Chapter 22

Question 1

What is lattice enthalpy (ΔlattHꝋ)?

Answer 1

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

A measure of the strength of the ionic bonds in a giant ionic lattice.

Question 2

Is lattice enthalpy endothermic or exothermic?

Answer 2

exothermic, as when ions are combined to form an ionic solid lattice there is an extremely large release of energy. The value will always be negative.

Question 3

What do we use to measure lattice enthalpy?

Answer 3

The Born- Haber cycle.

Question 4

What is enthalpy change (ΔH)?

Answer 4

The amount of heat energy transferred during a chemical reaction, at a constant pressure

Question 5

What is the enthalpy of formation (ΔfHꝋ)?

Answer 5

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions

Question 6

Is the enthalpy of formation (ΔfHꝋ) endothermic or exothermic?

Answer 6

The ΔHfꝋ can be endothermic or exothermic as the energy change is the sum of the bonds broken and formed, so the enthalpy change can have positive or negative values.

Question 7

What is ionisation enthalpy (ΔieHꝋ)?

Answer 7

The amount of energy required to remove an electron from a gaseous atom of an element to form a gaseous ion under standard conditions

Question 8

Is ionisation enthalpy (ΔieHꝋ) exothermic or endothermic?

Answer 8

Endothermic, energy is needed to overcome the attraction between an electron and the nucleus

Question 9

What is the first ionisation energy (ΔieHꝋ)?

Answer 9

Energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

Question 10

What is the standard enthalpy change of atomisation (ΔatHꝋ)?

Answer 10

The enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.

Question 11

Is the standard enthalpy change of atomisation (ΔatHꝋ) endothermic or exothermic?

Answer 11

Endothermic, energy is always required to break any bonds between the atoms in the element, to break the element into its gaseous atoms

Question 12

What is the first electron affinity (ΔEAHꝋ)?

Answer 12

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

Question 13

Do endothermic reactions go up or down in a born haber cycle?

Answer 13

Up, energy of substances increases.

so exothermic reactions go down..

Question 14

What states do atoms go through in the Born-Haber cycle (in order)?

Answer 14

The cycle shows all the steps needed to turn atoms into gaseous ions and from gaseous ions into the ionic lattice.

Question 15

What states do atoms go through in the Born-Haber cycle (in order)?

Answer 15

The cycle shows all the steps needed to turn atoms into gaseous ions and from gaseous ions into the ionic lattice.

Question 16

What is a successive electron affinity and are they endothermic/ exothermic?

Answer 16

When an atom has a charge like Mg2+ it has to go from Mg1+ to that 2+. Successive electron affinities are endothermic, a second electron is being gained by a negative ion (which repels it) so energy must be put in to force it on.

1st electron affinity goes down (exothermic) second goes up (endothermic).

Question 17

What’s the order of the enthalpy changes in a Born-haber cycle?

Answer 17

1- Enthalpy change of formation
2- Enthalpy change of atomisation (1st element)
3- Enthalpy change of atomisation (2nd element)
4- First ionisation energy
5- First electron affinity
6- lattice enthalpy

Question 18

How do you calculate lattice energy?

Answer 18

ΔHfꝋ = ΔHatꝋ + ΔHatꝋ + IE + EA + ΔHlattꝋ
rearrange..
ΔHfꝋ = ΔH1ꝋ + ΔHlattꝋ

ΔH1ꝋ = the sum of all of the various enthalpy changes necessary to convert the elements in their standard states to gaseous ions.

so its- ΔHlattꝋ = ΔHfꝋ - ΔH1ꝋ

Question 19

What is the standard enthalpy change of solution (ΔHsol)?

Answer 19

The enthalpy change when 1 mole of a solute dissolves in a solvent.

If the solvent is water- the ions from the ionic lattice finish surrounded with water molecules.

Question 20

What is the standard enthalpy change of hydration (ΔHhyd)?

Answer 20

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Question 21

What are the two processes that occur when a solid ionic compound dissolves in water?

Answer 21

1- the ionic lattice breaks up and forms separate gaseous ions. (Opposite of lattice energy)

2- The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.

Question 22

What are the two key factors that affect lattice enthalpy?

Answer 22

The charge of the ions that make up the crystalline lattice.

The radius of the ions that make up the crystalline lattice.

Question 23

How does the ionic radius increasing affect lattice energy and why?

Answer 23

The lattice energy becomes less exothermic.

This is because the charge on the ions is more spread out over the ion when the ions are larger.
The ions are also further apart from each other in the lattice. Therefore, the electrostatic forces of attraction between the oppositely charged ions in the lattice are weaker.

Question 24

How does the ionic charge increasing affect lattice energy and why?

Answer 24

The lattice energy gets more exothermic.

The greater the ionic charge, the higher the charge density.
This results in stronger electrostatic attraction between the oppositely charged ions in the lattice.

Question 25

What are the two key factors that affect lattice enthalpy of hydration?

Answer 25

It is affected by the amount that the ions are attracted to the molecules and this is controlled by the ionic charge and radius of the atoms.

Question 26

How does ionic radius decreasing affect the enthalpy change of hydration?

Answer 26

ΔHhydꝋ becomes more exothermic. Smaller ions have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution. Therefore, more energy is released when they become hydrated.

Question 27

How does ionic charge increasing affect the enthalpy change of hydration?

Answer 27

ΔHhydꝋ is more exothermic. Ions with large ionic charges have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution. So more energy is released when they become hydrated.

Question 28

What is entropy (S)?

Answer 28

The dispersal of energy within the chemicals making up the chemical system. The greater the entropy the greater the dispersal of energy and the greater the dispersal. greater entropy- means that the molecules are more disordered.

Question 29

What units are used for entropy?

Answer 29

JK-1mol-1

Question 30

Does entropy increase or decrease when ice melts (solid-> liquid)?

Answer 30

Entropy increases in changed in state that give a more random arrangement of particles.

Question 31

What is the standard entropy of a substance?

Answer 31

The entropy of one mole of a substance under standard conditions.

Question 32

How do you calculate entropy?

Answer 32

ΔSꝋ = ΣSproductsꝋ - ΣSreactantsꝋ Sum of the standard entropy of products - the sum of the standard entropy of reactants.

Question 33

What does it mean for a reaction to be feasible?

Answer 33

reactions can take place of their own accord, in other words, they are energetically favourable.

Question 34

What does feasibility take no account of? What is an issue with this?

Answer 34

feasibility takes no account of the rate of reaction and states only what is possible, not what actually happens. A feasible reaction might be incredibly slow, such as the rusting of iron - which may not be helpful.

Question 35

What equation is used to calculate if a reaction is feasible or not?

Answer 35

The Gibbs equation.

Question 36

What is the Gibbs equation?

Answer 36

ΔGꝋ = ΔHreactionꝋ - TΔSsystemꝋ free energy change = enthalpy change with surroundings - temperature in kelvin x entropy change of system.

Question 37

What is free energy change?

Answer 37

The overall change in energy during a chemical reaction.

Question 38

What 2 energies is the free energy change made up of?

Answer 38

1- the enthalpy change. (heat transfer between the chemical system and the surroundings) 2- the entropy change at the temperature of the reaction. (the dispersal of energy within the chemical system itself)

Question 39

For a reaction to be feasible should the free energy change be above or below 0?

Answer 39

There must be a decrease in free energy. ΔG < 0

Question 40

What units is ΔH and ΔS shown in?

Answer 40

ΔH- shown in KJmol-1 ΔS- shown in JKmol-1 so the units of ΔS need to be divided by 1000 to match KJ

Question 41

When ΔGꝋ is negative is the reaction feasible?

Answer 41

Yes

Question 42

When ΔGꝋ is positive is the reaction feasible?

Answer 42

No

Question 43

What is a limitation of using Gibs equation

Answer 43

doesn't account for rate of reaction- the actual rate could be very slow.