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http://i.ehow.com/images/GlobalPhoto/Articles/4856084/143193-main_Full.jpg. http://honolulu.hawaii.edu/distance/sci122/Programs/p21/image.gif. Determination of Enthalpy Changes Associated with a Reaction & Hess’s Law. Jae Lee Period 2 Experiment 13. Background Information.

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determination of enthalpy changes associated with a reaction hess s law

http://i.ehow.com/images/GlobalPhoto/Articles/4856084/143193-main_Full.jpghttp://i.ehow.com/images/GlobalPhoto/Articles/4856084/143193-main_Full.jpg

http://honolulu.hawaii.edu/distance/sci122/Programs/p21/image.gif

Determination of Enthalpy Changes Associated with a Reaction & Hess’s Law

Jae Lee

Period 2

Experiment 13

background information
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
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
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)
materials

http://www.hansatech-instruments.com/quantitherm.jpg

Materials
  • A calibrated temperature probe
  • Thermometer
  • Calorimeter
  • Water
  • 2.00 M HCl
  • 2.00 M NaOH
  • 2.00 M NH4CL
  • 2.00 M NH3

http://wolfesscience.com/images/lab-thermometer.jpg

more materials
More materials
  • Styrofoam cover
  • Polystyrene cups
  • 400 ml beaker
  • Glass Stirring Rod

http://www.slsonline.co.uk/asps/uploads/big/2148-1.jpg

http://www.crscientific.com/stir-rods.jpg

procedure
Procedure
  • 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
a graph of the heat capacity of the calorimeter
A graph of the heat capacity of the calorimeter.

Analysis

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
slide10
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

slide11
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

slide12
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)

slide13

(#2.)Temperature changes for each of the 3 reactionsCalculate 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
slide14

http://www.razor-gator.com/SkinAcidsPhotos/Bottle-HCl&NaOH.jpghttp://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

slide15
(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

slide16
(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

slide17
#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

slide18
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

slide19
#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%

http://shiar.nl/calc/tipics/ti83+01.gif

conclusion
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.