Chemistry 30 Lesson 1-09 Page 1 © 2012 T. de Bruin Chem 30 Investigation 10.A: Hess’s Law and the Enthalpy of Combustion of Magnesium Magnesium ribbon burns in air in a highly exothermic combustion reaction. (See equation (1) below.) A very bright flame accompanies the production of magnesium oxide, as shown in the photograph on the right. It is impractical and dangerous to use a simple calorimeter to determine the enthalpy change for this reaction. (1) Mg(s) + 1 2 O 2 (g) → MgO(s) DH = ? Instead, you will determine the enthalpy changes for two other reactions (equations (2) and (3) below). You will use these enthalpy changes, along with the known enthalpy change for another reaction (equation (4) below), to determine the enthalpy change for the combustion of magnesium. (2) MgO(s) + 2 HCl(aq) → MgCl 2 (aq) + H 2 O(ℓ) DH = ? (3) Mg(s) + 2 HCl(aq) → MgCl 2 (aq) + H 2 (g) DH = ? (4) H 2 (g) + 1 2 O 2 (g) → H 2 O(ℓ) + 285.8 kJ Notice that equations (2) and (3) occur in aqueous solution. You can use a simple calorimeter to determine the enthalpy changes for these reactions. Equation (4) represents the formation of water directly from its elements in their standard state (from your data book p.5). Question What is the molar enthalpy of formation of magnesium oxide? Safety Precautions • Hydrochloric acid is corrosive. Use care when handling it. • Be careful not to inhale the magnesium oxide powder. Materials • 1.00 mol/L HCl(aq) • MgO(s) powder • Mg ribbon (or Mg turnings) • simple calorimeter • 100 mL graduated cylinder • electronic balance • thermometer Figure 1 Magnesium ribbon reacts rapidly in dilute hydrochloric acid. With nested polystyrene cups, the enthalpy change can be determined by measuring the temperature change of the hydrochloric acid solution.
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Chemistry 30 Lesson 1-09 Page 1© 2012 T. de Bruin
Chem 30 Investigation 10.A: Hess’s Law and the Enthalpy of Combustion of Magnesium
Magnesium ribbon burns in air in a highly exothermic combustionreaction. (See equation (1) below.) A very bright flame accompaniesthe production of magnesium oxide, as shown in the photograph onthe right. It is impractical and dangerous to use a simple calorimeterto determine the enthalpy change for this reaction.
(1) Mg(s) +12
O2(g) → MgO(s) DH = ?
Instead, you will determine the enthalpy changes for two otherreactions (equations (2) and (3) below). You will use these enthalpychanges, along with the known enthalpy change for anotherreaction (equation (4) below), to determine the enthalpy change for the combustion of magnesium.
(2) MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(ℓ) DH = ?
(3) Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g) DH = ?
(4) H2(g) +12
O2(g) → H2O(ℓ) + 285.8 kJ
Notice that equations (2) and (3) occur in aqueous solution. You can use a simple calorimeter todetermine the enthalpy changes for these reactions. Equation (4) represents the formation of waterdirectly from its elements in their standard state (from your data book p.5).
Question
What is the molar enthalpy of formation of magnesium oxide?
Safety Precautions
• Hydrochloric acid is corrosive. Use care when handling it.• Be careful not to inhale the magnesium oxide powder.
Materials
• 1.00 mol/L HCl(aq)• MgO(s) powder• Mg ribbon (or Mg turnings)• simple calorimeter• 100 mL graduated cylinder• electronic balance• thermometer
Figure 1Magnesium ribbon reacts rapidly indilute hydrochloric acid. With nestedpolystyrene cups, the enthalpychange can be determined bymeasuring the temperature changeof the hydrochloric acid solution.
Chemistry 30 Lesson 1-09 Page 2© 2012 T. de Bruin
Procedure
Part 1: Determining ΔrH of Reaction (2)
1. Set up a simple calorimeter. Using a graduated cylinder, add 100 mL of 1.00 mol/L HCl (aq) to
the calorimeter. Caution: Hydrochloric acid can burn your skin.
2. Record the initial temperature, ti, of the HCl (aq).
3. Obtain no more than 1.00 g of MgO (s) powder. Record the exact mass.
4. Add the MgO (s) powder to the calorimeter containing the HCl (aq). Swirl the solution gently,
recording the highest temperature, tf, reached.
5. Dispose of the reaction solution down the sink with plenty of water and rinse out the Styrofoam
cup.
Part 2: Determining ΔrH of Reaction (3)
1. Using a graduated cylinder, add 100 mL of 1.00 mol/L HCl (aq) to the calorimeter.
2. Record the initial temperature, ti, of the HCl (aq).
3. Obtain a strip of magnesium ribbon no longer than 15 cm. Record the exact mass.
4. Add the Mg (s) to the calorimeter containing the HCl (aq). Swirl the solution gently, recording
the highest temperature, tf, reached.
5. Dispose of the solution down the sink with plenty of water and rinse out the Styrofoam cup.
Chemistry 30 Lesson 1-09 Page 3© 2012 T. de Bruin
Chem 30 Investigation 10.A: Hess’s Law and the Enthalpy of Combustion of Magnesium
Observations: Name:
Compound Mass (g) Volume ofHCl (aq) mL ti (˚C) tf (˚C)
MgO
Mg
Analysis1. Determine the molar enthalpy change of reactions (2) and (3).
Chemistry 30 Lesson 1-09 Page 4© 2012 T. de Bruin
2. Write thermochemical equations for reactions (2) and (3) and (4) using ΔH notation. Then,using Hess’s Law, determine the enthalpy change for equation (1)
3. Compare your result with the accepted value of ΔcH for magnesium (see p. 5 of your data book– it’s the formation of MgO). Calculate your percent error.
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