Thermodynamics

Question 1

enthalpy of formation

Answer 1

enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states under standard conditions - exothermic (usually)

Question 2

enthalpy of combustion

Answer 2

enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states under standard conditions - exothermic

Question 3

enthalpy of neutralisation

Answer 3

enthalpy change when 1 mole of water is formed in a reaction between an acid and an alkali under standard conditions - exothermic

Question 4

ionisation enthalpy

Answer 4

first: enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions
second: enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions
-endothermic

Question 5

electron affinity

Answer 5

First: enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
- exothermic
Second: enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
-endothermic

Question 6

enthalpy of atomisation

Answer 6

enthalpy change when one mole of gaseous atoms is produced from an element in its standard state - endothermic

Question 7

hydration enthalpy

Answer 7

enthalpy when one mole of gaseous ions become hydrated (dissolved in water) - exothermic

Question 8

enthalpy of solution

Answer 8

enthalpy change when one mole of an ionic solid is dissolved - varies

Question 9

bond dissociation enthalpy

Answer 9

enthalpy change when one mole of covalent bonds is broken in the gaseous state - endothermic

Question 10

lattice enthalpy of formation

Answer 10

enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gas phase - exothermic

Question 11

lattice enthalpy of dissociation

Answer 11

enthalpy change when one mole of a solid ionic compound is broken into its constituent ions in the gas phase - endothermic

Question 12

enthalpy of vaporisation

Answer 12

enthalpy change when one mole of a liquid is turned into a gas - endothermic

Question 13

enthalpy of fusion

Answer 13

enthalpy change when one mole of a solid is turned into a liquid - endothermic

Question 14

born-haber cycle

Answer 14

ionise metal –> down 2nd ea –> down
1st ea non-metal —> up
up
atomise non-metal lattice
up of
atomise metal formation
up
start line: elements in standard states
down (enthalpy formation)
base line: solid ionic compound <———————————-

Question 15

enthalpy change definition

Answer 15

change in heat content at constant pressure

Question 16

Hess’s law?

Answer 16

The enthalpy change for a reaction is independent of the route taken
A —> B = A—>C—>B

Question 17

enthalpy of formation hess law

Answer 17

elements enthalpy of formation always 0
arrows pointing up from constituent elements
sum of product enthalpy - sum of reactant enthalpy

Question 18

enthalpy of combustion hess law

Answer 18

arrows pointing down to oxides
reaction across top often enthalpy of formation

Question 19

bond enthalpy hess law

Answer 19

arrows pointing down to gas atoms
bond enthalpies are mean not specific for compound so not very accurate

Question 20

equation for enthalpy change and conversion to kJ/mol
(calorimetry)

Answer 20

q = mcAt
q: heat energy given out (J)
m: mass of substance heated (g)
AT: temperature rise (K)
c: specific heat capacity (J/g/K)
Then
/1000 for kJ
Finally
enthalpy change/mol = q/number of moles reacting

Question 21

problems with calorimetry calculations + solution

Answer 21

heat loss major problem - to reduce you can measure heat capacity of calorimeter as a whole

Question 22

enthalpy of solution hess cycle

Answer 22

gas ions —-> ionic solid (lattice enthalpy formation)
gas ions —-> dissolved (aq) ions (hydration enthalpy)

arrows pointing down from gas ions to

ionic solid ——-> dissolved (aq) ions (enthalpy of solution)

Question 23

trend in lattice enthalpy in regards to ionic bonding

Answer 23

greater magnitude of lattice enthalpy, greater ionic bonding and vice versa
compounds with smaller ion and/or greater charge have stronger attraction so greater lattice enthalpy

Question 24

how to find experimental lattice enthalpy?

Answer 24

born-haber cycle

Question 25

what is perfect ionic model?

Answer 25

theoretical calculation for lattice enthalpy that considers size, charge and arrangement of ions in lattice - assumes structure is perfectly ionic

Question 26

problem with perfect ionic model

Answer 26

often distortion of ions in ionic compound (not perfectly spherical) - they have covalent character bigger difference between theoretical and real value in general, the greater the covalent character still ionic bonding but some covalent properties such as low solubility, lower melting point/conductivity than expected

Question 27

entropy

Answer 27

Entropy (S) is how disordered something is and is measured in J/mol/K Gases have most entropy Entropy will naturally increase over time

Question 28

entropy

Answer 28

Entropy (S) is how disordered something is and is measured in J/mol/K Gases have most entropy Entropy will naturally increase over time

Question 29

how does entropy vary with temperature?

Answer 29

Zero at absolute zero and increases with temperature Big increases in entropy at state changes - greatest from liquid to gas as entropy substantially larger in gases than solids or liquids

Question 30

entropy change

Answer 30

change in S = [sum S products] - [sum S reactants] reactions with increase in entropy are favourable sometimes very small change so unable to tell if it will be positive or negative

Question 31

Gibbs free energy change

Answer 31

change in G combines change in H and change in S AG (kJ/mol) = AH (kJ/mol) - T (K) AS (J/mol/K) need to convert AS to kJ/mol/K Feasible reaction if AG less than or equal to 0 Depends on temperature if feasible or not AG = 0 is the point at which reaction becomes feasible If feasible still may not occur due to having very high activation energy