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12 terms

Enthalpy change definitions

Enthalpy change definitions for OCR A2 Chemistry.
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Lattice enthalpy
The enthalpy change accompanying the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
Hess' Law
States that if a reaction can take place by more than one route, and the initial and final conditions are the same, then the total enthalpy change is the same for each route.
Standard enthalpy change of formation (^Hf e.g. K(s)+ 1/2Cl2(g) --> KCl(s) (usually exothermic for an ionic compound))
The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
Standard enthalpy change of atomisation (^Hat e.g. 1/2Cl2(g) --> Cl(g) ENDOTHERMIC process because bonds have to be broken)
The enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state.
First ionisation energy (^HI1 - an endothermic process because electron being lost has to overcome attraction from the nucleus in order to leave the atom - K(g) ---> K+(g) + e-
The enthalpy change accompanying the removal of one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Second ionisation energy (^HI2 - an endothermic process because electron being lost has to overcome attraction from the nucleus - Ca+(g) --> Ca2+(g) + e-
The enthalpy change accompanying the removal of one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.
First electron affinity (^HEA1 - EXOTHERMIC process because electron is attracted into the outer shell of an atom by then nucleus. Cl(g) + e- --> Cl-(g))
The enthalpy change accompanying the addition of one electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
Second electron affinity (^HEA2 - ENDOTHERMIC process because electron is repelled by the 1- ion and this repulsion has to be overcome. O-(g) + e- --> O 2-(g)
Is the enthalpy change accompanying the addition of one electron to each ion in one mole of gaseous ions to form one mole of gaseous 2- ions.
Standard enthalpy change of solution (^Hs can be exothermic/endothermic e.g. KCl(s) + aq --> K+(aq) + Cl-(aq) )
The enthalpy change that takes place when one mole of a compound is completely dissolve in water under standard conditions.
Standard enthalpy change of hydration (^Hhyd e.g. K+(g) + aq --> K+(aq) EXOTHERMIC - because energy is released when ions form bonds with water molecules.
The enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions.
Standard enthalpy change of neutralisation (^Hneut)
The enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form one mole of water under standard conditions.
Standard enthalpy change of combustion (^Hc)
The enthalpy change that accompanies the reaction of a one mole of an element or compound with excess oxygen with reactants and products in their standard states.