15.7.1: Lecture Demonstrations

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Calorimeter Constant

To a styrofoam cup calorimeter containing 250 mL of water at 24.0 ^oC is added a known amount of heat as 250 mL of water from a second styrofoam cup at 32.0 ^oC. The final temperature, measured by a computer-interfaced thermistor, is 27.7 ^oC. The computer-generated plot of T vs. time should be projected^[1]. What is the calorimeter constant?

ΔT _calorimeter = 27.7 ^oC – 24.0 ^oC = 3.7 ^oC

ΔT _cold = 27.5 ^oC – 24.0 ^oC = 3.7 ^oC

ΔT_hot = 27.7 – 32.0 = -4.3 ^oC

q_hot = m x S.H. x ΔT = 250 g x 4.18 J/g^oC x -4.3^oC = -4494 J

q_cold = m x S.H. x ΔT = 250 x 4.18 x 3.7 ^oC = 3867 J

q_calorimeer + q_hot + q_cold = 0

q_calorimeter -4494 + 3867 = 0

q_calorimeter = 627 J.

C = Q_calorimeter/T = 627 J / 3.7 ^oC = 169 J/^oC

The Ammonium Nitrate "Cold Pack"^[2]

A styrofoam cup calorimeter contains 250 mL of water at 25.0^oC. Solid NH₄NO₃ (5 g ) is added, and the temperature falls to 23.8^oC. The computer-generated plot of T vs. time should be projected^[3]. The calorimeter is found to absorb 169 J to change its temperature 1 ^oC, so it is said to have a calorimeter constant of 169 J/^oC. What is the enthalpy change for the dissolution reaction?

q_water = m x S.H. x ΔT = 250 g x 4.18 J/g^oC x -1.2^oC

= -1254 J

q_calorim = C x ΔT = 169 x -1.2^oC = -203 J

q _tot = -1254 + -203 = -1457 J

q _rxn = +1457 J.

ΔH _rxn = q (kJ) / n (mol)

n = m / M = 5 g NH₄NO₃ / 80 g/mol = 0.057 mol

ΔH _rxn = q/n = 1.457 kJ / 0.057 mol = +25.6 kJ/mol

The reaction is spontaneous even though it is endothermic, because of the large positive entropy change resulting from water association with the separate ions in solution.

References

We used Vernier LoggerPro(R) software
J. Chem. Educ., 2004, 81 (1), p 64A
We used Vernier LoggerPro(R) software

Contributors

Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.

The Ammonium Nitrate "Cold Pack"[2]

References

Contributors

The Ammonium Nitrate "Cold Pack"^[2]