# Determining If Hess Law Works In A Demonstration Of Four Different Reactions

Better Essays
Hess’ Law Lab Report
The purpose of the experiment is to determine if Hess’ Law works in a laboratory experiment consisting of four different reactions. During the first reaction, cold water was placed into a calorimeter and continually mixed with a stir bar. Hot water was then added into the calorimeter while the probe recorded the temperature readings throughout the reaction. The temperature change was then used to find the specific heat of the calorimeter. The same process was used in the second, third, and fourth reactions of hydrochloric acid and sodium hydroxide, ammonium chloride and sodium hydroxide, and ammonia and hydrochloric acid respectively in order to determine the heats of reaction.
Chemical theories and concepts used in this experiment include
Tavg - Tmix = 53.0°C - 45.6°C = 7.4°C
Mass of Water: (100 mL)(1.00 g/1.00 mL) = 100 g
Heat Capacity of Calorimeter:
(100g)(4.184 J/g°C)(7.4°C) = 31.0 x 102 J / (45.6°C - 22.4°C) = 134 J/°C
Reaction 2
Mass of solution: (100 mL)(1.03 g/1.00 mL) = 103 g q = [(32.1°C - 21.5°C)(103g)(4.184 J/g°C)] + [(10.6°C)(134 J/°C)] = -5990 Joules
Moles of solution: (2.0 M)(0.05 L) = 0.1 moles
ΔHrxn = (-5990 Joules) (1 kJ / 1000 J) = -5.99 kJ / 0.1 mol = -59.9 kJ/mol
Reaction 3 q = [(21.9°C - 20.9°C)(103g)(4.184 J/g°C)] + [(1.0°C)(134 J/°C)] = -565 Joules
ΔHrxn = (-565 Joules) (1 kJ / 1000 J) = -0.565 kJ / 0.1 mol = -5.65 kJ/mol
Reaction 4 q = [(31.2°C - 21.1°C)(103g)(4.184 J/g°C)] + [(10.1°C)(134 J/°C)] = -5710 Joules
ΔHrxn = (-5710 Joules) (1 kJ / 1000 J) = -5.71 kJ / 0.1 mol = -57.1 kJ/mol
Calculation of ΔH Through Hess’ Law
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) ΔH = -59.9 kJ/mol
NH4Cl(aq) + NaOH(aq) → NH3(aq) + NaCl(aq) + H2O(l) ΔH = -5.65 kJ/mol
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) ΔH = -59.9 kJ/mol
NH3(aq) + NaCl(aq) + H2O(l) → NH4Cl(aq) + NaOH(aq) ΔH = 5.65 kJ/mol
HCl(aq) + NH3(aq) → NH4Cl(aq)
ΔH = -59.9 kJ/mol + 5.65 kJ/mol = -54.3