Breaking bonds between atoms or ions requires energy, making it an endothermic process. bonded particles + energy → separated particles Bond energy is the energy required to break a chemical bond. The stronger the bond, the greater the energy needed to break it. Forming bonds between atoms or ions releases energy, making it an exothermic process. separated particles → bonded particles + energy Bond energy is also the energy released when a bond is formed. The stronger the bond, the greater the energy released.

Endothermic Reactions 2 H2O(l) → 2 H2(g) + O2(g) ΔrH° = +571.6 kJ In any endothermic reaction, the energy required to break bonds (2 O─H bonds in H2O) is greater than the energy released when bonds are formed (O═O and H─H). breaking > making rH is positive Exothermic Reactions H2(g) + Cl2(g) → 2 HCl(g) ΔrH° = –184.6 kJ In any exothermic reaction, the energy required to break bonds (H─H and Cl ─Cl) is less than the energy released when bonds are formed (H─Cl). making > breaking rH is negative

Energy & Bonding If two atoms are chemically bonded, energy must be added to the atoms to break them apart. The energy added is stored in the separated atoms as potential energy.

Breaking bonds is endothermic! E.g., H2(g) + energy  2 H(g) Breaking bonds is endothermic! + Energy  Bond is broken! Energy stored!

When two atoms form a chemical bond, energy is released. Potential energy stored in the separated atoms is converted to kinetic energy as the bonds form. (The atoms get hotter!)

E.g., H(g) + F(g)  HF(g) + Energy Bond formation is exothermic! Bond forms: Energy released! Atoms get hot! 9p+ +  Cooler!

E.g., The Reaction * H2(g) + Energy  2 H(g) * Cl2(g) + Energy  2 Cl(g) **2 H(g) + 2 Cl(g)  2 HCl(g) + Energy * Endothermic! ** Exothermic!

Energy is absorbed in order to break the H-H and Cl-Cl bonds, but more energy is released when H-Cl bonds form. Therefore, the reaction is exothermic.

Homework: Read pgs. 532 – 534 pg. 534 Section 12.2 Questions #’s 1 – 9