KEY CONCEPT Life depends on chemical reactions.

Question When you hear the word chemical reaction, what comes to mind? Talk to your neighbor and give examples

Bonds break and form during chemical reactions. Chemical reactions change substances into different substances by breaking and forming chemical bonds. Ex. The demonstration in class and your body uses oxygen to break down glucose for energy. There are two parts to a chemical reaction: 1) Reactants: are the substances that are changed during a chemical reaction. (Left side of equation) 2) Products are the substances that are made by a chemical reaction. (Right side of the equation) We represent chemical reactions in the form of formulas, like this one:

Bond energy: is the amount of energy that is needed to break a bond. Energy is added to break bonds. Energy is released when bonds form. A reaction is at equilibrium when reactants and products form at the same rate. CO2 + H2O H2CO3

Chemical reactions release or absorb energy. All chemical reactions involve changes in energy In chemical reactions E is absorbed or released Weather a reaction releases or absorbs E depends on the bond E Activation energy is the amount of energy that is needed to start a chemical reaction.

There are two types of reactions that occur: Exothermic reactions: release more energy than they absorb. (Exo = outside) If the products have a lower bond E than the reactants, the reaction is exothermic. Excess energy is released by the reaction. (heart or light)

2) Endothermic reactions: absorb more energy than they release 2) Endothermic reactions: absorb more energy than they release. (Endo = inside) If the products have a higher bond E than the reactants, the reaction is endothermic. Energy is absorbed by the reaction.