thermodynamic lab

8/2/2019 Thermodynamic Lab

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Cauvel 1

Jacob Cauvel

Mrs. Shafer Pd. 4,5

October 13, 2009

Thermodynamics—enthalpy of Reaction and Hess’s Law

Objective

The point of this experiment was demonstrate Hess’s Law through a series of

chemical reactions. The values of heat change and enthalpy of reaction were both

calculated and compared between the reactions.

Data

Initial temperature (°C)

50.0 mL H2O—room temperature 19.8°C50.0 mL H2O—heated 70.0°C

Mixing Data

Time (sec) Temperature (°C) Time (sec) Temperature (°C)

20 39.3 120 38.5

40 38.9 140 38.4

60 38.8 160 38.3

80 38.8 180 38.2

100 38.5

T mix °C 39.25 qcal , J 2361.7

T ave , °C 44.9 C cal J/°C -121.42

Temperature versus Time after Mixing

37

37.25

37.5

37.75

3838.25

38.5

38.75

39

39.25

39.5

39.75

40

0 20 40 60 80 100 120 140 160 180

Time, Seconds

T e m p e r a t u

r e ,

d e g r e e s

c e

l s i u s



Cauvel 2

Initial temperature (°C)

50.0 mL 2.0 M HCL 20.1°C50.0 mL 2.0 M NaOH 18.4°C

Mixing DataTime (sec) Temperature (°C) Time (sec) Temperature (°C)

20 32.4 120 32.2

40 32.4 140 32.2

60 32.3 160 32.2

80 32.3 180 32.1

100 32.25

T mix °C 32.45 ∆ H , kJ/mol -30.53

qrxn, J -365.9305


31.9

32

32.1

32.2

32.3

32.4

32.5

0 20 40 60 80 100 120 140 160 180

Tme, Seconds

T e m

p e r a t u r e ,

d e g r e e s

c e l s i u s

Initial temperature (°C)50.0 mL 2.0 NH4Cl 18.5°C

50.0 mL 2.0 M NaOH 18.5°C

Mixing DataTime (sec) Temperature (°C) Time (sec) Temperature (°C)

20 19.8 120 19.8

40 19.8 140 19.85

60 19.8 160 19.9

80 19.8 180 19.9

100 19.8



Cauvel 3

T mix °C 19.76 ∆ H , kJ/mol -79.65

qrxn, J -7422.5071


19.65

19.7

19.75

19.8

19.85

19.9

19.95

0 20 40 60 80 100 120 140 160 180

Time, Seconds

T e m p e r a t u r e ,

D e g r e e s

C e l s i u s

Initial temperature (°C)50.0 mL 2.0 M NH3 18.9°C

50.0 mL 2.0 M HCl 18.9°C

Mixing Data

Time (sec) Temperature (°C) Time (sec) Temperature (°C)

20 30.7 120 30.440 30.6 140 30.3

60 30.5 160 30.3

80 30.5 180 30.2

100 30.4

T mix °C 30.71 ∆ H , kJ/mol -81.37

qrxn , J -3052.5307



Cauvel 4


29.9

30

30.1

30.2

30.3

30.4

30.5

30.6

30.7

30.8

0 2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

1 6 0

1 8 0

Time, Seconds

T e m p e r a t u r e ,

d e g r e e s

c e l s i u s

Series1

Linear (Series1)

Calculations



Cauvel 5

Questions

1. Write the net ionic equations for the three reactions involved in the experiment. Show

how the first two reactions are arranged algebraically to determine the third.

2. Calculate the value of ∆H for the third reaction for the values of ∆H determined for the

first two reactions using Hess’s Law.

3. Find the percent difference between the calculated and measured values of ∆H for the

third reaction.

1. What is meant by calorimetry?Calorimetry is the experimental measurement of heat produced in chemical and physical

processes. (definition from Chemistry the Central Science text book)

2. How does graphical analysis improve the accuracy of the data?



Cauvel 6

The graphical analysis improves the accuracy of the data by allowing a trend line to be

added which was used to calculate the Tmix.

3. The equation for calculating the heat evolved in each reaction is:

qrxn = -[9grams of solution x specific heat of solution x ∆Tsolution) + (Ccal x ∆Tsolution)

What is the meaning of the negative sign in front of the brackets?

When you calculate the equation without the negative, you are calculating the heatabsorbed (endothermic + ) but, when you add the negative, you are calculating the heat

given off (exothermic - ).

4. Do the lab results support Hess’s Law?Due to error during either the procedure or lab calculations, the results do not support

Hess’s Law.

5. How could the procedure be modified to achieve greater accuracy?The procedure could be modified to achieve greater accuracy by using a more effiecient

calorimeter that will not absorb heat from the reaction.

6. Find a table in a reference that lists standard heats of formation for the species

included in your net ionic equations. Use them to calculate ∆Hrxn for each of the

three net ionic equations. Do these values support Hess’s Law?Ammonia=-46.19kJ/mol

HCl=-92.3kJ/mol

Sodium Hydroxide=-469.15kJ/molAmmonium Choloride=-314.43kJ/mol

These values do support Hess’s Law because the first two net ionic equations add up to

equal the third equation.

Conclusion

In this experiment, a series of reactions were created, measured, and calculated todemonstrate Hess’s Law. The most significant source of air in this experiment occurred

with the calorimeter. Because some heat was lost during the reactions, and because it was

not accounted for in the calculations, the concluding data is off. This made it so that Hess’s

Law didn’t work out, however, if the lab was done properly, Hess’s law would haveworked, as proven by using a standard heats of formation chart.

thermodynamic lab

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