Combustion reactions release heat. Where does the heat come from?
Heat is a form of energy.
The total energy in the universe is constant, and the heat is released as a consequence of the chemical changes that occur during combustion.
The reactants and products of a chemical change have the same numbers and types of atoms, but the atoms are linked by different types of chemical bonds in the products. A chemical reaction can be viewed as breaking the bonds in the reactants and forming different bonds in the products.
Breaking of chemical bonds consumes energy. Formation of chemical bonds releases energy.
The energy consumed or released depends on the atoms in the chemical bond and the type of chemical bond.
Strong bonder bonds release more energy when they are formed and require more energy to break.
The overall energy change for a reaction depends on how much energy is consumed in breaking the bonds and then released in re-making the bonds.
Heat is a form of energy.
The total energy in the universe is constant, and the heat is released as a consequence of the chemical changes that occur during combustion.
The reactants and products of a chemical change have the same numbers and types of atoms, but the atoms are linked by different types of chemical bonds in the products. A chemical reaction can be viewed as breaking the bonds in the reactants and forming different bonds in the products.
Breaking of chemical bonds consumes energy. Formation of chemical bonds releases energy.
The energy consumed or released depends on the atoms in the chemical bond and the type of chemical bond.
Strong bonder bonds release more energy when they are formed and require more energy to break.
The diagram is typical for a combustion reaction.
More energy is released on forming the bonds in products than was consumed in breaking the bonds.
The difference between the two (shown by the double-headed arrow) is released as heat.
More energy is released on forming the bonds in products than was consumed in breaking the bonds.
The difference between the two (shown by the double-headed arrow) is released as heat.
E | Energy consumed in breaking bonds |  reactants |  products | Energy released in making bonds |
heat released |  |