Chapter 22 - Enthalpy and Entropy

Question 1

Define lattice enthalpy

Answer 1

Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

e.g Na+ (g) + Cl- (g) = NaCl (s)
gaseous ions solid ionic lattice

Question 2

What kind of energy change will lattice enthalpy always be?

Answer 2

lattice enthalpy is Exothermic and hence Enthalpy change is negative because bond forming is exothermic.

Question 3

What is the Born-Haber cycle?

Answer 3

A way of indirectly calculating lattice enthalpy from other known energy changes because it cant be calculated directly.

Question 4

What is the lattice formation and what energy change is it?

Answer 4

lattice formation is Changing gaseous ions into the solid ionic lattice and its Exothermic.

gaseous ions to ionic lattice

Question 5

Define standard enthalpy change of formation

Answer 5

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

Na (s) + 1/2 Cl2 (g) = NaCl (s)
reactants to one mole of a product

Question 6

Define standard enthalpy change of atomisation

Answer 6

The standard enthalpy change of atomisation is the enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

Na (s) = Na (g)
1/2 Cl2 (g) = Cl (g)

Question 7

Define first ionisation energy

Answer 7

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Na(g) = Na+ (g) + e-

Question 8

Why is ionisation energy always endothermic?

Answer 8

ionisation energy is endothermic because Energy is required to overcome the attraction between a negative electron and a positive nucleus.

Question 9

construct a born-haber cycle for NaCl?

Answer 9

Question 10

what does a general born-haber cycle look like?

Answer 10

Question 11

Define first electron affinity

Answer 11

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Cl (g) + e- = Cl- (g)

Question 12

Why is first elctron affinity always exothermic?

Answer 12

Electron being added is attracted in towards the nucleus.

Question 13

what do group 2 compounds require in there born haber cycle?

Answer 13

Born-haber cycles for group two compounds require both first and second ionisation energies.

Question 14

what do oxides and sulfides require in born haber cycles?

Answer 14

born-haber cycles for oxides and sulfides require first and second electron affinities/

O+ e- = O-
O- + e- = O 2-

Question 15

why is the second electron affinity endothermic?

Answer 15

the second electron affinity is negative because

1) an electron is being gained to a negatively charged atom which repels s there the same charge
2) Energy is needed to force the negatively charged electron onto the ion

Question 16

Why are successive electron affinities endothermic?

Answer 16

The negative ion repels the additional electron, so energy is required to force the electron onto it.

Question 17

Define standard enthalpy change of solution

Answer 17

standard enthalpy change of solution is The enthalpy change that takes place when one mole of a solute dissolves in a solvent under standard conditions.

Na+Cl- + aq= Na+ (aq) + Cl- (aq)
solute to dissolved

Question 18

What processes take place when a solid ionic compound dissolves in water?

Answer 18

when a solid ionic compound dissolves in water
the Ionic lattice breaks up
Water molecules are attracted to and surround the ions.

Question 19

What are the energy changes involved in these processes of a solid ionic compound dissolving in water?

Answer 19

When a solid ionic compound dissolves in water, the Ionic lattice is broken up forming separate gaseous ions.
gaseous ions interact with polar water molecules forming hydrated aqueous ions.
This is Called enthalpy change of hydration.

Question 20

Define enthalpy change of hydration

Answer 20

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Na+ + aq = Na + (aq)

Question 21

Describe the effect of ionic size on lattice enthalpy and melting point

Answer 21

as Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes less negative ( less exothermic)
Melting point decreases as less energy is needed to overcome the decreased attraction

Question 22

Describe the effect of ionic charge on lattice enthalpy and melting point

Answer 22

as Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative (more exothermic)
Melting point increases as more energy is needed to overcome increased attraction

Question 23

Describe the effect of ionic size on hydration enthalpy

Answer 23

as Ionic size increases
Attraction between ion and water molecules decreases
Hydration energy less negative

Question 24

Describe the effect of ionic charge on hydration enthalpy

Answer 24

Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative

Question 25

Why is hydration enthalpy an inaccurate method of predicting solubility?

Answer 25

Many compounds with endothermic enthalpy changes of solution are soluble.

Question 26

Define entropy

Answer 26

entropy is a measure of the dispersal of energy within the chemicals in a chemical system.

Question 27

What are the units of standard entropy?

Answer 27

J/K/mol

Question 28

How does entropy relate to physical states?

Answer 28

In order of size of entropy:
Gases > liquids > solids
solid has the least entropy and gases the most

Question 29

Define standard entropy

Answer 29

The entropy of one mole of a substance, under standard conditions : pressure of 100kpa and temp of 298 k
(Always positive)

Question 30

How is an entropy change calculated for a reaction?

Answer 30

entropy Change= products entropy sum- reactants entropy sum

Question 31

What is feasibility?

Answer 31

Whether a reaction is able to happen and is energetically feasible (or spontaneous). if products have less energy than reactants its ‘energetically feasible’

Question 32

What is the equation for free energy change?

Answer 32

ΔG = ΔH - TΔS

Question 33

What is the requirement for the free energy change such that a reaction is feasible?

Answer 33

ΔG < 0

there must be A decrease in free energy for it to happen.

Question 34

Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?

Answer 34

High activation energy resulting in a very slow rate.

Question 35

what does a general born-haber cycle look like?

Answer 35

Question 36

what are zwitterions?

Answer 36

zwitterions have both a negative and positive charge but are neutral overall

Question 37

table with bond enthalpy, bond dissociation and definitions?

Answer 37

j