# Hess Law Lab Report

Topics: Thermodynamics, Enthalpy, Observational error Pages: 3 (702 words) Published: January 22, 2013
LAB REPORT ON VERIFICATION OF HESS’S LAW
Our purpose of doing this lab was to prove the Hess’s law correct. Hess’s law suggests that the enthalpy change of a reaction must be equal to the sum of the enthalpy changes of the related reactions which lead to the original reactions. The following are the reactions at the lab; 1) NaOH ( s) NaOH (aq)

2) NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l)
3) NaOH (s) + HCl (aq) NaCl (aq) + H2O (l)
As explained before, Hess’s Law states that the enthalpy change of reaction three (ΔH3) should be equal to the sum of the enthalpy changes of the first two reactions ( ΔH1 + ΔH2 ). As seen at the equations, the solid NaOH dissociates into its ions, after that it is neutralized by HCl. DATA COLLECTION AND PROCESSING

Temperature Data Table for Trial 1
Trial 1 | Initial Temperature / °C ± 0.5 °C| Final Temperature / °C ± 0.5 °C| Reaction 1| 24 °C| 30 °C|
Reaction 2| 25 °C| 31 °C|
Reaction 3| 24 °C| 35 °C|

Specific heat capacity: 4.18 J g-1 °C-1
Density of the solutions: 1 g cm-3
Volume of solutions: 100 cm3 ± 0.1 cm3
Molar mass of NaOH = 40 g mol-1

Temperature Data Table for Trial 2
Trial 2 | Initial Temperature / °C ± 0.5 °C| Final Temperature / °C ± 0.5 °C| Reaction 1| 23 °C| 28 °C|
Reaction 2| 24 °C| 30 °C|
Reaction 3| 23.5 °C| 36 °C|

Qualitative Data: At the first reaction, nothing happened as we added the NaOH pellets because the added particles were too small for it to make a physical change, no physical changes were observed at the second and third reactions.

EVALUATION
Following are the results for our experiment;
ΔH1 = -46.0 kJ mol-1 ± 8.5 kJ mol-1
ΔH2 = -50.2 kJ mol-1 ± 8.5 kJ mol-1
ΔH3 = -98.3 kJ mol-1 ± 8.4 kJ mol-1
ΔH3 = -96.2 kJ mol-1 ± 17 kJ mol-1 (this is the value that has to be found according to Hess’s Law The literature values for ΔH1 and ΔH2 are provided by our teacher. The values are -40 kJ mol-1 and -57 kJ mol-1 respectively, making the...