Constant Pressure Calorimetry

Constant-pressure Calorimetry Definition

Let's begin by looking at the definition of calorimetry.

We use a calorimeter to do this as it’s a calibrated and insulated device.

The temperature we measure from the calorimeter is what’s used to find out the amount of heat transferred to or from a substance in a chemical reaction.

Important things to know to understand how a calorimeter works are:

What a system and surroundings are.

When an exothermic reaction occurs in a calorimeter, the heat created (Q) is transferred from the system to its surroundings making Q negative.

When an endothermic reaction occurs in a calorimeter, the heat created (Q) is transferred from the surroundings to the system making Q positive.

Constant-pressure Calorimetry Formula

In calorimetry, we assume that the energy released and absorbed by the surroundings is the same as the energy released and absorbed by the system.

For example, if we wanted to determine the heat change (enthalpy) or heat of fusion for ice melting within the calorimeter we can assume that the energy absorbed by ice while melting is equal to the energy in value but opposite in sign to the energy released by water inside the calorimeter at constant pressure (1 atm). Please see Figure 1, below:

Q_ice = -Q_surroundings

Another easier way to think about this is if you have a chocolate bar, and you give it to your crying sibling. You released or lost your chocolate bar to your younger sibling who has gained or absorbed your chocolate bar. In this case, you are the surroundings and the chocolate bar is the energy from the surroundings being transferred to the system, which is your younger sibling.

In giving your sibling your chocolate bar, you have performed an equal exchange of energy with opposite signs (as you lost the chocolate bar while your sibling gained it simultaneously).

Q_{sibling chocolate bar} = -Q_{your chocolate bar}

Figure 1: Constant-pressure calorimetry, exothermic versus endothermic processes. Daniela Lin, Study Smarter Original

This means that the net heat change is zero as Q_ice + Q_surroundings = 0 or Q_{sibling chocolate bar} + Q_{you without chocolate bar} = 0.

This can also be translated to Q_reaction + Q_solution = 0.

This is due to the law of conservation of energy.

Overall, exothermic reactions result in a negative Q as heat is transferred from the system (sodium hydroxide crystals in the first experiment depicted in the figure above) to the surroundings (water) leading to a temperature increase.

In contrast, endothermic reactions result in a positive Q as heat is transferred from the surroundings (water) to the system (ice cubes in the second experiment depicted in the figure above) leading to a temperature decrease.

Now that we understand the basics concerning calorimetry we can go over the equation.

Figure 2: Enthalpy change equation. Daniela Lin, Study Smarter Originals.

Enthalpy or the heat of change equation is the formula we can use to find the amount of heat transferred in a calorimeter.

Constant-pressure Calorimetry Importance

Constant-pressure calorimetry is important because we use it to measure the amount of heat gained or lost during a chemical reaction.

Knowing this allows us to know whether a reaction is exothermic, releasing heat, or endothermic, absorbing heat.

In chemistry, it’s important for scientists to understand all of this because heat transfer plays a crucial role in machine efficiency, material sciences, etc.

Some real-world applications of calorimetry include controlling homeostasis in humans, counting the number of calories humans consume, and other thermal changes necessary to maintain our survival.

Constant-pressure Calorimetry Examples

Now that we understand how important constant-pressure calorimetry is, we understand the need to use the formula to calculate the heat changes associated with it.

We can now move on to reading about examples involving constant-pressure calorimetry:

Constant-pressure vs Constant-volume Calorimetry

Throughout this entire article, we’ve been addressing constant-pressure calorimetry. A coffee cup or a double styrofoam cup with a stopper on top is an example of constant-pressure calorimetry.

Figure 3: Constant-pressure calorimeter example. Daniela Lin, Study Smarter Originals.

A constant-pressure calorimeter keeps the system isolated from other surroundings like air, dust, etc. The two holes in the cover of the cup allow us to stir the mixture to make sure it's dissolved and to measure the temperature change when the pressure is constant.

But did you know that the constant-pressure calorimeter is not the only type of calorimeter?

Well, from the title above you probably guessed that the other type of calorimetry we’ll talk about is the constant-volume calorimeter.

Except it keeps the volume constant and not the pressure.

A constant-volume calorimeter is also called a bomb calorimeter and this is because it’s a container that can withstand a large pressure change that can occur in chemical reactions.

Similar to a constant-pressure calorimeter, a bomb calorimeter also is a sealed system. But instead of a more simple setup it consists of a large enclosed steel vessel usually shaped like a bomb that contains the system (reactants), water in which the large steel vessel is submerged in, a thermometer, a stirrer, and lastly an ignition wire.

Examples of when we use a constant-volume calorimeter include in combustion reactions.

You’ve reached the end of the article, and now you should understand the importance of a constant-pressure calorimeter, how to calculate enthalpy changes in a calorimeter and the differences between a constant-pressure and constant-volume calorimeter.

For more practice and examples click on our flashcards associated with this article.