Thermodynamics

Question 1

What is enthalpy change?

Answer 1

The heat energy change at a constant pressure

Question 2

What does Hess’s law state?

Answer 2

The enthalpy change of a chemical reacyion is independent to the route taken

Question 3

What is the enthalpy of lattice dissociation?

Answer 3

The enthalpy change to separate one mole of a solid ionic compound into its gaseuous ions

Question 4

What is the symbol of enthalpy lattic of dissociation?

Answer 4

ΔH ⦵ LD

Question 5

Is the enthalpy of lattice dissociation positive or negative? Why?

Answer 5

It is positive, as bonds are being broken

Question 6

What is enthalpy lattice pf formation?

Answer 6

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions

Question 7

What is the symbol for enthalpy lattice of formation?

Answer 7

ΔH ⦵ LF

Question 8

Is the enthalpy lattice of formation positive or negative?

Answer 8

Negative, as bonds are formed so energy is required

Question 9

What is the relationship between enthalpy lattic of dissociatipn and formation?

Answer 9

They will be equal in magnitude buthave the opposite sign (if one is negative the other is positive)

Question 10

What is the standard enthalpy lf formation? And its symbol

Answer 10

The enthalpy change when one mole of a compound is formed from its constituent elements, when all reactants and products are in their standard state

ΔH ⦵ f

Question 11

What is the standard enthalpy of atomisation and its symbol?

Answer 11

The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state

ΔH ⦵ at

Question 12

What is bond enthalpy?

Answer 12

The heat energy required to break one mole lf a given covalent bond in a molecule in the gaseous state

Question 13

What is ionisatipn energy?

Answer 13

The energy rewuired to remove one mole of electrons from one mole of atoms on teh gaseous state.

Question 14

What is the symbol for the first and second ionisation energy?

Answer 14

ΔH ⦵ IE1/2

Question 15

What is the second ionisation energy?

Answer 15

The enthalpy change when one mole of 2+ gaseous ions are formed from one mole lf gaseous 1+ ions

Question 16

What is the first electron affinity and its symbol?

Answer 16

ΔH ⦵ EA1
The enthalpy change when one mole of geseous 1- ions are formed from one mole of gaseous atoms

Question 17

What is the second electron affinity and its symbol?

Answer 17

ΔH ⦵ EA2
The enthalpy change when one mole of 2- ions are formed from one mole of 1- ions in the gasupus state

Question 18

What are born harber cycle diagrams?

Answer 18

Diagrams that use hess’s law to calculate the enthalpy of lattice dissociation and formatiom

Question 19

Using born harber cycle diagrams, what is teh equation for the enthalpy of lattice formation?

Answer 19

ΔH ⦵ formation - ( sum of all other enthalpy changes)

Question 20

Using the born harber cycle diagrams, what is the equation for enthalpy of lattice dissociation?

Answer 20

—ΔH ⦵ LF
Or
(Sum of all other enthalpy changes) - ΔH ⦵ formation

Question 21

What is the perfect ionic model?

Answer 21

A model where ions are perfect spheres with only electrostatic forces of attraction

Question 22

Why might a value from the born harber cycles not be accurate?

Answer 22

Born harber cycles assume that the ionic lattice has the ‘perfect ionic model’ when in reality they sometimes have some covalent characters

Question 23

How do theoretical and experimental (from the born harber cycle) values indicate how perfect an ionic model is?

Answer 23

A perfect ionic model will have very similar experimental and theoritical values, so will have pure ionoc bonding

Question 24

What are two characteristics of perfect ionic models?

Answer 24

Both ions are spherical
The charge within the model is evenly distributed

Question 25

How do ionic models show partial covalent bonding?

Answer 25

a positive ion attracts electrons in the negative ion, because it is small and highly charged. Therefore the ions are no longer spherical and the charge is not evenly distributed

Question 26

What is polarisatipn in ionic compounds?

Answer 26

When the ions in a compound are distorted (no longer spherical) because the positive ipn attracts the electrons in the negative ion

Question 27

What affects the polarising power of ions in an ionic lattice?

Answer 27

Positive ions that are higher in charge and smaller are best at polarising negative ions Negative ions that are higher in charge and larger are the easiest to polarise

Question 28

What characteristics do ionoc compounds that have covalent characteristics show?

Answer 28

They have low solubility Melting points and electrical conductivity are lower than expected

Question 29

What is the enthalpy of solution and its equation?

Answer 29

The enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution ΔH ⦵ sol

Question 30

What is the enthalpy of hydration and its symbol?

Answer 30

The enthalpy change when one mole of aqueous ions are formed from one mol of geseous ions

Question 31

What factors affect the lattice enthalpy lf formation and how?

Answer 31

Size of the ion- if size increases, attraction between oppositely charged ions decreases so the enthalpy is less exothermic Charge- if more charged, attraction between opposite ions increases so the enthalpy is more exothermic

Question 32

What factors affect affect the hydration enthalpy and how?

Answer 32

Size lf ion- if the size of the ion increases, the attraction between the delta positove hydrogen in water and the negative ion decreases So enthalpy is less exothermic Charge lf ion- if charge increases, the attraction between water and ion increases so the enthalpy is more exothermic

Question 33

What is entropy? And what is its symbol?

Answer 33

S- a measure of the disorder in a system

Question 34

Is entropy positive or negative? Why?

Answer 34

There is always a degree of disorder because particles always have energy, therefore the values for S are always positive

Question 35

What are the units for entropy?

Answer 35

J mol-1 k-1 Joules/ mol/ kelvin

Question 36

What four factors determine the entropy of a substance?

Answer 36

State of matter, dissolving, mixtures, number of particles

Question 37

How does state of matter affect entropy of a substance?

Answer 37

Solids are ordered, so have a low entropy Gases are very disordered so have a high entropy

Question 38

What is the difference of entropy between a solution and a solid and solvent?

Answer 38

The solution is more disordered, so the entropy is higher

Question 39

How does the number of particles affect entropy?

Answer 39

As the number of particles increases, disorder increases so entropy increases

Question 40

What is the equation for entropy change of a reaction?

Answer 40

Entropy change = total entropy of products- total entropy of reactants

Question 41

What does a positive entrophy change indicate?

Answer 41

An increase in disorder

Question 42

What does a negative entrophy change indicate?

Answer 42

A decrease in disorder

Question 43

Is a negative or positive entropy change more favourable in a reaction?

Answer 43

A positive entropy change is more favourable

Question 44

What are units for enthalpy change?

Answer 44

KJmol-1

Question 45

Why is a negative enthalpy change favourable?

Answer 45

Because heat is given out, which causes the entropy of the surroundings to increase. This is favourable

Question 46

What is the equation for the enthalpy change of a reaction?

Answer 46

Enthalpy of formation of products - enthalpy of formation of reactants

Question 47

What is Gibbs free energy and feasibility?

Answer 47

States whether a reaction is feasable or not at any given temperature

Question 48

What is the equation for gibbs free energy change? What are its units?

Answer 48

∆G = ∆H - T ∆S

Question 49

Using Gibbs free energy, when will a reactipn be feasable?

Answer 49

When ∆G is zero or negative

Question 50

Why can an endothermic reaction take place spontaneously at room temperature?

Answer 50

A spontaneous reaction will only occur if ∆G is 0 or below 0 If ∆H positive (and so unvarourable) yet the T ∆S of the reaction is also positive and greater than ∆H, a negative ∆G will still form

Question 51

How do you calculate the temperature at which a reaction becomes feasable?

Answer 51

A reaction becomes feasable when ∆G is zero or below. Take ∆G as 0: T= ∆H/ ∆S

Question 52

What temperature is required when ∆H is favourable yet ∆S is unfavourable? Why

Answer 52

Low temp H is negative and S is negative For G to be negative, - TS must be smaller in magnitude than H

Question 53

What temperature is required when ∆H is unfavourable yet ∆S is favourable? Why

Answer 53

High temperature S is positive and H is positive For G to be negative, TS must be larger than H

Question 54

Why might a reaction not occur,despite it being favourable?

Answer 54

Because its activation energy is too high and the rate of reaction is too slow

Question 55

When answering a question comparing theoretical and experimental values, what use you state?

Answer 55

The bonds present If not perfect ionic model, state it has covalent characteristics If it is perfect ionic model, it only shows ionic bonds

Question 56

What is the equation for enthalpy lattice of formation?

Answer 56

Enthalpy of solution = enthalpy lattice of dissociation + enthalpy of hydration (of all the products)

Question 57

How does of having a perfect ionic model effect the enthalpy of lattice formation?

Answer 57

Covalent characteristic causes the bonds holding the lactic together to be stronger Therefore the lattice of formation is more exothermic

Question 58

What is the equation for the enthalpy of hydration?

Answer 58

= enthalpy of solution - enthalpy of lattice dissociation