enthalpy and entropy

Question 1

define enthalpy of formation

Answer 1

enthalpy change when one mole of a compound is formed from its elements in standard states under standard conditions: 298K and 100kPa

Question 2

define enthalpy change of first ionisation

Answer 2

wen 1 mole of gaseous ions is formed from one mole of gaseous atoms

Question 3

is ionisation endo or exo thermic

Answer 3

always endothermic as energy needed to overcome attraction between electron and nucleus

Question 4

define enthalpy change of atomisation

Answer 4

enthalpy change when 1 mole of gaseous atoms is formed from its elements in its standard states under standard conditions

Question 5

is standard enthalpy change of atomisation endothermic or exothermic?

Answer 5

always endothermic as energy is always required to break bonds between atoms in the element

Question 6

what exactly is lattice enthalpy

Answer 6

• measure of the strength of ionic bonding in a giant ionic lattice

Question 7

define lattice enthalpy as a formation process

Answer 7

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

Question 8

define enthalpy change of solution

Answer 8

• enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions

Question 9

define standard enthalpy change of hydration

Answer 9

enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions. ​

Question 10

enthalpy change of hydration of potassium chloride equations

Answer 10

K+(g) + aq → K+(aq)​

Cl-(g) + aq → Cl-(aq)

Question 11

enthalpy change of solution of potassium chloride equation

Answer 11

KCl(s) + aq → K+(aq) + Cl-(aq)

Question 12

what happens when a solid dissolves in water

Answer 12

• ionic lattice breaks up (reverse of lattice enthalpy)
• water molecules surround ions (enthalpy change of hydration)
• magnitiude of these two processes results in the enthalpy change of solution being exothermic or endothermic.​

Question 13

what factors affect lattice energy

Answer 13

• ionic charge and radius

Question 14

how does ionic radius affect lattice energy

Answer 14

• becomes less exothermic as ionic radius increases as charge on ions is more spread out over the ion when ions are larger
• ions are also further apart from each other in lattice so electrostatic forces of attraction between oppositely charged ions in lattice is weaker

Question 15

how does ionic charge affect lattice energy

Answer 15

• gets more exorthermic as ionic charge of ions increases. greater ionic charge means higher charge density
• this means stronger electrostatic attraction between oppositely charged ions in lattice, and as a result its more exothermic

Question 16

factors that affect enthalpy of hydration

Answer 16

• ionic charge and radius

Question 17

how does ionic radius affect enthalpy of hydration

Answer 17

• becomes more exothermic with decreasing ionic radii. smaller ions have a greater charge density resulting in stronger ion-dipole attractions between water molecules and ions in solution so more energy is released when they become hydrated and become more exothermic

Question 18

how does ionic charge affect enthalpy of hydration

Answer 18

• more exothermic with larger ionic charges as they have a greater charge density resulting in stronger ion-dipole attractions between water molecules and ions in solution, so more energy is released when they become hydrated and it becomes more exothermic

Question 19

how would you predict solubility of an ionic compound

Answer 19

if the sum of the hydration enthalpies is larger than the magnitude of the lattice enthaloy, then overal enthalpy change of solution will be exothermic, and so compound should disolve
- but solubility also depends on temperature and entropy

Question 20

which state has smallest entropy and which has greatest

Answer 20

solids have the smallest entropies, and gases have the greatest

Question 21

what is entropy

Answer 21

measure of disorder in particles. the number of ways particles can arrange themselves
units are J/K/mol.
the greater the entropy value, the greater the energy is spread our per kelvin per mol

Question 22

what does an increase in entropy mean for the system

Answer 22

• it means that the system has become more energetically stable

Question 23

second law of thermodynamics

Answer 23

• entropy of the universe is always increasing

Question 24

what is the standard entropy of a substance?

Answer 24

the entropy of one mole of a substanse under standard conditions (100kPa and 298K)
- they are always positive

Question 25

how to calculate entropy change of a reaction

Answer 25

sum of standard entropy of products - sum of standard entropy of reactants

Question 26

based on energy, what determines whether a reaction can happen or not

Answer 26

• a reaction can happen if the products have a lower overall energy than the reactants

Question 27

what is feasibility

Answer 27

whether a reaction is able to happen and if it is energetically feasible (spontaneous)

Question 28

what is the gibbs equation

Answer 28

free energy change = enthalpy change with surroundings - (temperature in K * entropy change of system)

Question 29

how do you know reaction is feesible using the gibbs equation

Answer 29

free energy change has to be less than 0

Question 30

the lattice enthalpy of sodium oxide is more exothermic than potassium oxide. explain why. (2 marks)

Answer 30

• Na+ ions have a smaller ionic radius/ r smaller than the K+ ions.
• this means they have higher electron density and so they have a stronger attraction to O2- ions than the K+ ions

Question 31

the first ionisation energy of sodium is more endothermic than that of potassium, explain why

Answer 31

sodium has a smaller atomic radius and fewer shells
so the nuclear attraction increases/ the outer electrons experience more attraction