17.1 Enthalpy change Flashcards
What is an enthalpy change
What are the standard conditions
When you measure a heat change at constant pressure
298K , 100kpa
What is the enthalpy of formation
∆fH
What would the value of this be for an element
The enthalpy change when 1 mol of a compound is formed from its constituent elements under standard conditions, when they are in their standard states
It would be zero because it takes no energy to form a naturally occurring compound
What is enthalpy of combustion
∆cH
The enthalpy change when 1 mol of substance is burned completely in oxygen, under standard conditions with reactants and products in standard states
What is standard enthalpy of atomisation
∆atH
The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state, under standard conditions
What is first ionisation energy
∆iH
The standard enthalpy change when 1 mol of gaseous atoms are converted into 1 mol of 1+ ions
What is the second ionisation energy
The enthalpy change when 1 mol of gaseous 1+ ions are converted into 1 mol of gaseous 2+ ions
So a mole of electrons are removed
What is the first electron affinity
∆eaH
The enthalpy change when mole of gaseous atoms are converted into one mole of gaseous 1- ions
What is the second electron affinity
∆eaH
The enthalpy change when one mole of gaseous 1- ions are converted into 1 mol of gaseous 2- ions
What is lattice enthalpy of formation
∆LH
Why is the value always negative for this
The standard enthalpy change when one mole of solid ionic compound is made from its gaseous ions
. When a lattice forms, new bonds are formed so it releases energy making it exothermic
What is Lattice enthalpy of dissociation
Why is the value always positive for this
The standard enthalpy change when one mole of solid ionic compound is dissociated into its constituent gaseous ions
When a lattice is broken, the bonds break so require energy for this, so take in energy from surroundings so it is endothermic
What is enthalpy of hydration
∆hydH
The standard enthalpy change when 1 mole of gaseous ions are converted into aqueous ions
eg Na+(g) + aq —> Na+(aq)
What is enthalpy of solution
∆solH
The standard enthalpy change when one mole of solute dissolves completely in enough solvent, so there is no more enthalpy change on further dilution
What is mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds
What is ionic bonding
Electrons are transferred from metal atoms to non metal atoms, so ions are formed which have stable outer shells of electrons
The ions arrange themselves into an ionic lattice so that the opposite charged ions are next to each other
