PSEB SS Chemistry flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Mole definition

One mole contains 6.022 × 10²³ elementary entities (Avogadro constant). It is the amount of substance whose mass in grams equals its relative formula mass.

Molar volume of gas

At STP (273 K, 1 atm) one mole of any ideal gas occupies 22.4 dm³. At 100 kPa the value is about 22.7 dm³ mol⁻¹.

Ideal gas equation

PV = nRT relates pressure, volume, amount and temperature of a gas. R equals 8.314 J K⁻¹ mol⁻¹ when P is in Pa and V in m³.

Empirical formula

The simplest whole-number ratio of atoms in a compound. It is found by dividing each element's moles by the smallest value obtained.

Limiting reagent

The reactant that is completely consumed first, fixing the maximum product yield. Compare mole ratios with the balanced equation to identify it.

Percentage yield

Actual yield divided by theoretical yield, multiplied by 100. It is below 100 percent because of side reactions, losses in transfer and incomplete reaction.

First law of thermodynamics

Energy is conserved: ΔU = q + w, where q is heat absorbed by the system and w is work done on it. Internal energy is a state function.

Standard enthalpy of formation

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar. Elements in standard state have zero value.

Hess's law

The total enthalpy change of a reaction is independent of the route taken. It allows ΔH to be calculated from formation or combustion data.

Entropy and spontaneity

ΔG = ΔH − TΔS. A reaction is spontaneous when ΔG is negative; this needs favourable enthalpy, favourable entropy, or a suitable temperature.

Le Chatelier's principle

When a system at equilibrium is disturbed, it shifts in the direction that opposes the change. Temperature alters K; concentration and pressure do not.

Equilibrium constant Kc

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power, at equilibrium. Its value depends only on temperature.

pH of weak acid

For a weak monoprotic acid of concentration c and dissociation constant Ka, pH = ½(pKa − log c), assuming dissociation is small compared with c.

Buffer solution

A mixture of a weak acid and its conjugate base (or weak base and conjugate acid) that resists pH change on adding small amounts of acid or alkali.

Electrode potential

The potential of a half-cell measured against the standard hydrogen electrode, which is assigned 0.00 V. More positive values mean greater tendency to be reduced.

Faraday's laws

The mass deposited or liberated at an electrode is proportional to charge passed and to molar mass divided by electrons transferred: m = MIt/(nF), F = 96500 C mol⁻¹.

VSEPR theory

Electron pairs around a central atom repel and adopt positions maximising separation. Lone pairs repel more strongly than bond pairs, reducing bond angles.

Hybridisation types

sp gives linear 180° geometry, sp² gives trigonal planar 120°, and sp³ gives tetrahedral 109.5°. The number of electron domains determines the hybridisation.

Ionisation energy trend

First ionisation energy increases across a period as nuclear charge rises and radius falls, and decreases down a group as shielding and distance increase.

First-order half-life

For a first-order reaction t½ = 0.693/k, independent of initial concentration. Each half-life reduces the remaining reactant concentration by half.

IUPAC naming priority

The principal functional group with highest priority determines the suffix: carboxylic acid, ester, amide, aldehyde, ketone, alcohol, amine, alkene, alkyne, halide.

Markovnikov's rule

In addition of HX to an unsymmetrical alkene, hydrogen adds to the carbon bearing more hydrogens, giving the more stable carbocation intermediate and major product.

Titration mole relationship

Moles of analyte equal molarity times volume in dm³. Use the balanced equation's coefficients to convert between acid and base moles before finding concentration.

Primary standard criteria

A primary standard is highly pure, stable, non-hygroscopic, readily soluble and of high molar mass to reduce weighing error. Examples include oxalic acid and KHP.

Burette reading precision

Burette readings are recorded to two decimal places, that is, to 0.05 cm³. The titre is the final reading minus the initial reading.

Concordant titres

Titres that agree within about 0.10 cm³ of each other. The mean of concordant values is used; the rough titration is excluded from the average.

KMnO4 end point

In acidified permanganate titrations the end point is a persistent pale pink colour. The solution is acidified with dilute sulfuric acid, not hydrochloric acid.

Starch indicator timing

In iodometric titrations starch is added near the end point, when iodine colour fades to pale straw, because starch–iodine complex forms strongly at low iodine concentration.

Flame test colours

Characteristic flame colours: sodium golden yellow, potassium lilac, calcium brick red, barium apple green, copper blue-green, lithium crimson.

Sulfate ion test

Add dilute hydrochloric acid then barium chloride solution. A white precipitate of barium sulfate, insoluble in dilute acid, confirms sulfate ions.

Chloride ion test

Acidify with dilute nitric acid and add silver nitrate solution. A white precipitate of silver chloride that dissolves in ammonia confirms chloride ions.

Melting point purity test

A pure crystalline solid melts sharply over a narrow range of 1–2 °C. Impurities lower the melting point and broaden the observed range.

Retardation factor Rf

Rf equals the distance travelled by the solute divided by the distance travelled by the solvent front. It is always less than one and depends on the solvent used.