If anhydrous ammonium nitrate is added to water at room temperature, the temperature of the solution — Thermochemistry and Calorimetry Chemistry Question
Problem Context
If anhydrous ammonium nitrate is added to water at room temperature, the temperature of the solution decreases. However, if anhydrous calcium chloride is added to water at room temperature, the temperature of the solution increases. Design and carry out an experiment to determine what mass of ammonium nitrate must be added along with 10.0 g of CaCl2 to 100. mL of water so that the final temperature of the solution is the same as the initial room temperature of the water. You will be asked to describe the method you developed to solve this problem.
Give a brief description of your experimental plan. Include a sketch of the equipment you will use and the steps you plan to take to solve this problem.
Model Answer
Plan: State:
a. Add mass CaCl2 to recorded amount of H2O
b. Record initial and final temperatures to obtain ∆T for CaCl2 sample.
c. Add mass NH4NO3 to recorded amount of H2O.
d. Record initial and final temperatures to obtain ∆T for NH4NO3 sample
e. Repeat 2-3 times.
Sketch the apparatus as directed in the instructions.
Record your data and other observations.
Model Answer
Data: Record:
a. mass CaCl2 and volume of water.
b. Ti and Tf for CaCl2
c. mass NH4NO3 and volume of water
d. Ti and Tf for NH4NO3
Amounts of solids added should be large enough to get a reasonable value for ∆T
e. results of a “verification” experiment to check prediction.
Replication: multiple trials
Sample Data:
Trial 1 / Trial 2
Mass NH4NO3: 5.005 g / 5.073 g
Volume H2O: 50.0 mL / 51.9 mL
Initial temperature: 21.5 °C / 21.5 °C
Final temperature: 14.8 °C / 15.0 °C
∆T (NH4NO3): –6.7 °C / –6.5 °C
mass CaCl2: 5.064 g / 5.187 g
volume H2O: 50.0 mL / 52.0 mL
Initial temperature: 21.4 °C / 21.0 °C
Final temperature: 33.7 °C / 34.0 °C
∆T (CaCl2): 12.3 °C / 13.0 °C
What is the mass of ammonium nitrate that must be added along with 10.0 g of calcium chloride to 100 mL of water and so that the temperature of the resulting solution is the same as that of the water? Show your methods clearly.
Model Answer
Calculations:
Calculate:
a. ∆T/g CaCl2 and ∆T/g NH4NO3 for the same amount of H2O or q/g CaCl2 and q/g NH4NO3 for the same amount of H2O
b. ratio of grams of NH4NO3 to CaCl2
c. amount of NH4NO3 needed for 100 grams of H2O
Sample Calculations:
For NH4NO3, note that the solution became cooler as the NH4NO3 dissolved. This means the dissolution process is absorbing heat from the water.
qaverage for NH4NO3 = 2.79 × 10^2 J/g or 279 J/g
This is the heat absorbed from the water as the NH4NO3 dissolves.
For CaCl2, note that the solution became warmer as the CaCl2 dissolved. This means the dissolution process is releasing heat to the water.
qaverage for CaCl2 = -5.27 × 10^2 J/g or -527 J/g
This is the heat released to the water as the CaCl2 dissolves.
Ratio of heats: qCaCl2 = -qNH4NO3, so the ratio is:
527 J / g CaCl2 / 279 J / g NH4NO3 = 1.89 g NH4NO3 / 1.00 g CaCl2
Conclusion:
For 10.0 g CaCl2: 10.0 g CaCl2 × (1.89 g NH4NO3 / 1.00 g CaCl2) = 18.9 g NH4NO3