Chemistry, Class 11 and 12 formula book

“Everything through moles”

How to start any numerical

Grams, litres and particle counts cannot be compared directly. Convert each to moles first, use the balanced equation, then convert back to whatever the question wants.

“Molarity has volume, molality has mass”

Telling the two apart

Molarity is per litre of solution, molality per kilogram of solvent. Only molality is safe when the temperature changes.

“Smallest share wins”

Finding the limiting reagent

Divide each reactant's moles by its coefficient in the balanced equation. The smallest answer is the limiting reagent, and it sets the yield.

“n plus l, then n”

The order in which orbitals fill

Lower n + l fills first. If two are equal, the one with the smaller n goes first. That single rule puts 4s before 3d without memorising the diagonal chart.

“Bus seats”

Hund's rule

People take empty seats on a bus before sitting next to someone. Electrons fill each orbital of a subshell singly before any pairs up.

“Half and full are happiest”

Chromium and copper

Cr is 3d⁵4s¹ and Cu is 3d¹⁰4s¹, not the expected 3d⁴4s² and 3d⁹4s², because half-filled and fully filled subshells are extra stable.

“Kelvin or nothing”

Every gas law

Every gas calculation needs absolute temperature. Using °C in Charles's law is the single most common error in the chapter.

“Light gases are quick”

Graham's law

Rate goes as one over the square root of molar mass. Hydrogen diffuses four times faster than oxygen, because 32 over 2 is 16 and its square root is 4.

“a for attraction, b for bulk”

The van der Waals constants

a corrects the pressure for intermolecular attraction; b corrects the volume for the space the molecules themselves take up.

“Negative means it goes”

Reading ΔG, ΔH and ΔS

Negative ΔG means spontaneous. Negative ΔH means heat released. Positive ΔS means more disorder. Only ΔG decides whether a reaction happens.

“Both negative, cold helps; both positive, heat helps”

When temperature decides

If ΔH and ΔS are both negative, low temperature makes ΔG negative. If both are positive, high temperature does. If signs differ, temperature cannot change the answer.

“Break costs, make pays”

Bond enthalpy calculations

Breaking bonds always absorbs energy and forming them always releases it. Bonds broken minus bonds formed gives ΔH, in that order.

“Solids and liquids sit out”

Writing an equilibrium constant

Pure solids and pure liquids never appear in K. Their concentration does not change, so it is absorbed into the constant.

“Push it, it pushes back”

Le Chatelier's principle

Add a reactant and the reaction moves forward; raise the pressure and it shifts to the side with fewer gas molecules; heat an exothermic reaction and it goes backwards.

“Buffer is pK_a plus the log of the ratio”

Henderson-Hasselbalch

When salt and acid are equal the log is zero, so pH equals pK_a. That is the best buffering point, and it is worth remembering on its own.

“s squared, 4s cubed, 108s to the five”

Solubility product expressions

AB gives s², AB₂ gives 4s³, A₂B₃ gives 108s⁵. The coefficients come from the stoichiometry, not from memory alone.

“OIL RIG”

Oxidation and reduction

Oxidation Is Loss of electrons, Reduction Is Gain. Say it once at the start of every redox question and the agents follow.

“The agent suffers the opposite”

Naming the agents

The oxidising agent is itself reduced; the reducing agent is itself oxidised. Whatever the agent does to the other, it has the reverse done to itself.

“KMnO₄ is 5 in acid, 3 in neutral”

n-factor of permanganate

In acidic medium manganese goes from +7 to +2, so five electrons. In neutral or faintly alkaline medium it goes to +4, so three.

“Count particles, not kind”

What colligative means

One mole of sugar and one mole of NaCl do not behave alike, because NaCl gives two particles. It is the number of particles in solution that matters, never their identity.

“i is how many it breaks into”

The van't Hoff factor

NaCl gives 2, CaCl₂ gives 3, glucose gives 1. If the solute associates instead, as benzoic acid does in benzene, i falls below 1.

“Salt on ice, sugar in the pot”

Which way each effect goes

Adding solute lowers the freezing point and raises the boiling point. Salt melts ice on a road; sugar syrup boils above 100 °C.

“Red Cat, An Ox”

What happens at each electrode

REDuction at the CAThode, ANode is OXidation. True in both galvanic and electrolytic cells, even though the signs of the electrodes swap.

“Cathode minus anode”

Calculating cell potential

E°_cell = E°_cathode − E°_anode, with both written as reduction potentials. Never reverse a sign and subtract as well; do one or the other.

“0.0591 over n”

The Nernst equation at room temperature

At 298 K the factor 2.303RT/F is 0.0591 volts. Memorise the number and the equation becomes one line.

“Order from experiment, molecularity from the step”

Telling them apart

Order is measured and can be zero or fractional. Molecularity is counted from a single step and is always a whole number.

“First order forgets where it started”

Half-life

For first order, t_1/2 = 0.693/k, whatever the initial concentration. For zero order the half-life depends on it.

“A catalyst lowers the hill”

What a catalyst does

It provides a path with lower activation energy. Both forward and reverse rates rise by the same factor, so the equilibrium position does not move.

“1, 2, 4”

Atoms per unit cell

Simple cubic 1, body centred 2, face centred 4. Almost every solid state numerical starts by picking the right Z.

“Edge, face, body: 2, √2, √3”

Relating a and r

Simple cubic touches along the edge, fcc along the face diagonal, bcc along the body diagonal. a = 2r, a√2 = 4r, a√3 = 4r.

“Schottky slims, Frenkel shuffles”

The two defects

Schottky removes a pair of ions, so density falls. Frenkel just moves an ion inside, so density is unchanged.