THERMODYNAMICS

Question 1

^H f

Answer 1

The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states.

Question 2

^H c

Answer 2

The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.

Question 3

^H at

Answer 3

The enthalpy change when one of gaseous atoms is formed from an element in its standard state.

Question 4

^H BE

Answer 4

The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds.

Question 5

1st ^H i

Answer 5

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge.

Question 6

2nd ^H i

Answer 6

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge.

Question 7

1st ^H aff

Answer 7

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge under standard conditions.

Question 8

2nd ^H aff

Answer 8

The standard enthalpy change when one mole of electrons is added to a mole of gaseous ions each with a single negative charge, to form a mole of ions each with a two negative charge.

Question 9

^H LF

Answer 9

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

Question 10

^H Hyd

Answer 10

The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.

Question 11

^H Sol

Answer 11

The standard enthalpy change when one mole of solute dissolves in enough solvent to form a solution in which the ions are far enough apart not to interact with each other.

Question 12

^H LD

Answer 12

The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions.

Question 13

C(s) + O2(g) à CO2(g) is an example of:

Answer 13

enthalpy change of formation

Question 14

C2H5OH(l) + 3O2(g) à 2CO2(g) + 3H2O(l) is an example of

Answer 14

^H combustion

Question 15

½ Br2(l) à Br(g)

Answer 15

atomisation

Question 16

Mg(s) à Mg(g)

Answer 16

atomisation

Question 17

CH4(g) à H(g) + CH3(g)

Answer 17

bond enthalpy

Question 18

how are bond enthalpy and atomisation linked

Answer 18

AT x2 = BE

Question 19

which two enthalpies are linked

Answer 19

bond enthalpy and atomisation

Question 20

Mg2+(g) + 2Br-(g) à MgBr2(s)

Answer 20

lattice formation

Question 21

MgBr2(s) à Mg2+(g) + 2Br-(g)

Answer 21

lattice dissociation

Question 22

NaCl(s) à Na+(g) + Cl-(g)

Answer 22

lattice dissociation

Question 23

Cl-(g) + aq à Cl-(aq)

Answer 23

hydrolysis

Question 24

NaCl(s) + aq à Na+(aq) + Cl-(aq)

Answer 24

solution

Question 25

equation for ^H

Answer 25

∆H = ∑Bonds Broken – ∑Bonds Formed

Question 26

factors of lattice enthalpies

Answer 26

• Charge on the ions
• Size of the ions (ionic radius)

Question 27

why does charge and radius affect lattice enthalpy

Answer 27

As the ionic radius increases:

• The attraction between the ions decreases
• The lattice enthalpy becomes less negative (less exothermic)

Question 28

NaCl has a theoretical value of ∆HLF = -777 kJmol-1. Experimentally this value has been determined to be -780 kJmol-1
what does this mean?

Answer 28

the lattice is almost purely ionic bonding

Question 29

why can the theoretical ^H differ from the actual ^H

Answer 29

actual has covalent character

Question 30

comparing covalent character

Answer 30

The Zn2+ ion is quite small with quite a strong +ve charge.

The Se2- ion is large with a diffuse electron cloud.

The two will ionically bond because of the electrostatic attraction.

The Zn2+ ion is strongly polarising

The electron cloud about the Se2- ion becomes distorted

Some of the electron density is shared.

This is covalent character

Question 31

what is covalent character

Answer 31

an atoms ability to attract a pair of electrons in a covalent bond

Question 32

what is the perfect ionic model

Answer 32

The term ‘Perfect Ionic Model’ refers to compounds which display purely ionic bonding with no covalent character.

Question 33

what can be calculated with the perfect ionic model

Answer 33

theoretical lattice enthalpies

Question 34

features of the perfect ionic model include

Answer 34

• Ions act as point charges
• Ions are perfect spheres which cannot be distorted
• The ions show purely ionic bonding with no covalent character.

Question 35

why affects a reaction occuring

Answer 35

Temperature

• Enthalpy – most reactions which are feasible are exothermic because the reactants want to release energy to the environment. This means the products will be lower in energy and more stable.
• The third factor which determines whether a reaction is feasible or not is called ENTROPY.

Question 36

what is entropy

Answer 36

disorder

Question 37

if entropy decreases

Answer 37

less disorder

Question 38

if entropy increases

Answer 38

more disorder

Question 39

what has the highest entropies

Answer 39

gases

Question 40

does entropy increase or decrease when gas is produced

Answer 40

increases- more disorder

Question 41

does entropy increase or decrease when fewer moles are produced

Answer 41

decreases- less disorder

Question 42

formula for entropy change

Answer 42

∆Sᶿ = ∑Sᶿ (products) - ∑Sᶿ (reactants)

Question 43

formula for Gibbs

Answer 43

∆G = ∆H - T∆S

Question 44

what do you need to do to ^s when calculating Gibbs

Answer 44

x 1x10-3

Question 45

when is a reaction feasible

Answer 45

^G< 0