Hess’s Law

Objective :Determine the enthalpy of combustion of Mg using Hess’s Law.

Notes :

  • 2H2(g) + O2(g)  2H2O(l) H = -571.6kJ
  • Cp of coffee cup is given as 10.0J/C.
  • Cp of solution will be estimated to be the same as that of water, 4.184J/(gC)
  • We are assuming a constant pressure (H = q)

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Procedure :

1. We will have to pick which equipment to use to measure the temperature from various equipments depending on what’s available.

  1. Obtain 2 nested styrofaom cups that will function as your calorimeter for this experiment. Carefully measure out exactly 25.0 ml of HCl (density = 1.02 g/ml) and pour it into your calorimeter.
  • What is the function of a calorimeter?
  1. Measure and record the mass of a magnesium strip (~0.15 g), using an analytical balance.
  • What does it mean if the magnesium strip is not shinny?
  1. Slide the temperature probe into the small notch cutout on the cover and place the probe into the HCl. Stir the HCl with the probe to maintain a uniform temperature throughout the solution. Wait until the temperature stabilizes.
  • What would be the significance of covering the calorimeter?
  1. Roll the magnesium ribbon into a loose ball. Collect temperature readings. After a‘initial temperature’ is determined, slide the cover aside and drop the ball of magnesium into the calorimeter. Slide the cover back into place. Continue stirring and reading temperature until enough data points are taken until you have the ‘final temparature’.
  • What would happen to the reaction if the some of Mg is not submerged in the solution?
  1. Perform Steps 2-5now using ~0.25 g of magnesium oxide in place of the magnesium strips.

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Prelab Questions :

(PL1) Write a balanced equation for combustion of Mg. Make sure to include states.

(PL2) Write a balanced equation for a reaction between Mg and HCl, and between MgO and HCl. Make sure to include states.

(PL3) If the solution is getting hotter, is the reaction endothermic or exothermic?

(PL4) Write out series of equations that would ADD up to the combustion of Mg in Hess’s Law format. Use the reactions between Mg and HCl , the reaction between MgO and HCl , and other equation as needed.

Hess’s Law FORMAT example

A + B C + D

B + D  E

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Then A + 2B C + E

(PL5) Why can’t we just measure the enthalpy of combustion of Mg directly? i.e. Why are we using Hess’s Law?

Analysis:

Do the following analysis for Mg and HCl reaction.

A1. Clearly label everything on your Excel graph including initial temperature, final temperature and T.

A2. Determine the moles of Mg and moles of HCl. Determine which substance is the limiting reactant in the reaction.

A3. Identify what is (are) gaining heat and what is (are) losing heat.

A4. Calculate the enthalpy change for the reaction (Hrxn) in kJ in the amount used. Then convert it to kJ/mol of ???. Be very careful and think about which mole you are referring to.

A5. Repeat A1 – A4 for the reaction of MgO and HCl.

A6. Copy (or correct) the set of equations from (PL4). Put corresponding enthalpy information next to each equation you have in proper unit. See summary section for hint.

A7. Use Hess’s Law, determine the enthalpy of combustion of Mg. Watch out for unit.

PostlabQuestions:

(Q1) What would happen to the final answer (what is final answer?) if significant amount of heat was lost to the surrounding during the first reaction?

Summary :

Show all equations that would add to combustion of Mg with enthalpy information.

For example

A + B C + D 30.kJ/mol

B + D  E 60.kJ/mol

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Then A + 2B C + E 90.kJ/mol