Lesson Explainer: Measuring Enthalpy Changes Chemistry

In this explainer, we will learn how to perform calorimetry experiments and use the results to calculate the enthalpy change for a chemical reaction.

Most chemical reactions, including those occurring in living organisms, will release energy or absorb energy when they take place. According to the law of conservation of energy, chemical reactions must lead to the conversion of energy from one category to another. Energy is transformed from one place to another, and it is neither created nor destroyed.

In thermodynamics, the reaction of interest is contained within an area that we call the system. The area outside the system, which is where we make observations, is called the surroundings. An example of this terminology is shown below, where a reaction may be taking place inside a beaker.

An open system can exchange energy and matter with its surroundings, a closed system can exchange only energy with its surroundings, and an isolated system can exchange neither energy nor matter with its surroundings. An isolated system does not interact with its surroundings. A sealed thermos bottle that is thermally, mechanically, and electrically insulated from its surroundings is an example of an isolated system.

Chemical reactions that transfer energy as heat to the surroundings are described as exothermic reactions. Combustion reactions are examples of exothermic reactions. When a chemical reaction takes place in a system that absorbs energy as heat from the surroundings, it is described as an endothermic reaction. The gel ice packs that are contained in first aid kits contain chemicals that produce an endothermic reaction when mixed together.

The amount of heat that is transferred between the reaction system and the surroundings can be measured experimentally. To be able to calculate the energy transferred in these processes, we need to be able to relate the energy transferred to the amount of heating or cooling that is observed in the experiment.

Heat is the transfer of thermal energy between molecules and atoms in a system. This transfer of thermal energy is caused by a difference in temperature. Heat and temperature are closely associated, but they are fundamentally different. Temperature is the measure of the average kinetic energy of molecules. It can also be used to express how hot or cold a substance is.

When a substance is heated, energy is transferred to the molecules and atoms within the substance. The kinetic energy of these atoms and molecules is increased, and the temperature of the substance rises. The temperature change depends on the heat capacity of the substance. The heat capacity, , is defined as where is the quantity of energy measured in joules and is the temperature change in kelvins, or degrees Celsius. Since the size of a kelvin, K, is the same as that of a degree Celsius, the temperature change will be the same in either case.

Since heat capacity depends on the amount of substance being heated or cooled, it is known as an extensive property. The heat capacity may be used as an intensive property, in which case it is called the specific heat capacity. The specific heat capacity is defined as the heat capacity divided by the mass of the sample:

Specific heat capacity is measured in units of joules per kelvin per gram, or joules per degree Celsius per gram. Specific heat capacity is often simply referred to as specific heat.

Some values for the specific heat of different substances are seen in the table below. Liquid water has a relatively high specific heat when compared to other substances. If heat energy is supplied at a constant rate to liquid water, it will take longer to heat up through a given temperature when compared with the same mass of iron.

If a known mass of a substance is heated at constant pressure so that no work is done against the surroundings, the quantity of heat energy gained can be calculated using the formula where is the energy, measured in joules.

The mass of the substance, , is measured in grams. , which is pronounced “delta T,” is the temperature change of the substance. can be calculated by subtracting the final temperature from the starting temperature and is measured with a thermometer. Numerically, will have the same value whether the final and starting temperatures are both measured in degrees Celsius or in kelvins.

The specific heat capacity, , is formally measured in units of . However, it is more common to use units of grams instead to give units of .

In a laboratory, the temperature change caused by a reaction can be measured using a simple device known as a calorimeter. Experiments that measure the heat transferred by reactions are called calorimetry experiments.

A simple calorimeter consists of an insulated reaction container, a stirrer, and a thermometer. A polystyrene cup, supported in a beaker, with a close-fitting lid is often used as an insulated container for the reaction. The container is usually filled with a predetermined mass of water, which contains the reactants. A diagram of a simple calorimeter is shown below.

This type of calorimeter can be used to measure the heat energy change taking place in a reaction that happens in aqueous solution. Neutralization reactions and redox reactions may be studied using this type of apparatus. In this type of simple calorimeter, the insulated container is at atmospheric pressure, and the reaction takes place at constant pressure.

Example 3: Determining the Suitability of Experimental Apparatus for a Calorimetry Experiment

The diagram below shows the experimental setup for a simple calorimeter to measure the enthalpy change in certain reactions. For which type of reaction would this experimental apparatus not be suitable for measuring the change in enthalpy?

1. Dissolution
2. Combustion
3. Displacement
4. Neutralization
5. Precipitation

Answer

The type of calorimeter displayed in the diagram above is most suitable for measuring the enthalpy change for reactions that take place in aqueous solution, or in liquid media.

Dissolution reactions take place when a solute dissolves in a liquid solvent. An example of this is when an ionic compound, such as ammonium nitrate, dissolves in water to form hydrated ions.

Although displacement reactions can occur between solid reactants, they usually take place in water. An example of this type of reaction would be magnesium powder displacing copper in an aqueous copper sulfate solution.

Neutralization reactions take place between an acid and a base. The acid and the base are usually dissolved in water. In this case, the basic solution may be referred to as an alkali. These reactions are often encountered during a titration experiment in which an acid/base indicator is used.

Precipitation reactions involve the formation of an insoluble product from a combination of solutions. In these reactions, the solvent is frequently water.

All of the reactions described so far could be conducted inside a simple polystyrene-cup calorimeter, insulated with cotton wool.

Combustion reactions, which produce a flame of some sort, need a good supply of oxygen to ensure complete combustion. It would not be possible to heat a polystyrene cup directly with a flame, as it will catch fire, along with the cotton wool insulation. The polystyrene cup is not a good conductor of heat, and the oxygen supply to the flame would be limited inside the apparatus shown. The apparatus shown is not suitable for calorimetry experiments involving combustion. The correct answer is B.

For reactions such as the combustion of fuel, a different type of calorimeter is needed. Here, a constant-volume or combustion calorimeter is used. A simple diagram of a combustion calorimeter is show below.

Like the simple calorimeter, the combustion calorimeter contains water, a stirrer, and a thermometer. However, since the sample needs to be combusted, it cannot be placed directly into the water. Instead, the sample is placed inside a chamber, which can then be ignited remotely using the ignition wires.

As this chamber is also known as a bomb cell, the entire calorimeter is often referred to as a bomb calorimeter.

When using both the simple and bomb calorimeters, certain assumptions are made. These include the maximum temperature reflecting the amount of heat given off and that no heat escapes from the calorimeter to the surroundings. Finally, we also assume that the calorimeter itself, as well as the other equipment present such as the stirrer and thermometer, do not absorb any of the heat.

The bomb calorimeter is often used to find the calorific value of nutritional foods.

The joule (J) and the calorie (cal) can both be used as units of energy. The joule is the SI unit for energy. A joule is defined as the amount of thermal energy required to raise the temperature of 1 g of water by .

Joules can be converted into calories if they are divided by a value of 4.184, and calories can be converted into joules if they are multiplied by a value of 4.184.

The following conversion factors can also be used to convert between joules (J) and calories (cal):

One thousand calories can be defined as a kilocalorie (kcal). The relationship between the calorie and the kilocalorie is

An enthalpy change for a chemical reaction is usually calculated with units of kilojoules per mole. This is often referred to as the molar enthalpy change. The heat energy transferred during the chemical reaction must be linked to the number of moles of particles involved in the chemical reaction. For solid compounds, the moles reacting can be calculated from the mass of the limiting reagent used. For liquid solutions, the moles reacting can be calculated from the concentration of the solution and the volume taken. It is important that the molar enthalpy change is given the correct sign. For exothermic reactions, the molar enthalpy change is given a negative sign. For endothermic reactions, the molar enthalpy change is given a positive sign.

From experimental results, the molar enthalpy change can be calculated using the following equation: where is the number of moles reacting for the limiting reagent in the balanced chemical equation.

It is important to realize the limitations of calorimetry experiments carried out using simple apparatus in a laboratory. The calorimeter is not a perfectly isolated system, and heat transfer to the surrounding air and surfaces is inevitable. As a result, the measured temperature change will usually be less than it would be if there were no heat transfers. This will lead to the calculated value for a molar enthalpy change being lower than the actual value.

When combustion experiments are conducted, heat transfer to the surrounding air is more significant and large experimental errors may be encountered. Incomplete combustion frequently occurs in simple laboratory experiments. Incomplete combustion experiments leave sooty deposits on calorimeters. Less energy is released by the combustion reaction in these conditions. This will lead to lower experimental values for molar enthalpies of combustion, compared with accepted data, measured under standard conditions.

Let’s summarize what we have looked at in this explainer.