1.8 ThermoDynamics

Question 1

define enthalpy change of formation

Answer 1

• enthalpy change when one mole of a compound is formed from its elements under standard conditions, 298K and 100kPa, all reactants and products being in their standard states

Question 2

define enthalpy of atomisation

Answer 2

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

Question 3

define bond dissociation enthalpy

Answer 3

• enthalpy change when one mole of a covalent bond is broken into two gaseous atoms.

Question 4

define first ionisation enthalpy

Answer 4

• enthalpy change to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a +1 charge

Question 5

define second ionisation enthalpy

Answer 5

• enthalpy change to remove one mole of electrons from one mole of gaseous 1+ ions to produce gaseous 2+ ions

Question 6

define first electron affinity

Answer 6

• enthalpy change that occurs when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous ions with a 1- charge

Question 7

define second electron affinity

Answer 7

• enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions

Question 8

define enthalpy of lattice formation

Answer 8

• enthalpy change when one mole of an ionic crystal lattice is formed from its constituent ions in gaseous form

Question 9

define enthalpy of lattice dissociation

Answer 9

• enthalpy change when one mole of an ionic crystal lattice is seperated into its constituent ions in gaseous form

Question 10

define enthalpy of hydration

Answer 10

• enthalpy change when one mole of gaseous ions become aqueous ions

Question 11

define enthalpy of solution

Answer 11

• enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well seperated and do not interact with one another

Question 12

explain why first electron affinity is exothermic

Answer 12

• ion is more stable than the nucleus
• attraction between nucleus and electron

Question 13

explain why second electron affinity is endothermic

Answer 13

• energy is required to overcome repulsive force between negative ion and electron

Question 14

what do the strength of enthalpy of lattice formations depend on?

Answer 14

• the sizes of the ions
• the charges on the ion

Question 15

how do the charge of the ions affect enthalpy of lattice formation?

Answer 15

• the greater the charge on the ion, the stronger the electrostatic attraction between the oppositely charged ions
• this results in a higher enthalpy of lattice formation

Question 16

how do the sizes of the ions affect enthalpy of lattice formation?

Answer 16

• the smaller the ionic radii, the closer the ions can get to each other
• increasing strength of electrostatic attraction
• leading to a higher lattice enthalpy

Question 17

what is the perfect ionic model?

Answer 17

• theoretical model that assumes ions are perfect spheres, purely ionic and the attractions are 100% electrostatic

Question 18

when does the perfect ionic model fail?

Answer 18

• when compounds contain covalent character
• when compounds contain large ions with low charge density

Question 19

what is meant by a spontaneous process?

Answer 19

• a process that will proceed on its own without any external influence

Question 20

what is entropy?

Answer 20

• a measure of the disorder of randomness in a system
• the higher the entropy, the greater the disorder and the more ways the system can be arranged

Question 21

why does the entropy increase when ice melts?

Answer 21

• water molecules are more disordered and have more freedom of movement than that in a solid form

Question 22

what is the order of most entropy in terms of states

Answer 22

gas > liquid > solids

Question 23

when can an increase in entropy occur?

Answer 23

• when there is a change of state from solid or liquid to gas
• when there is a significant increase in number of molecules between products and reactants

Question 24

how can we calculate change in entropy and what are the units?

Answer 24

sum of the entropy of products - sum of the entropy of reactants

units are JK^-1mol^-1

Question 25

do elements in their standard states have zero entropy?

Answer 25

• no
• only perfect crystals at absolute zero will have zero entropy

Question 26

what is Gibbs free energy change?

Answer 26

• thermodynamic quantity used to predict weather a process or reaction will occur spontaneously at a constant temperature and pressure.

Question 27

what is the formula for Gibbs Free Energy?

Answer 27

^G = ^H - T^S

^ = change
^H = change in enthalpy
^S = change in entropy
T = temperature K

Question 28

what are units for Gibbs Free Energy?

Answer 28

^G = kJmol^-1

^H = kJmol^-1

^S = JK^1mol^-1

divide ^S by 1000

Question 29

what are the conclusions of Gibbs Free Energy?

Answer 29

if ^G > 0, reaction is not feasible

if ^G < 0, reaction is feasible

if ^G = 0, reaction is at equilibrium

Question 30

how can i calculate the temp when a reaction will become feasible?

Answer 30

• find T when ^G = 0

Question 31

what is the effect on temperature on feasibility if ^H is exothermic

Answer 31

if ^S > 0, reaction is feasible at all temperatures, increasing temp makes it more feasible

if ^S < 0, reaction is feasible only at low temps, and increasing temp can make it non feasible

Question 32

what is the effect on temperature on feasibility if reaction is endothermic?

Answer 32

if ^S > 0, reaction becomes feasible at higher temps

if ^S < 0, reaction is never feasible nomatter what the temp is

Question 33

how can we plot a graph of Gibbs Free Energy?

Answer 33

y = mx + c

y = ^G

m = -^S

c = ^H

x intercept = T when reac is feasible

Question 34

what is the equation for enthalpy of solution?

Answer 34

enthalpy of solution = enthalpy of lattice dissociation + the sum of enthalpies of hydration

Question 35

why is enthalpy of dissociation enthalpy endothermic?

Answer 35

• ions are held together by strong electrostatic forces
• lattice dissociation involves breaking these ionic bonds to seperate the ions
• energy is required to break these bonds

Question 36

why are enthalpy of hydration of ions exothermic?

Answer 36

• ions are surrounded by water molecules in the solution
• water molecules interact with ions via ion dipole interaction
• negative ions attracted to delta positive H ions on polar water molecules
• positive ions are attracted to delta negative oxygen on polar water molecules