3.1.8.1 - born-haber cycles

Question 1

if starting from LiCI (s) how do you create born haber cycles going up
then going down

Answer 1

enthalpy of formation (exo) -> Li(S) + 1/2CL2(g)

enthalpy of atomisation of chlorine Li(s) + cl(g)

enthalpy atomisation of lithium Li(g) + cl (g)

1st ionisation energy of lithium li+(g) + cl(g) + e-

going down..

1st electron affinity of chlorine (exo)
Li+(g) + Cl-(g)

lattice enthalpy of formation of lithium chloride
LiCI (s)

Question 2

steps going up __ ____ ____ as the steps going down

Answer 2

steps going up is the same as the steps going down

Question 3

when calculating born haber cycles
when you go w the arrow you …
when you go agianst the arrow you…

Answer 3

when you go with the arrow, keep the sign the same
when you go against the arrow change the sign

Question 4

down arrows are ..

Answer 4

exo

Question 5

up arrows are ..

Answer 5

endo

Question 6

if calc enthalpy for 1/2 cl2 and values given is for cl2 what do you do

Answer 6

half the enthalpy

Question 7

The greater the value of the lattice enthalpy, the ….

Answer 7

stronger the ionic bonds

Question 8

Smaller ions and higher charge ions give stronger ionic bonding
because

Answer 8

Smaller ions are more closely packed in the lattice, so are more
attracted to each other, resulting in high lattice enthalpy.
* Ions with high charge have stronger electrostatic forces of attraction
between the ions which results in high lattice enthalpy

Question 9

assumptions of perfect ionic model

Answer 9

✴The bonding in the compound is completely ionic.
✴The ions are regarded as perfect spheres + are not distorted (i.e. have
their charge evenly distributed around them).

Question 10

if expirmental value is diff from theortical value it indicates..

Answer 10

compound doesnt follow perfect ionic model - there is some covalent character to the compound

Question 11

when is their covalent character in ionic compounf

Answer 11

a small and highly charged cation can distort a large anion so
that there is covalent character to the ‘ionic compound’. This means that
ions are distorted and not spherical (That IS assumed by the perfect
ionic model)
positive ion polarises negative ion

Question 12

the more polarisation we get…

Answer 12

the more covalent character there will be

Question 13

covalent character to a compound leads to …

Answer 13

This leads to a difference between the values of the experimental
lattice enthalpy and the theoretical lattice enthalpy. The greater the
difference in the values, the greater the covalent character in the
compound.

Question 14

define enthalpy change of solution

Answer 14

the enthalpy change when one mol of an ionic sub is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution

Question 15

2 things that need to occur for a substance to dissolve

Answer 15

substance bonds must break (endo)
new bonds formed between solvent and substance (exo)

Question 16

explain how a substance is dissolved

Answer 16

ionic lattice in solid form
substance bonds broken to create to create free moving

Question 17

steps of dissolving a solid ionic lattice

Answer 17

1) ionic lattice in solid form
+ water
2) substance binds broken to create free moving ions
3) bonds formed between ions and water - ions are hydrated

Question 18

most ionic compounds dissolve in _____ solvents

Answer 18

most ionic compounds dissolve in polar solvents

Question 19

how do ionic compounds dissolve in polar solvents

Answer 19

the delta positive H is attracted to negative ions and the delta neg oxygen is attracted to positive ions, the struc starts to break down

Question 20

the water molecules surrond the ions in a process called …

Answer 20

hydration

Question 21

what must happen for hydration to occur

Answer 21

new bonds formed must be the same strength or greater than those broken
if not then the sub is very unlikely to dissolve. soluble sub tend to have exothermic enthalpies of solution bc of this

Question 22

enthalpy of hydration

Answer 22

1 mole of aqueous ions are formed from gaseous ions
Na+ (g) + aq –> Na+(aq)
Cl-(g) + aq –> Cl-(aq)

Question 24

extra steps needed for born haber cycle of group elements

Answer 24

Second ionisation energy of Mg needs to be included because Mg forms 2+
ions.
* Atomisation of Cl is multiplied by 2 as there are 2 moles of Cl for
every 1 mole of Mg
* First electron affinity of Cl is multiplied by 2 as there are 2 moles
of Cl- formed in the ionic lattice.

Question 25

the bigger the difference in lattice enthalpy

Answer 25

the more polarisation you have
the bigger the lattice enthalpy