Thermodynamics

Question 1

define enthalpy change

Answer 1

the heat energy change at constant pressure

Question 2

define standard enthalpy of formation

Answer 2

the enthalpy change when one mole of a substance is formed from its elements under standard conditions with all substances in their standard states

Question 3

define standard enthalpy of combustion

Answer 3

the enthalpy change when one mole of a substance is completely combusted in excess oxygen under standard conditions with all substances in their standard states

Question 4

define mean bond enthalpy

Answer 4

the enthalpy change when one mole of a given covalent bond is broken averaged over a range of compounds

Question 4

define standard enthalpy of atomisation

Answer 4

the enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state

Question 5

define bond dissociation enthalpy

Answer 5

the enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms

Question 6

define first ionisation energy

Answer 6

the 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

the 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 lattice formation enthalpy

Answer 8

the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions

Question 8

define first electron affinity

Answer 8

the enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 9

define second electron affinity

Answer 9

the enthalpy change when one mole of electrons is added to one mole of 1- ions to form one mole of gaseous 2- ions

Question 10

why is the second electron affinity of any ion always endothermic

Answer 10

due to the repulsion between the negative ion and negative electron, energy is required to overcome it

Question 11

why is the second ionisation of an ion greater than the first

Answer 11

it is more difficult to remove an electron from a positive ion due to the stronger electrostatic attraction

Question 11

define lattice dissociation enthalpy

Answer 11

the enthalpy change to separate one mole of solid ionic compound into its gaseous ions

Question 12

how to calculate the standard enthalpy of atomisation for diatomic substances

Answer 12

bond dissociation enthalpy/2

Question 12

why is the first electron affinity exothermic

Answer 12

due to the electrons attraction to the nucleus

Question 13

what does Bend-Mex mean

Answer 13

• breaking bonds is endothermic
• making bonds is exothermic

Question 14

how does the strength of attraction correlate with the lattice formation enthalpy

Answer 14

• the stronger the attractions, the more exothermic the lattice formation enthalpy is

Question 15

how does the strength of attraction correlate with the lattice dissociation enthalpy

Answer 15

• the stronger the attractions, the more endothermic the lattice dissociation enthalpy is

Question 16

how does ion charge affect lattice enthalpy

Answer 16

• the higher the charge on the ion, the smaller the ionic radius will be meaning they are closer together in the lattice leading to a stronger ionic bond
• stronger attraction means more energy is given out on the formation and more energy is needed to dissociate the lattice

Question 17

how does the size of ion affect lattice enthalpy

Answer 17

• the smaller the ionic radius the closer together they are in the lattice leading to a stronger ionic bond
• stronger attraction means more energy is given out on the formation and more energy is needed to dissociate the lattice

Question 18

what is the perfect ionic model

Answer 18

• a model that assumes all ions are perfect spheres with purely ionic attraction

Question 19

how can you use the theoretical and experimental lattice enthalpy values to predict the ionic shape of a compound

Answer 19

• if the experimental value is the same as the theoretical there is no covalent character and it is the perfect ionic model
• if the experimental value differs from the theoretical it is due to the covalent character

Question 20

how is a covalent character formed

Answer 20

• the cation is much smaller than the anion and/or is highly charged
• the cation attracts the anion’s electrons causing the anion to become distorted and more covalent i.e. polarised

Question 21

how does a covalent character affect the strength of an ionic bond

Answer 21

- with a covalent character, the ionic bond is stronger

Question 22

how does polarisation affect the experimental enthalpy of lattice formation

Answer 22

the experimental enthalpy of lattice formation will be more exothermic than the theoretical value

Question 23

how does polarisation affect the experimental enthalpy of lattice dissociation

Answer 23

stronger ionic bond therefore more endothermic

Question 24

what does the enthalpy of solution mean

Answer 24

- when one mole of solid dissolves in water to form aqueous ions

Question 25

what is enthalpy of hydration

Answer 25

- the enthalpy change when one mole of gaseous ions becomes aqueous ions

Question 26

why is the enthalpy of hydration exothermic

Answer 26

Due to the attraction between… • the positive ion and the partially negative O of water • the negative ion and the partially positive H of water.

Question 27

how to calculate enthalpy of solution

Answer 27

enthalpy of lattice dissociation + enthalpy of hydration

Question 28

what affects the value of the enthalpy of hydration

Answer 28

- the stronger the attraction between the ion and part of the water molecule, the more exothermic the enthalpy of hydration

Question 29

what is entropy

Answer 29

- a measure of disorder in a system

Question 30

which factors affect entropy

Answer 30

- the state of the substances ( gas has most disorder, solid has least disorder) - the number of particles/moles ( more moles = greater disorder)

Question 31

how to calculate entropy change

Answer 31

- entropy change = ∑entropy of products - ∑entropy of reactants

Question 32

how to calculate gibbs free energy

Answer 32

- ΔG = ΔH - TΔS

Question 33

how to know if a reaction is feasible at a given temperature

Answer 33

- if the gibbs free energy is less than or equal to 0, reaction is feasible - if the gibbs free energy is more than 0, reaction is not feasible