enthalpy change of a chemical reaction gives information about the relative bonding energies of the products and reactants. If the product substances experience stronger bonding forces than the reactants do–if the potential energy of the products is lower than that of the reactants–the reaction process will release the energy difference as heat and/or work, and we call it an exothermic reaction.
Methods
We used short strips of magnesium ribbon (cleaned with steel wool to remove most of the surface oxide) and 1.00M sulfuric acid solution provided by the stockroom. We weighed the magnesium on the laboratory balance, and measured the acid solution with a Class A 5-mL transfer pipette. To check the temperatures, we used liquid-in-glass emergent stem thermometers with a range of –10°C to +150°C. The wall clock was used to monitor elapsed time.
The major piece of equipment used is the ice calorimeter, first described in 1780 by Antoine
Lavoisier and Simon de la Place1, and modified for use in this course2. The device
Enthalpy
D. F. Nachman
6/23/2010
Abstract: An ice calorimeter was used to study the reaction of magnesium metal and 1.00M sulfuric acid solution: Mg(s) + H2SO4(aq) →MgSO4(aq) + H2(g). We found the experimental molar enthalpy of reaction to be ΔH = –355 ± 17 kJ/mol at 0°C, 24% lower than the textbook value of ΔH° = –466.9 kJ/mol, reported at 25°C.
Introduction
Whether a chemical reaction occurs spontaneously or is driven by an outside force, it almost always exchanges energy with the surroundings. Energy exchange can occur as work or as heat flow. When a reaction occurs under constant-pressure conditions, we call the heat portion of the energy exchanged the enthalpy change of the reaction. Because the work can be hard to measure accurately, and because the work often represents only a minor fraction of the total energy change, we often focus only on the heat form of energy exchange. Measuring the enthalpy change of a chemical reaction gives information about the relative bonding energies of the products and reactants. If the product substances experience stronger bonding forces than the reactants do–if the potential energy of the products is lower than that of the reactants–the reaction process will release the energy difference as heat and/or work, and we call it an exothermic reaction.
Methods
We used short strips of magnesium ribbon (cleaned with steel wool to remove most of the surface oxide) and 1.00M sulfuric acid solution provided by the stockroom. We weighed the magnesium on the laboratory balance, and measured the acid solution with a Class A 5-mL transfer pipette. To check the temperatures, we used liquid-in-glass emergent stem thermometers with a range of –10°C to +150°C. The wall clock was used to monitor elapsed time.
The major piece of equipment used is the ice calorimeter, first described in 1780 by Antoine
Lavoisier and Simon de la Place1, and modified for use in this course2. The device
References: 1 Antoine Lavoisier, Elements of Chemistry, Robert Kerr, trans., Part III, Chapter III, Description of the Calorimeter, Dover Publications, New York, 1965. 2 Thermochemistry lab document, Figure 1. Ice calorimeter apparatus, Mesa Community College CHM152LL website, www.physci.mc.maricopa.edu/Chemistry/CHM152, accessed 1/11/2011. 3 Darrell D. Ebbing , Steven D. Gammon, General Chemistry, 9th ed., Houghton Mifflin, Boston, 2009, p. 426. 4 “Appendix C: Thermodynamic Quantities for Substances and Ions at 25°C” in Darrell D. Ebbing, Steven D. Gammon, General Chemistry, 9th ed., Houghton Mifflin, Boston, 2009, p. A-10