# Thermodynamics: Enthalpy of Neutralization and Calorimetry

Thermodynamics: Enthalpy of Neutralization/ Calorimetry

Introduction

The First Law of Thermodynamics states that energy can be neither created nor destroyed. This means that energy, instead of ‘disappearing’, is either transformed, transferred, dispersed, or dissipated. When energy is lost by a system, it will be acquired by the surroundings. Heat can be described as the amount of energy needed to cause the temperature of a substance to rise and it is transferred from warmer areas to cooler ones. In order to be able to measure the change in heat or enthalpy of a reaction, a colorimeter can be used. The calorimeter was first introduced in the 18th century and can be used with any procedure that involves the flow heat between a system and it’s surroundings (CACT). It is capable of measuring the heat created or exchanged after a reaction has occurred  in a system with a constant pressure.

A calorimeter can be used to find the specific heat of a substance or even the heat of neutralization between a base and an acid. A basic calorimeter is composed of two Styrofoam cups (to provide insulation and prevent heat from entering or exiting the system), a lid covered with aluminum, a thermometer, and a stirrer. Since the calorimeter will still release heat, the first step is to find the heat capacity of it which is the heat absorbed by the calorimeter.

In order to find the heat capacity of a substance, the item must be weighed, heated, dropped into a calorimeter filled with cool water, and allowed to cool for a minute. During this whole process data will be collected including the weight of the item and the water, the temperature of the water and of the item; this information will form the base for the formula through which the specific heat of the item can be measured. In this experiment the specific heat for a unknown metal will be calculated. This specific heat will then be compared to the true value which suggests how accurate the calorimeter is.

Likewise, in order to find the amount of heat released by a chemical reaction that leads to neutralization, solutions of each the acid and the base are formed that have an equal molarity. These two are then mixed inside the calorimeter and stirred for two minutes and then the temperature of the solution taken. All the necessary data is collected and then the heat of neutralization is obtained. In this experiment the base was NaOH and the acid HCl. The experiment goes on to test the specific heat of NaCl in a similar manner as that of  the metal cylinder.

Methods

To make the calorimeter, two Styrofoam cups are placed together. The lid is made out of a circular piece of cardboard that is as big as the mouth of the cups and covered with aluminum foil only on the top; the lid should also have a hole in the middle where the stirrer and thermometer can be introduced. See figure 1 for a picture of the set up.

To find the heat capacity the calorimeter is first weighed (note: calorimeter must also be weighed after any new substance is added). Next, 50 mL of distilled water are added inside and have the temperature is taken. Another 50 mL of water is placed on a heating plate until it reaches 65 C°, taken off, then its temperature is taken and allowed to rest until the temperature stops increasing (this temperature is recorded), and added into the calorimeter along with the cooler water. The lid is placed and the water is stirred for about a minute and afterwards the temperature of the water is taken again (during the one minute, the thermometer should be cooled in a cup filled with room temperature water). With this information the Heat Capacity can be calculated through the formula: (m ∙ΔTs)hot = (m ∙ΔT  s)cold + (Heat Capacity  ΔT).

Now with the heat Capacity of the calorimeter, the specific heat of the metal cylinder can be found. The calorimeter is weighed (again, the calorimeter must be weighed after any new substance is added), and 70 mL of distilled water are added (after it’s temperature is taken). The cylinder is placed in a beaker filled with enough water to cover it, placed on a heating plate until the temperature reaches 95C°, taken off, allowed to rest until the temperature stops rising (this temperature is also recorded), and placed into the calorimeter. The metal and water are stirred for a minute and then the temperature of the water with the metal is taken again and recorded. With this information, the following formula can be used to find the specific heat of the metal cylinder: (m ∙ΔTs)metal = (m ∙ΔT  s)water + (Heat Capacity  ΔT)calorimeter .

To find the heat of neutralization between NaOH and NaCl, first two 2 M, 50.0 mL solutions are made for NaOH and NaCl respectively. The temperatures of each of the solutions are taken and recorded. The same steps as before are followed including weighing the calorimeter before and after anything is added. The solutions are then poured into the calorimeter one at a time, covered, and stirred for two minutes. At the end of the minutes the temperature of the solution is taken again. Measure the weight of calorimeter and of the NaCl. In order to find the heat of ...