Reactions, Rates, and Equilibrium

Atoms let go of old bonding partners and grab new ones — the same atoms rearranged. Nothing is created or destroyed, which is why equations must balance.

Atoms are conserved: 1 C, 4 H and 4 O appear on both sides. Only the arrangement — and the energy stored in bonds — changes.

Exothermic: new bonds are stronger/lower energy, so surplus energy escapes as heat and light (a gas flame). Endothermic: new bonds are weaker, so energy is absorbed from the surroundings (instant cold packs).

The upfront energy needed to stretch and break existing bonds before new ones form. It's the 'lip' a boulder must be nudged over — it's why methane and oxygen can sit together until a spark arrives.

Molecules must (1) collide, (2) collide hard enough to clear the activation barrier, and (3) collide in the right orientation.

Steeply, not linearly — the fraction of molecules with enough energy rises sharply. Rule of thumb: +10 °C roughly doubles the rate (milk: a week in the fridge, an afternoon on the counter).

Only the surface can react, so grinding a solid multiplies contact area. A log is hard to light; sawdust from the same log can explode — the cause of grain silo explosions.

Offers an alternative route with a lower activation barrier — a tunnel through the hill instead of a climb over it — and emerges unchanged. Platinum in catalytic converters; enzymes in your body.

One where products can collide and turn back into reactants, written with a double arrow, e.g. N₂ + 3H₂ ⇌ 2NH₃.

When forward and reverse rates become equal so amounts stop changing. It isn't dead — molecules still react furiously in both directions, the traffic is simply balanced.