Thermodynamics

Question 1

Exothermic processes

Answer 1

Bond forming
– enthalpy change

Question 2

Endothermic processes

Answer 2

Bond breaking
+ enthalpy change

Question 3

Enthalpy change of formation
+ formation of NaCl

Answer 3

The enthalpy change when 1 mole of a substance is formed from its component elements in their standard states

Na(s) + 1/2Cl2(g) –> NaCl(s)

Question 4

Enthalpy change of lattice formation + Formation of NaCl

Answer 4

The enthalpy change when 1 mole of an ionic compound is formed from its component ions in a gaseous state

Na+(g) + Cl-(g) –> NaCl(s)

Question 5

What factors affect strength of enthalpy change of lattice formation

Answer 5

Charge on ion
Size of ion

greater charge + smaller ion = more exothermic

Charge > size

Question 6

What energy change is lattice formation

Answer 6

Exothermic, forms ionic bond

Question 7

Enthalpy change of atomisation + atomisation of Na + Cl

Answer 7

The enthalpy change when 1 mole of gaseous atoms are FORMED

Na(s) –> Na(g)
1/2Cl2(g) –> Cl(g)

Question 8

What type of energy change is atomisation

Answer 8

Endothermic, breaking bonds of attractions

Question 9

1st ionisation energy
+ 1st ionisation energy of Na

Answer 9

The enthalpy change to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Na(g) –> Na+(g) + e-

Question 10

What type of energy change is ionisation energy

Answer 10

Endothermic, overcoming electrostatic bond between nucleus and outer electron

Question 11

What factors affect strength of ionisation energy

Answer 11

Nuclear charge, size of ion, electron shielding

Question 12

Bond dissociation energy + Bond dissociation energy of Cl2

Answer 12

The enthalpy change when 1 mole of covalent bonds are broken in the gaseous state

Cl2(g) –> 2Cl(g)

Question 13

1st electron affinity + 1st electron affinity of Cl

Answer 13

The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms to make 1 mole of gaseous ions with a 1- charge

Cl(g) + e- –> Cl-(g)

Question 14

What type of energy change is 1st electron affinity

Answer 14

Exothermic, forming electrostatic attraction between nucleus and ion

Question 15

2nd electron affinity + 2nd electron affinity of O-

Answer 15

The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms with a 1- charge to make 1 mole of gaseous ions with a 2- charge

O-(g) + e- –> O^2-(g)

Question 16

What type of energy change is 2nd electron affinity

Answer 16

Endothermic, although a bond is being formed, negatively charged electrons are being added to a negatively charged ion which causes forces of repulsion, requiring energy to form the bond

Question 17

What are perfect ionic compounds

Answer 17

Ionic compounds with :
Perfect spheres
No covalent character

Question 18

What is meant by covalent character?

Answer 18

Where the electron clouds in an ionic bond overlap, forming a weak covalent bond

Question 19

If covalent character is present in an ionic bond, what will happen to the lattice enthalpy?

Answer 19

It will become more exothermic

Question 20

What 2 factors affect the strength of an ionic bond?

Answer 20

Ionic charge
Ionic radius

Question 21

What does theroretical lattice enthalpy assume?

Answer 21

That the ionic compounds resemble the perfect ionic model

Question 22

What does it mean if the theoretical and experimental value of lattice enthalpy are similar?

Answer 22

Compound obeys the perfect ionic model

Question 23

What does it mean if the theoretical and experimental value of lattice enthalpy are not similar?

Answer 23

The bonding is stronger and so contains covalent character, does not obey perfect ionic model

Question 24

Why do some ionic compounds contain covalent character?

Answer 24

If the positve ion is polarising, the electron cloud is pulled from the negative ion, distorting the ion and overlapping clouds

Question 25

What factors increase the polarising ability of the + ion in an ionic compound?

Answer 25

Small ionic radius Large ionic charge

Question 26

What factors increase the polarisability of the - ion in an ionic compound?

Answer 26

Larger ionic radius Low ionic charge

Question 27

When considering factors affecting the polarising of an ionic bond, is the charge or radius of the ions more significant?

Answer 27

Charge is a more significant factor to consider.

Question 28

What is entropy?

Answer 28

The measure of disorder in a system

Question 29

What factors affect the entropy of a system

Answer 29

States - Gas>liquid>solid (gas causes greater entropy) Greater Mr = greater entropy Number of species/ more species = greater entropy

Question 30

How is entropy change calculated?

Answer 30

Entropy of products - reactants

Question 31

What is the gibbs free - energy equation?

Answer 31

ΔG = ΔH - TΔS T must be in kelvin ΔS must be KJ K-1 mol-1

Question 32

What does ΔG mean + its units

Answer 32

If the value is 0 or below, a reaction is feasible at that temperature Kjmol^-1

Question 33

Why may some reactions not occur even if the reaction is deemed feasible?

Answer 33

Activation energy is too high

Question 34

How can you use a graph to calculate gibbs free energy?

Answer 34

ΔG = ΔH - T ΔS Y = -mx + c ΔG = - ΔST + ΔH Where -ΔS = gradient

Question 35

What is enthalpy of solution + Enthalpy of solution of MgCl2

Answer 35

The enthalpy change when 1 mole of an ionic substance is completely dissolved in water to infinite dilution MgCl2(s) --> Mg^2+(aq) + 2Cl-(aq)

Question 36

What is enthalpy of hydration + enthalpy of hydration of Mg^2+

Answer 36

The enthalpy change when 1 mole of gaseous ions is diluted to 1 mole of aqueous ions Mg^2+(g) --> Mg^2+(aq)

Question 37

What is lattice dissociation enthalpy + equation using MgCl2

Answer 37

The enthalpy change when 1 mole of an ionic solid is converted into gaseous ions MgCl2(s) --> Mg^2+(g) + 2Cl-(g)

Question 38

What is the formula to find enthalpy change of solution?

Answer 38

ΔH solution = ΔH L.D + ΔH hydration of both ions

Question 39

What does it mean if the enthalpy change of L.D is more than hydration?

Answer 39

Solution enthalpy will be endothermic

Question 40

What does it mean if the enthalpy change of L.D is less than hydration

Answer 40

Solution enthalpy will be exothermic

Question 41

What factors can affect hydration enthalpy?

Answer 41

Size of ion - smaller means more exothermic Charge of ion - greater means more exothermic

Question 42

What factors can affect lattice dissociation

Answer 42

SIze of ion - smaller means more endothermic (stronger electrostatic forces need more energy to overcome) Charge of ion - larger means more endothermic (requires more energy to overcome)

Question 43

Why are ionic compounds with very endothermic lattice dissociation enthalpies insoluble?

Answer 43

Lots of energy needed to break the electrostatic forces.

Question 44

What is the nature of attraction when talking about enthalpy of hydration

Answer 44

The forces between the partial charges on the water molecule and the ions

Question 45

How does the enthalpy of solution affect the solubility of the compound?

Answer 45

If enthalpy of solution exothermic, solubility decreases as temperature increases If enthalpy of solution is endothermic, solubility increases with temperature