Unit 1: Section 6 - Thermodynamics CDS *

Question 1

what is enthalpy change?

Answer 1

the heat energy transferred in a reaction at constant pressure

Question 2

what are standard conditions?

Answer 2

298 K

100 kPa

Question 3

what is the enthalpy change for exothermic and endothermic reactions?

Answer 3

exothermic - negative ΔH, heat is given out

endothermic - positive ΔH, heat is taken in

Question 4

what is the enthalpy change of formation?

Answer 4

the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 5

what is bond dissociation enthalpy?

Answer 5

the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecule are broken

Question 6

what is the enthalpy change of atomisation?

Answer 6

the enthalpy change when 1 mole of monatomic gaseous atoms is formed from an element/compound in its standard state

Question 7

what is the first ionisation energy?

Answer 7

the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms

Question 8

what is the second ionisation energy?

Answer 8

the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions

Question 9

what is the first electron affinity?

Answer 9

the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms

Question 10

what is the second electron affinity?

Answer 10

the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions

Question 11

what is the enthalpy change of hydration?

Answer 11

the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions

Question 12

what is the enthalpy change of solution?

Answer 12

the enthalpy change when 1 mole of an ionic compound dissolves into its aqueous ions

Question 13

what is the lattice enthalpy of formation?

Answer 13

the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

Question 14

what is lattice enthalpy of dissociation?

Answer 14

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions

Question 15

what does Hess’s law state?

Answer 15

the total enthalpy change of a reaction is always the same, no matter which route is taken

Question 16

what is a born-haber cycle?

Answer 16

start with the enthalpy of formation (elements in standard states –> ionic lattice)
then use enthalpy of atomisation and ionisation ( turn into gas, and turn diatomic molecules into monatomic e.g. Cl2 –> 2Cl then remove electrons)
use electron affinity (electron from ionisation added to other atom)
then lattice enthalpy of formation (turns gaseous ions into ionic lattice)

Question 17

how to work out theoretical lattice enthalpy?

Answer 17

doing some calculations on the purely ionic model of a lattice.

Question 18

what is the purely ionic model of a lattice?

Answer 18

it assumes that all ions are spherical, and have their charge evenly distributed around them

Question 19

what is experimental lattice enthalpy?

Answer 19

finding lattice enthalpy from experiments, the value is often different to theoretical because most ionic compounds have some covalent character

Question 20

what is covalent character?

Answer 20

the positive and negative ions in a lattice aren’t usually exactly spherical.
positive ions polarise neighbouring negative ions to different extents, the more polarisation, the more covalent the bonding

Question 21

what does it mean if a ionic compound has covalent character?

Answer 21

the bonding in the lattice is stronger than expected, because covalent bonds are stronger than ionic.
experimental enthalpy will be a lot higher than theoretical

Question 22

what happens when a solid ionic lattice dissolves?

Answer 22

bonds between ions break to give free ions - endothermic
bonds between ions and water are made - exothermic
enthalpy change of solution is the overall effect on the enthalpy of these

Question 23

why can water molecules bond to ions?

Answer 23

oxygen is more electronegative than hydrogen, so it draws electrons towards itself, creating a dipole.
positive hydrogen can form bonds with negative ions
negative oxygen form bonds with positive ions

Question 24

when do substances dissolve?

Answer 24

substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in.
so soluble substances tend to have exothermic enthalpies in solution

Question 25

how can you use Hess’s law to work out enthalpy of solution?

Answer 25

solution - converts lattice to aqueous ions
hydration - converts gaseous ions to aqueous ions
lattice dissociation - converts gaseous ions into the lattice
a triangle can be constructed
ΔSOL = ΔHYD(+ve ion) + ΔHYD(-ve ion) - ΔLEFORM

Question 26

how to tell if a substance is insoluble in water?

Answer 26

if the enthalpy of solution is positive and very big, so very endothermic, the substance is insoluble

Question 27

what is entropy? (S)

Answer 27

a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
the more disordered the particles, the higher the entropy

Question 28

what factors affect entropy?

Answer 28

physical state affects entropy

more particles means more entropy

Question 29

why does physical state affect entropy?

Answer 29

solid particles vibrate about a fixed point - not much disorder - low entropy
gas particles move fast wherever they want - most disorder - highest entropy

Question 30

why do more particles mean more entropy?

Answer 30

the more particles, the more ways their energy can be arranged

Question 31

what makes a reaction feasible?

Answer 31

substances tend to disorder because it makes them more energetically stable, so they move to increase their entropy, this means an endothermic reaction can happen without energy

Question 32

how to calculate entropy change?

Answer 32

entropy of products - entropy of reactants

Question 33

what is the standard entropy of a substance?

Answer 33

the entropy of 1 mole of that substance under standard conditions

Question 34

what does it mean if total entropy of a reaction increases?

Answer 34

it is feasible for the reaction to occur, but it is not guaranteed

Question 35

what is free energy change?

Answer 35

a measure used to predict whether a reaction is feasible

Question 36

what is ΔG when a reaction is feasible?

Answer 36

when ΔG ≤ 0

then a reaction is feasible

Question 37

what is the equation for Gibbs free energy change?

Answer 37

ΔG = ΔH - TΔS
ΔH = enthalpy change (kJ/mol)
ΔS = entropy change (J/mol/K)/1000 --> (kJ/mol/K)
T = temperature in kelvins
must all have same units so ΔS / 1000

Question 38

when is a reaction always feasible?

Answer 38

when the reaction is exothermic and has a positive entropy change

Question 39

when is a reaction never feasible?

Answer 39

when the reaction is endothermic and has a negative entropy change

Question 40

how to calculate the temperature that a reaction becomes feasible?

Answer 40

when ΔG is 0 a reaction becomes feasible
so when
T = ΔH/ΔS
the reaction becomes feasible

Question 41

what do the direction of the arrows show in the Born Haber cycle?

Answer 41

the direction of energy, if its pointing up its endothermic

down means exothermic

Question 42

what happens if there is a 2- ion in a born Haber cycle?

Answer 42

the 1st electron affinity will go below the equation of the gaseous ions then back up in the second electron affinity
1st is exothermic
2nd is endothermic - repulsion between electron and negative ion means 2nd Ea requires energy to add the electron

Question 43

how to know which ionic bond is stronger?

Answer 43

the bigger the ions, the weaker the ionic bonds
the higher the charge, the stronger the bond
charge takes priority over size

Question 44

what does the graph of entropy by temperature look like?

Answer 44

as temperature increases, entropy increases
when a change of state happens, entropy increases dramatically (vertical line), this change is bigger going from liquid to gas then solid to liquid

Question 45

when does a change of state become feasible?

Answer 45

ΔG = 0

this is when a substance melts or boils

Question 46

How are diatomic molecules treated differently in born-Haber cycles?

Answer 46

The atomisation energy is needed twice e.g. to turn Br2 into 2 Br
They will also need 2x ionisation energy or 2x electron affinity