Thermodynamics Lab

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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)
2042.0
4041.7
6041.5
8041.3
10041.0
12040.7
14040.5
16040.2
18040.0

Tmix = 42.2 °CTavg = 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)
2036.7
4036.9
6036.8
8036.6
10036.5
12036.3
14036.1
16036.0
18035.8

Tmix = 37.1 °Cqrxn = -6193.54 JΔH = -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)
2025.2
4025.2
6025.2
8025.2
10025.1
12025.1
14025.1
16025.0
18025.0

Tmix = 25.3°C...
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