2.4 Chemical Reactions KEY CONCEPT Life depends on chemical reactions.

2.4 Chemical Reactions Bonds break and form during chemical reactions. Chemical reactions change substances into different ones by breaking and forming chemical bonds. –Reactants are changed during a chemical reaction. –Products are made by a chemical reaction. 6O 2 + C 6 H 12 O 6 6 CO 2 + 6H 2 O REACTANTS PRODUCTS glucoseoxygencarbon dioxidewater

2.4 Chemical Reactions Bonds have different strengths: some are strong, some are weak. –Energy is added to break bonds. –Energy is released when bonds form. Does this reaction ultimately release energy or use energy? Bond energy is the amount of energy to break a bond. 6O 2 + C 6 H 12 O 6 6 CO 2 + 6H 2 O

2.4 Chemical Reactions A reaction is at EQUILIBRIUM when reactants and products form at the same rate. CO 2 + H 2 O H 2 CO 3 Reactions can be a two-way street: Sometimes products convert back to reactants Which way the reaction moves depends on the conditions CO2 is a waste product in your body. It forms carbonic acid (H 2 CO 3 ) with water. Outside your cells there is a lot of CO 2 (cells respire), so H 2 CO 3 is formed. In your lungs there is little CO 2 (aah, fresh air!), so H 2 CO 3 is broken down.

2.4 Chemical Reactions Chemical reactions release or absorb energy, but most reactions need some energy to get started. Activation energy is the amount of energy that needs to be absorbed to start a chemical reaction.

2.4 Chemical Reactions Exothermic reactions release more energy than they absorb. –Reactants have higher bond energies than products. –Excess energy is released by the reaction. exo = out therm = heat Exothermic reaction releases heat/energy

2.4 Chemical Reactions Endothermic reactions absorb more energy than they release. –Reactants have lower bond energies than products. –Energy is absorbed by the reaction to make up the difference. endo = in therm = heat Endothermic reaction takes in heat/energy