Advanced Chemistry: Structure, Properties, Energy and Equilibrium

Structure: how atoms are arranged

Atoms join by sharing or moving electrons. Each atom in a molecule keeps electron pairs (bonds and lone pairs) as far from each other as possible, because electron pairs repel. This simple idea (called VSEPR) gives the shape.

Common shapes

• Linear: CO₂ (180 degrees).
• Bent: H₂O (about 104.5 degrees), because two lone pairs push the bonds closer.
• Pyramid: NH₃ (about 107 degrees), one lone pair.
• Tetrahedral: CH₄ (109.5 degrees), four bonds, no lone pair.

Bigger structures follow the same ideas: a diamond is one giant network of carbon atoms, a salt crystal is a repeating grid of ions.

Properties: what shape and charge decide

Some atoms pull shared electrons harder; this pull is called electronegativity. If the pulls inside a molecule do not cancel, one end is slightly negative and the other slightly positive. The molecule is polar. If the pulls cancel, as in straight CO₂ or tetrahedral CH₄, the molecule is non-polar.

What this changes

• Polar molecules attract each other more, so water boils at 100 °C while methane boils near −162 °C.
• Like dissolves like: salt and sugar dissolve in water; oil does not.
• Giant networks (diamond, silica) are very hard and melt at very high temperatures; metals conduct because electrons move freely.

Energy: activation energy, heat and catalysts

To start a reaction, old bonds must first loosen. This needs a minimum push called the activation energy (E_a). Think of a ball that must climb a hill before it can roll down.

Heat of reaction

If the products are lower in energy than the reactants, heat is released (exothermic, ΔH negative). If they are higher, heat is taken in (endothermic, ΔH positive). ΔH does not depend on the hill height.

Catalysts

A catalyst gives the reaction an easier path with a lower hill. The reaction becomes faster, but the starting and ending energies, and so ΔH, do not change. A catalyst is not used up.

Equilibrium: when a reaction seems to stop

Many reactions are reversible: A ⇄ B. At the start only the forward reaction goes. As B builds up, the backward reaction speeds up. At equilibrium the two rates are equal, so the amounts of A and B stay constant. Nothing has stopped; both reactions still run (it is dynamic).

Equilibrium constant

For A ⇄ B, K = [B] / [A]. A large K means mostly products; a small K means mostly reactants. K changes only with temperature.

Le Chatelier's idea

If you disturb a system at equilibrium (add more A, change pressure or temperature), it shifts to reduce the disturbance. A catalyst does not shift the equilibrium; it only gets you there faster.

Try it: see polarity with a comb

Rub a plastic comb on dry hair. Open a tap so a thin stream of water falls. Bring the comb near the stream: the water bends towards it, because water molecules are polar. Now try the same with a thin stream of cooking oil: it barely moves. In the 3D, pick water and carbon dioxide in turn and compare the readout.

Key formulas and definitions

• Bond angles: CO2 180, NH3 about 107, H2O about 104.5, CH4 109.5 (degrees)
• Exothermic: products lower than reactants, dH < 0
• Endothermic: products higher than reactants, dH > 0
• For A <-> B: K = [B] / [A]
• At equilibrium: rate forward = rate backward

Worked examples

Common mistakes

• Thinking a catalyst changes the amount of product at equilibrium. It only changes how fast equilibrium is reached.
• Saying a reaction stops at equilibrium. Both forward and backward reactions continue at equal rates.
• Thinking polar bonds always make a polar molecule. Shape can cancel the pulls (CO2).
• Mixing up activation energy and dH. One is the hill height, the other is the difference between start and end.