Experiment 14 Enthalpy of Hydration. Purposes and Goals The purpose of this experiment is to use Hess’s Law to determine the enthalpy change (ΔH) for.

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Presentation transcript:

Experiment 14 Enthalpy of Hydration

Purposes and Goals The purpose of this experiment is to use Hess’s Law to determine the enthalpy change (ΔH) for the hydration of an anhydrous compound.

Introduction Conservation of energy Hess’s Law of Heat Summation Alternative view of Hess’s Law: ΔH does not depend on how I get from reactants to products.

I can use this idea to determine ΔH for a reaction which is hard to study directly. Example: graphite  diamond C(s, gr)  C(s,di)

A thermochemical cycle ΔH 1 C(s,gr) + O 2 (g) CO 2 (g) ΔH 2 ΔH 3 C(s,di) + O 2 (g)

 H 1 =  H 2 +  H 3 The direct conversion of graphite to diamond (corresponding to  H 2 ) is hard to measure, but the two combustions (  H 1 and  H 3 ) are easy to measure. We can determine  H 2 indirectly from  H 1 and  H 3.  H 2 =  H 1 -  H 3

Your experiment ΔH 1 MgSO 4 (s) + xs H 2 O(l) Mg 2+ (aq) + SO 4 2- (aq) + 7 H 2 O(l) ΔH 2 ΔH 3 MgSO 4. 7 H 2 O(s) + xs H 2 O(l)

Calorimetry Calorimetry---measuring heat flow Fundamental idea: Q reaction = -Q solution Q solution = C ΔT C = (mass of solution) x (specific heat of the solution)

Safety Aprons and glasses Thermometers are fragile and expensive; don’t use a thermometer as a stirring rod!

Procedure Work in pairs. Needed equipment: 100-mL graduated cylinder, stirring rods, thermometers, 2 thermometer clamps, 2 calorimeter cups, ring stand, ring. Same setup as Expt. 6 (CHEM 1031 lab).

One run with MgSO 4, two with hydrate. Weigh empty calorimeter cup. Record mass on data sheet. Weigh solute (about 7.50 g of MgSO 4, g of hydrate). Idea: equal moles of solute in samples.

Add 100 mL water to cup; measure temperature every 30 seconds for five minutes. Add MgSO 4 ; use stirring rod (NOT THERMOMETER) to dissolve solute quickly. Measure temperature every minute for fifteen minutes. After run is over, measure and record mass of cup plus solution.

Weigh second calorimeter cup; add water and measure temperature for five minutes as before. After 5 minutes, add MgSO 4. 7 H 2 O. Measure temperature for 15 minutes as before. Measure and record mass of cup and solution.

Discard solution (sink); add 100 mL of water and repeat run. After 5 minutes, add second portion of MgSO 4. 7 H 2 O. Measure temperature for 15 minutes as before. Measure and record mass of cup plus solution; you will use the same mass of empty cup for both runs.

Calculations Draw a graph of temperature vs time for your experimental data. Each run is a separate graph. Extrapolate the temperature lines before and after adding solute to the time at which solute was added.

 T = T final – T initial C = mass of solution x specific heat of solution (specific heat of solution = J g -1 deg -1 ) Q reaction = - C  T (Good model: p of lab manual!)

Average  H values for solution of hydrate. ΔH 1 MgSO 4 (s) + xs H 2 O(l) Mg 2+ (aq) + SO 4 2- (aq) + 7 H 2 O(l) ΔH 2 ΔH 3 MgSO 4. 7 H 2 O(s) + xs H 2 O(l)

You have calculated the heats of solution of MgSO 4 (  H 1 ) and of MgSO 4. 7 H 2 O (  H 3 ). The enthalpy of hydration (  H 2 ) is  H 2 =  H 1 -  H 3.