Name: _____________________
Period: ___
Lab: Thermochemistry
Introduction:
In this experiment, the enthalpy of several reactions will be determined using the method of calorimetry.
The reactions that will be considered are the following:
1: NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
2: NH4Cl (aq) + NaOH (aq) → NH3 (aq) + NaCl (aq) + H2O (l)
3: NH3 (aq) + HCl (aq) → NH4Cl (aq)
All of the reactions in this experiment are exothermic. As heat is given off for each reaction, the temperature of the mixture will
increase. The amount of heat given off by the reaction can be calculated according to the following equation:
∆H = mc∆T
Where: m = mass (g)
c = specific heat capacity (The specific heat capacity of each mixture can be approximated as that of water, 4.18 J/g ºC.)
∆T = the change in temperature (ºC)
Given the volume and concentration of each solution, the number of moles for each substance in the reaction can be calculated,
therefore, the enthalpy of each reaction can be calculated in kJ/mol. The experimental values can be compared to the actual
values, which can be calculated using the heats of formation.
Procedure:
(1) Obtain 100 mL of 0.20 M NaOH, 100 mL of 0.20 M HCl, 50 mL of 0.20 M NH3, and 50 mL of 0.20 M NH4Cl.
(2) Construct a calorimeter by placing two Styrofoam cups together. Determine the mass of the empty calorimeter and record.
Obtain a calorimeter cover and a thermometer.
(3) Measure 50 mL of NaOH into a beaker. Determine the initial temperature. Measure 50 mL of HCl into another beaker.
Determine the initial temperature. Record. (The temperature of both solutions should be the same. If it is not, use the average
temperature as the initial temperature)
(4) Combine the NaOH and HCl into the Styrofoam cup calorimeter. Cover the calorimeter and use a thermometer to stir.
Record the highest temperature reached by the mixture. Determine the mass of the mixture and the calorimeter (without cover).
Record. Calculate the mass of the mixture. Record.
(5) Discard the mixture in the sink. Rinse and dry the beakers, calorimeter, and thermometer.
(6) Measure 50 mL of NH4Cl into a beaker. Determine the initial temperature. Measure 50 mL of NaOH into another beaker.
Determine the initial temperature. Record. (The temperature of both solutions should be the same. If it is not, use the average
temperature as the initial temperature)
(7) IN THE FUMEHOOD: Combine the NH4Cl and NaOH into the Styrofoam cup calorimeter. Cover the calorimeter and use
a thermometer to stir. Record the highest temperature reached by the mixture. Determine the mass of the mixture and the
calorimeter (without cover). Record. Calculate the mass of the mixture. Record.
(8) Discard the mixture in the sink. Rinse and dry the beakers, calorimeter, and thermometer.
(9) Measure 50 mL of HCl into a beaker. Determine the initial temperature. Measure 50 mL of NH3 into another beaker.
Determine the initial temperature. Record. (The temperature of both solutions should be the same. If it is not, use the average
temperature as the initial temperature)
(10) Combine the HCl and NH3 into the Styrofoam cup calorimeter. Cover the calorimeter and use a thermometer to stir.
Record the highest temperature reached by the mixture. Determine the mass of the mixture and the calorimeter (without cover).
Record. Calculate the mass of the mixture. Record.
(11) Discard the mixture in the sink. Rinse and dry the beakers, calorimeter, and thermometer.
Data:
Mass of empty calorimeter: _______ g
Reaction
NaOH + HCl
NH4Cl + NaOH
NH3 + HCl
Initial
Temperature (ºC)
Final
Temperature (ºC)
∆T (ºC)
Mass of mixture +
calorimeter (g)
Mass of
mixture (g)
Questions:
(1) Using the change in temperature and the mass of each mixture, determine the heat given off by each reaction in Joules.
(2) In all reactions, there are an equal number of moles of each substance. Determine the number of moles.
(3) Calculate the enthalpy of each reaction in kJ/mol. Give the correct sign.
(4) (a) Show that reaction 3 can be obtained from reactions 1 and 2 using Hess’ Law.
(b) Calculate the enthalpy for reaction 3 using Hess Law. How does this value compare to the enthalpy calculated for the
reaction in question 3?
(5) Using the heats of formation, calculate the actual enthalpy for each reaction.
Substance
NaOH (aq)
HCl (aq)
NH4Cl (aq)
NH3 (aq)
NaCl (aq)
H2O (l)
Hºf (kJ/mol)
-470
-167
-300
-80
-407
-286
Conclusion:
Give your experimental result for the enthalpy of each reaction.
Error Analysis:
Calculate the percent errors for your results. The actual value for the enthalpy is that obtained from the heats of reaction.
Describe the main source of error in this experiment and explain the excepted effect on the results.