Browse All 960 Chemistry Terms A–Z

Elimination Reaction Stoichiometry & Chemical Reactions Dimensionless A reaction in which two substituents are removed from adjacent atoms, generating a multiple bond. R–CH₂–CHX–R → R–CH=CH–R + HX

Ellingham Diagram Inorganic & Coordination Chemistry kJ/mol A plot of the standard free energy of formation of metal oxides against temperature, used to choose a reduction route. ΔG° = ΔH° − TΔS°

Empirical Formula Foundations of Chemistry Dimensionless The chemical formula giving the simplest whole-number ratio of atoms of each element in a compound. ratio = (mass % ÷ A_r), normalised

Emulsion Solutions & Colligative Properties nm A colloidal dispersion of one liquid as fine droplets throughout another liquid with which it is immiscible.

Enantiomer Organic Chemistry degrees One of a pair of stereoisomers that are non-superimposable mirror images of each other. (+) and (−), or R and S

End Point Acids, Bases & Salts mL The point in a titration at which the indicator changes colour, signalling that the reaction is judged complete.

Endergonic Reaction Thermochemistry & Thermodynamics kJ/mol A reaction with a positive Gibbs free energy change, which requires an input of energy to proceed. ΔG > 0

Endothermic Reaction Stoichiometry & Chemical Reactions kJ/mol A reaction that absorbs energy from the surroundings, giving a positive enthalpy change. ΔH > 0

Energy Profile Diagram Chemical Kinetics & Catalysis kJ/mol A plot of potential energy against reaction coordinate showing reactants, transition states, intermediates and products.

Enthalpy Thermochemistry & Thermodynamics kJ/mol A thermodynamic state function equal to internal energy plus the product of pressure and volume. H = U + pV

Enthalpy Level Diagram Thermochemistry & Thermodynamics kJ/mol A diagram plotting the enthalpies of reactants and products to show the direction and size of the enthalpy change.

Enthalpy of Atomisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of gaseous atoms is produced from an element in its standard state. ΔH_at°

Enthalpy of Dilution Thermochemistry & Thermodynamics kJ/mol The enthalpy change when a solution is diluted from one concentration to another by adding solvent. ΔH_dil

Enthalpy of Fusion Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a solid melts to a liquid at its melting point. ΔH_fus

Enthalpy of Neutralisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when an acid and a base react to form one mole of water under standard conditions. H⁺ + OH⁻ → H₂O, ΔH = −57.1 kJ

Enthalpy of Reaction Thermochemistry & Thermodynamics kJ/mol The heat absorbed or released when a reaction occurs at constant pressure in the molar amounts of the balanced equation. ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants)

Enthalpy of Solution Solutions & Colligative Properties kJ/mol The heat absorbed or released when one mole of a substance dissolves completely in a large excess of solvent. ΔH_sol = ΔH_lattice + ΔH_hyd

Enthalpy of Sublimation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a solid converts directly into vapour. ΔH_sub = ΔH_fus + ΔH_vap

Enthalpy of Vaporisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a liquid becomes vapour at its boiling point. ΔH_vap

Entropy Thermochemistry & Thermodynamics J/(mol·K) A measure of the dispersal of energy and the number of microscopic arrangements available to a system. S = k ln W

Entropy Change of the Surroundings Thermochemistry & Thermodynamics J/K The entropy change experienced by the surroundings when they absorb or release heat from a system. ΔS_surr = −ΔH_sys / T

Enzyme Biochemistry & Polymer Chemistry Dimensionless A biological catalyst, almost always a protein, that accelerates a specific biochemical reaction.

Enzyme Catalysis Chemical Kinetics & Catalysis Dimensionless Catalysis by protein molecules that bind substrates at a specific active site and lower the activation energy dramatically.

Enzyme Specificity Biochemistry & Polymer Chemistry Dimensionless The ability of an enzyme to select one substrate, or one class of substrate, and catalyse one particular reaction.

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