Lab #3 - Molar Heat of Dissociation and Fusion
Title page:
Purpose:
Include title of the lab, name of you and your partner, date and
course code.
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To observe and determine the molar heats during physical processes. /1
Hypothesis: Using a calorimeter the molar enthalpies of dissociation & fusion can be
accurately calculated.
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Materials:
NaOH(s)
plastic lid
H2O(l)
H2O(ice)
thermometer balance
styrofoam cup
paper towel stirring rod
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Procedure:
Part A- NaOH Dissociation
1. Place exactly 100 ml of water into a styrofoam cup.
2. Use the thermometer to measure and record the initial temperature of H2O.
3. Weigh approximately 2 grams of sodium hydroxide pellets.
4. Drop the pellets into the calorimeter and quickly close the lid.
5. Stir the solution with the stirring rod
6. Record the “peak temperature”. Run 3 trials.
7.
Part B- KNO3 Dissociation
1. Wash and dry previous materials.
2. Set up calorimeter.
3. Measure 30 ml H2O and 3 g of KNO3 pour it into the calorimeter.
4. Measure the initial temperature of the HNO3.
5. Stir the solution with the stirring rod
6. Record the “peak temperature”. Run 3 trials.
Part C- Enthalpy of H2O Fusion
1. Wash and dry previous materials.
2. Obtain 100 ml of heated water.
3. Add the 100 ml of heated water to the styrofoam cup and measure the temp.
4. Dry some ice with a paper towel and add approximately 15 g of ice.
5. Stir until all of the ice melts.
6. Measure and record the final temperature.
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Observations: Include these tables and any other useful observations.
Table 1- NaOH Dissociation
Trial 1
Trial 2
Mass of water ( = 1.0 g/mL)
Initial temperature of H2O (oC)
Final temperature of H2O (oC)
Mass of sodium hydroxide pellets (g)
Moles of sodium hydroxide (mol)
Energy absorbed by water (KJ)
Actual Molar enthalpy of NaOH (KJ/mol)
Trial 3
Lab #3 - Molar Heat of Dissociation and Fusion
Table 2- KNO3 Dissociation
Trial 1
Trial 2
Mass of water ( = 1.0 g/mL)
Initial temperature of H2O (oC)
Final temperature of H2O (oC)
Mass of sodium KNO3 (g)
Moles of sodium KNO3 (mol)
Energy absorbed by water (KJ)
Actual Molar enthalpy of KNO3 (KJ/mol)
Table 3- Enthalpy of H2O Fusion
Trial 1
Trial 2
Mass of water ( = 1.0 g/mL)
Mass of ice (g)
Initial temperature of water (oC)
Final temperature of water (oC)
Moles of ice present (mol)
Energy absorbed by the water (KJ)
Actual Molar enthalpy of fusion (KJ/mol)
Table 4- Summary Table
Hd
KNO3
Average actual molar enthalpy (KJ/mol)
Theoretical molar enthalpy (KJ/mol)
% Error
Hd
NaOH
Trial 3
Trial 3
Hfusion
H2O
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Calculations:
1. Determine the energy released during each trial using Q=mCpTf - Ti) where
m is the mass of water and Cp is 4.18 J/goC. Determine the molar enthalpy of
dissociation Hd by dividing the energy released by the number of moles of
water. State the answer in KJ/mol.
2. For Part C use mICECp,ice(Tf –TiICE) + nHfusion = mH2OCp,H2O(Tf-Ti,H2O) to
determine the Hfusion in KJ/mol for all 3 trials.
3. Take the average molar enthalpies for Parts A-C.
4. Calculate % error in molar enthalpy by using the average actual molar
enthalpy and the equation below.
% error =
Theoretical Molar Enthalpy - Actual Molar Enthalpy
x 100%
Theoretical Molar Enthalpy
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Results:
 Restate the purpose.
 Did your data agree with the hypothesis?
 Write the thermochemical equations for dissociation and fusion.
 State whether each process is endothermic or exothermic and explain why.
 What are some sources of error and how did they affect the results?
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Conclusion:
 Summarize in 2-3 sentences the major findings and observations of this
experiment
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