Enthalpy Flashcards
enthalpy of formation
enthalpy change when one mole of a substance is formed form its constituent elements with all substances in their standard conditions
- exothermic (generally)
enthalpy of combustion
enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states
- exothermic
enthalpy of neutralisation
enthalpy change when 1 mole of WATER is formed in a reaction between an acid and an alkali under standard conditions
- exothermic
ionisation enthalpy (first)
enthalpy change when one mole of gaseous atoms loosed one election to from one more of gaseous 1+ ions
- endothermic
ionisation enthalpy (second)
enthalpy change when one mole of gaseous 1+ ions loosed one election to from one more of gaseous 2+ ions
- endothermic
electron affinity (first) exo/endo?
enthalpy change when each atom in one mole of gaseous atoms gains one electron to become 1- ions
- exothermic
electron affinity (second)
enthalpy change when each atom in one mole of gaseous 1- ions gains one electron to become 2- ions
- endothermic
enthalpy of atomisation
enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
- endothermic
hydration enthalpy
enthalpy change when one mole of gaseous ions becomes hydrated (dissolves in water)
- exothermic
enthalpy of solution
enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that all the dissolved ions are well separated and do not interact with each other (dissociated ions)
-varies
bond dissociation enthalpy
enthalpy change when one mole of covalent bonds is broken in the gaseous state
- endothermic
lattice enthalpy of formation
enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in GAS state
- exothermic
lattice enthalpy of dissociation
enthalpy change when one mole of solid ionic compound is broken up into its constituent ions in their GAS state
- endothermic
enthalpy of vaporisation
energy change when one mole of a liquid is turned into a gas
- endothermic
enthalpy of fusion
energy change when one mole of a solid is turned into a liquid
- endothermic
average bond enthalpy
the average enthalpy change that takes place when breaking by homiletic fission 1 mol of a given type of bond in the molecules of a gaseous species
bond dissociation enthalpy
the enthalpy change when 1 mol of a given bond in the molecules of a gaseous species is broken by homiletic fission
overall enthalpy change (eq)
[sum of enthalpy of formation product] - [sum of enthalpy of formation of reactants]
Hess’ law
if a reaction takes place by 2 routes, and starting and finishing conditions are the same the total enthalpy change will be the same for each route
symbol for enthalpy
H
units of enthalpy
kJmol-1
value of ΔH when reaction is exothermic
negative
value of ΔH when reaction is endothermic
positive
enthalpy
energy stored within the bonds
standard lattice enthalpy
energy change when 1 mole of an ionic lattice is formed from gaseous ions under standard conditions
standard conditions
298K
100Pa
standard lattice enthalpy is a measure of.... more negative (exothermic) the value the...
ionic bond strength
the stronger the bonding
smaller the ionic radius of the ions involved in lattice enthalpy…
more exothermic - higher charge density and smaller radius means ions can sit closer together making attractions between ions stronger
higher the charge of the ions involved in lattice enthalpy…
more exothermic - more energy released when lattice forms as stronger electrostatic attraction between ions
standard state
physical state under standard conditions
bond making….. energy
releases (exothermic)
bond breaking…. energy
requires (endothermic)
collision theory
in order for parties to react, they must collide in the right direction and orientation with a minimum amount of kinetic energy
(eq) percentage error
= error in measurement / measurement taken x 100
gibs free energy
ΔG = ΔH-TΔS
value of gibs energy when reaction is feasible
negative
why might calculated enthalpy change may not be the same as experimental value?
because values used are averaged across range of molecules
exact values depend on particular environment/molecule
potential disadvantages of catalysts (2)
- toxic
- expensive
average bond enthalpy is a measure of …
average strength of a covalent bond
enthalpy of reaction
enthalpy change accompanying reaction in molar quantities shown in chemical equation under standard conditions with all reactants and products in standard states
q =
Mc Δ T
experimental method for investigating ΔH combustion of methanol
- add methanol to spirit burner and weigh
- place unde breaker of 150cm3 water - measured usingg measuring cylinder
- record initial temp of water
- light spirit burner and leave for 3 mins
- extinguish flame and immediately record highest temp reached
- reweigh spirit burner
what does q stand for in q=mcΔt
energy change
experimental method for investigating ΔHr for 2 liquids/solutions
use polystyrene cup
entropy
dispersal of energy in a system and how disordered it is
standard entropy
entropy of one mol of substance under standard conditions
units of entropy
J/K/Mol
feasibility
whether a reaction can happen or not
free energy
overall energy change during a chemical reaction
free energy equation
ΔG = ΔH - TΔS
Why might some reactions not appear to take place even though free energy is negative?
because rate isn’t taken into account - rate may be so slow reaction does not appear to be happeneing
factors affecting lattice enthalpy
- ionic radius
- ionic charge
when (in terms of enthalpy) will a substance dissolve?
hydration>lattice
predict difference in hydration enthalpies of F- and Cl-
F- will me more negative
as smaller atomic radius
smaller sized ion has greater attraction to H2O
another way of calculating ΔH
q (energy) / moles
why is first election affinity sometimes positive
it takes energy to overcome repulsive intermolecular forces
why may experimental enthalpy value be different to predicted?
- some heat lost to surroundings not just to water
- incomplete combustion - CO not CO2
- evaporation of methanol from wick
- non standard conditions
bond enthalpies are always…
ENDOthermic
to dissolve..
HYRDATION enthalpy»_space;> LATICE enthalpy
entropy equation
products - reactants
free energy
overall energy change In a reaction
limitations of feasibility w gibbs (2)
some reactions may not appear to occur as…
- large activation energy
- slow rate
