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Experiment 14. Enthalpy of Hydration. Purposes and Goals. The purpose of this experiment is to use Hess’s Law to determine the enthalpy change ( Δ H) for the hydration of an anhydrous compound. Introduction. Conservation of energy Hess’s Law of Heat Summation

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experiment 14

Experiment 14

Enthalpy of Hydration

purposes and goals
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
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.
slide4
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
A thermochemical cycle

ΔH1

C(s,gr) + O2(g) CO2(g)

ΔH2ΔH3

C(s,di) + O2(g)

slide6
DH1 = DH2 + DH3
  • The direct conversion of graphite to diamond (corresponding to DH2) is hard to measure, but the two combustions (DH1 and DH3) are easy to measure. We can determine DH2 indirectly from DH1 and DH3.
  • DH2 = DH1 - DH3
your experiment
Your experiment

ΔH1

MgSO4(s) + xs H2O(l) Mg2+(aq) +

SO42-(aq)

+ 7 H2O(l)

ΔH2ΔH3

MgSO4.7 H2O(s) + xs H2O(l)

calorimetry
Calorimetry
  • Calorimetry---measuring heat flow
  • Fundamental idea: Qreaction = -Qsolution
  • Qsolution = C ΔT
  • C = (mass of solution) x (specific heat of the solution)
safety
Safety
  • Aprons and glasses
  • Thermometers are fragile and expensive; don’t use a thermometer as a stirring rod!
procedure
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).
slide11
One run with MgSO4, two with hydrate.
  • Weigh empty calorimeter cup. Record mass on data sheet.
  • Weigh solute (about 7.50 g of MgSO4, 15.35 g of hydrate). Idea: equal moles of solute in samples.
slide12
Add 100 mL water to cup; measure temperature every 30 seconds for five minutes.
  • Add MgSO4; 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.
slide13
Weigh second calorimeter cup; add water and measure temperature for five minutes as before.
  • After 5 minutes, add MgSO4.7 H2O. Measure temperature for 15 minutes as before. Measure and record mass of cup and solution.
slide14
Discard solution (sink); add 100 mL of water and repeat run. After 5 minutes, add second portion of MgSO4.7 H2O. 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
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.
slide18
DT = Tfinal – Tinitial
  • C = mass of solution x specific heat of solution (specific heat of solution = 3.837 J g-1 deg-1)
  • Qreaction = - C DT
  • (Good model: p. 14-3 of lab manual!)
slide19
Average DH values for solution of hydrate.

ΔH1

MgSO4(s) + xs H2O(l) Mg2+(aq) +

SO42-(aq)

+ 7 H2O(l)

ΔH2ΔH3

MgSO4.7 H2O(s) + xs H2O(l)

slide20
You have calculated the heats of solution of MgSO4 (DH1 ) and of

MgSO4.7 H2O (DH3).

  • The enthalpy of hydration (DH2) is

DH2 = DH1 - DH3.