Energy in chemical reactions Chapter 3.3

Chemical reactions release or absorb energy Bond Energy: the energy associated with chemical bonds. Breaking a bond between atoms requires a certain amount of energy.

Exothermic reaction If more energy is released when the products form than is needed to break the bonds in the reactants, then energy is released during the reaction. Exothermic Reaction: a reaction in which energy is released

Exothermic reactions Produce an increase in temperature Energy is released as heat or light Combustion reactions are exothermic Glow sticks, fireflies, some species of fish and jellyfish

Endothermic reaction If more energy is required to break the bonds in the reactants than is released when the products from, then energy must be added to the reaction. Endothermic Reaction: a reaction in which energy is absorbed

Endothermic reactions Produce a decrease in temperature Energy is absorbed from the surroundings Photosynthesis, where plants absorb the energy from the sun to make sugar

Exothermic Reaction Reactants  Products + Energy Endothermic Reaction Reactants + Energy  Products

Exothermic and endothermic work together If an exothermic reaction goes in the opposite direction, it becomes an endothermic reaction, and vise versa. The energy in sunlight is stored by endothermic reactions. When it is needed, it is released by exothermic reactions. This forms a cycle of energy storage and use on Earth.