module 5: thermodynamics

Question 1

define enthalpy change of atomisation

Answer 1

the enthalpy change when 1 mol of gaseous monoatomic element is formed from elements in its standard state

Question 2

define lattice enthalpy

Answer 2

the enthalpy change when 1mol of an ionic solid is made from the gaseous ions

represents ionic lattice bonding strength, always exothermic

Question 3

outline how you would structure a born-haber cycle

Answer 3

1st ionisation: e-
2nd ionisation: 2e-

Question 4

what factors can affect lattice enthalpy and why

Answer 4

ionic charge and ionic radius

high ionic radius = low lattice enthalpy
smaller ions can get closer and have stronger attraction

high ionic charge = high lattice enthalpy
greater charge means electrostatic attraction is stronger

Question 5

what is charge density

Answer 5

it’s dependent on ionic radius and ionic charge, a higher charge density gives rise to a greater lattice enthalpy

Question 6

what is entropy (S)

Answer 6

a measure of the disorder of energy/how spread out it is

Question 7

how does an equation show whether entropy has increased or decreased

Answer 7

gases are more disordered than liquids and solutions which are more disordered than solids as they have more movement
so entropy has increased if there is a higher number of moles of items that are more disordered and vide versa for decreased entropy

increase in entropy: CaCO3(s) → CaO(s) + CO2(g)

Question 8

what is meant by feasible or spontaneous reactions

Answer 8

the idea that reactions are energetically favourable and therefore should occur
but it may not take place because of a high activation energy etc.
they will always have an entropy change > 0

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Question 9

how are exothermic reactions linked to entropy

Answer 9

exothermic reactions transfer heat to surroundings and so increase the energy disorder of the surroundings and so increasing entropy of surroundings

Question 10

how are endothermic reactions linked to entropy

Answer 10

in endothermic reactions thermal energy is transferred from the surroundings into the system
this decreases energy disorder and so entropy of the system

Question 11

how can the total entropy change for any process be calculated

Answer 11

for any process, total entropy change is equal to entropy change of system plus entropy change of surroundings

Question 12

how is the standard entropy change (ΔS) of a system calculated

Answer 12

ΔS = entropy of products - entropy of reactants

Question 13

what equation calculates Gibbs free energy change

Answer 13

if deltaG is negative that means that the reaction is feasible/spontaneous

Question 14

what do you assume when finding the temperature at which a reaction would be spontaneous

Answer 14

that deltaH and deltaS aren’t affected by temperature

Question 15

how do you know what sign should be on a value that was calculated using an energy cycle?

Answer 15

look at the arrow direction
down: negative bc it’s exothermic
up: positive bc it’s endothermic

Question 16

what do you refer to when asked about how enthalpy changes differ in a question that has previously mentioned lattice enthalpy

Answer 16

ionic radius
lattice enthalpy
how each type of enthalpy change involved would be affected

Question 17

define enthalpy change of hydration

Answer 17

the enthalpy change when 1 mole of gaseous ions dissolve in water to form an infinitely dilute solution

Question 18

define enthalpy change of solution

Answer 18

enthalpy change when 1mole of ionic substance dissolves in sufficient water to form an infinitely dilute solution

can be endo/exothermic

Question 19

how is an equation to show enthalpy of solution written

Answer 19

Question 20

how is an equation to show enthalpy of hydration written

Answer 20

Question 21

is enthalpy of solution exothermic or endothermic

Answer 21

can be either

Question 22

is enthalpy of hydration endothermic or exothermic

Answer 22

exothermic because it’s the energy released when an attraction forms between ions and water molecules

Question 23

explain enthalpy of hydration

Answer 23

an ionic solid dissolves in water, forming positive and negative ions to form ion-dipole attractions with water
the O is attracted to positive ions whilst the H is attracted to negative ions

Question 24

what is the relationship between enthalpy of solution and enthalpy of hydration

Answer 24

Question 25

how do you discuss comparitive lattice enthalpies

Answer 25

discuss lattice enthalpy in terms of being more exothermic in comparison between the two ionic compounds

Question 26

how is one O written in born-haber before atomisation

Answer 26

1/2O2

Question 27

what should you always include in first vs second electron affinity

Answer 27

+ e- in the first one
not in the second because they’ve both been used up now

Question 28

which way would an arrow for enthalpy of solution go in a hess diagram

Answer 28

down

Question 29

outline how you would draw an energy cycle linking enthalpy change of hydration, solution and lattice enthalpy

Answer 29