Unit 5: Thermochemistry Bond Energy

Bond Energy Breaking chemical bonds requires energy Forming new chemical bonds releases energy

Calculating Enthalpy from Bond Energy Recall that enthalpy (H) is the heat energy absorbed or lost in a reaction To determine H from bond energies: H = ΣHbonds broken – ΣHbonds formed H = ΣHreactant bonds– ΣHproduct bonds Used when H of the entire molecule is not known – this is just an approximation (although a good one)

Calculating Enthalpy from Bond Energy How do you use the bond energies to calculate the enthalpy? You must draw Lewis structures for each molecule! Determine the total number of valence electrons Make sure each atom has 8 valence electrons, either through lone pairs or bonds Remember, hydrogen only has 2 valence electrons

Bond Energy Example #1 2 H2(g) + O2(g)  2 H2O(g) Determine H for the following reaction: 2 H2(g) + O2(g)  2 H2O(g)

Bond Energy Example #1 2 H2(g) + O2(g)  2 H2O(g) Determine H for the following reaction: 2 H2(g) + O2(g)  2 H2O(g)

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) Bond Energy Example #2 Calculate ΔH for the following reaction: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

Bond Energy Exit Slip 6H2 (g) + N2 (g) → 2NH3 (g) Calculate ΔH for this reaction: 6H2 (g) + N2 (g) → 2NH3 (g)