Bond Energy Calculations Flashcards

All 6 cards in this deck

What is the formula for the overall energy change of a reaction from bond energies?

Energy change = (total energy to break bonds in reactants) − (total energy released making bonds in products).

What are the steps to calculate the energy change of a reaction from bond energies?

e.g. $\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$; H–H = 436, Cl–Cl = 244, H–Cl = 431 kJ mol$^{-1}$

1. Energy to break bonds: $436 + 244 = 680$ kJ mol$^{-1}$.
2. Energy released making bonds: $2 \times 431 = 862$ kJ mol$^{-1}$.
3. Energy change $= 680 - 862 = -182$ kJ mol$^{-1}$ (exothermic).

What does the sign of a calculated energy change tell you about a reaction?

Negative = exothermic (energy released); positive = endothermic (energy taken in).

What units are used for bond energies and for the energy change of a reaction?

kJ mol$^{-1}$.

What are the steps to find an unknown bond energy from the overall energy change?

e.g. $\mathrm{H_2 + Cl_2 \rightarrow 2HCl}$, energy change $-180$ kJ mol$^{-1}$; H–H = 436, Cl–Cl = 244 kJ mol$^{-1}$

1. Total energy to break bonds: $436 + 244 = 680$ kJ mol$^{-1}$.
2. Energy released making bonds = bonds broken − energy change $= 680 - (-180) = 860$ kJ mol$^{-1}$.
3. Divide by the number of those bonds: $860 \div 2 = 430$ kJ mol$^{-1}$ for H–Cl.

True or false? In the products $\mathrm{2H_2O}$ there are 4 O–H bonds to count in a bond energy calculation.

True. Multiply the bonds in one molecule by the balancing number in the equation.