PSEB SS Chemistry key terms

The amount of substance containing exactly 6.022 × 10²³ representative particles (atoms, ions, or molecules). It links mass in grams to particle number and to molar volume of gases at stated conditions.

The number of particles in one mole, 6.022 × 10²³ per mole. It converts between counted particles and amount in moles, and appears in calculations of molar mass, gas volume, and concentration.

The mass of one mole of a substance in grams per mole, numerically equal to its relative formula mass. It converts a weighed mass into moles, which is the first step in most stoichiometric calculations.

The quantitative relationship between amounts of reactants and products as shown by a balanced equation. Coefficients give mole ratios used to predict masses, volumes, or concentrations in a reaction.

The reactant that is completely consumed first and therefore fixes the maximum amount of product formed. Identified by dividing each reactant's moles by its coefficient and comparing the results.

The actual mass of product obtained expressed as a percentage of the theoretical mass calculated from stoichiometry. Values below 100 percent arise from side reactions, incomplete reaction, and losses during transfer or purification.

The volume occupied by one mole of a gas, about 22.4 dm³ at standard temperature and pressure. It allows conversion between moles of gas and volume, provided conditions are stated.

The relation pV = nRT linking pressure, volume, amount of gas, and absolute temperature through the gas constant R. Real gases approach it at low pressure and high temperature.

The pressure a single gas in a mixture would exert if it alone occupied the whole container at the same temperature. It equals its mole fraction times the total pressure in an ideal mixture.

The number of moles of one component divided by the total number of moles in the mixture. It is dimensionless, always below one, and the sum of all mole fractions equals one.

Concentration expressed as moles of solute per cubic decimetre (litre) of solution. Because it depends on solution volume, it varies slightly with temperature as the solution expands or contracts.

A solution whose concentration is known accurately, prepared by weighing a primary standard or by diluting a stock solution in calibrated glassware. It is essential for reliable volumetric titration.

A volumetric procedure in which a solution of known concentration is added from a burette until the reaction with a measured volume of analyte is just complete, as signalled by an indicator or instrument.

The stage of a titration at which the amount of titrant added exactly matches the stoichiometric amount required to react with the analyte. It is a theoretical point, not necessarily the observed colour change.

The observable change, usually a colour change of an indicator, that signals the analyst to stop adding titrant. A well-chosen indicator makes the end point closely match the equivalence point.

A highly pure, stable, non-hygroscopic substance that can be weighed accurately and used to standardise another solution directly. Examples include potassium hydrogen phthalate and anhydrous sodium carbonate.

Loss of electrons by a species, accompanied by an increase in oxidation number. In redox reactions oxidation is always coupled with reduction, so electrons lost by one species are gained by another.

Gain of electrons by a species, accompanied by a decrease in oxidation number. The oxidising agent accepts these electrons, while the reducing agent supplies them.

A formal charge assigned to an atom in a compound by treating bonding electrons as belonging to the more electronegative atom. It tracks electron transfer and helps balance redox equations.

A reaction in which oxidation numbers change because electrons are transferred between species. Such reactions can be balanced by the ion-electron method or by oxidation-number changes.

A species that accepts electrons from another substance and is itself reduced. Common examples are acidified potassium manganate(VII), potassium dichromate(VI), and chlorine.

A species that donates electrons to another substance and is itself oxidised. Examples include hydrogen, carbon, zinc metal, and iron(II) salts.

The potential difference, measured in volts, of an electrode relative to a standard hydrogen electrode under stated conditions. Its sign indicates the tendency of the half-cell to undergo reduction.

The reference half-cell with hydrogen gas at one atmosphere bubbling over platinum in 1 mol dm⁻³ acid at 298 K. Its potential is defined as exactly zero volts, so all other potentials are relative to it.

A device converting chemical energy into electrical energy, or the reverse, through two half-cells joined by a salt bridge or membrane. Electrons flow through the external circuit while ions migrate internally.

The decomposition of an electrolyte by passing a direct current through it, driven by an external power supply. Products at each electrode depend on ion discharge potentials and electrolyte concentration.

The electric charge carried by one mole of electrons, approximately 96500 coulombs per mole. It links the quantity of electricity passed during electrolysis to the amount of substance deposited or liberated.

A tube containing a concentrated inert electrolyte, such as potassium nitrate, that connects two half-cells. It completes the circuit and maintains electrical neutrality without allowing bulk mixing of the solutions.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the position of equilibrium shifts so as to oppose that change, provided temperature remains the only altering factor for K.

The state of a reversible reaction in which forward and reverse rates are equal, so concentrations remain constant while both reactions continue. It requires a closed system and constant temperature.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium. It is constant at a given temperature and indicates how far a reaction proceeds.

A logarithmic measure of hydrogen ion concentration: pH equals the negative logarithm to base ten of the hydrogen ion concentration in mol dm⁻³. Values below 7 are acidic and above 7 alkaline at 298 K.

A solution that resists change in pH when small amounts of acid or alkali are added, usually a weak acid with its conjugate base. It works because the components neutralise added H⁺ or OH⁻ ions.

The reaction of ions of a dissolved salt with water, producing an acidic or alkaline solution. Salts of weak acids or weak bases hydrolyse appreciably, which determines the pH of their solutions.

A weak acid or base whose colour differs markedly between its protonated and deprotonated forms. Its colour-change interval must overlap the steep part of the titration curve for accurate results.

The heat energy exchanged with the surroundings at constant pressure during a reaction, given the symbol ΔH. Exothermic reactions have negative values; endothermic reactions have positive values.

The total enthalpy change of a reaction is independent of the route taken, provided initial and final states are the same. It allows unknown enthalpy changes to be calculated from known ones.

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and one atmosphere. Values are tabulated and used with Hess's law to find reaction enthalpies.

The experimental measurement of heat exchanged in a physical or chemical change, using a calorimeter and a measured temperature rise. Results depend on careful correction for heat losses to surroundings.

The energy needed to raise the temperature of one gram of a substance by one kelvin, measured in joules per gram per kelvin. For water it is about 4.18 J g⁻¹ K⁻¹, a value used in calorimetry calculations.

The minimum energy that colliding particles must possess for a reaction to occur. It determines reaction rate and is lowered by a catalyst, which provides an alternative pathway.

A substance that increases the rate of a reaction by providing a lower-energy pathway, without being consumed overall. It does not alter the position of equilibrium or the enthalpy change of the reaction.

The change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area, and catalysts, and is studied by measuring properties such as gas volume or colour.

The model stating that reacting particles must collide with sufficient energy and correct orientation for a reaction to occur. It explains how concentration, temperature, and surface area affect rate.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different structures and properties. Types include chain, position, functional group, and stereoisomerism.

An atom or group of atoms that gives a family of organic compounds its characteristic reactions and properties. Examples are hydroxyl, carbonyl, carboxyl, amino, and halogen groups.

A family of organic compounds with the same functional group and general formula, in which successive members differ by one CH₂ unit. Members show a gradual change in physical properties.

A representation showing which atoms are bonded to which, and by what bond type, without necessarily showing every bond as a line. It distinguishes isomers that share a molecular formula.

Identification of the components present in a sample, for example by flame tests, precipitation reactions, or gas evolution. It answers what is present, not how much is present.

Determination of the amount or concentration of a component in a sample, using techniques such as titration, gravimetry, or instrumental measurement. It answers how much is present.

An insoluble solid formed when two solutions are mixed and a reaction produces a sparingly soluble compound. Its colour and solubility behaviour help identify ions in qualitative analysis.

Separation of an insoluble solid from a liquid by passing the mixture through a porous medium that retains the solid. The liquid passing through is the filtrate and the retained solid is the residue.

Purification in which a solid is dissolved in a minimum of hot solvent and recovered as crystals on cooling. Impurities stay in the mother liquor, giving a purer product.

Separation based on differences in boiling point, in which vapour from a heated liquid is condensed and collected. Fractional distillation separates miscible liquids with close boiling points.

A separation technique in which components distribute differently between a stationary phase and a moving phase. It is used for identification and purification of mixtures, including coloured pigments and amino acids.

The digits in a measured value that carry meaningful information about its precision. Results of calculations should be reported to the same precision as the least precise measurement used.

The difference between an experimental value and the accepted value, expressed as a percentage of the accepted value. It quantifies accuracy, while precision refers to closeness of repeated measurements.