Bond energies

The board is bookkeeping, an accounting model: it breaks every old bond first and makes the new ones after. In a real reaction, bonds break and form together as molecules collide, but the energy change overall is the same, because it depends only on the bonds at the start and at the end.

Bond energies such as C–H are averages over many different molecules, so ΔH worked out from them is close to the measured value, not exactly it. They are for gases: here every substance, water too, is a gas.

The models are not to scale, and their colours only label the elements. Methane lies flat, as in its displayed formula: a real methane molecule is a tetrahedron. The moves are slowed down.

With reduced motion, each move finishes at once.