Enthalpy Defintiosn Flashcards
Enthalpy of formation
The enthalpy change:
when 1 mole of a compound is formed
from its elements
under standard conditions
Enthalpy of neutralisation:
The enthalpy change:
when acid and alkali react together
under standard conditions
to form 1 mole of water
nb: when an acid and an alkali neutralise each other heat is always evolved
Enthalpy change of atomisation
The heat energy:
needed to produce 1 mole of gaseous atoms
from the element
in its standard state
Enthalpy of vaporisation:
The heat energy:
needed to turn 1 mole of a liquid into a gas at its boiling point under standard conditions (1 atm)
Enthalpy change of combustion:
The enthalpy change:
when 1 mole of a substance is completely burned in oxygen under standard conditions
enthalpy of combustion will always be negative
therefore the heat energy evolved when 1 mole of a substance is burned completely in oxygen under STANDARD CONDITIONS
Enthalpy change of ionisation energy
The energy required to remove 1 mole of electrons from one mole of gaseous atoms to form 1 mole of gaseous +1 ions
Enthalpy of solution:
The enthalpy change:
when 1 mole of an IONIC substance dissolves in sufficient water to give an infinitely dilute solution
Enthalpy change of hydration:
The enthalpy change:
when 1 mole of gaseous ions dissolve in sufficient water to give an infinitely dilute solution
Enthalpy change of electron affinity:
The enthalpy change (heat evolved) when 1 mole of gaseous atoms acquires 1 mole of electrons to form 1 mole of gaseous 1- ions
Enthalpy change of lattice energy:
The heat energy change
when 1 mole of solid ionic lattice
is formed from its scattered gaseous ions
nb: lattice energies are exothermic therefore describe lattice energies as being more or less exothermic
standard enthalpy change of reaction
This is the enthalpy change (heat evolved or absorbed) when substances react under standard conditions in quantities given by the equation for the reaction
second electron affinity
is the energy needed to add a second electron to each ion in 1 mole of gaseous 1- ions to give gaseous 2- ions.
nb: second electron affinities are positive. energy is needed to force an electron onto an already negative ion
what sign are hydration enthalpies always?
Hydration enthalpies are always negative. Heat is evolved due to the formation of attractive forces between the ions and the water molecules
does the enthalpy of hydration depend on the type of charge or number of charge?
the enthalpy of hydration refers equally to positive or negative ions and is irrespective to the number of charges
how to calculate enthalpy of solution (you cannot work it out directly)?
draw arrow from ionic solid to ions and label this lattice energy (regardless of which definition of lattice energy you use)
this is done as you are showing ‘how much heat energy is needed to split the lattice up before you recover the heat again as the ions become hydrated’
