Chemistery

Here's a question that has cost more marks than almost any other in bonding: what is the shape of water? Tetrahedral or bent? Both answers appear in textbooks, and both are right — to two different questions. Once you see which is which, you'll never lose the mark again. The short answer: electron geometry is the arrangement of all electron domains around the central atom, lone pairs included. Molecular geometry is the arrangement of the atoms only , with lone pairs left out of the name. They're identical when there are no lone pairs, and different every time there are. Quick comparison at a glance Feature Electron geometry Molecular geometry Also called Electron-domain geometry, electron-pair geometry Molecular shape, shape of the molecule What it counts Bonds and lone pairs Bonded atoms only Number of possibilities 5 (linear → octahedral) Many more — each electron geometry branches Determined by Total electron domains Domains minus lone pairs Sets the ideal ...

A Lewis structure is flat. A real molecule is not. Water isn't a straight line, methane isn't a cross, and ammonia isn't a triangle — and the theory that gets you from the flat drawing to the real three-dimensional shape, in about thirty seconds per molecule, is VSEPR. The short answer: VSEPR stands for Valence Shell Electron Pair Repulsion , and it says that the groups of electrons around a central atom push each other as far apart as possible. Count the groups, arrange them at maximum separation, and you have the molecule's shape. What VSEPR actually claims The whole theory rests on one idea: electrons repel electrons . Every region of electron density around a central atom — every bond and every lone pair — is negatively charged, so they all shove each other away. They settle into whatever arrangement puts them at the greatest possible angular distance from one another, and the atoms come along for the ride. That's it. There's no orbital mathematics inv...

You've drawn the Lewis structure. Every atom has its octet, the dots are all placed — and then the question asks for the shape, and suddenly it matters which pairs are which. Getting lone pairs and bonding pairs straight is the single step between a correct dot diagram and a correct molecular shape. The short answer: a bonding pair is a pair of electrons shared between two atoms, holding them together — it's what a line in a structural formula represents. A lone pair is a pair of electrons sitting on one atom only, bonded to nothing, and because it's held by a single nucleus it spreads out more and pushes harder on everything around it. Quick comparison at a glance Feature Bonding pair Lone pair Also called Shared pair, bond pair Non-bonding pair, unshared pair How many nuclei attract it Two One Shown in a structure as A line (or two dots between atoms) Two dots on a single atom Shape in space Pulled tight between two nuclei Fatter and closer to its own atom Rep...

You have an equilibrium. You poke it. Which way does it move? There's one rule that answers this for every disturbance you'll be asked about, and it fits in a sentence. The short answer: Le Chatelier's principle states that if a system at equilibrium is disturbed, the position of equilibrium shifts in the direction that partially opposes the change. Add something and the system consumes some of it; heat it and it absorbs some of that heat. What "opposes the change" actually means The system doesn't undo the change — it can't. It partially offsets it. Add 1 mol of a reactant and the shift will use up some of that mol, so the new equilibrium has more of it than before, just less than 1 mol more. Think of it as pushing back, not resetting. The three disturbances 1. Concentration You do this Equilibrium shifts Because Add a reactant Right (towards products) To use some of it up Remove a reactant Left To replace some of it Add a product Left To...

Diamond is not the most stable form of carbon. Graphite is. Every diamond on Earth is slowly, inevitably turning into pencil lead — and will finish somewhere around the heat death of the universe. That gap between should and does is the whole of this post. The short answer: thermodynamics tells you whether a reaction is energetically favourable and how far it will go; kinetics tells you how fast it gets there. A reaction can be thermodynamically favourable and kinetically impossible at room temperature — which is why diamonds survive. Quick comparison at a glance Feature Thermodynamics Kinetics Question answered Will it happen? How far? How fast will it happen? Key quantities ΔH, ΔS, ΔG, K Rate, activation energy (Eₐ) Depends on Only the start and end states The pathway between them Shown on an energy profile by The difference in height between reactants and products The height of the hump Changed by a catalyst? No Yes Changed by temperature? Yes (ΔG and K both shift) ...

Equilibrium sounds like the reaction has stopped. It hasn't — and that misunderstanding costs more marks in this topic than anything else. The short answer: chemical equilibrium is the state a reversible reaction reaches in a closed system when the forward and reverse reactions are happening at the same rate , so the concentrations of everything stop changing. It is dynamic : both reactions continue indefinitely, they just cancel out. What "dynamic" actually means Picture a sealed flask. At the start there's only reactant, so the forward reaction is fast and the reverse can't happen at all. As product builds up, the forward rate falls (reactant is being used) and the reverse rate rises (there's now product to react back). Sooner or later the two rates meet. From that moment on, product is being made exactly as fast as it's being destroyed, so the amounts stay constant — even though, at the molecular level, nothing has slowed down at all. An analogy...