Thermodynamics

Question 1

define enthalpy change of formation

Answer 1

enthalpy change when 1 mole of substance/compound is formed from its constituent elements in their standard states under standard conditions

Question 2

define lattice enthalpy of formation

Answer 2

enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

Question 3

define lattice enthalpy of dissociation

Answer 3

the enthalpy change when 1 mole of a solid ionic compound is dissociated into its gaseous ions under standard conditions

Question 4

define enthalpy change of dissociation

Answer 4

enthalpy change when 1 mole of bonds of the same type of molecule in the gaseous state is broken

Question 5

define enthalpy change of 1st ionisation

Answer 5

enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms

Question 6

define enthalpy change of atomisation

Answer 6

enthalpy change when 1 mole of gaseous atoms is made from an element in its standard state

Question 7

define enthalpy change of 2nd ionisation

Answer 7

enthalpy change when 1 mole of gaseous 2+ ions are made from 1 mole of gaseous 1+ ions

Question 8

define 1st electron affinity

Answer 8

enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms

Question 9

define 2nd electron affinity

Answer 9

enthalpy change when 1 mole gaseous 2- ions are made from 1 mole of gaseous 1- ions

Question 10

What are born-haber cycles?

Answer 10

useful to calculate lattice enthalpies

Question 11

why can theoretical and experiment lattice enthalpies be different?

Answer 11

different depending on how ‘purely ionic’ the compound is
Perfectly ionic model:
- ions are perfectly spherical
- the charge is evenely distributed in this sphere

if experimental value is differently it shows the compound does not follow the perfectly ionic model and has some covalent characteristics

Question 12

why does the experimental value sometimes have some covalent characteristics?

Answer 12

positive ion distorts the charge distribution in the negative ion, therefore the positive ion polarises the negative ion

the more polarisation, the more covalent character there will be

Question 13

define enthalpy of solution

Answer 13

enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution

Question 14

what is needed for a substance to dissolve?

Answer 14

substance bonds must break (endo)
new bonds formed between the solvent and the substance (exo)

Question 15

what occurs when a substance is dissolved in a solvent?

Answer 15

substance bonds are broken to create free moving ions
bonds are formed between ions and water, the ions become hydrated

most ionic compounds are dissolved in solvents such as H2O, the delta positive H is attracted to negative ions, delta negative O is attracted to positive ions

Question 16

what must happen for hydration to occur?

Answer 16

the new bonds formed must be the same strength or greater than those broken, if not then the substance is unlikely to dissolve

Question 17

how can the enthalpy of solution be calculated?

Answer 17

from knowing the lattice dissociation enthalpy and the enthalpy of hydration

Question 18

how do you calculate the enthalpy of a solution?

Answer 18

enthalpy of lattice dissociation + enthalpy of hydration

Question 19

what is entropy?

Answer 19

a measure of disorder, the more disordered the substance, the greater its entropy value. entropy will increase as temperature increases as the particles gain energy and move further apart, becoming more disordered

Question 20

which state has the greatest entropy?

Answer 20

gases

Question 21

how do you calculate the overall entropy change?

Answer 21

S (total) = S products - S reactants

Question 22

what is gibbs free energy?

Answer 22

allows entropy change to be found without needing to measure the effects on the surroundings

Question 23

what is the equation for gibbs free energy? (G)

Answer 23

G = H - T.S

Question 24

what value must the Gibbs free energy be in order for a reaction to be feasible?

Answer 24

negative or zero

Question 25

how does temperature effect the gibbs energy value and feasibility?

Answer 25

H and S both positive = will make T.S greater therefore will be feasible at certain temperatures H negative, S positive = makes G become more negative, therefore always feasible H positive, S negative = no effect G will always be positive, so never feasible H and S both negative, usually feasible at lower temperatures

Question 26

how would you rearrange the equation to find the temperature at which gibbs free energy is zero?

Answer 26

T = H/S

Question 27

what must be done in order to get the correct gibbs value?

Answer 27

enthalpy change must be converted from kJmol-1 to Jmol-1, to do this x1000