Hydrogen reacts with chlorine to give hydrogen chloride. Bond energies are 436 kJ for H to H, 243 kJ for Cl to Cl and 432 kJ for H to Cl, all per mole. What is the enthalpy change?

1. Making two moles of hydrogen chloride means breaking one mole of H to H and one mole of Cl to Cl bonds. 2. Energy in for breaking is 436 plus 243, which is 679 kilojoules. 3. Two moles of H to Cl bonds then form, releasing 2 times 432, which is 864 kilojoules. 4. The enthalpy change is bonds broken minus bonds formed, so 679 minus 864. 5. That gives minus 185 kilojoules, and the negative sign marks an exothermic reaction. 6. Plus 185 kilojoules comes from subtracting in the wrong order, which flips a real release into a false absorption. 7. Minus 247 kilojoules comes from forming only one mole of H to Cl bonds instead of two. 8. Plus 1111 kilojoules comes from adding all three bond energies rather than subtracting the ones formed. _Source: OpenStax Chemistry (CC BY 4.0), Chs 7 and 17, section 7.5 Strengths of Ionic and Covalent Bonds_