22 - Enthalpy And Entropy

Question 1

What does lattice enthalpy measure

Answer 1

Ionic bond strength

• energy given out when gaseous ions combine to make a solid lattice

Question 2

Standard lattice enthalpy definition

Answer 2

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

Question 3

What are standard conditions

Answer 3

298k
25c

100kpa

Question 4

The more negative the lattice enthalpy the…

Answer 4

Stronger the bonding

Question 5

Standard enthalpy change of formation definition

Answer 5

The enthalpy change that takes place when one mole of a compound is formed from its elements in their standard states under standard conditions

Question 6

enthalpy change of formation equation

Answer 6

The normal reaction equation

Elements in standard state - > solid compound

Question 7

Standard enthalpy change of atomisation definition

Answer 7

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions

Question 8

enthalpy change of atomisation

Is it endothermic or exothermic

Answer 8

Endothermic - because bonds are broken to form gaseous atoms

Elements in standard state -> gaseous atoms

Question 9

First ionisation energy

Answer 9

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ atoms

Gaseous atom -> gaseous ion

Question 10

First ionisation

Endo or exo

Answer 10

Endo

Energy required to overcome attraction between a negative electron and the positive nucleus

Question 11

First electron affinity definition

Answer 11

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 12

First electron affinity

Endo or exo

Answer 12

Exo

- the electron added is attracted in towards the nucleus

Question 13

Why is the second successive electron affinity endothermic

Answer 13

A second electron is gained by a negative ion, energy is need to force the electron onto the negative ion, due to repulsion

Question 14

Enthalpy change of solution definition

Answer 14

The enthalpy change that takes place when one mole of a solute dissolves in a solvent under standard conditions

Ionic compound + aq -> aqueous ions

Question 15

What is the experimental determination of the enthalpy change of solution

Procédure

Answer 15

1) weigh out sample of KCl
2) pour 25cm3 of distilled water into a plastic cup in a beaker
3) measure initial temp
4) tip all of KCl in cup - stir till dissolved, temp is constant
5) record temp to nearest 0.5c

Question 16

What is the experimental determination of the enthalpy change of solution

Calculations

Answer 16

1) use q=mct 
C= 4.18
Mass of water - (1gcm-3 x volume used)
Mass of solution - (water + solid)
- energy = kJ

2)find moles of solid compound

3) calculate enthalpy change in solution
- 0.05 mol of KCl gains 278kj energy from solution - 1 mol would gain

Energy / mol

Question 17

What is the dissolving process steps

Answer 17

1) the bonds between the ions break to give gaseous ions - endo = Lattice enthalpy

2) the separate gaseous ions interact with polar water to form hydrated aqueous ions
= enthalpy change of hydration

Question 18

Enthalpy change of hydration definition

Answer 18

Enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions

Question 19

Enthalpy change of hydration

Équation

Answer 19

Can be exo or endo - depending on relative size of LE

Gaseous ions + aq -> aqueous ions

Question 20

List 2 factors affecting Lattice enthalpy

Answer 20

Ionic size

Ionic charge

Question 21

Affect of ionic charge on LE

Answer 21

• ionic radius increases
• lower charge density
• ions further apart in lattice
• attraction between ions decrease
• lattice enthalpy less negative
• melting point decreases

Question 22

Effect of ionic charge on LE

Answer 22

• ionic charge increases
• more energy released when ionic lattice forms
• attraction between ions increases
• LE becomes more negative
• mp increases

Question 23

List factors affecting hydration

Answer 23

Ionic size

Ionic charge

Question 24

Ionic size affect on hydration enthalpy

Answer 24

• ionic radius increases
• lower charge density
• attraction between ion and water molecules decreases
• hydration enthalpy is less negative

Question 25

Ionic charge on hydration enthalpy

Answer 25

- ionic charge increases - attraction with water molecules increases - stronger electro station attraction - more energy released when the bonds are made - hydration enthalpy more negative

Question 26

Entropy definition

Answer 26

The dispersal of energy and disorder within chemicals making up the chemical system

Question 27

The higher the entropy (positive) ...

Answer 27

The more disordered a system

Question 28

What is entropy the measure of

Answer 28

The number of ways that particles can be arranged The number of ways that the energy can be shared out between the particles

Question 29

How physical state affects entropy

Answer 29

Solid -> liquid -> gas Gases: Randomness increases, most disorder, have the greatest entropy

Question 30

Why more particles means more entropy

Answer 30

The more particles The more ways their energy can be arranged Entropy increases because the number of moles increases Energy is spread out more

Question 31

Why are some reactions feasible

Answer 31

Substances like disorder, they’re more stable | Particles move to increase their entropy

Question 32

What does thermodynamically stable mean

Answer 32

When a substance reaches its maximum entropy state Won’t react further without energy input

Question 33

How to calculate entropy change

Answer 33

Change in S = S of products - S reactants

Question 34

What does a positive entropy mean

Answer 34

The reaction is likely to be feasible React without the addition of energy

Question 35

Why does negative free energy not guarantee a reaction

Answer 35

Free energy change doesn’t tell you about rate of reaction It might have a high activation energy or happen very slowly

Question 36

What is free energy

Answer 36

The overall change in energy during a chemical reaction Tells you if a reaction is feasible or not

Question 37

What 3 factors determine whether a process will take place

Answer 37

Température - kelvin Entropy - j K-1mol-1= convert by dividing by a thousand Enthalpy - kjmol-1

Question 38

Free energy change equation

Answer 38

G = H - tS

Question 39

What two types of energy make up free energy change

Answer 39

The enthalpy change - heat transfer between the chemical system and the surroundings Entropy change at temperature of reaction - dispersal of energy within chemical system

Question 40

Limitations of predictions made for feasibility

Answer 40

- high activation energy | - slow rate of reaction