Section 6 - Thermodynamics Flashcards
Define enthalpy change
The heat energy transferred in a reaction at constant pressure
What are standard conditions for enthalpy change?
298K and 100kPa
What type of reaction has a positive enthalpy change?
Endothermic
What type of reaction has a negative enthalpy change?
Exothermic
Define enthalpy change of formation
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard cpmdotopms
Define bond dissociation enthalpy
The enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken
Define enthalpy change of atomisation of an element
The enthalpy change when 1 mole of gaseous artoms is formed from an element in its standard state.
Define enthalpy change of atomisation of a compound
The enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms
Define first ionisation energy
The enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms
Define second ionisation energy
Enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions
Define first electron affinity
Enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms
Define second electron affinity
Enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions
Define enthalpy change of hydration
Enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions
Define enthalpy change of solution
Enthalpy change when 1 mole of solute is dissolved in enough solvent that no furhter enthalpy change occurs on further dilution
Define lattice enthalpy of formation
Enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions under standard conditions
Define lattice enthalpy of dissociation
Enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions
What is Hess’ Law?
Enthalpy change is independent of route taken
Why are theoretical lattice enthalpies different from experimental values?
Theoretical lattice enthalpies are based on the assumption that all ions are spherical and have their charges evenly distributed.
Usually slight polarisation occurs between neighbouring positive and negative ions so most ionic compounds have some covalent character and aren’t exactly spherical.
Why does dissolving have an enthalpy change?
The bonds between ions break which is endothermic
Bonds form between the ions and water which is exothermic
The enthalpy change of solution is the overall effect of these
Why can water molecules bond to ions?
Oxygen is more electronegative than hydrogen so water is polar. The H can bond to the negative ions and the O can bond to the positive ions.
Why do soluble substances tend to have exothermic enthalpies of solution?
Substanes tend to only dissolve if the energy released is roughly the same as or greater than the energy taken in.
What is entropy?
A measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between particles.
What factors affect entropy?
- physical state
- amount of particles
Do stable substances have more or less disorder?
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