Shiken premium Upgrade Banner

Chemical reactions involve something called bond-making and breaking. This process needs energy to work. Sometimes, the energy needed to break the old bonds is different from the energy needed to make new bonds. This difference in energy is either released or absorbed by the reaction. This is what we call energetics. It's a way of studying how energy flows in a chemical process. So, what makes a reaction release or absorb energy? Keep reading to find out!

In this article, we're going to talk about the energetics of chemical reactions. We'll explain the difference between enthalpy and heat. You'll learn about enthalpy change, bond enthalpy, calorimetry, and Hess’ Law. We'll also talk about the energetics of nuclear reactions. Finally, we'll explain energetic efficiency and quality. If you're interested in how energy works in chemical reactions, this article is for you! And don't worry, we'll explain everything in simple terms so it's easy to understand.

What is energetics in chemistry?

Energetics is a part of chemistry that focuses on energy changes in chemical reactions, while thermodynamics looks at how these changes affect a chemical system's properties. There are four laws of thermodynamics, but we'll only talk about the first one here: energy cannot be created or destroyed, only converted from one form to another.

There are different types of energy, but in chemical energetics, we're only concerned with chemical energy and heat energy. Chemical energy is stored in bonds between atoms, while heat energy is released or absorbed when bonds are broken or made.

When a chemical process takes place, the chemical energy stored in the bonds between atoms often converts to heat energy that gets released. A reaction will also absorb heat energy from the surroundings to break the bonds in the compounds. By looking at the overall balance of the absorption and evolution of heat energy in a chemical reaction, we can tell whether the process is endothermic or exothermic. If you want to learn more about the different types of energy, you can check out thermodynamics.

What are endothermic and exothermic reactions?

When bonds break in a chemical reaction, heat energy is absorbed from the surroundings. Conversely, when new bonds form, heat energy is released. If more bonds are broken than formed, the overall result is an absorption of energy. We call these reactions endothermic. On the other hand, if more bonds are formed than broken, there is an overall release of energy, and we call these reactions exothermic.

In other words, endothermic reactions absorb energy, while exothermic reactions release energy. This all depends on the ratio of bonds broken to bonds formed. Breaking bonds requires energy and is an endothermic process, while forming bonds releases energy and is an exothermic process. Later, we'll look at enthalpy diagrams to understand the balance between energy release and absorption.


Energetics of bond-making and bond-breaking
Energetics of bond-making and bond-breaking

In energetics, you will learn how heat energy transfers in and out of a chemical reaction. More importantly, you will discover enthalpy. What is enthalpy? What is so special about it and how is enthalpy different from heat? Let us explore further.

What is enthalpy?

Enthalpy (H) is a measure of the thermal energy stored in a system, expressed in kJ mol-1. In energetics, we refer to the reaction mixture as the system, while everything outside of it is the surroundings. When a reaction absorbs energy, the total enthalpy of the system increases, and when it releases energy, the total enthalpy decreases. We can't directly measure the total enthalpy in a system, but we can measure changes in enthalpy that occur during a chemical reaction.

Enthalpy change (∆H) is the amount of heat energy that is either absorbed or released during a chemical reaction while the pressure remains constant. We use the Greek letter delta '∆' to signify 'change in.' Enthalpy is a state function variable, which means its value remains the same regardless of how we get there. This allows us to calculate enthalpy change for different routes of a reaction, as long as we start with the same reactants and end up with the same products. Enthalpy and heat are often used interchangeably. However, they're not exactly the same. Heat is a form of energy that flows between systems due to a temperature difference, while enthalpy is the thermal energy stored in a system. Heat is energy in motion, while enthalpy is an essential part of a system. Heat comes and goes and is not a fundamental part of a system, but enthalpy is. When heat enters or leaves a system, it results in a change in enthalpy. At a constant pressure, heat flow directly relates to enthalpy. This is why we can calculate the change in enthalpy if we know the value of heat, which we can measure using calorimetry.

Enthalpy diagrams

Enthalpy diagrams, also known as energy diagrams, are used to illustrate how enthalpy changes during a chemical reaction.

We draw the reactants and products at different heights that correspond to their energy levels, and use arrows to show the bond-breaking and bond-making processes. When bonds break, energy is absorbed, so we draw an upward arrow. Meanwhile, when bonds form, energy is released, so we draw a downward arrow. Enthalpy, as a measure of energy, is equivalent to energy change. The balance between bond-making and bond-breaking determines whether a reaction is endothermic or exothermic, which we can see clearly in an enthalpy diagram. Enthalpy diagrams are useful because they allow us to quickly determine whether a reaction is endothermic or exothermic.


How to draw an enthalpy diagram
How to draw an enthalpy diagram

To illustrate this further, have a look at another enthalpy diagram below. It shows the combustion of methane (CH4 + 2O2 → CO2 + 2H2O). We can see from the energy diagram that the energy released in the formation of new bonds is greater than the energy absorbed to break bonds. That makes the combustion of methane an exothermic reaction.


Enthalpy diagram- the combustion of methane
Enthalpy diagram- the combustion of methane

Bond enthalpy, or bond energy, is the amount of energy required to break one mole of a specific covalent bond in the gas phase. In an energy diagram, the energy absorbed to break a bond is the bond enthalpy.

When a bond is formed, the same amount of energy is released as when it was broken. We can use bond enthalpies to calculate the change in enthalpy during a reaction. To do so, we use an average of all the bond enthalpies of the same bond in different environments, called the mean bond enthalpy. Using mean bond enthalpies in calculations allows us to predict the enthalpy change of a reaction based on the bonds that are formed and broken. You can learn more about bond enthalpy in the Bond Enthalpy topic.


We have mentioned before that if we know the value of q (heat flow), we can calculate the enthalpy change in a reaction.

Calorimetry is a method to measure the enthalpy changes that occur during a chemical reaction.

Simply seal the reacting chemicals inside the container with a thermometer to record any temperature change. We can use this change in temperature to calculate the enthalpy change of the reaction by using the following equation:

q = mcΔT

q is the energy transferred, measured in joules (J). m is the mass of the water in grams (g).c is the specific heat capacity (J g-1 ºC-1).ΔT is the temperature change measured in Kelvin (K) or degree Celsius (ºC).


A simple calorimeter
A simple calorimeter

Specific heat capacity (c) is the energy required to increase the temperature of 1g of a given liquid by 1 °C. For example, water has a specific heat capacity of 4.18 Jg-1 ºC-1. You can learn more about specific heat in Thermal Physics.

You can learn how to conduct a calorimetry experiment and use the results to calculate enthalpy change in Calorimetry. Sometimes it is not possible to use calorimetry to calculate enthalpy changes. When we have this problem we use Hess' Law.

What is Hess’ law?

Hess' Law is a fundamental principle in thermodynamics that states that the total enthalpy change for a reaction is independent of the route of the chemical reaction. This means that the change in enthalpy is a state function that is pathway independent. If we know an alternate route for a reaction, we can figure out the enthalpy change using Hess' Law.

For example, if we have a chemical reaction from reactants A to products B, we can go directly from A to B or go through an intermediate, C. According to Hess' Law, the enthalpy change in reaction A → B is equal to the sum of enthalpy changes in reaction A → C and C → B. This principle can also be applied to physical processes, not just chemical reactions. Now, let's explore the energetics of nuclear reactions and energetic efficiency.

Energetics of nuclear reactions

In nuclear reactions, there is a visible change in mass that occurs alongside energy changes. This relationship between mass and energy is explained by Albert Einstein's equation E = mc2, which is also known as the mass-energy equivalence. The constant c2 (the speed of light squared) links the changes in mass and energy.

In nuclear reactions, the energy changes are much larger than those in chemical reactions, and the mass change is observable. We can use this mass change to calculate the energy changes in nuclear reactions using the relationship between an atomic mass unit (AMU) and megaelectronvolts (MeV).

When a nucleus forms, energy is released as nuclear binding energy, which is the energy needed to bind protons and neutrons together to make the nucleus. The loss in mass that occurs during nuclear reactions is referred to as mass defect, where the mass of individual subatomic particles is more than that of the nucleus. To learn more about nuclear binding energy and mass defect, you can visit the Binding Energy topic.

Energetic efficiency

Energy is a fundamental aspect of our world, and it can be converted into various forms like movement, heat, electricity, and sound. However, what we really want energy to do is work, and this can be anything from moving a car to baking a cake. Getting energy to do the work we desire is not always easy, as some of the energy input may be lost as wasted energy, such as sound or heat.

Energy efficiency is a measure of how much of the energy supplied to a system or reaction gets converted to the type of work energy we want it to do. Energy efficiency is expressed as a percentage ratio of the energy output to the energy input in a given system. Efficient processes are more sustainable since they use fewer resources, produce less waste, and are cheaper.

Chemists strive to make their reactions more efficient since it is beneficial in terms of cost, sustainability and resource usage. One example of an energy-efficient process in organic synthesis is the metathesis method. This method uses a catalyst that is energy-efficient to promote high atom economy reactions at standard pressure and temperature. You can learn more about metathesis in Organic Synthesis.

Energy Quality

Energy quality is a measure of how easily we can convert one form of energy to another. High-quality energy is energy that can be easily converted to do work, while low-quality energy is energy that cannot be easily used to do work. The perception of energy quality depends on what we want the energy to do.

For example, electrical energy is considered high-quality energy because of its efficiency and ability to be converted into other forms of energy. LED bulbs are high-quality energy light bulbs because they convert electrical energy more efficiently into light energy. On the other hand, old-fashioned filament bulbs are low-quality energy light bulbs because most of the energy supplied to them gets dispersed as heat energy.

Some scientists define high-quality energy as energy that can be transformed into other kinds of energy without much loss, while others define it as energy that can be used to do high-quality tasks. Heat energy from burning fuel is considered low-quality energy by some scientists because it can only do simple things like boil water. In summary, energy quality is a subjective concept that depends on the desired outcome and purpose of the energy.

In energetics, energy is either absorbed or released in chemical reactions. Endothermic reactions absorb energy when more bonds are broken than made, while exothermic reactions release energy when more bonds are made than broken. Bond enthalpy is the amount of energy required to break one mole of a covalent bond in the gas phase, and it can vary from compound to compound due to the effects of other atoms in the molecule. Mean bond enthalpies are used to account for this variation.

Enthalpy change (∆H) is the amount of heat energy absorbed or released in a chemical reaction under constant pressure. For endothermic reactions, ΔH is positive, and for exothermic reactions, ΔH is negative. Calorimetry is a method to measure the enthalpy changes that occur during a chemical reaction by measuring the change in temperature. The enthalpy change can be calculated using the equation: q = mcΔT. Hess' law states that the total enthalpy change for a reaction is independent of the route taken. This means that the enthalpy change for a reaction can be calculated by adding the enthalpy changes of the steps involved in the reaction, regardless of the intermediate steps taken. Overall, energetics is concerned with the energy changes that occur during chemical reactions, and understanding these changes is important in fields such as chemistry, biochemistry, and materials science.


What is energetics in chemistry?

In chemistry, energetics is what we call the study of the flow of energy in and out of a chemical reaction.

What topics are included in energetics? 

In A-Level chemistry, energetics includes: Enthalpy changes Hess’ Law Calorimetry Bond enthalpy

Is energetics part of a broader field within chemistry?

Yes, it is part of thermodynamics, or thermochemistry.

What is calorimetry?

Calorimetry is a method to measure the enthalpy changes that occur during a chemical reaction, by measuring the change in temperature.

What happens to energy in a chemical reaction?

Energy is either absorbed or released in a chemical reaction.

Join Shiken For FREE

Gumbo Study Buddy

Explore More Subject Explanations

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
Join 20,000+ learners worldwide.
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime