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
ΔH1
C(s,gr) + O2(g)
ΔH2
C(s,di) + O2(g)
CO2(g)
ΔH3
 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
ΔH1
MgSO4(s) + xs H2O(l)
+ 7 H2O(l)
ΔH2
Mg2+(aq) +
SO42-(aq)
ΔH3
MgSO4. 7 H2O(s) + xs H2O(l)
Calorimetry
• Calorimetry---measuring heat flow
• Fundamental idea: Qreaction = -Qsolution
• Qsolution = 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 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.
• 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.
• 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.
• 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
• 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.
Typical Results MgSO4
36
temperature (C)
34
32
30
28
26
24
22
20
0
5
10
time (min)
15
20
Typical Results MgSO4.7H2O
22.5
temperature (C)
22
21.5
21
20.5
20
19.5
0
5
10
time (min)
15
20
 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
Qreaction
DH 
moles_solute
• (Good model: p. 14-3 of lab manual!)
• Average DH values for solution of
hydrate.
ΔH1
MgSO4(s) + xs H2O(l)
+ 7 H2O(l)
ΔH2
Mg2+(aq) +
SO42-(aq)
ΔH3
MgSO4. 7 H2O(s) + xs H2O(l)
• 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.