The Energy Change of Reactions Flashcards

All 6 cards in this deck

Is breaking bonds in the reactants exothermic or endothermic?

Endothermic — energy must be supplied to break bonds.

What happens to energy when bonds in the products are formed?

Energy is released (bond making is exothermic).

How is the overall energy change of a reaction found from bond energies?

Overall energy change = energy needed to break bonds in the reactants − energy released forming bonds in the products, in kJ/mol.

In an exothermic reaction, how do the bond breaking and bond making energies compare?

The energy released from forming new bonds is greater than the energy needed to break existing bonds.

True or false? In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released when new bonds form.

True. That is why energy is taken in overall and the energy change is positive.

What are the steps to calculate the overall energy change from bond energies?

e.g. H–H + Cl–Cl $\rightarrow$ 2 H–Cl (H–H $436$, Cl–Cl $243$, H–Cl $432$ kJ/mol)

1. Add the bond energies of all bonds broken: $436 + 243 = 679$.
2. Add the bond energies of all bonds made: $2 \times 432 = 864$.
3. Energy change = broken − made $= 679 - 864 = -185$ kJ/mol, so exothermic.