Temperature, pressure, Kp and Le Chatelier’s principle

Collision theory says that molecules react only if they collide with enough energy to break bonds. The smallest energy that works is the activation energy, and molecules that have at least this energy are the activated molecules. At any temperature only a fraction of the molecules are activated. Heating makes the molecules move faster and raises this fraction, so more collisions succeed in each second and the rate rises. For many reactions the rate roughly doubles for each 10 °C rise: say “often”, never “always”. Two questions must be kept apart. Speed: heating makes a reaction faster in both directions, so equilibrium is reached sooner. Position: where the equilibrium settles depends on the heat of the forward reaction. If the forward reaction is exothermic, heating shifts the position backward and lowers the yield, and cooling does the opposite. A faster reaction is therefore not the same as a larger yield. On paper, 2NO₂ ⇌ N₂O₄ + heat shows this: cooling shifts the position toward colourless N₂O₄ and heating toward brown NO₂. The brown gas is poisonous and is never made at home.