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Enthalpy
AQA A-level chemistry thermodynamics year 13
| Term | Definition |
|---|---|
| Enthalpy | A measure of the heat content of a substance |
| 3 types of enthalpy change from AS | Enthalpy of combustion Enthalpy of formation Bond enthalpy |
| First type of calculation | Enthalpy of formation: enthalpy change = elements UP to reactants and products, reactants to products |
| Second type of calculation | Enthalpy of combustion: reactants and products DOWN to oxides, reactants to products |
| Third type of calculation | Bond enthalpy: reactants and products DOWN to gas atoms, reactants to products. Elements must be gaseous before going down to gas atoms |
| Enthalpy of neutralisation | Enthalpy change when solutions of acid and an alkali react together under standard conditions to produce 1 mole of water |
| Enthalpy change of a solution | Enthalpy change when one mole of an ionic solid is dissolved infinitely in water so that the ions are separated and do not interact with each other |
| Hess cycle for enthalpy of solution | Gas ions going UP to ionic solid and dissolved ions, ionic solid to dissolved ions. Ionic solid is lattice enthalpy of formation and dissolved ions is hydration enthalpy |
| Enthalpy of hydration | Enthalpy change when one mole of gaseous ions becomes hydrated I.e. in water. This is exothermic and works especially well for smaller, positively charged ions |
| Lattice enthalpy of formation | Enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase |
| Lattice enthalpy of dissociation | Enthalpy change when one mole of solid ionic compound is broken up into its constituent ions in the gas phase |
| Enthalpy of formation | Enthalpy change when one mole of substance is formed from its constituent elements with all substances in their standard states under their standard conditions |
| Enthalpy of combustion | Enthalpy change when one mole of substance undergoes complete combustion in oxygen with all substances in standard states under standard conditions |
| Ionisation enthalpy (1st and 2nd) | 1st ionisation: the enthalpy change when one mole of gaseous atoms loses an electron to produce gaseous 1+ ions 2nd ionisation: the enthalpy change when one mole of gaseous 2+ ions is produced from a mole of 1+ ions |
| Electron affinity (1st and 2nd) | 1st ionisation: the enthalpy change when one mole of gaseous atoms gains an electron to produce gaseous 1- ions 2nd ionisation: the enthalpy change when one mole of gaseous 2- ions is produced from a mole of 1- ions |
| Enthalpy of atomisation | Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state |
| Bond disassociation enthalpy | Enthalpy change when one mole of covalent bonds is broken in the gaseous state |
| Enthalpy of vapourisation | Enthalpy change when one mole of a liquid is turned into a gas |
| Enthalpy of fusion | Enthalpy change when one mole of a solid is turned into a liquid |
| Lattice enthalpy indication | The magnitude of the lattice enthalpy of a compound indicates the strength of the ionic bonding, the bigger the lattice enthalpy the stronger the bonding. Generally, compounds with smaller ions have higher charges and stronger attractions |
| Born-Haber cycle | A cycle that includes all the enthalpy changes in the formation of an ionic compound, it allows the measurement of certain unknown enthalpies |
| Order of Born-Haber cycle | 1. Constituent elements in standard state may be atomised upwards or formed directly (downwards) 2. Metals are ionised to cations (upwards) 3. Non-metals undergo electron affinity to anions (downwards) 4. Lattice enthalpy of formation (downwards) |
| Why we use mean bond enthalpies | Because the bond enthalpy of 2 atoms can vary because the bonds will be broken in different environments |
| Perfect ionic model | A theoretical representation used to describe bond enthalpies in ionic compounds according to the model that all ions have complete outer shells and fully transferred electrons with no sharing of electrons |
| What similarities between lattice enthalpy of formation and perfect ionic model values mean | Similarities mean the compound is closer to having a perfect ionic structure, differences mean the compound is has more covalent character |
| Covalent character | The extent to which two atoms share electrons in a chemical bond |
| Distorted ions | Ions that do not have a perfectly symmetrical arrangement of electrons around its nucleus |
| Fajan’s rules: explanation | The electron cloud of a negative ion can become polarised by a small positively charged ion. The bigger and lower charged the negative ion is compared to the positive ion the more polarised the molecule becomes leading to covalent character |
| Fajan’s rules: sodium halides | Very similar theoretical and experimental values. Structure more closely resembles the perfect ionic model. Electron density models confirm the ions are almost perfect spheres with little electron density between them |
| Fajan’s rules: silver halides | Very different theoretical and experimental values. Ions aren’t discreet spheres and some electron density is concentrated between the ions. Not a true covalent bond but strong covalent character. Not always a complete transfer of electrons |
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