Thermodynamics

Question 1

Define enthalpy change

Answer 1

Heat energy change at constant pressure

Question 2

Define enthalpy of lattice formation

Answer 2

Enthalpy change when 1 mol of solid ionic compound is formed from its gaseous ions

Question 3

How would you calculate Entropy change for a reaction (delta S)

Answer 3

Sum of products - reactants

Question 4

How do you calculate Gibbs free energy?

Answer 4

AG = AH - TAS

H= enthalpy change
S= entropy change
T= temp

Question 5

When is G (gibbs) feasible and not feasible

Answer 5

G is 0 or less than 0 (negative) = feasible

G is more than 0 = not feasible

Question 6

How would you calculate what temperature the reaction becomes feasible

Answer 6

T = AH/ (AS/1000)

Question 7

Define enthalpy of hydration

Answer 7

Enthalpy change when one mole of gaseous ions becomes aqueous ions

Question 8

Define enthalpy of atomisation

Answer 8

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

Question 9

Define electron affinity

Answer 9

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

Question 10

Define mean bond dissociation enthalpy

Answer 10

Enthalpy change when one mole of covalent bonds is broken with all species in the gaseous state

Question 11

Define enthalpy change of combustion

Answer 11

Enthalpy change when 1 mol of a compound is burned completely in O2 with all reactants and products in their standard states under standard conditions

Question 12

What is Hess’ law

Answer 12

The enthalpy change for a chemical reaction is independent of the route taken

Question 13

What’s the perfect ionic model

Answer 13

• assumes bonding is 100% ionic (has no covalent character)
• all ions are perfect spheres

Question 14

According to the perfect ionic model, what would be the difference between LDE values

Answer 14

There would be no difference between the experimental and theoretical

Question 15

Why does LDE decrease

Answer 15

The size of the negative ion increases

Question 16

What does a bigger difference between LDEexp and LDEtheo mean

Answer 16

The bigger the difference between the values the more covalent character

Question 17

What effect does the covalent character have on bonds

Answer 17

It causes the strength of the bonds to increase

Question 18

What does the difference between the experimental LDE and theoretical/perfect ionic LDE mean

Answer 18

• because there is a difference it shows there is some degree of covalent character

Question 19

How do you work out activation energy (Ea) in the Arrhenius equation

Answer 19

Ea = RT(lnA-lnk)

Question 20

If a reaction is carried out at a higher temperature how does it effect (AG) delta G

Answer 20

TAS gets bigger

Question 21

How does disorder effect entropy (delta s)

Answer 21

An increase in disorder and entropy will lead to a positive entropy change

Question 22

When will an increase in entropy occur

Answer 22

• state change from solid/liquid to gas
• increase in number of molecules between products and reactants

Question 23

How do AS and AH effect AG/feasibility

Answer 23

• increased entropy (+ve AS) and exothermic (-ve AH) will make AG -ve so will be feasible

Question 24

When temp increases what happens to the feasibility if entropy (AS) decreases/is negative

Answer 24

AG is more likely positive so reaction is not feasible so won’t occur

Question 25

When temp increases what happens to the feasibility if entropy (AS) increases/is positive

Answer 25

AG is negative so reaction is feasible

Question 26

When temp increases what happens to the feasibility if entropy (AS) is 0

Answer 26

AG will not change much

Question 27

How does larger ion size effect enthalpy of formation

Answer 27

- Bigger ion means smaller charge density - so enthalpy of formation will be less negative/exothermic - because there is a weaker attraction between ion and halide

Question 28

Define enthalpy of solution

Answer 28

Enthalpy change when 1 mol of an ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions

Question 29

Define enthalpy of lattice dissociation

Answer 29

Enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in their gaseous states

Question 30

Define enthalpy change of formation

Answer 30

The energy transferred when 1 mol of a compound is formed from its elements under standard conditions, and all reactants and products in their standard states

Question 31

Define mean bond enthalpy

Answer 31

Enthalpy change needed to break 1 mol bonds averaged over a range of molecules

Question 32

Why is electron affinity endothermic

Answer 32

The ion and the electron both have a negative charge so energy is required to overcome repulsion

Question 33

How do you work out AH from a graph

Answer 33

Y intercept

Question 34

How do you work out AS from a graph

Answer 34

gradient x 1000

Question 35

How do you work out what temperature a reaction will become feasible from a graph

Answer 35

The temperature at the X intercept and above will be feasible

Question 36

How do you work out bond enthalpy

Answer 36

Reactants - products

Question 37

What’s the difference between enthalpy of solution and hydration

Answer 37

Solution - (s) —> (aq) Hydration - (g) —> (aq)