topic 13b: entropy Flashcards
why does enthalpy changed alone not determine whether reactions occur
endothermic reactions can still occur
what is entropy
measure of disorder of a system
order of priority when considering entropy
- state (gas>liquid>solid)
- increase in number of molecules
explain entropy changes for states
- solid lowest entropy as particles can only vibrate about fixed positions. lowest disorder
- liquid more as can slide over each other, but confined by container. more disorder
- gases the most as free to move beyond confines of container. most diosrder
- boiling is a bigger increase in entropy than melting
unit of entropy
J/K/Mol
perfect crystal
- zero entropy
- at abosulte 0 (0K)
delta s system
- entropy products - entropy reactants
delta s surroundings
- -delta h (j) / T
total entropy change (delta s total)
delts s system + delta s surroundings
if a reaction is spontaneous then … (delta s total)
is POSITIVE
delta G
delta H - T delta s system
if negative then feasible
delta g unit
j or kj / mol
delta g during physical phase changes
0
eg melting boiling etc
delta g in terms of equilibrium
delta g = -RT lnK
EVEN IF A REACTION IS FEASIBLE, WHY MIGHT IT STILL NOT OCCUR
- activatoin energy barrier too high
what does it mean for a reastion to be spontaneous
delta s total is positive
Explain how lattice energy values, together with other data, can be used to
predict the solubility of ionic compounds.
- delta h solution = -LE + delta h hydration
- the more exothermic the more soluble
- depednds on SIZE and CHARGE of ions
why night you not get MgCl3 but you can get CoCl3
- 3rd ionisation energy for Mg too endothermic
- as electron removed from x subshell
assumption when changing temp in delta s
- delta s system not affected by temperature change
why does volume decrease in hydration
- shorter bonds between ion and water
