Some things make heat when they change.
This heat can feel very warm. It can even make fires. Hand warmers use this to keep us warm. It is a neat way to use energy. Do you like being warm?
Some things make heat when they change.
This heat comes from stored energy. Burning gas is one way this happens.
Hand warmers also use this to stay warm. They use iron to make heat. Some changes can even make fires. This can be very bright and hot. It is a way to release energy. It is a very powerful thing.
Some changes make heat. Scientists call these exothermic reactions. In these reactions, stored energy is let out as heat.
You can see this when fuel burns. This is called combustion. Burning natural gas is one example.
When things burn, they release power. This can be very strong. Sometimes, these changes cause fires or explosions.
Nature uses these changes in a smart way. Living things use a process called respiration. This lets out energy to make ATP. ATP helps life work.
You can use this science at home, too. Hand warmers use iron to make heat. They use a set of steps called oxidation. This makes the warmers feel hot.
Scientists measure this heat in a lab. They use a tool called a calorimeter. This tool tracks how much heat moves. They often measure the change in enthalpy. Enthalpy is a way to talk about energy. In these reactions, the enthalpy change is negative. This means energy left the system.
Some chemical changes release energy. Scientists call these exothermic reactions. In these reactions, stored energy is let out as heat.
This happens because the energy in the starting materials is higher than in the end products. The extra energy must go somewhere. It usually leaves the system as heat. This is why many reactions feel hot to the touch.
Most spectacular classroom demonstrations are exothermic. They are often also exergonic. This means they also release Gibbs energy.
How does this work step by step? First, the reactants start with a certain amount of enthalpy. Enthalpy is basically the energy in a chemical system. During the reaction, the substances change into new products. These products have less enthalpy than the original reactants. The difference between these two amounts is the enthalpy change, or ΔH. In an exothermic reaction, this change is always a negative value. This negative value shows that heat moved out of the system.
Scientists use special tools to study these changes. They use a device called a calorimeter. One type is a bomb calorimeter. They also use a reaction calorimeter to monitor heat flow. These tools track the heat moving in or out of a vessel. This happens at a constant pressure. Scientists measure the energy in Joules per mole. This helps them see exactly how much heat was released.
There are many real examples of this science. The thermite reaction is a famous one. It uses iron(III) oxide and aluminium to make heat. This reaction is so strong it can melt iron.
Another example is the burning of hydrogen. When hydrogen burns, it releases 483.6 kJ/mol. Combustion of natural gas is also exothermic. This involves methane and oxygen.
Even hand warmers use this science. They use the oxidation of iron to create warmth.
You can see these reactions in many places. Nature uses them in a very controlled way. Living things use aerobic respiration to release energy. This process helps create ATP for life.
This is different from a fire or an explosion. Uncontrolled reactions like those can be wasteful. It is hard to capture all that released energy. When you use a hand warmer, you are using a safe version. It is a way to use chemistry to stay warm.
An exothermic reaction is a chemical process that releases energy. This energy usually leaves the system in the form of heat. In the field of thermochemistry, scientists define these reactions by their enthalpy change. Enthalpy is essentially the total energy contained within a chemical system. For an exothermic reaction, the standard enthalpy change, written as ΔH, is negative. This negative value tells us that the system lost energy to its surroundings.
To understand how this happens, we must look at the energy levels of the substances involved. Every reaction begins with reactants, which are the starting materials. These reactants possess a specific amount of enthalpy. As the reaction proceeds, the reactants transform into new substances called products. In an exothermic reaction, the products have less enthalpy than the original reactants. Because the products hold less energy, the extra energy must be released. This released energy is what we feel as heat.
Scientists often distinguish between exothermic and exergonic reactions. An exothermic reaction focuses on the change in enthalpy, or ΔH. An exergonic reaction is defined by the International Union of Pure and Applied Chemistry (IUPAC) through the Gibbs energy change, written as ΔG. This change represents the overall energy available to do work. Most strongly exothermic reactions are also exergonic. This occurs because the enthalpy change makes a major contribution to the Gibbs energy change.
There are many different types of exothermic reactions in our world. Combustion is a very common type, which involves burning a fuel. For example, the combustion of a hydrocarbon like methane is exothermic. This reaction involves methane reacting with oxygen to produce carbon dioxide and water. Another famous example is the thermite reaction. In this process, aluminium reduces iron(III) oxide. This reaction releases enough heat to produce molten iron.
Hand warmers also use this science by using the oxidation of iron to create warmth.
We can observe these energy changes through various chemical equations. When hydrogen burns, the reaction is 2H2 (g) + O2 (g) → 2H2O (g). This specific reaction has a ΔH of −483.6 kJ/mol. The negative sign confirms it is exothermic. For reactions involving gases, scientists can approximate the enthalpy change using bond energies. They do this by subtracting the total bond energy of the products from the total bond energy of the reactants.
Measuring these energy shifts requires precise laboratory instruments. Scientists use a process called calorimetry to measure heat production or absorption. One common tool is the bomb calorimeter, which is used to study reactions in a closed system. Another instrument is the reaction calorimeter. This device monitors the heat flow into or out of a reaction vessel at constant pressure. The energy change is measured in Joules per mole. This allows researchers to calculate the standard enthalpy change accurately.
Exothermic reactions are vital to both technology and life. Nature uses combustion in a highly controlled way through aerobic respiration. This process allows living things to capture released energy to form ATP. This is very different from uncontrolled reactions like fires or explosions. Uncontrolled reactions are often considered wasteful because the energy is hard to capture. By studying these processes, we learn how to better manage energy in everything from fuels to biological systems.
Where will your curiosity go next?
🖼️ Images & Media (3)
More to explore
✨ What else?
Related topics you might enjoy
🔬 Go deeper
More advanced topics to explore
🪜 Step back
Simpler topics to build understanding
What is Nepedia?
A free, ad-free encyclopedia for children. Every article is written at five reading levels, so the same page works for a five-year-old and a fifteen-year-old — use the level switcher above to see this one change. No account needed to read.