2014 AP Chemistry Lab
Enthalpy, Entropy, and Free Energy
Objective: You will measure the ΔrxnHo for an endothermic reaction, and calculate the values of ΔrxnSo and ΔrxnGo. You will interpret the results.
Procedure:
1.Use a graduated cylinder to measure 90 – 100 mL of 1.0 M HCl(aq) into a Styrofoam cup calorimeter as shown at right. Record the volume of the solution to the nearest ± 0.1 mL, and measure the temperature to the nearest ± 0.1 oC.
2.Onto a sheet of weighing paper or in a weighing boat, weigh out a sample of 4 – 5 g of solid sodium hydrogen carbonate [NaHCO3(s)], recording its mass to the nearest ± 0.01 g.
3.Quickly add the solid NaHCO3 to the 1.0 M HCl(aq), stirring constantly and recording the final temperature obtained (when the temperature is no longer changing). Record your observations.
4.Upon completion, the reaction mixture may be discarded down the drain.
Calculations:
1.Write the balanced equation for the reaction that occurs. Calculate the number of moles of each reactant initially present, the moles of each reactant consumed in the reaction, and the moles of reaction that occurs.
2.Using your observations, predict the sign (+ or -) of the ΔrxnHo, ΔrxnSo, and ΔrxnGo for the reaction equation given above. Justify each of your responses.
3.Approximating the density and specific heat of the 1.0 M HCl(aq) as the values for pure water (why is this approximation justified?), calculate the mass of the reaction mixture, the temperature change of the reaction mixture during the reaction, and the amount of heat released/absorbed by the reaction. Be careful to use the correct sign (+ or -) and include appropriate units. You may find the following equations useful:
qsolution = Cp×m×ΔT
qrxn = - qsolution
4.Calculate the experimental ΔrxnHo from ΔrxnHo = qrxn / (moles of rxn).
Substance / ΔfHo (kJ mol-1) / Sfo (J mol-1 K-1)CO2(g) / -393 / 214
Cl-(aq) / -167 / 57
H+(aq) / 0 / 0
NaHCO3(s) / -951 / 102
Na+(aq) / -240 / 59
H2O(l) / -286 / 70
5.Use the values given in the table at right to calculate the predicted or accepted values of ΔrxnHo and ΔrxnSo.
6.Calculate the % error in your experiment and give one plausible explanation for your error. Be sure that your error accounts for the + or – sign of the error.
7.Calculate the value of ΔrxnGo for the equation given. Assume that the temperature is the average value of the initial and final values. Write the equilibrium expression and calculate the value of the equilibrium constant, Keq. As usual, be careful to use consistent and correct units in your calculations.
ΔrxnGo = ΔrxnHo – TΔrxnSo