thermodynamics

Question 1

what is bond dissociation enthalpy

Answer 1

enthalpy change when one mole of a bond/ covalent is broken to give separated atoms with everything in the gase state

Question 2

what is the standard enthalpy of formation

Answer 2

the standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its constituent elements in its standard states under standard conditions

Question 3

what are ionisation energies

Answer 3

the first ionisation energy of an element is the energy required to remove one mole of electrons from one mole of gaseous atoms of the elements

Question 4

what is the enthalpy of lattice formation

Answer 4

the enthalpy change when 1 mole of ionic solid is formed its gaseous ions

Question 5

what is the enthalpy of dissociation

Answer 5

the enthalpy change when 1 mole of ionic solid is converted to its gaseous ions

Question 6

what happens when we know the lattice enthalpy

Answer 6

when we know the enthalpy of lattice, we can predict the strength of the ionic bonding

Question 7

how can we predict which ionic compound is more exothermic

Answer 7

It is whichever has the stronger ionic bonding is more exothermic

Question 8

what are the factors that effect the strength of ionic bonding

Answer 8

• the size of the ions
• the change of ionic bonding

the strength of ionic bonding increases as:

• the ions get smaller
• the ions get more highly charged

Question 9

how do we find lattice enthalpy

Answer 9

we can’t find it directly as it is too difficult but we can do it indirectly using a Hess cycle - which can be neatly drawn as a Born Haber cycle

we can’t find it directly because:

• there isn’t any equipment capable of forming and reacting gaseous ions
• heating an ionic compound results in gaseous ion pairs, not individual ions

Question 10

what are the stages of the Born Haber cycle

Answer 10

1) enthalpy of formation
2) atomisation
3) ionisation
4) electron affinities

Question 11

what is atomisation

Answer 11

when 1 mole of gaseous atoms are formed from an element in its standard state

Question 12

how can we work out the enthalpy of atomisation

Answer 12

the enthalpy of atomisation = 1/2 of bond dissociation enthalpy

this applies when:
we are breaking 1/2 mole of a molecule e.g. Cl2

forming separate atoms
everything is in gaseous states in its standard state

only when the elements are covalently bonded in their standard state

Question 13

what is electron affinity

Answer 13

the enthalpy change when 1 mole of electrons are added to 1 mole of gaseous atoms

Question 14

endothermic or exothermic

Answer 14

1) lattice enthalpy = exothermic
2) enthalpy of formation = exothermic
3) ionisation = endothermic
4) electron affinities = EA1 exothermic EA2=endothermic

Question 15

why is the first electron affinity exothermic

Answer 15

the first electron affinity is exothermic because as the electron gets closer to the positive nucleus energy is being given out

Question 16

why is the second, third e.t.c electron affinity endothermic

Answer 16

the electron is being added to a negatively charged ion

therefore, the repulsion outweighs the attraction of the ions nucleus
so the overall force between the anion and the electron is repulsive

so energy is required to add that electron in

Question 17

what does Hess Law state

Answer 17

the enthalpy of a reaction is independent of the route the reaction takes

Question 18

why are the theoretical and experimental values for lattice enthalpy different

Answer 18

theoretical and experiment values enthalpies can be different depending on how “purely ionic” the compound is

the experimental value is derived from Born Haber cycles and take into account the covalent characteristics of the salt/ionic compound

Question 19

what are the theoretical lattice enthalpies

Answer 19

theoretical lattice enthalpies are calculated from data assuming a PERFECTLY IONIC MODEL OF A LATICE

Question 20

what is the perfectly ionic model

Answer 20

1. ions that are perfectly spherical - there is no distortion
2. The charge is evenly distributed in this sphere - this is called point charge

Question 21

what are the practical lattice enthalpies

Answer 21

the experimental value is derived from Born Haber cycles and take into account the covalent characteristics of the salt/ionic compound

this number is often different from the theoretical value

Question 22

what does the difference between the theoretical and practical enthalpy imply

Answer 22

This tells us that the compound being experimented on doesn’t follow the perfectly ionic model and has some covalent characteristic

e.g.
most of the time the positive ion distorts the charge distribution in the negative ion. We say the positive ion POLARISES the negative ion

Question 23

the more polarisation(…)

Answer 23

the more polarisation of the ionic bonds, the more electron density is shared between ions, the covalent character increases and so the difference between the theoretical and experimental values increases

Question 24

how can we figure out whether a substance is purely ionic

Answer 24

comparing theoretical and practical lattice enthalpy we can see how much a substance is purely ionic

in all of the examples, the experimental value is always more exothermic than the theoretical value

Question 25

what does a large difference in theoretical and experimental values show

Answer 25

it shows that we have some covalent characteristics being displayed This is caused by larger distortions in the negative ion The bigger the difference between the lattice enthalpy, the more polarisation you have The greater the covalent character

Question 26

what happens when the cation is next to the anion

Answer 26

when a cation is next to an anion the electron cloud of the anion is distorted towards the anion

Question 27

As the amount of polarisation increases (...)

Answer 27

- more electron density is shared between ions - the amount of covalent character increases - the difference between experimental and theoretical enthalpy increases

Question 28

what is the polarising power determined by

Answer 28

the charge of the positive ion the size of the positive ion Larger amount of polarisation - higher polarising power - higher polarising power of the stated cation and/or higher polarisability of the anion

Question 29

what is common exam question in regards to silver halide

Answer 29

although silver cation is large with a 1+ charge, ALL SILVER HALIDES have a large difference between their theoretical and experimental lattice energies SO WHAT WE NEED TO SAY IS: AgBr has a larger difference in theoretical and experimental lattice energies AgBr has a large amount of covalent character

Question 30

why are covalent characteristics good

Answer 30

covalent character can increase the strength of bonding in an ionic compound since experimental lattice enthalpies account for covalent character while theoretical lattice enthalpies don't, experimental lattice enthalpies are always more exothermic or negative than the theoretical ones

Question 31

what are factors that affect polarisation

Answer 31

1) polarising power | 2) polarisablity of the anion

Question 32

if an anion has a high polarisability (...)

Answer 32

some anions electron clouds are more easier to distort than others this is because(...) the larger and more highly charged the ion is, the easier it is to distort/ higher polarisability sometimes we are unable to predict the polarisability

Question 33

what is the enthalpy of the solution

Answer 33

the change when 1 mole ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution -dissolves in enough solvent to form an infinitely dilute solution e.g. take a solid, add water until we have just dissolved the ionic compound

Question 34

what must happen before something dissolves

Answer 34

1. substances bonds must break (endothermic) 2. new bonds formed between the solvent and substance (exothermic) e. g. adding an ionic salt to water ionic lattice in solid form substance bonds are broken to create free moving ions bonds are formed between ions and water. The ions are therefore hydrated

Question 35

what do most ionic compounds dissolve in

Answer 35

polar solvents like H2O The delta positive hydrogen is attracted to the negative ions and the delta negative oxygen is attracted to positive ions The structure, therefore, begins to break down

Question 36

what must happen before the ionic structure is broken down and hydration occurs

Answer 36

for this to happen, new bonds formed must be the same strength or greater than those broken If not, it is unlikely to dissolve therefore, enthalpies for soluble substances are exothermic essentially, if the energy for the first process (endothermic - enthalpy of solution) is greater than the energy in the second process (exothermic- enthalpy of hydration ) the substance will not dissolve

Question 37

what is entropy

Answer 37

entropy is the measure of disorder in a system entropy (S) is the number of ways energy can be shared out between particles It also tells us the ways in which particles can be arranged by disorder we mean randomness

Question 38

the more disorder there is ....

Answer 38

the more disorder there is the higher the level of entropy e.g. disorder increases from solid, liquid to a gas

Question 39

why do solids have the lowest level of disorder

Answer 39

solids have the lowest level of disorder, particles are arranges neatly in rows

Question 40

why do liquids and gases have higher entropy than solids

Answer 40

liquids and gases are more disordered to put liquids into a state of order you need to apply energy. Particles want to be the lowest energy possible therefore entropy requires little energy

Question 41

what affects entropy

Answer 41

the number of particles also affect entropy change If a reaction is in the same state but more moles are produced, the entropy increases This is because there are more ways energy can be distributed

Question 42

how can a reaction occur even when it is enthalpically unfavourable

Answer 42

a reaction can be spontaneous (feasible) even if it is enthalpically unfavourable (e.g. endothermic) because a reaction will tend towards more disorder and hence increase entropy -changes in entropy can sometimes change in enthalpy e.g. a reaction is very enthalpically unfavourable as it is very endothermic the number of mole increases and so the reaction is entropically favourable starting with 2 solids but making gas and a liquid

Question 43

how can we work out entropy

Answer 43

the entropy of products - entropy of reactants most reactions will not occur unless the entropy change is positive entropy is represented by symbol S the units are in joules per kelvin per mol

Question 44

what is Gibbs free energy

Answer 44

Gibbs free energy tells us if a reaction is feasibly or not

Question 45

how do we work out Gibbs free energy

Answer 45

enthalpy - (entropy change x temperature)

Question 46

when is a reaction feasible

Answer 46

a reaction is feasible in theory if Gibbs is negative or | zero

Question 47

even if a reaction is feasible, you may not observe a reaction occurring why?

Answer 47

this is because the activation energy is too high or the rate of the reaction is very slow

Question 48

when is a reaction feasible at any temperature

Answer 48

if a reaction is exothermic (negative enthalpy - delta H) and has a positive entropy change, the delta G (Gibbs free energy) is always negative since: G = H – TS these reactions are feasible at any temperature

Question 49

when is a reaction never feasible at any temperature

Answer 49

if a reaction is endothermic (positive enthalpy - delta H) and has a negative entropy change, the delta G (Gibbs free energy) is always positive since: G = H – TS these reactions are never feasible at any temperature

Question 50

when is a reaction feasible only at certain temperatures

Answer 50

if delta H is positive (endothermic) and S is also positive then the reaction will only be feasible at a certain temperature. This is because: G = H – TS

Question 51

how can we find if the temperature when gibbs equals 0

Answer 51

by rearranging this equation: G = H – TS to this T= H/S This is because when G = 0 H=TS

Question 52

What is on the y axis and what is on the x-axis in a Gibbs free energy graph

Answer 52

temperature (K) is on the x free energy change is on the y the y-intercept of this graph is delta H the gradient of this graph is -S

Question 53

how do you work out the enthalpy of a solution

Answer 53

lattice dissociation + enthalpy of hydration of positive ios

Question 54

what is the enthalpy of hydration

Answer 54

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