topic 13b: entropy

Question 1

why does enthalpy changed alone not determine whether reactions occur

Answer 1

endothermic reactions can still occur

Question 2

what is entropy

Answer 2

measure of disorder of a system

Question 3

order of priority when considering entropy

Answer 3

1. state (gas>liquid>solid)
2. increase in number of molecules

Question 4

explain entropy changes for states

Answer 4

1. solid lowest entropy as particles can only vibrate about fixed positions. lowest disorder
2. liquid more as can slide over each other, but confined by container. more disorder
3. gases the most as free to move beyond confines of container. most diosrder
4. boiling is a bigger increase in entropy than melting

Question 5

unit of entropy

Answer 5

J/K/Mol

Question 6

perfect crystal

Answer 6

• zero entropy
• at abosulte 0 (0K)

Question 7

delta s system

Answer 7

• entropy products - entropy reactants

Question 8

delta s surroundings

Answer 8

• -delta h (j) / T

Question 9

total entropy change (delta s total)

Answer 9

delts s system + delta s surroundings

Question 10

if a reaction is spontaneous then … (delta s total)

Answer 10

is POSITIVE

Question 11

delta G

Answer 11

delta H - T delta s system
if negative then feasible

Question 12

delta g unit

Answer 12

j or kj / mol

Question 13

delta g during physical phase changes

Answer 13

0
eg melting boiling etc

Question 14

delta g in terms of equilibrium

Answer 14

delta g = -RT lnK

Question 15

EVEN IF A REACTION IS FEASIBLE, WHY MIGHT IT STILL NOT OCCUR

Answer 15

• activatoin energy barrier too high

Question 16

what does it mean for a reastion to be spontaneous

Answer 16

delta s total is positive

Question 17

Explain how lattice energy values, together with other data, can be used to
predict the solubility of ionic compounds.

Answer 17

• delta h solution = -LE + delta h hydration
• the more exothermic the more soluble
• depednds on SIZE and CHARGE of ions

Question 18

why night you not get MgCl3 but you can get CoCl3

Answer 18

• 3rd ionisation energy for Mg too endothermic
• as electron removed from x subshell

Question 19

assumption when changing temp in delta s

Answer 19

• delta s system not affected by temperature change

Question 20

why does volume decrease in hydration

Answer 20

• shorter bonds between ion and water