Using the bond-energy method, if 100 kJ of bonds are broken and 80 kJ of bonds are formed, what is the energy change?

• A. -20 kJ
• B. 20 kJ
• C. 0 kJ
• D. 180 kJ

Answer: (B)

Bond-energy calculations compare energy needed to break bonds with energy released when bonds form. Net energy change = energy broken minus energy formed. Here, 100 kJ are required to break bonds and 80 kJ are released when bonds form, so the net change is 100 − 80 = +20 kJ. The positive sign means the system absorbs energy (endothermic). The other options don’t fit because they would imply different balances: a negative value would mean more energy is released than broken, zero would require equal energies, and 180 kJ would come from simply adding the two amounts, which isn’t how the energy change is defined.