Thermodynamics Lab

Thermodynamics- Enthalpy of Reaction and Hess’s Law
Objectives:
1. To calculate the heat of reaction of a given reaction using the concepts derived from Hess’s Law.
Pre-lab Questions:
1. Define Heat of Reaction.
The enthalpy change associated with the completion of a chemical reaction.
2. Define Specific Heat.
The energy it takes to raise the temperature of 1 gram of a substance by one degree Celsius.
3. Calculate the heat of reaction assuming no heat is lost to the calorimeter. Use correct significant figures.
Q = c x m x t q = (4.18)(1.02 g/ml x 50ml )(3.9 oC) = -831 J
4. In problem 3 above, the calorimeter has a heat capacity of 8.20 J/goC. If a correction is made to account for heat absorbed by the calorimeter, what is the heat of reaction?
Qrxn = -(qcal + qsol) qrxn = -(32.0 + 831) = 863 J
5. Calculate the enthalpy of reaction for the formation of 1 mole of AB in solution.
0.60 M x .025 L = .015 Moles A, B, and therefore AB (1:1:1)

Enthalpy of reaction = -0.831 KJ / .015 moles AB = - 58 KJ/mol.
Data Tables:
Part 1: Determination of the Heat Capacity of the Calorimeter
Initial Temperature 50.0 mL H2O—room temperature: 21.6 °C 50.0 mL H2O—heated: 70.0 °C
Mixing Data
Time (seconds) Temperature (°C) 20 42.0
40 41.7
60 41.5
80 41.3
100 41.0
120 40.7
140 40.5
160 40.2
180 40.0

Tmix = 42.2 °C Tavg = 45.8 °C qcal =1504 J Ccal = 73.0 J/°C

Part 2: Determination of Heats of Reaction
Reaction 1
Initial Temperature 50.0 mL 2.0 M HCl - 25.3 °C 50.0 mL 2.0 M NaOH - 24.3 °C
Mixing Data
Time (seconds) Temperature (°C)
20 36.7
40 36.9
60 36.8
80 36.6
100 36.5
120 36.3
140 36.1
160 36.0
180 35.8

Tmix = 37.1 °C qrxn = -6193.54 J ΔH = -61.9 kJ/mol

Reaction 2
Initial Temperature 50.0 mL 2.0 M NH4Cl - 24.9 °C
50.0 mL 2.0 M NaOH - 24.4 °C
Mixing Data
Time (seconds) Temperature (°C)
20 25.2
40 25.2
60 25.2
80 25.2
100 25.1
120 25.1
140 25.1
160 25.0
180 25.0

Tmix =