Enthalpy and entropy

Question 1

What is the definition of lattice enthalpy

Answer 1

Enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions
Always exothermic, measures the strength of ionic bonding in a giant ionic lattice

Question 2

Are ionic solid compounds stable?

Answer 2

Tend to be very stable

Question 3

How does stability arise from ionic bonds

Answer 3

Strong electrostatic attraction between oppositely charged ions in a lattice

Question 4

Born-Haber cycles

Answer 4

Like a Hess cycle where the lattice enthalpy cannot be measured directly

Question 5

Standard enthalpy change of formation

Answer 5

Enthalpy change that occurs when 1 mole of a compound is formed from its elements under standard conditions
The compound will always be an ionic compound in its solid lattice

Question 6

Standard enthalpy change of atomization

Answer 6

Enthalpy change that takes place for the formation of 1 mole gaseous atoms from the element in its standard state under standard conditions
Always endothermic, bonds broken to form gaseous atoms

Question 7

Electron affinity

Answer 7

Opposite of ionization energy

Question 8

1st electron affinity

Answer 8

Enthalpy change that takes place when 1 electron is added to each atom in a mole of gaseous atoms to form 1 mole of gaseous 1- ions
Always exothermic

Question 9

Determination of lattice enthalpies

Answer 9

Start from standard states
Formation of gaseous atoms
Formation of gaseous ions
Form lattice

From standard states
Formation of lattice from elements

Question 10

Considerations to make when determining lattice enthalpies

Answer 10

If an ion has a charge greater than 1, multiply IE and EA

If there is a diatomic atom, multiply everything by a multiple of 2

Question 11

Successive electron affinities

Answer 11

1st EA, exothermic

2nd EA, endothermic

Question 12

Why is the 2nd successive electron affinity endo?

Answer 12

2nd electrons is being gained by a -ve ion

Energy must be put in to force the -ve electron onto -ve ion

Question 13

Definition of enthalpy change of solution

Answer 13

Enthalpy change that takes place when 1 mole of solute dissolves in a solvent to form aqueous ions
Can be exo or endo

Question 14

How do ions act in solution?

Answer 14

Partial charges in water are attracted to +ve -ve ions
Lattice dissolved into separate ions
Ions are now surrounded by water to form aqueous ions

Question 15

Determination of enthalpy change in solution

Answer 15

Calculate q=mc∆t/1000
m=mass of water and solute as its the enthalpy change of the solution we want

E/Moles=kj

Question 16

What mass is used in enthalpy solution calculations?

Answer 16

Water+solute

as its the solution that changes temperature

Question 17

What processes occur during dissolving?

Answer 17

Ionic lattice breaks up

Water molecules are attracted to and surround ions

Question 18

What energy changes are involved in dissolving

Answer 18

Ionic lattice broken up, forming separate gaseous ions,

Separate gaseous ions interact with polar water molecules too from hydrated aqueous ions, enthalpy change of hydration

Question 19

Energy cycle in hydration

Answer 19

From ionic lattice
Formation of gaseous ions (opposite of lattice enthalpy)
Hydrate ions from gaseous ions to form aqueous ions

Question 20

Considerations to make when forming enthalpy cycles

Answer 20

If 2 of the same ion is used, multiply by 2
Ensure all species are balanced
Consider arrow direction

Question 21

Properties of ions compounds

Answer 21

Tend to have high melting and boiling points
Soluble in polar solvents
Conduct electricity when molten or in aqueous solution

Question 22

Factors affecting lattice enthalpy

Answer 22

Ionic size

Ionic charge

Question 23

Effect of ionic size on lattice enthalpy

Answer 23

As ionic radius increases
Attraction between ions decrease
Lattice enthalpy less -ve
Melting point decreases

Question 24

Effect of ionic charge on lattice enthalpy

Answer 24

As ionic charge increases
Attraction between ions increases
Lattice energy becomes more -ve
Melting point increases

Question 25

Combination of both effects from alkali metals to the right

Answer 25

Increasing charge Decreasing size Both result in more attraction

Question 26

Combination of both effects from noble gases to the left

Answer 26

Increasing charge Increasing size Opposing effects

Question 27

How to predict melting points

Answer 27

Magnitudes of lattice enthalpy give a good indication of melting points Other factor such as the packing of ions may need to be considered

Question 28

Factors affecting hydration

Answer 28

Similar to lattice enthalpies Ionic size Ionic charge

Question 29

Effect of ionic size on hydration enthalpy

Answer 29

Ionic radius increases Attraction between ion and water molecules decreases Hydration energy less -ve

Question 30

Effect of ionic charge on hydration enthalpy

Answer 30

Ionic charge increases Attraction between ion and water molecules increases Hydration energy more -ve

Question 31

Predicting solubility

Answer 31

To dissolve a compound, attraction between ions in ionic lattice must be overcome Requires a quantity of energy equal to lattice enthalpy Water molecules are attracted to +ve and -ve ions, surrounding them and releasing energy equal to hydration enthalpy If sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy, enthalpy change of solution will be exothermic, should dissolve However, some endothermic enthalpy changes are soluble, also depends on temperature and entropy

Question 32

What is entropy?

Answer 32

Dispersal of energy within the chemicals making up the chemical system JK-1 mol-1

Question 33

What has the greatest enthalpy between the states of matter?

Answer 33

In increasing amounts of entropy Solid Liquid Gas

Question 34

What affects the amount of entropy?

Answer 34

Melting and boiling points increase randomness of particle movement, energy more spread out +S

Question 35

Entropy at and above 0K

Answer 35

At 0K, there would be no energy, all substances have an entropy value of 0 Above 0K, energy becomes more dispersed among particles, +ve entropy

Question 36

Predicting entropy changes

Answer 36

If physical states change, entropy changes | If no of gas moles changes, entropy changes

Question 37

Standard entropy

Answer 37

Entropy of 1 mole of substance under standard conditions JK-1mol-1 Always +ve

Question 38

Calculating entropy changes

Answer 38

Entropy change=Entropy of products-entropy of reactants

Question 39

Feasibility of reactions

Answer 39

Used to describe whether a reaction is able to happen and is energetically feasible

Question 40

Free energy

Answer 40

Overall change during a chemical reaction is the free energy change ∆G made up of 2 energies Enthalpy change ∆H, heat transfer between chemical systems and surroundings Entropy change at temperature of reaction T∆S, dispersal of energy within chemical system itself

Question 41

Gibbs equation

Answer 41

∆G=∆H-T∆S Temperature in K ∆S must match kJ in ∆H, divide ∆S by 1000

Question 42

Conditions for feasibility

Answer 42

∆G must be less than 0

Question 43

Working out minimum temperature for feasibility

Answer 43

T=∆H/∆S | Convert K to C

Question 44

Limitation of predictions made for feasibility

Answer 44

Does not account for activation energies or rate of reaction