By knowing the value of the bond energy, we can predict if a chemical reaction will be exothermic or endothermic. If the bonds in the products are stronger than the bonds in the reactants, then the products are more stable and the reaction will give off heat (exothermic).
Exothermic chemical reactions release energy as heat, light, electrical or sound (or all four). Usually when someone says it’s an exothermic reaction, they usually just mean energy is being released as heat.
Some release heat gradually (for example, a disposable hand-warmer), while others are more explosive (like burning magnesium). The energy comes from breaking the bonds within the chemical reaction.
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Other chemical reactions will just sit there and do nothing, unless you add energy to it first. These types of reactions need to absorb energy in order to react, so you’ll notice a temperature drop when the reaction takes place (a disposable ice pack, for example, is a chemical reaction that takes place using the energy from the water, so it makes the water colder when it uses this energy).
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