energy Flashcards
define standard lattice enthalpy
enthalpy change when 1 mole of ionic lattice is formed from its gaseous ions under standard conditions
what is lattice enthalpy a measure of ?
ionic bond strength
stronger ionic bonds have a more … lattice enthalpy
more negative
explain why smaller / more charged ions have more -ve lattice enthalpy :
- greater charge density of the ions involved
- so stronger electrostatic attraction between ions
- more -ve lattice enthalpy
what is Hess’s law ?
total enthalpy change of a reaction is always the same, no matter what route
what is entropy a measure of ?
number of ways that particles can be arranged (disorder)
the more disordered…
the higher the entropy is
why do solids have a low entropy ?
vibrate at a fixed point – no random arrangements
why do gases have a high entropy ?
constantly moving – random arrangements
what are the factors of entropy ?
- physical state (gas is high)
- no. of moles (more moles is high)
what is the symbol for entropy ?
S
what are the units for entropy ?
J / K / mol
delta S =
S of products - S of reactants
what does feasible mean ?
a reaction that will occur spontaneously (without addition of energy)
what is thermodynamically stable ?
when a substance reaches maximum entropy state
what is the symbol for free energy ?
G
what does delta G tell you ?
whether a reaction is feasible or not
a reaction is feasible if delta G is …
negative
delta G =
delta H - ( T x delta S)
what are the units for free energy ?
J / mol
is the reaction feasible when delta G = 0 ?
just feasible, so temp at which reaction becomes feasible can be worked out by rearranging the equation
when will delta G always be +ve ?
what does it mean ?
delta H is +ve & delta S is -ve
not feasible
when will delta G always be -ve ?
what does it mean ?
delta H is -ve & delta S is +ve
feasible
why is a reaction not guaranteed if it is theoretically feasible ?
- high Ea
- very slow reaction
what is the equation for enthalpy change ?
q = mc delta T
standard enthalpy change of reaction
EC when reaction occurs in molar quantities same as chemical equation
standard enthalpy change of formation
EC when 1 mol of a compound is formed form its elements in their standard states
standard enthalpy change of combustion
EC when 1 mol of substance is completely burned in oxygen
standard enthalpy change of neutralisation
EC when acid + alkali to form 1 mol of water
soluble substances tend to have a …. enthalpy change of solution
exothermic (negative)
if temp increases, what happens to E.C. of sol?
E.C. is -ve
- energy change
