Exothermic reactions are common in daily life and involve the release of heat energy. Some typical everyday examples include:
In each case, heat is released as a product, making it easy to relate to real-life exothermic processes.
Endothermic reactions absorb heat from their surroundings. A common example in real life is:
These processes all involve heat absorption and feel cold to the touch.
Exothermic reactions have many practical uses because they release useful heat energy. They are commonly used in:
These examples show how exothermic processes are vital for everyday comfort, industry, and biology.
Respiration is an exothermic reaction used by all living things to release energy from food. In this process:
The equation for cellular respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (Heat)
Here are 10 exothermic reaction examples with their chemical equations:
Each reaction releases heat energy as a product.
Exothermic reactions release heat energy to their surroundings, while endothermic reactions absorb heat from the environment.
You can easily distinguish them by the energy flow and change in temperature in the surroundings.
Yes, all combustion reactions are exothermic because they involve burning a substance in oxygen and always release heat and sometimes light.
This is why combustion is used for heating and power generation.
You can identify an exothermic reaction by observing a rise in temperature or heat during the process.
This confirms the release of energy in exothermic processes.
Dissolving common salt (NaCl) in water is generally an endothermic process. The solution absorbs heat from the surroundings, making it feel cold. However, dissolving other salts (like CaCl2) can be exothermic.
Sometimes the heat released in an exothermic reaction is spread quickly or not concentrated, so the object may actually feel cool or neutral to touch. Factors include:
This is why not every exothermic process feels hot in practice.
When making a new substance from other substances, chemists say either that they carry out a synthesis or that they synthesize the new material. Reactants are converted to products, and the process is symbolized by a chemical equation. For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). Fe(s) + S(s) → FeS(s) The plus sign indicates that iron reacts with sulfur. The arrow signifies that the reaction “forms” or “yields” iron sulfide, the product. The state of matter of reactants and products is designated with the symbols (s) for solids, (l) for liquids, and (g) for gases.
Balancing atoms in chemical reactions explained Two people making change for five dollars as a representation of how matter is conserved in a chemical reaction.
Chemists ordinarily work with weighable quantities of elements and compounds. For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams of iron sulfide. Because matter is not created or destroyed in a chemical reaction, the total mass of reactants is the same as the total mass of products. If some other amount of iron is used, say, one-tenth as much (5.585 grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of sulfur were initially present with 5.585 grams of iron, then 28.859 grams of sulfur would be left over when the reaction was complete. The reactant that is completely consumed first and that therefore limits the amount of product that can be formed is known as the limiting reagent (or limiting reactant).
The reaction of methane (CH4, a major component of natural gas) with molecular oxygen (O2) to produce carbon dioxide (CO2) and water can be depicted by the chemical equation CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Here another feature of chemical equations appears. The number 2 preceding O2 and H2O is a stoichiometric factor. (The number 1 preceding CH4 and CO2 is implied.) This indicates that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equation is balanced because the same number of atoms of each element appears on both sides of the equation (here one carbon, four hydrogen, and four oxygen atoms). Analogously with the iron-sulfur example, we can say that 16 grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide and 36 grams of water. That is, 80 grams of reactants will lead to 80 grams of products.
The ratio of reactants and products in a chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that matter is conserved in chemical processes, and calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is called the molar mass) and represents 6.022140857 × 1023 atoms (Avogadro’s number). One mole of iron contains 55.847 grams; one mole of methane contains 16.043 grams; one mole of molecular oxygen is equivalent to 31.999 grams; and one mole of water is 18.015 grams. Each of these masses represents 6.022140857 × 1023 molecules.