Exothermic and Endothermic Reactions

Exothermic reactions An exothermic reaction is a chemical change that releases energy. Where does the energy that is being released come from? The energy released comes from the breaking of the bonds of the reactants. Every time a bond is broken, energy is released. Every time a bond is formed, energy is consumed. The energy consumed to form a bond is equal to the energy released when it is broken. The release or absorption of energy is proportional to the number of bonds that need to be broken or formed. Thus, single bonds will require/release less energy than double bonds and double bonds will require/release less energy than triple bonds. See Table 4.12 and 4.13 (page 115) Type of bond and Energy

Exothermic reactions

 Activation energy: Energy needed for the reaction to take place.  Difference of energy for these processes (Endo and Exothermic reactions) refers to initial (Reactants) and final (Products) states.  Energy is released because the energy consumed by the formation of the bonds of the products is lower than that released from the breaking of the bonds of the reactants.

Endothermic reactions An endothermic reaction is a chemical change that absorbs energy. Energy is absorbed because the energy consumed by the formation of the bonds of the products is higher than that released from the breaking of the bonds of the reactants.

Endothermic reactions

Energy of the reaction CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (g) CH 4 : 4 (C-H bonds) = 4 x (414 kJ) = 1656 kJ O 2 : 2 (O=O bonds) = 2 x (498 kJ) = 996 kJ Energy required to break the bonds of the reactants: 2652 kJ (Activation Energy) CO 2 : 2 (C=O bonds) = 2 x (741 kJ) = 1482 kJ H 2 O: 2 (2 O-H bonds) = 4 x (464 kJ) = 1856 kJ Energy released by the formation of the products: 3338 kJ Reaction Energy: E (absorbed by reactants) – E (released by products) Reaction Energy: 2652 kJ – 3338 kJ Reaction Energy: -686 kJ

Energy of the reaction Graphing Energy of the reaction: 0 E 2652 t R P EaEa

Activation Energy