?Hess Law
Abalaziz AlyahyaLab partner: Jacob
Instructor: Pihl
Date: 11/25/2018
Introduction
The critical objective of this report was to consequently determine whether if the Sodium hydroxide and hydrochloric acid is endothermic or exothermic and whether the enthalpy change (?H) of the reaction between NaOH and HCL can be calculated using Hess Law. There are three variables that was considered during this experiment and those are; controlled variable which was considered as the time recorded during the reaction of each experiment, dependent variable which was the temperature change (?T) recorded in the reaction of each experiment and independent variable which was the actual and the substance amount of the reaction chemicals.

Experiment procedure
Reaction one;
100cm3 of distilled water was measured and placed in a glass beaker on a stirrer. The room temperature of the water was recorded.

4.0g of Sodium hydroxide was added into the water, there was various on both of the pastilles hence the desired temperature values was reached.

In a continuous stirring the pastilles was dissolved completely the ?T was then recorded in a constant time period of 30 seconds each.

Reaction two;
50cm3 of 2.0 mol/dm3 of HCL was measured and the solution placed on the stirrer, then the temperature measured up.

2.0mol/dm3 some of volume of Sodium hydroxide was added into the solution.

The temperature of the reaction was then recorded in a continuous time of 30 seconds within a time period of 150 seconds.

Reaction three
55cm3 of 2.0mol/dm3 hydrochloric acid was measured using a measuring tube plus 45.0cm3 of distilled water and the placed in stirrer and the temperature of the solution recorded.

2.0grams of sodium hydroxide was added in the solution.

During the continuous stirring the pastilles was dissolved in this case and the temperature of the reaction recorded in grace period of 30 seconds within a time limit of 150 seconds
Reaction one; NaOH (aq) + H2O (aq) NaOH+ H2O
Temperature of the solution reaction in a time period of 30 sec
Time(sec) Temperature (0c)
0 20.0
30 21.1
60 22.0
90 22.4
120 23.0
150 23.4
Graph one of temperature change during the reaction.

lefttop
Calculation of ?H during the reaction
?H= 23.4-20.0= 3.4k
100cm3=100g=0.1kg
Heat energy = 0.1kg*4.18c*3.4k= 1.4212kj
Sodium hydroxide amount is 1/40 =0.025
This reaction showed as exothermic as the ?H was
-1.4212*(1/0.025)= -56.848kj/ml
Reaction two; HCL +NaOH
The reaction temperature change.

Time (sec) Temperature (0)
0 20.1
30 25.3
60 25.5
90 25.3
120 25.2
150 25.1
Graph on temperature change during the reaction

Calculation of ?H
?T=25.3-20.1= 5.2K
50cm3= 50g= 0.05kg
Heat energy produced during the reaction= 0.05kg*4.18c*5.2k=1.0868kJ
Since the reaction was exothermic the enthalpy change was;
-1.0868*(1/0.025)= -43.472Kj/mol
Reaction three; HCL+H2O+NaOH
The table showing time difference’
Time (sec) Temperature (0)
0 19.2
30 22.1
60 24.5
90 26.3
120 28.1
150 29.0
Graph showing temperature change during the reaction

Calculations of the enthalpy change during the reaction.

Enthalpy change = 29.0-19.2= 9.8k
55cm3=55g= 0.055kg
Heat energy= 0.055kg*4.18c*9.8k=2.25302Kj
The HCL amount =0.025mol
-2.25302*40= -96.1208kJ/mol
The enthalpy change of the three reactions using the hess law
?H of reaction one = -56.848
?H of reaction two = -43.472
?H of reaction three= X
Using Hess law it is possible to calculate the enthalpy change of reaction three
?H1+?H2= ?H3
-56.848+-43.472= -100.32Kj/mol=?H3
Discussion
According to this experiment that was conducted in accordance with the Hess law using a calorimeter. For the determination of the neutralization heat process of the strong acid and the strong base which were reacted together; hydrochloric acid which was a strong acid and sodium hydroxide which was a strong base, a 2 directional reaction was conducted that produced sodium chloride which was a salt, in this situation the reaction was measured using calorimeter.According to the graphs above for the reactions one, two and three; there was a clear indication that the reactions were both exothermic, since the graphs shows that as the temperature rises also time surpasses. According to the graph of the reaction two, above the curve decreases within the first 30 seconds of the reactions. This consequently happened since the reaction was only active on the first 30 seconds and then the solution started cooling since the reaction come at end.

The ?H of the reaction was calculated, then after that the ?H3 was again calculated using the Hess law and the difference compared as seen bellow. The ?H3 calculated using Hess law showed a slightly difference from the first calculated one. This difference was accorded to be due to the slightly measurement of the reactants and weight of the substances used during the reaction.

The calculated reaction
-96.1208kJ/mol
The calculated change using Hess Law
-100.32Kj/mol
For a precise condition of using Hess Law in the calculation of ?H of a reaction three, it was determined that Hess Law can be used to give a precise and exact ?H of a reaction in any given experiment. This depends only on the preciseness of the calculated before enthalpy change of the reaction one and two.

In this situation for a more accurate results to be achieved, certain conditions should be met during the experiment; an accurate experimental materials, and also the temperatures the substances should be recorded before the reaction starts. Hence if this is not done the rate of reaction can be affected. Moreover to make the experiment and the collected data more accurate the double boiler should be used, this help to contain the in initial temperatures of both reactions.
According to the experiment the reactions that took place were all exothermic and also it was determined that Hess law can be used to provide an accurate enthalpy change of reactions, although it can only be accurate given raw data that is accurate.
Reference
J.Kotz, P. Treichel, J. Townsend (2009) Chemistry ; Chemical reactivity 7th ed. Instructors Editions; Brooks/Cole.

Greenbowe, T.J. and Meltzer, D.E. (2003). “Student learning of thermodynamic context of solution Calorimetric. ‘ International Journal of Science Education, 25(7), 779-800.

Bird, Tony. 1987, Penuntum Praktikum Kimia Fisik untuk University. Experiment in Physical Chemistry. Jakarta; Gramedia.