Chemical Reactions · The Micro World

Effective collisions

Molecules move and collide constantly, yet almost no collisions lead to reaction (faint grey sparks).

Only a collision that meets two conditions is an effective collision (golden flash):

① the energy along the line of impact ≥ the activation energy Ea; ② the orientation is right: H₂ and I₂ must meet side by side to break the old bonds and form two H–I bonds at once.

Temperature and activated molecules

At one temperature, molecules do not all share the same energy; they follow the Maxwell–Boltzmann distribution (white curve).

Molecules with energy ≥ Ea are activated molecules; they glow orange on the stage.

Raise the temperature and the curve flattens and widens: the fraction of activated molecules rises sharply, and so do effective collisions. That is why reactions speed up when heated.

Catalysis

The platinum surface at the bottom is a catalyst. Molecules adsorbed on it have weakened bonds and aligned orientations, so the reaction takes a path with lower activation energy (green dashed line).

At the same temperature, more collisions clear the lower barrier: green flashes cluster on the catalyst surface.

A catalyst does not change ΔH, and it is itself unchanged by the reaction.

Concentration and pressure

The more molecules per unit volume, the more collisions per unit time, and the more effective collisions.

Push the piston to compress the gas, or add reactants, and watch the collision frequency and reaction rate change.

Note: the fraction of activated molecules is unchanged; what changes is their number per unit volume.

Reaching equilibrium

2NO₂ (red-brown) ⇌ N₂O₄ (colourless).

At first there is only NO₂: the forward reaction is fast and the reverse slow. As N₂O₄ builds up, the reverse reaction speeds up.

When the forward rate = reverse rate, concentrations stop changing and chemical equilibrium is reached. At the molecular level both reactions never stop: this is a dynamic equilibrium.

Starting from NO₂ or from N₂O₄, the system reaches the same equilibrium.

Shifting equilibrium

When conditions change, equilibrium shifts in the direction that counteracts the change (Le Chatelier's principle).

Heating: shifts toward the endothermic direction (more NO₂), so the colour darkens.

Compression: shifts toward fewer gas molecules (more N₂O₄). Look closely: the colour first darkens (higher concentration), then lightens a little.

Chain reaction

A mixture of H₂ and Cl₂ barely reacts in the dark, but explodes under strong light.

Light splits Cl₂ into two chlorine atoms Cl· (radicals, the glowing spheres). Each Cl· takes an H from H₂, forming HCl and H·; the H· takes a Cl from Cl₂, regenerating Cl·…

One radical can trigger thousands of reactions while releasing a lot of heat, so the reaction accelerates. This is a chain reaction.