Energy and Chemical Reactions Using Bond Enthalpies to calculate energy changes in a reaction 1

Bond Enthalpies The energy to required to break a covalent bond in the gaseous phase is called a bond enthalpy. Bond enthalpy tables give the average energy to break a chemical bond. Actually there are slight variations depending on the environment in which the chemical bond is located (see page 307) 2

Bond Enthalpy Table The average bond enthalpies for several types of chemical bonds are shown in the table below: 3

Bond Enthalpies By comparing the energies for breaking bonds in the reactants and forming bonds in the products one can approximate the enthalpy change for a reaction 4

Bond Enthalpies Energy is required to break chemical bonds (endothermic). Therefore when a chemical bond is broken it carries a positive value Energy is released when chemical bonds form (exothermic). When chemical bonds are formed their energy change carries a negative value 5

Exothermic reaction: more energy is released in forming the bonds than in breaking the bonds.

The endothermic reaction absorbs more energy breaking bonds than it releases in bond formation.

Bond Enthalpy Calculations Example 1: Calculate the enthalpy change for the reaction N2 + 3 H2  2 NH3 1. Draw the structural formulas 2. List the number and types of each kind of bond broken and formed 3. Bond breaking is endothermic, so add these 4. Bond formation is exothermic, so subtract these 5. Add the result to find the enthalpy of the overall reaction. 8

Bonds broken N=N = 945 H-H 3(435) = 1305 Total = 2250 kJ Bonds formed 2x3 = 6 N-H 6(390) = - 2340 kJ Net enthalpy change = + 2250 - 2340 = - 190 kJ

Another problem Example 2: Calculate the enthalpy change for the reaction O2 + 2 H2  2H2O

Add bonds broken and subtract bonds formed Bonds broken (add) O = O 495 2 H-H 2(432) Bonds formed (subtract) 4 H-O 4(467) ∆H = -509 kJ

Homework Read section 5.3 Page 312 #1-4 Page 313 #1-3