2.Do your results agree with Hess’s law to within 1%? 5%? 10%? Suggest reasons for any discrepancy.
Name: C ALCULATIONS : FOLLOW THE SIGNIFICANT FIGURES CONVENTION & SHOW UNITS FOR EACH QUANTITY . Reaction 1: solution mass = mass of H 2 O + mass of NaOH heat absorbed q = m·s· ∆ T : moles of NaOH = mass of NaOH/molar mass of NaOH molar enthalpy of reaction, Δ H rxn = – q ÷ moles of NaOH Reaction 2: solution mass = mass of HCl solution + mass of NaOH solution heat absorbed q = m·s· ∆ T : moles of NaOH = volume of NaOH solution × molar concentration of NaOH solution molar enthalpy of reaction, Δ H rxn (kJ/mol NaOH): Reaction 3: solution mass = heat absorbed q = m·s· ∆ T: moles of NaOH: molar enthalpy of reaction, Δ H rxn (kJ/mol NaOH):
Thermochemistry III: Heat of Combustion of Magnesium OBJECTIVES: • Further explore quantitative aspects of constant-pressure calorimetry • Use Hess’s law for indirect determination of reaction enthalpy DISCUSSION: In the Thermochemistry II experiment, you tested the validity of Hess’s law. In this experiment, you will use this principle to make an indirect determination of the enthalpy of reaction ( ∆ H rxn ) for the combustion of magnesium ribbon: Mg( s ) + 1 / 2 O 2 ( g ) → MgO ( s ) (1) This reaction releases so much enthalpy that the magnesium metal incandesces at white heat. It will not be practical to measure the reaction enthalpy in your foam cup calorimeter, but you can use Hess’s law and a combination of known and measurable reactions to indirectly determine the molar heat of magnesium combustion in oxygen. The other reactions appear below: H 2 ( g ) + 1 / 2 O 2 ( g ) → H 2 O ( l ) (2) Mg ( s ) + 2 HCl ( aq ) → MgCl 2 ( aq ) + H 2 ( g ) (3) MgO ( s ) + 2 HCl ( aq ) → MgCl 2 ( aq ) + H 2 O ( l ) (4) The first of these, reaction (2), is the formation reaction of liquid water, which has a known standard molar enthalpy value: ∆ H f ° = –285.8 kJ · mol –1 . Finding the appropriate combination of reactions (2)–(4) will allow you to use Hess’s law for reaction (1). PROCEDURE: Reaction 3: Mg ( s ) + 2 HCl ( aq ) → MgCl 2 ( aq ) + H 2 ( g ) . 1. Place a calorimeter cup inside a 250-mL beaker as before. Measure out 100.0 mL of 1.00 M HCl solution into the cup. Lower the temperature probe into the solution. 2. Weigh a 0.5-g sample of magnesium ribbon, and record the mass to the full available precision of the balance. 3. Record the temperature every 20 s. After you have obtained three readings at the same temperature, add the weighed magnesium ribbon to the solution. Stir with a glass rod while the temperature climbs. Once the temperature passes its maximum and starts to drop, suspend data collection. If you can’t tell, collect data for 460 s. 4. Dispose of the product solution in the proper waste container.
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