Born haber (Thermodynamics)

Question 1

what is enthalpy of lattice formation

Answer 1

Standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituents ions in gaseous form

Question 2

what is enthalpy of lattice dissociation

Answer 2

Standard enthalpy change when 1 mole of an ionic crystal lattice is separated into its constituents in gaseous form

Question 3

what is the enthalpy change of formation

Answer 3

Energy transferred when 1 mile of the compound is formed from its elements under standard conditions ( 298k and 100kpa) , all reactants and products being in their standard states

Question 4

what is enthalpy of atomisation

Answer 4

Is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard states

Question 5

what is bond dissociation enthalpy

Answer 5

Standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms

Question 6

what is the first ionisation enthalpy

Answer 6

Enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge

Question 7

second ionisation enthalpy

Answer 7

the enthalpy change to remove 1 mole of electrons from one mole of gaseous +1 ions to produce one mole of gaseous 2+ ions

Question 8

what is the first electron affinity

Answer 8

Enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge

Question 9

what is the second electron affinity

Answer 9

the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions

Question 10

what is enthalpy change

Answer 10

heat energy at constant pressure

Question 11

EQ : In terms of the forces acting on particles , suggest one reason why the first electron affinity of oxygen is an exothermic process (1)

Answer 11

There’s an attractive force between the nucleus of an O atom and an external electron

Question 12

EQ : Explain why the second ionisation energy of calcium is greater than the first ionisation energy of calcium

Answer 12

electron removed is closer to the nucleus

Question 13

what is the enthalpy of hydration of an ion

Answer 13

enthalpy change when 1 mole of a gaseous ion forms aqueous ions
e.g. Mg2+ (g) => Mg2+ (aq)

Question 14

what is an equation to help with the definition of electron affinity

Answer 14

X(g) + e- => X- (g)

Question 15

what is a perfect ionic lattice

Answer 15

• ions are in perfect sphere
• Only electrostatic attraction

Question 16

what is enthalpy change of combustion

Answer 16

The energy change when 1 mole of a substance is burned in excess oxygen with all reactants and products in their standard states

Question 17

what is enthalpy of solution

Answer 17

enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another
e.g NaCl (s) + aq => Na+(aq) + Cl-(aq)

Question 18

how do you work out enthalpy change of solution

Answer 18

Enthalpy of lattice dissociation + sum of enthalpy of hydration

Question 19

how do you work out lattice dissociation enthalpy from the value of lattice formation enthalpy

Answer 19

reverse the + or - symbol

Question 20

why in a born haber cycle when calculating for electron affinity the answer has to be a minus

Answer 20

because it’s an exothermic process , energy is released when Cl gains an electron

Question 21

EQ : explain why there is a difference between the hydration enthalpy of the magnesium and sodium ions

Answer 21

magnesium ion is smaller and more charged
magnesium ion attracts water more strongly

Question 22

how could the conditions for the reaction be changed to make the reaction feasible

Answer 22

If temp is lowered , G will become more negative because -TS will be less positive

Question 23

why is the second electron affinity endothermic for oxygen

Answer 23

electron and ion both have negative charges so repel each other

Question 24

why is the first electron affinity exothermic for atoms that form negative ions

Answer 24

Ion is more stable than the atom and there is an attraction between nucleus and electron

Question 25

for enthalpy of hydration why is it an exothermic process

Answer 25

- bonds are made between the ions and the water molecules

Question 26

what factors does the strength of an enthalpy of lattice formation depend on

Answer 26

- sizes of the ions • the larger the ions the less negative the enthalpy of lattice formation as ions are larger the charges become further away so have a weaker attractive force - Charges on the ion • the bigger the charge the greater the attraction between ions so stronger lattice enthalpy

Question 27

when is there a tendency towards covalent character in ionic substances

Answer 27

positive ion is small positive ion has multiple charges negative ion is large negative ion has multiple reactive charges

Question 28

EQ : explain why evaporation of water is spontaneous even though this change is endothermic . Refer to change in arrangement of water molecules and entropy change (4)

Answer 28

- molecules become more disordered when water evaporates - entropy change is positive - T delta S > delta H - delta G < 0

Question 29

if you increase delta S does delta G become more or less positive than 0

Answer 29

Delta G becomes less positive so feasible

Question 30

EQ : A reaction is endothermic and is accompanied by a positive entropy change . Which statement best describes the feasibility of the reaction (1)

Answer 30

It is feasible above a certain temperature

Question 31

when will delta G always be negative

Answer 31

When there’s an increasing entropy and is exothermic ( decrease in delta H)

Question 32

What if a reaction involves a decrease in entropy what will happen to delta G if we increase temp

Answer 32

increasing temp will make it less likely that Delta G is negative delta G > 0

Question 33

what if the reaction has an entropy change close to zero what will happen to delta G if we increase temperature

Answer 33

Will not have a large effect on the feasibility of the reaction as -T delta S will be small and Delta G will not change much

Question 34

Why are hydration enthalpies exothermic

Answer 34

Energy given out when water molecules bond to metal ions . - Negative ions are electro statically attracted to delta positive hydrogens on the polar water molecules and the positive ions are electro statically attracted to delta negative oxygen on the polar water molecules

Question 35

what increases the hydration enthalpy

Answer 35

The higher the charge density ( smaller ions or ions with larger charges )

Question 36

EQ : which statement best describes the reason why silver iodide is insoluble in water

Answer 36

It’s enthalpy of solution is endothermic

Question 37

In enthalpy of hydration , is the substance more likely to be soluble if the enthalpy of solution is exothermic or endothermic

Answer 37

Exothermic

Question 38

EQ : At standard pressure , nitrogen dioxide condenses at 21.2 degrees celsius . Explain the effect that condensation has on its entropy (2)

Answer 38

- molecules in a liquid are less free to move around than molecules in a gas - less disorder - entropy decreases

Question 39

What’s the value of delta S if the graph is negative

Answer 39

Positive delta S

Question 40

what’s the value of a positive graph on Delta S

Answer 40

Delta S is negative

Question 41

EQ : the enthalpy of solution of sodium chloride is slightly endothermic at 298K . Explain why sodium chloride is reasonably soluble in water at room temp (3)

Answer 41

- Dissolving gives free ions in solution so increases disorder - entropy change increases - this means that the reaction can be feasible / delta G can be negative even when delta H is positive

Question 42

what is the equation when working out lattice enthalpy of dissociation ( arrow pointing up) in a born haber

Answer 42

Enthalpy of formation + enthalpy of lattice dissociation = sum of values

Question 43

what is the equation when working out lattice enthalpy of formation ( arrow pointing down ) in a born haber

Answer 43

enthalpy of formation = enthalpy of lattice formation + sum of values

Question 44

EQ : explain why theoretical enthalpy of lattice dissociation is different from the experimental value calculated using a born haber cycle (2)

Answer 44

- experimental lattice enthalpy includes covalent character - theoretical lattice enthalpy assumes that it’s perfectly ionic

Question 45

EQ : in terms of electrostatic forces , suggest why the electron affinity of fluorine has a negative value (2)

Answer 45

- attraction between proton and electron - energy is released