Start writing a post


Thermochemistry is all about studying the heat released and absorbed during chemical reactions. Read on!

Gloria Sun

Hi there! So I know the previous notes that I’ve given you in this chemistry series have been difficult. They really are, even for me. That’s why teaching it and explaining it in my own words is so helpful for me, and why I’m being persistent in continuing the material. Thankfully, thermochemistry is easier and more intuitive than Chemical Kinetics (last week’s stuff).

Thermochemistry is all about studying the heat released and absorbed during chemical reactions. Read on!

Note: The study of how fast a reaction goes is kinetics. Thermodynamics do not tell you how long it will take for a reaction to go to completion!!

We study thermochemistry because it’s useful! When we do combustion reactions for fuel, we can understand how much energy a mole of a gas can release when burned in oxygen. When we eat food, we release energy in each step of the breakdown, which is used by our body for metabolic processes.

What’s more important, we can find what conditions will optimize the yields of products.

So, thermochemistry is a subset of thermodynamics – it deals with forms of energy and their interconversions.


The system is some portion of the universe arbitrarily chosen for consideration (i.e. a beaker where the reaction is taking place).

The surroundings are everything in the universe not included in the system.

Universe = system + surroundings

This “equation” always cracks me up when I see it. What a simple, idealistic way of viewing the world!

Now, a system can be (1) open, (2) closed, or (3) isolated.

  1. An open system allows for the exchange of heat, matter, and work.
  2. A closed system allows exchange of work and heat but not matter.
  3. An isolated system does not allow the exchange of matter, work, or heat.

Since in thermochemistry, we’re dealing with energy, what exactly is it? It’s a rather abstract concept – unlike matter, it can’t be seen and there are different forms. Basically it’s the capacity to do work.

Potential and kinetic energy are the two main forms of energy. I see thermal energy as a subset of kinetic energy, because it’s the energy associated with the motion of atoms and molecules, and chemical as a form of potential.


Energy can be converted from one form into another, but it cannot be created or destroyed. The total quantity of energy in the universe is thus a constant. This observation is the law of conservation of energy, the First Law of thermodynamics.

In the form of an equation, the change in internal energy for a closed system is equal to the heat plus work:

ΔE = E final – E initial = q + w

So, heat and work are two ways of increasing the internal energy of the system.

ΔE refers to the change in internal energy, E. It’s the sum of the kinetic and potential energies of the individual particles of a system.

Heat (q) is the energy transfer resulting from a temperature difference.

Work (w) is the action of a force through a distance (force × distance). Here, w is the work done on the system.

Sign convention: Heat absorbed by a system is positive; heat released by a system is negative. If work is done on a system, work is positive; if work is done by the system, work is negative.

So, based on the first law of thermodynamics, in an isolated system, ΔE should be zero.

The thermodynamic property (state function) is a property whose change depends only on the initial and final states of the system.

Examples of state functions include energy, enthalpy, entropy, Gibbs free energy, pressure, volume, and temperature.

Examples of properties that are NOT state functions include heat and work.

All right. The heat of reaction is the heat released or absorbed during a chemical reaction.

This can be either exothermic – it gives off heat to the surroundings – or endothermic – it absorbs heat from the surroundings.

Note: During endothermic reactions (especially in isolated systems), the temperature of the system decreases because the reaction uses the energy abstracted from the reactants to form products.

These chemical reactions are conducted in either constant volume (isochoric) or constant pressure (isobaric) conditions.

Most (99%) of the reactions we do for educational purposes are isobaric – the pressure doesn’t change, while the volume of the reacting system usually changes. To do a isochoric reaction, most commonly we use bomb calorimeters, which allows the reaction to take place in a sealed container with rigid walls that do not expand or contract.

For the two conditions, the amount of heat absorbed or given out is different for a reaction, so we wish to understand the difference in heat change.


Need a song to get you through the rest of the post?

PV work is mechanical (not electrical, for example) and is the work associated with the volume change of a system.

w = -PΔV

where P is the constant external pressure of the reacting system and

ΔV = V final – V initial

given that the external pressure is constant (isobaric reaction).

In general, the change in internal energy of a closed system is

ΔE = q + w = q + w PV + w non-PV

Since by definition, a constant volume process has a “w PV = 0”, assuming that all work is PV work, for an isochoric process:

q = ΔE

Therefore, we can measure the change in internal energy by measuring the heat absorbed or given out in a constant volume chemical reaction (assuming all work is PV-Work)


Because q + w = ΔE, the heat of the reaction for a constant pressure process is:

q = ΔE + PΔV

So we can describe a constant pressure process, it’s convenient to have another state function enthalpy, H:

H = E + PV

So enthalpy isn’t easily visualized – it’s a mathematical construct. ΔH is the enthalpy of the products minus the enthalpy of the reactants.

For any reaction:

ΔH = ΔE + Δ(PV)

For a constant pressure reaction:


The heat of a reaction at constant pressure equals the change in enthalpy of the reacting system, if the work involves only pressure-volume work, so that

q = ΔE

ΔH is positive for an endothermic reaction and negative for an exothermic reaction.

Finally, a thermochemical equation is a chemical equation that includes the enthalpy change. For example:

2 H2(g) + O2(g) → 2 H2O(l) ΔH = -571.6 kJ


We don’t want to tabulate heats of reaction for all temperatures and pressures, so we have defined thermodynamic standard states.

The standard state of a substance is its pure form at 1 bar pressure and a temperature of 25 C.

The standard enthalpy change is the enthalpy change for reactants in their standard state going to products in their standard state. We use a superscript zero to denote standard state conditions. These are usually given in a table.

There are different types of standard enthalpies of reactions.

  • Enthalpy of Combustion – the standard enthalpy change accompanying the combustion of 1 mole of a substance in oxygen.
  • Enthalpy of Formation – the standard enthalpy change accompanying the formation of 1 mole of pure substance from its elements in their most stable forms, with all substances in their standard states.
    The standard enthalpies of formation of all elements in their most stable form are zero.


The enthalpy change for any chemical reaction is the same, regardless of the path by which the reaction occurs. (This law is a consequence of the fact that enthalpy is a state function.)

Image Source: ChemGuide

That’s saying that there is more than one pathway for a reaction (via a different intermediate, for example).

The overall reaction enthalpy is the sum of the reaction enthalpies of the individual reactions into which a reaction may be divided.

This is sooo good! Because otherwise, we wouldn’t be able to calculate enthalpy changes that are difficult to impossible to measure!

Now, there are two methods for finding ΔH.

Here’s an example 0f (1) the algebraic method:

Image Source: OK State

(2) Alternatively, we can write Hess’s law as:

so that the enthalpy change of a reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants.


The bond dissociation enthalpy is defined as the standard reaction enthalpy for the process in which the A-B bond is broken homolytically:

A-B(g) -> A(g) + B(g)

where A and B could be atoms or groups of atoms.

Bond enthalpies are derived from the heats of formation of the species involved in the definition of bond enthalpy or from spectroscopic measurements.

These bond energies are always positive!

When bond enthalpies are given in a table, this is their mean value averaged from many different molecules. This is useful because they allow us to make estimates of enthalpy changes.


Calorimetry is the experimental method of determining heats of reactions.

Image Source: Annenberg Learner

The heat capacity (C) of an object is the amount of heat required to raise the temperature of the substance by 1 K. The SI unit of heat capacity is J/K.

The specific heat (c) is the heat capacity per unit mass. The SI unit is J/kg/K, and you can find these values in tables.

q = mcΔT

If we know the specific heat capacity and the mass of a substance, then the change in the sample’s temperature will tell us the amount of heat (q) that has been absorbed or released in a process.

Great job! Hope you enjoyed the music, and hope you enjoyed learning about thermochemistry. As always, if you have questions, feedback, or anything you want me to know, leave it in the comments! I always check.


Report this Content
This article has not been reviewed by Odyssey HQ and solely reflects the ideas and opinions of the creator.
Student Life

100 Reasons to Choose Happiness

Happy Moments to Brighten Your Day!

A man with a white beard and mustache wearing a hat

As any other person on this planet, it sometimes can be hard to find the good in things. However, as I have always tried my hardest to find happiness in any and every moment and just generally always try to find the best in every situation, I have realized that your own happiness is much more important than people often think. Finding the good in any situation can help you to find happiness in some of the simplest and unexpected places.

Keep Reading...Show less

Remember The True Meaning of Christmas

“Where are you Christmas? Why can’t I find you?”

A painting of the virgin Mary, the baby Jesus, and the wise men

It’s everyone’s favorite time of year. Christmastime is a celebration, but have we forgotten what we are supposed to be celebrating? There is a reason the holiday is called Christmas. Not presentmas. Not Santamas. Not Swiftmas. Christmas.

boy standing in front of man wearing santa claus costume Photo by __ drz __ on Unsplash

What many people forget is that there is no Christmas without Christ. Not only is this a time to spend with your family and loved ones, it is a time to reflect on the blessings we have gotten from Jesus. After all, it is His birthday.

Keep Reading...Show less
Golden retriever sat on the sand with ocean in the background
Photo by Justin Aikin on Unsplash

Anyone who knows me knows how much I adore my dog. I am constantly talking about my love for her. I attribute many of my dog's amazing qualities to her breed. She is a purebred Golden Retriever, and because of this I am a self-proclaimed expert on why these are the best pets a family could have. Here are 11 reasons why Goldens are the undisputed best dog breed in the world.

Keep Reading...Show less

Boyfriend's Christmas Wishlist: 23 Best Gift Ideas for Her

Here are the gifts I would like to ask my boyfriend for to make this season unforgettable.

Young woman opening a Christmas gift

Recently, an article on Total Sorority Move called 23 Things My Boyfriend Better Not Get Me For Christmas, was going around on social media. I hope the author of this was kidding or using digital sarcasm, but I am still repulsed and shocked by the lack of appreciation throughout this article. I would like to represent the girlfriends out there who disagree with her standpoint -- the girlfriends who would be more than happy to receive any of these gifts from their boyfriends.

Keep Reading...Show less
Two teenage girls smiling

The 2000s were a time that many young adults today can look back on, joyfully reminisce and somewhat cringe at the trends and the fads that we all used to love and adore. Here's a list of things from the golden 2000s that will have one feeling nostalgic about all of those times.

Keep Reading...Show less

Subscribe to Our Newsletter

Facebook Comments