Chemistery

Two-thirds of the way into every thermodynamics unit, the same question hits: "Wait — is enthalpy the disorder one, or is that entropy?" They're the two quantities that together decide whether a reaction happens, and mixing them up scrambles everything downstream. The short answer: enthalpy change (ΔH) measures the heat a process releases or absorbs at constant pressure, in kJ/mol. Entropy change (ΔS) measures how much more spread out energy and matter become, in J/(mol·K). Nature favours lower enthalpy and higher entropy — and when the two disagree, temperature settles the argument. Quick comparison at a glance Feature Enthalpy (H) Entropy (S) What it tracks Heat content; energy released or absorbed Dispersal of energy and matter; number of ways to arrange Symbol for change ΔH ΔS Units kJ/mol J/(mol·K) — note the joules, not kilojoules Favourable direction Negative (heat released) Positive (more spread out) Everyday signal Flask feel...

You've felt enthalpy changes your whole life: the campfire that warms your hands, the cold pack that soothes a sprain. Chemistry just puts a number and a sign on what your skin already knows — and that number is where thermochemistry calculations begin. The short answer: enthalpy change (ΔH) is the heat a system absorbs or releases during a process at constant pressure, usually reported in kJ/mol. A negative ΔH means heat is released (exothermic); a positive ΔH means heat is absorbed (endothermic). What enthalpy actually is Every substance carries a store of chemical energy — in its bonds, its particle motion, its interactions. Chemists call the heat-related bookkeeping of that store at constant pressure the substance's enthalpy (H) . You can never measure H itself, and you never need to: what reactions reveal is the change , ΔH = H(products) − H(reactants). If the products end up lower in enthalpy than the reactants, the difference left the system as heat: ΔH < 0,...

Every thermochemistry question starts with an unspoken decision: which part of the world are we talking about? Get that boundary wrong and every sign in the problem flips. Students lose more marks to this quiet setup step than to any equation. The short answer: the system is the part of the universe you're studying — usually the reacting chemicals themselves. The surroundings are everything else that can exchange energy or matter with it: the solvent, the flask, the air, the thermometer, you. Together they make up the universe: universe = system + surroundings. Quick comparison at a glance Feature System Surroundings What it is The chemicals or process being studied Everything outside the system Example in a beaker reaction The reacting substances Water, beaker, bench, air, thermometer Where ΔH refers to Energy change of the system — Exothermic reaction Releases energy Absorbs that energy — warms up Endothermic reaction Absorbs energy Supplies...

Leave a metal spoon and a mug of water in the sun and the spoon gets hot long before the water does. Same sunshine, very different responses — because every substance has its own price tag for warming up. The short answer: specific heat capacity (c) is the amount of energy needed to raise the temperature of 1 g of a substance by 1 °C, measured in J/(g·°C). The heat involved in any temperature change is calculated with q = mcΔT — mass × specific heat capacity × temperature change. What specific heat capacity actually means Think of specific heat capacity as a substance's thermal stubbornness . A high value means the substance soaks up a lot of energy for each degree it warms — and hands a lot back as it cools. A low value means its temperature swings easily. Some benchmark values worth recognizing: Substance c in J/(g·°C) Water (liquid) 4.18 Ethanol 2.44 Ice 2.09 Aluminum 0.897 Iron 0.449 Copper 0.385 Lead 0.128 Water's 4.18 is unusu...

A sparkler burns at over 1000 °C, yet a stray spark on your hand barely stings. A bathtub of water at 40 °C could warm you up for an hour. If temperature told the whole story, the sparkler should be far more dangerous — so clearly heat and temperature are not the same thing. The short answer: heat is energy being transferred from a hotter object to a colder one, measured in joules (J). Temperature is a measure of the average kinetic energy of the particles in a substance, measured in °C or kelvin — it tells you which way heat will flow, not how much energy there is. Quick comparison at a glance Feature Heat Temperature What it is Energy in transit between objects Measure of average particle kinetic energy Symbol q T Units joules (J), kilojoules (kJ), calories °C, K (kelvin), °F Depends on amount of substance? Yes — more matter can transfer more energy No — a drop and a bucket of boiling water are both 100 °C Measured with Calorimeter (calculated, ...

"Shorter bonds are stronger bonds" is one of the first rules of thumb you're handed, and it's a good one — right up until you meet the fluorine molecule, whose bond is short and feeble. Knowing why the rule usually works, and where it doesn't, is the difference between remembering a slogan and understanding it. The short answer: bond length is the average distance between the two bonded nuclei, measured in picometres. Bond energy is how much energy it takes to break that bond, measured in kilojoules per mole. Between the same pair of atoms, shorter reliably means stronger — across different pairs of atoms, it doesn't have to. Quick comparison at a glance Feature Bond length Bond energy What it measures Distance between the two nuclei Energy needed to break the bond Typical units Picometres (pm) or ångströms (Å) kJ mol⁻¹ Also called Bond distance Bond enthalpy, bond strength (a bond dissociation energy is the value for one specific bond in one specif...

Draw ozone, O₃, and you're forced into an awkward choice: one O–O single bond and one O=O double bond. Which oxygen gets the double bond? Measure the real molecule and the answer is neither — both bonds are exactly the same length. The Lewis structure isn't wrong so much as too crude, and resonance is the patch. The short answer: resonance describes a molecule whose real electron arrangement can't be drawn as a single Lewis structure, so we draw two or more and treat the truth as an average of them. The electrons involved aren't stuck between two atoms — they're delocalised , spread over three or more. What resonance actually is When a molecule has more than one valid Lewis structure that differs only in where the electrons sit — the atoms stay put — the real molecule is none of them. It's a single, unchanging structure called the resonance hybrid , in which those electrons are smeared over the whole set of positions at once. The individual drawings are c...

Both numbers sit on the same atom in the same molecule, and both look like charges. Yet carbon in CO has a formal charge of −1 and an oxidation number of +2 — opposite signs on the same atom. That isn't a contradiction. They're answers to two different questions, and each one is deliberately, usefully wrong in its own way. The short answer: formal charge splits every bonding pair straight down the middle, giving one electron to each atom, and asks what charge each atom would then carry. Oxidation number does the opposite extreme — it gives both electrons of every bond to the more electronegative atom. Formal charge is for picking the best Lewis structure; oxidation number is for tracking redox. Quick comparison at a glance Feature Formal charge Oxidation number How bonds are split Evenly — one electron each Entirely to the more electronegative atom Assumption made Bonds are perfectly covalent Bonds are perfectly ionic Electronegativity used? No Yes — it decides who ta...

Carbon's outer shell holds one 2s orbital and three 2p orbitals — four orbitals of two different shapes and energies. So why are all four C–H bonds in methane identical, the same length and the same strength, pointing at perfect 109.5° angles? Something must be evening them out. That something is hybridization. The short answer: hybridization is the mixing of an atom's valence s and p orbitals into a new set of identical hybrid orbitals that point in the directions the bonds actually need. Mix one s with one p and you get two sp orbitals; one s with two p gives three sp² ; one s with three p gives four sp³ . What hybridization actually means Take methane. If carbon bonded using its raw orbitals, you'd expect three bonds at 90° from the three p orbitals and one different bond from the s orbital. Real methane has four identical bonds at 109.5°. The atomic orbitals plainly aren't being used as they come. Hybridization is the bookkeeping that fixes this. Before bond...

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...