thermodynamics

Question 1

what is Hess’ Law

Answer 1

enthalpy change for a reaction is independent of the route taken

Question 2

define standard enthalpy of formation

Answer 2

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

Question 3

what is the standard enthalpy of an element

Answer 3

0

Question 4

define standard enthalpy of combustion

Answer 4

enthalpy change when one mole of substance is completely burnt in excess oxygen

Question 5

define standard enthalpy of atomisation

Answer 5

enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

Question 6

define first ionisation energy

Answer 6

enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 6

define first ionisation energy

Answer 6

enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 7

define second ionisation energy

Answer 7

enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

Question 8

define first electron affinity

Answer 8

enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions

Question 9

define second electron affinity

Answer 9

enthalpy change when on emole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions

Question 10

define lattice enthalpy of formation

Answer 10

enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions

Question 11

define lattice enthalpy of dissociation

Answer 11

enthalpy change when one mole of solid ionic lattice is dissociated into its gaseous ions

Question 12

define enthalpy of hydration

Answer 12

enthalpy change when one mole of gaseous ions become hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)

Question 13

define enthalpy of solution

Answer 13

enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution

Question 14

define mean bond dissociation enthalpy

Answer 14

enthalpy change when one mole of covalent bonds if broken with all species in the gaseous state

Question 15

what is a born-haber cycle

Answer 15

thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in their standard states (enthalpy of 0)

Question 16

what factors affect the lattice enthalpy of an ionic compound

Answer 16

size of ions, charge on the ions

Question 17

how can you increase the lattice enthalpy of a compound

Answer 17

smaller ions since the charge centres will be closer together

increased charge, since there will be a greater electrostatic force of attraction between the oppositely charged ions

increasing the charge on the anion has a much smaller effect than increasing the charge on the cation, since increasing anion charge also has the effect of increasing ionic size

Question 18

how can born-haber cycles be used to see if compounds could theoretically exist

Answer 18

use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable

was used to predict the existence of the first noble gas containing compound

Question 19

what actually happens when a solid is dissolved in terms of interactions of the ions with water molecules

Answer 19

break lattice –> gaseous ions; dissolve each gaseous ion in water

aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)

Question 20

what is the perfect ionic model

Answer 20

assumes that ions are perfectly spherical and that there is an even charge distribution (100% polar bonds)

act as point charges

Question 21

why is the perfect ionic model often not accurate

Answer 21

ions are not perfectly spherical
polarisation often occurs when small positive ions or large negative ions are involved, so the ionic bond gains covalent character
some lattices are not regular and the crystal structure can differ

Question 22

which kinds of bonds will be the most ionic

Answer 22

between large positive ions and small negative ions

Question 23

define the terms spontaneous and feasible

Answer 23

if a reaction is spontaneous and feasible, it will take place of its own accord; does not take account of rate of reaction

Question 24

is a reaction with a +ve or -ve enthalpy more likley to be spontaneous

Answer 24

-ve since exothermic

Question 25

define entropy

Answer 25

randomness/disorder of a system high value entropy = more disordered

Question 26

what units is entropy

Answer 26

JK-1mol-1

Question 27

what is the second law of thermodynamics

Answer 27

entropy of an isolated system always increases as it is overhwlemingly more likely for molecules to be disordered than ordered

Question 28

is a reaction with positive or negative entropy change more likley to be spontaneous

Answer 28

+ve reactions always try and increase the amount of disorder

Question 29

how would you calculate entropy change for a reaction

Answer 29

entropy change = sum of product entropy - sum of reactant entropy

Question 30

gibbs free energy equation

Answer 30

change in gibbs free energy = enthalpy change - (temperature x entropy change)

Question 31

what does the value for gibbs free energy

Answer 31

if G<0, reaction is feasible if G=0, reaction is JUST feasible if G>0, reaction is not feasible

Question 32

what is the significance of the temperature at which G=0

Answer 32

temperature at which reaction becomes feasible in Kelvin

Question 33

what are the limitations of using G as an indicator of whether a reaction will occur

Answer 33

gibbs free energy only indicates if a reaction is feasibke it does not take into account the rate of reaction (kinetics of the reaction). in reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring

Question 34

if the reaction is exothermic and entropy increases, what is the value of G and what does this mean

Answer 34

G is always negative, so reaction is always feasible - product favoured

Question 35

if the reaction is endothermic and entropy decreases, what is the value of G and what does this mean

Answer 35

G always positive, so reaction is never feasible , reactant favoured

Question 36

if reaction is exothermic and entropy decreases, what is the value of G

Answer 36

temperature dependent

Question 37

if the reaction is endothermic and entropy increases, what is the value of G and what does this mean

Answer 37

temperature dependent

Question 38

why is entropy zero at 0K

Answer 38

no disorder - molecules/atoms are not moving or vibrating and cannot be arranged in any other way maximum possible state of order

Question 39

what are the two key things to look out for to decide if entropy increases/decreases/stays relatively the constant

Answer 39

number of moles - more moles made - increase in entropy - going from solid - liquid/gas or liquid - gas

Question 40

how is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant

Answer 40

the heat that is given out escapes to the surroundings