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Determination of Enthalpy
Changes Associated with a
Reaction & Hess’s Law
Jae Lee
Period 2
Experiment 13
Background Information
• Thermochemistry – study of heat chances
& transfers associated with chemical
reactions
• Hess’s law states that at constant
pressure, the enthalpy change for a
process is not dependent on the reaction
pathway, but is dependent only upon the
initial and final states of the system.
• ΔH is used to write the change in heat
Equations you should know and will
use..
• NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
• NH4Cl(aq)+NaOH(aq)
NH3(aq)+NaCl(aq)+H2O(l)
• NH3(aq) + HCl(aq)  NH4Cl(aq)
Purpose of Lab
• To determine the enthalpy change that
occurs when a strong base, NaOH
(Sodium Hydroxide), reacts with a strong
acid, HCL (Hydrochloric acid)
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Materials
• A calibrated
temperature probe
• Thermometer
• Calorimeter
• Water
• 2.00 M HCl
• 2.00 M NaOH
• 2.00 M NH4CL
• 2.00 M NH3
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More materials
• Styrofoam cover
• Polystyrene cups
• 400 ml beaker
• Glass Stirring Rod
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http://www.crscientific.com/stir-rods.jpg
•http://www.chem.ualberta.ca/~ngee/ExptG1.gif
Procedure
1.
2.
3.
4.
5.
6.
7.
8.
9.
Place 50.0 mL of room temp. distilled water in calorimeter
Use calibrated temperature probe to determine/ record
temperature
Add 50.0 mL of warm distilled water to calorimeter
Record temperature every 30 seconds for 3 min.
Clean beaker for next experiment
Measure temp chance water 50.0 ml of 2.00 M HCl is added to
50.0 ml of 2.00 M NaOH
Record temperature change every 30 sec. for 3 min.
Repeat step 7 but with 2.00 M NH4Cl with 2.00 M NaOH
Repeat step 7 but with 2.00 M NH3 with 2.00 M HCl
Analysis
A graph of the heat capacity of the calorimeter.
From the graph, determine the heat capacity of the
calorimeter. The specific gravity of water at 23.0 C is 0.998
and at 61.0 C
•Chart created from Experiment 13: Determination of Enthalpy Changes
Associated with a Reaction and Hess’s law
• Extrapolating the regression line to the Y
axis (0 seconds) gives a temperature of
41.4 °C at the moment the room
temperature and warm water were mixed
• Average temperature of room temperature
and warm water:
• 23.0 °C + 61.0 °C = 42.0 °C
2
• Ccalorimeter = qcalorimeter
(Tmix – Tinitial)
• qcalorimeter = -qwater
• Qwater = (mass water)*(specific heat)*(Tmix – Tavg)
• At 23.0°C: 50.0 mL H2O x 0.983 g*mL-1
1
= 49.9 g H2O
• At 61.0°C: 50.0 mL H2O x 0.983 g*mL-1
1
= 49.1 g H2O
• Total mass = 49.9 g + 49.1 g = 99.0 g H2O
=(99.0 g)*(4.18 J/g * °C)*(41.4°C –
42.0°C)
= -2.5x102 J Heat gained by calorimeter =
-qwater = 2.5x 102 J
Ccalormeter = qcalorimeter
2.5 x 102 J =
14 J* °C
(Tmix-Tinitial) (41.4 °C – 23.0 °C)
(#2.)
Temperature changes for each of the 3 reactions
Calculate the heat evolved in the reaction (kJ/mol
of product). Assume the density of each solution =
1.00gxmL-1
•Chart created from Experiment 13: Determination of Enthalpy Changes
Associated with a Reaction and Hess’s law
http://www.razor-gator.com/SkinAcidsPhotos/Bottle-HCl&NaOH.jpg
(a)
• HCl + NaOH
qrxn = -[(masssol’n) x (specific heat sol’n) x (ΔTsol’n)] + (Ccalorimeter x ΔTsol’n)
volumesol’n x molarity
= [100g x 4.18J/g·°C x (3.56°C – 23.0°C)] + (14 J/°C x 12.5°C)
0.500L x 2.0 mole/L
=
.
-5.4 x 103
0.0500L x 2.0 mol/L
= - 54 kJ/mol
(b)
• NH4Cl + NaOH
qrxn = -[(masssol’n) x (specific heat sol’n) x (ΔTsol’n)] + (Ccalorimeter x ΔTsol’n)
volumesol’n x molarity
= [100g x 4.18J/g·°C x (24.1°C – 22.9°C)] + (14 J/°C x 1.2°C)
0.500L x 2.0 mole/L
=
-5.2 x 102 J
0.0500L x 2.0 mol/L
= - 5.2 kJ/mol
.
(c)
• NH3 + HCl
qrxn = -[(masssol’n) x (specific heat sol’n) x (ΔTsol’n)] + (Ccalorimeter x ΔTsol’n)
volumesol’n x molarity
= [100g x 4.18J/g·°C x (33.1°C – 23.0°C)] + (13.6 J/°C x 10.1°C)
0.500L x 2.0 mole/L
=
.
-4.4x 103 J
0.0500L x 2.0 mol/L
= - 44 kJ/mol
#3
Write the net ionic equation, including the
ΔH’s, for the 1st two reactions studied &
rearrange the equation(s) in order to
created the 3rd reaction and the ΔH value
H+(aq) + OH-(aq)  H2O (l)
NH3(aq) + H2O (l)  NH4+ (aq) + OH-(aq)
NH3(aq) + H+(aq)  NH4+ (aq)
ΔH = -54 kJ · mol-1
ΔH = +5.2 kJ · mol-1
ΔH = -49 kJ · mol-1
#4
• Use calculator to calculate the %error
between the measured ΔH & the
calculated ΔH % error
• = observed–theoretical x 100%
theoretical
= -44-(-49) x 100%
-491
= -10%
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Conclusion
After doing this lab experiment, I was able to
determine the enthalpy change that occurs
when a strong base reacts with a strong
acid. By using Hess’s law, I am able to
produce a third reaction and find the
change in heat that happens throughout
the experiment.