17. Thermodynamics

Question 1

Define the standard molar enthalpy of formation

Answer 1

• the enthalpy change
• when one mole of a compound is formed from its constituent elements under standard conditions
• all reactants and products in their standard states

Question 2

Define the standard molar enthalpy of combustion

Answer 2

• enthalpy change
• when one mole of substance is completely burnt in oxygen
• all reactants and products in their standard states

Question 3

Define the standard enthalpy of atomisation

Answer 3

• the enthalpy change
• which accompanies the formation of one mole of gaseous atoms
• from the element in its standard state
• under standard conditions

Question 4

Write an equation for the formation of water

Answer 4

H2 (g) + 1/2 O2 (g) —> H2O (l)

Question 5

Write an equation for the combustion of methane

Answer 5

CH4 (g) +2O2 (g) —-> CO2 (g) + H2O (l)

Question 6

Write an equation for the atomisation of magnesium and then chlorine

Answer 6

Mg (s) —-> Mg (g)

1/2Cl2 (g) —-> Cl (g)

Question 7

Define the first ionisation energy

Answer 7

• standard enthalpy change
• when one mole of electrons
• is removed from one mole of gaseous atoms
• leaving one mole of +1 ions

Question 8

Write an equation for the first ionisation of Mg

Answer 8

Mg (g) —> Mg+ + e-

Question 9

Define the second ionisation energy

Answer 9

• standard enthalpy change
• when one mole of electrons
• is removed from one mole of gaseous +1 ions
• forming one mole of +2 ions

Question 10

Write an equation for the second ionisation energy of Mg

Answer 10

Mg+ (g) —-> Mg2+ (g) + e-

Question 11

Define first electron affinity

Answer 11

• standard enthalpy change
• when a mole of electrons
• is added to a mole of gaseous atoms
• forming one mole of -1 ions

Question 12

Define second electron affinity

Answer 12

• standard enthalpy change
• when a mole of electrons
• is added to a mole of gaseous -1 ions
• forming one mole of -2 ions

Question 13

Write an equation for the first electron affinity of oxygen

Answer 13

O (g) + e- —-> O- (g)

Question 14

Write an equation for the second electron affinity of oxygen

Answer 14

O- (g) +e- —–> O2- (g)

Question 15

Define lattice enthalpy of formation

Answer 15

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

Question 16

Write an equation for the lattice enthalpy of formation of NaCl

Answer 16

Na+ (g) + Cl- (g) —->NaCl (s)

Question 17

Define enthalpy of lattice dissociation

Answer 17

• standard enthalpy change
• when one mole of solid ionic compound
• dissociated into its gaseous ions

Question 18

Write an equation for the lattice dissociation of NaCl

Answer 18

NaCl (s) —> Na+ (g) + Cl- (g)

Question 19

Define enthalpy of hydration

Answer 19

• standard enthalpy change
• when water molecules surround
• one mole of gaseous ions

Question 20

Write an equation for the hydration of Na+

Answer 20

Na + (g) +aq —> Na+ (aq)

Question 21

Define enthalpy of solution

Answer 21

• standard enthalpy change
• when one mole of solute dissolves completely in sufficient solvent
• to form a solution in which the molecules or ions are far enough apart
• to not interact with each other

Question 22

Write an equation for the enthalpy of solution of NaCl

Answer 22

NaCl (s) + aq —> Na+ (aq) + Cl- (aq)

Question 23

Define mean bond enthalpy

Answer 23

• standard enthalpy change
• when one mole of gaseous molecules
• each break a covalent bond to form
• two free radicals
• averaged over a range of compound

Question 24

What is a Born-Haber cycle

Answer 24

A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound

Question 25

Write all the equation that has the equivalent enthalpy to the formation of NaCl (Na (s) + 1/2Cl2 (g) ---->NaCl (s)

Answer 25

Atomisation of Na: Na (s) + 1/2Cl2 (g) ---> Na (g) + 1/2Cl2 (g) Atomisation of Cl: Na (g) + 1/2Cl2 (g) -----> Na(g) + Cl (g) First ionisation energy of Na Na(g) + Cl (g) ----> Na + (g) + e- + Cl (g) First electron affinity of Cl: Na + (g) + e- + Cl (g) ----> Na+ (g) + Cl- (g)

Question 26

What is the trend in lattice enthalpies (ion size)

Answer 26

Larger ions lead to smaller lattice enthalpies

Question 27

Why does larger ions lead to smaller lattice enthalpies

Answer 27

The oppositely charges do not approach each other as closely when the ions are larger so less attraction

Question 28

What is the trend in lattice enthalpies (charge size)

Answer 28

Lattice enthalpy increases with the size of the charge

Question 29

Why does lattice enthalpy increase with the size of the charge

Answer 29

Ions with higher charge give out more energy when they come together

Question 30

Enthalpy of hydration has the same enthalpy trend as what

Answer 30

Lattice enthalpy

Question 31

The dissolving of ionic compounds in water is the sum of what three processes

Answer 31

- breaking the ionic lattice (to give separate ions- the lattice dissociation enthalpy has to be put in) - hydrating the positive ions (enthalpy of hydration is given out) - hydrating the negative ions (enthalpy of hydration is given out)

Question 32

Write the all the equations equivalent enthalpy change for the solution of NaCl (NaCl (s) + aq ---> Na+ (aq) + Cl- (aq) )

Answer 32

NaCl (s) ---> Na + (g) + Cl- (g) Na+ (g) + aq + Cl- (g) ---> Na+ (aq) + Cl- (g) Na + (aq) + Cl- (g) + aq ---> Na+ (aq) + Cl- (aq)

Question 33

Why in some compounds are there large discrepancies between the experimental and theoretical values for lattice formation enthalpy

Answer 33

Covalent character

Question 34

Describe how covalent character occurs

Answer 34

- when the positive ion is relatively small and high charge - and the negative ion is relatively large and has a high negative charge - the positive ion can approach more closely to the negative ions electron cloud - and distort them towards the positive ion - the positive ion can easily distort the negative as the negative has a large size so electrons far from nucleus -and large negative charge means lots of negative charge to distort - this means more electrons than expected are concentrated between the two ions - the negative ion is said to be polarised

Question 35

What factors increase polarisation and therefore covalent character

Answer 35

positive ion: small size, high charge | negative ion: large size, high charge

Question 36

What do the words feasible or spontaneous describe

Answer 36

Describes reactions which could take place of their own accord

Question 37

Reactions often occur spontaneously when the enthalpy change is

Answer 37

Negative

Question 38

What is the order of randomness of the states

Answer 38

Solid

Question 39

What is entropy

Answer 39

A numerical measure of disorder in a chemical system

Question 40

Apart from negative enthalpy change what else makes a reaction feasible

Answer 40

An increase (positive) entropy

Question 41

How do you calculate entropy change

Answer 41

Entropy of the products - entropy of the reactants

Question 42

What two factors govern the feasibility of a reaction

Answer 42

- enthalpy change | - entropy change

Question 43

What is Gibbs free energy

Answer 43

A quantity that combines enthalpy and entropy change

Question 44

If the change in G is negative...

Answer 44

then the reaction is feasible

Question 45

If the change in G is positive

Answer 45

then the reaction is not feasible

Question 46

What is the equation for ΔG

Answer 46

ΔG= ΔH -TΔS

Question 47

What happens when ΔG is 0

Answer 47

This is the temperature which the reaction is just feasible

Question 48

Why might spontaneous reactions not occur (such as the oxidation of graphite)

Answer 48

- the reaction takes place so slowly that for practical purposes it does not occur at all - when this happens we say graphite is thermodynamically unstable but kinetically stable