Chapter 22: enthalpy and entropy (22.1 - 22.3)

Question 1

what does lattice enthalpy measure

Answer 1

the strength of the ionic bonding in a giant ionic lattice

Question 2

define lattice enthalpy (Δlatt H)

DEFINITION REQUIRED

Answer 2

the enthalpy change the accompanies the formation of 1 mole of an ionic compounds from its gaseous ions under standard conditions.
It is always exothermic

Question 3

how do you work out lattice enthalpy (not experimentally)

Answer 3

born-haber cycles (see page 346 in textbook)

Question 4

define standard enthalpy of formation (Δf H)

Answer 4

the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 5

define standard enthalpy of atomization (Δatt H)

Answer 5

the enthalpy change that takes place for the formation of 1 mole of gaseous atoms from the element in its standard state

Question 6

define first ionization energy (ΔIE1 H)

DEFINITION REQUIRED

Answer 6

the enthalpy change required to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Question 7

define first/second electron affinity (ΔEA1 H / ΔEA2 H)

Answer 7

Energy required for each atom/ion in 1 mole of gaseous atoms/1- ions to gain 1 electron to form 1 mole of gaseous 1- ions/2- ions

Question 8

difference between first and second electron affinity

Answer 8

EA1 is exothermic (-ΔH)

EA2 is endothermic (+ΔH)

Question 9

factors affecting lattice enthalpy

Answer 9

ionic radius: smaller ions = stronger attraction = larger lattice enthalpy

ionic charge: larger charge = stronger attraction = larger lattice enthalpy

Question 10

define enthalpy change of solution (Δsol H)

DEFINITION REQUIRED

Answer 10

the enthalpy change that takes place when 1 mole of compound is dissolved in water under standard conditions .

Can be endothermic or exothermic.

Question 11

what happens when an ionic compound dissolves

Answer 11

• ionic lattice breaks up forming gaseous ions (opposite of Δlatt H)
• gaseous ions interact with polar water molecules to form hydrated aqueous ions (Δhyd H)

Question 12

how is water attracted to the ions which are being dissolved

Answer 12

𝛿- oxygen attracted to positive ion

𝛿+ hydrogen attracted to negative ion

Question 13

equation to work out energy change

Answer 13

q = mcΔT

q joules, m grams, c given, ΔT in K (change in temp)

Question 14

define enthalpy change of hydration (Δhyd H)

DEFINITION REQUIRED

Answer 14

the enthalpy change that accompanies the dissolving of gaseous ions in water forming 1 mole of aqueous ions

Question 15

general properties of ionic compounds

Answer 15

high melting and boiling points
soluble in polar solvents
conduct electricity when molten or in aqueous solution

Question 16

factors effecting hydration

Answer 16

ionic radius: smaller ions = stronger attraction between ion and water molecule = hydrations energy more negative

ionic charge: larger charge = stronger attraction = more negative hydration energy

Question 17

what is entropy

Answer 17

a measure of the dispersal of energy in a system

units: J/K/mol

Question 18

greater the entropy means

Answer 18

greater disorder / greater dispersal of energy
eg. solid -> liquid is increase in entropy
1 mol gas -> 2 mol gas is increase in entropy

Question 19

why do reactions tend towards higher entropy

Answer 19

energy has a natural tendency to spread out rather than be concentrated in one place

Question 20

equation for entropy change ΔS°

Answer 20

ΔS° = ΣS°(products) - ΣS°(reactants)

Question 21

what is feasibility

Answer 21

A reaction is ‘energetically feasible’ if the free energy change (ΔG) is negative

Depends on entropy change, enthalpy change and temperature

Question 22

equation for free energy ΔG

Answer 22

ΔG = ΔH - TΔS
(ΔH= enthalpy change of reaction. K= temp in K. ΔS= entropy change of reaction)
remember to convert ΔS into kJ as its standardly in J

Question 23

units of ΔG

Answer 23

kJ/mol

Question 24

limitations on ΔG for determining if a reaction will happen

Answer 24

• takes no account of the kinetics (rate of reaction, activation energy)
• assumes standard conditions