Lab Report: Experimental Verification of Hess's Law

Categories: Chemistry

Report, Pages 3 (553 words)

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Abstract

In this experiment, we aimed to validate Hess's Law by calculating the heat of reaction using coffee cup calorimeters. We investigated the reactions involving NaOH, aqueous NaOH, and hydrochloric acid. The heat generated during the dissociation of NaOH was determined to be -47.2 kcal/mol, while the heat of reaction between NaOH(aq) and HCl(aq) was found to be -54.6 kcal/mol. By combining these values, we predicted a heat of -101.8 kcal/mol and experimentally obtained -118.4 kcal/mol, which confirmed our hypothesis.

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Introduction

The empirical determination of the change in enthalpy of a reaction is facilitated through the use of coffee cup calorimetry. Enthalpy, often expressed as Q + PV (where Q is the heat change, P is pressure, and V is volume), can be effectively measured as the change in temperature (Q = mcΔT), where m is the mass, c is the heat capacity, and ΔT is the change in temperature, given that pressure remains constant. By calculating the heat of dissociation in NaOH(s) to NaOH(aq) and the heat of the acid-base reaction between sodium hydroxide and hydrochloric acid, we can utilize Hess's Law to determine the heat of the reaction: NaOH(s) + HCl(aq) → H2O(l) + NaCl(aq).

This study analytically calculates the heats of all three reactions, allowing us to hypothesize that the heats of the individual reactions can be summed to obtain the heat of the third reaction.

Methods and Materials

We employed coffee cups with a plate lid as our calorimeter, upon which the temperature probe was placed.

Our initial step involved recording measurements, weights, and temperature changes. Subsequently, we dissolved NaOH pellets into room temperature water and monitored the temperature change. Temperature readings were taken every 0.5 seconds for 1 minute or until the temperature stabilized after pellet addition. Utilizing these temperature readings, we identified the critical temperature at which the exothermic reaction ceased providing sufficient heat to counteract cooling. The final experiment involved the reaction between NaOH(aq) and HCl(s). Due to the rapid nature of this reaction, we recorded temperatures every second. Waste solutions were disposed of in designated containers within the fume hood. The calorimeter and all reusable equipment were thoroughly rinsed and dried. Given the use of acids and bases, safety protocols were followed, including the availability of spill kits and proper disposal of NaOH(aq) waste.

Materials:

Coffee cup calorimeter
Analytical balance
Glass stirring rod
Thermal probe
50 ml measuring cylinders
Beaker

Results and Discussion

The experimental results revealed a decrease in the water temperature from 43.0°C to 39.1°C over 190 seconds. In contrast, the temperature of sodium hydroxide increased from 17.1°C at 10 seconds to 20.9°C at 280 seconds, while hydrochloric acid exhibited a significant temperature rise from 18.0°C to 29.3°C during the same time interval.

The experimental results are presented in the following table:

Time (s)	Water Temperature (°C)	Sodium Hydroxide Temperature (°C)	Hydrochloric Acid Temperature (°C)
0	43.0	17.1	18.0
190	39.1	20.9	29.3

To further analyze the data:

Calculate the changes in enthalpy for all three reactions.
Perform all calculations using the provided spreadsheet dedicated to the experiment.

The primary objective of this experiment was to verify Hess's law, which states that the sum of the enthalpy changes of two reactions is equal to the enthalpy change of a third reaction. Our results substantiate this principle by combining the enthalpy changes of the individual reactions.