Topic 13 - Energetics II

Question 1

meaning enthalpy

Answer 1

measure of heat content of a substance

Question 2

meaning enthalpy changed

Answer 2

change in heat content at constant pressure

Question 3

what are the standard conditions

Answer 3

100 kPa and 25°C

Question 4

is the enthalpy change negative/positive in:
- exothermic reactions
- endothermic reactions

Answer 4

• exothermic: negative
• endothermic: positive

Question 5

What does Hess’s law state

Answer 5

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

Question 6

definition for enthalpy of combustion

Answer 6

enthalpy change when 1 mole of a substance undergoes complete combustion in oxygen with all substances in standard states
- exothermic (-ive)

Question 7

definition for enthalpy of formation

Answer 7

enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standards states
- exothermic (-ive) for most substances

Question 8

definition for enthalpy of neutralisation

Answer 8

enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
- exothermic (-ive)

Question 9

definition for ionisation enthalpy

Answer 9

first ionisation energy: enthalpy change when each atom in 1 mole of gaseous atoms loses 1 mole of electrons to form 1 mole of gaseous 1+ ions
- endothermic (+ive)
second ionisation energy: enthalpy change when eah ion in 1 mole of gaseous 1+ ions loses 1 mole of electrons to form 1 mole of gaseous 2+ ions
- endothermic (+ive)

Question 10

definition for electron affinity

Answer 10

first electron affinity: enthalpy change when each atom in 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous 1- ions
- exothermic (-ive)
second electron affinity: enthalpy change when each ion in 1 mole of gaseous 1- ions gains 1 mole of electrons to form 1 mole of gaseous 2- ions
- endothermic (+ive)

Question 11

definition enthalpy of atomisation

Answer 11

enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard form
- endothermic

Question 12

definition hydration enthalpy

Answer 12

enthalpy change when 1 mole of gaseous ions become hydrated
- exothermic

Question 13

definition enthalpy of solution

Answer 13

enthalpy change when 1 mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other
- varies

Question 14

definition bond dissociation enthalpy

Answer 14

enthalpy change when 1 mole of covalent bonds is broken in the gaseous state, under standard conditions
- endothermic

Question 15

definition lattice enthalpy of formation

Answer 15

enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase
- exothermic

Question 16

definition lattice enthalpy of dissociation

Answer 16

enthalpy change when 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase
- endothermic

Question 17

definition enthalpy of vaporisation

Answer 17

enthalpy change when 1 mole of a liquid is turned into a gas
- endothermic

Question 18

definition enthalpy of fusion

Answer 18

enthalpy change when 1 mole of a solid is turned into a liquid
- endothermic

Question 19

draw the standard enthalpy of solution cycle

Answer 19

triangle:
- gas ions -> ionic solid -> dissolved ions
- gas ions -> dissolved ions

• gas ions -> ionic solid = lattice enthalpy of formation
• ionic solid -> dissolved solid = enthalpy of solution
• gas ions -> dissolved ions = hydration enthalpy

Question 20

what is the difference between the experimental lattice enthalpy and the theoretical lattice enthalpy

Answer 20

• the experimental lattice enthalpy is calculated using the Born-Haber cycle
• the theoretical lattice enthalpy is calculated using theoretical calculation based on the purely ionic model
• the theoretical lattice assumes that the structure is perfectly ionic, e.g. the ions or spherical and have their charge density evenly distributed
• the greater the difference between the experimental and theoretical lattice enthalpies, the greater the covalent character due to the distorted ions

Question 21

what does it mean when there is some distortion of the ions in an ionic compound

Answer 21

• the ions are not perfectly spherical
• lot of distortion = ions have some covalent character

Question 22

describe the distortion in cations and anions

Answer 22

• Cations are small, so are very good at distorting anions. Anions are large and are easier to distort.
• The cation pulls the electron cloud of the anion toward itself, pulling it out of its normal spherical shape

Question 23

describe the characteristics of ionic compounds with some covalent character

Answer 23

• low solubility in water
• not very high melting point
• not very high electrical conductivity

Question 24

meaning entropy

Answer 24

a measure of disorder
- the greater the entropy, the more disordered something is
- J mol^-1 K^-1
- gases have the most entropy and solids have the least entropy
- substances will naturally want to become more disordered

Question 25

how does temperature affect the disorder of a substance

Answer 25

- at 0K, the entropy of a substance is 0 J mol^-1 K^-1 - the higher the temperature, the faster the particles vibrate, so the greater the entropy

Question 26

formula for total entropy change and formula for entropy of surroundings and system

Answer 26

- entropy of system = (enthalpy change of products) - (enthalpy change of reactants) - entropy of surroundings = - enthalpy change / temperature (K) - total enthalpy change = enthalpy of system + enthalpy of surroundings

Question 27

state the 2 formula's for Gibbs free energy change

Answer 27

G = enthalpy change - temperature(K) * entropy change G = - R(8.31) * T (K) * lnK (log of equilibrium constant) - if G is less than or equal to 0, then the reaction is feasible

Question 28

meaning feasible

Answer 28

a reaction can happen. The reaction may still not happen due to a high activation energy

Question 29

meaning spontaneous

Answer 29

when the reaction is thermodynamically possible

Question 30

describe the feasibility above and below the melting point

Answer 30

below the melting point, melting is not feasible as G is positive. At the melting point, G = 0 so melting becomes feasible and the substance melts

Question 31

describe how ionic charge and ionic radius affects the lattice energy

Answer 31

- a higher ionic charge means more energy is released when the ionic lattice forms, due to stronger electrostatic forces between the ions. The more energy released = more negative lattice energy, so the overall lattice energy for the compound is more exothermic - the smaller the ionic radii, so greater the charge density. This causes a stronger electrostatic attraction between ions, as they sit closer together. So, more exothermic lattice energy

Question 32

why do magnesium halides have more covalent character in their ionic bonds that sodium halides

Answer 32

- the sodium halides fit the ionic model well, because the sodium cation has a very small ionic charge (+1), so can't pull electrons from the anion towards itself, so the charge is evenly distributed around the ions. So, there's almost no polarisation - the magnesium halide does not fit the ionic model as well, because the magnesium anion has a larger ionic charge (+2), so can pull the electrons from the anion towards itself, polarising the bond. This means that the charge is not as evenly distributed

Question 33

meaning polarisation

Answer 33

in an ionic compound, when the ionic charge on the cation attracts electrons towards it from the anion

Question 34

explain why small cations and large anions cause more polarised ionic bonds, so more covalent character

Answer 34

- small cations have a high charge and are very polarising, because they have a high charge density. So, the cation pulls the electrons towards itself - large anions with a high charge are polarised more easily, because their electrons are further away from the nucleus. So, there is more repulsion between electrons and the electrons can be pulled more easily towards the cations - the anion's electron charge cloud will be dragged towards the positive cation

Question 35

what happens when a solid ionic lattice dissolved in water

Answer 35

- can only dissolved in water if the energy released is greater than the energy taken in. So, substances tend to have exothermic enthalpies of solution - the bonds between the ions break (endothermic) - the bonds between the ions are made (exothermic) - enthalpy of hydration

Question 36

how do ionic charge and ionic radius affect the enthalpy of hydration

Answer 36

- ions with a high charge are better at attracting water molecules. This is because the electrostatic attraction between the ions and the water molecules is stronger. This means more energy is released when the bonds are made, so more exothermic (greater) enthalpy of hydration - smaller ions have a higher charge density. So, they attract the water molecules better and have a more exothermic enthalpy of hydration

Question 37

draw how the water molecules are attracted to anions and cations

Answer 37

- in anions, hydrogen of water molecules attached to the ion, and oxygens faces out - in cations, the oxygen of water molecules attached to the ion, and hydrogen faces out

Question 38

how does physical state affect entropy

Answer 38

solid < liquid < gas - gas has a high disorder, as the gas molecules have a random arrangement, so high entropy - solids have a low disorder, as particles vibrate in a fixed position and their is hardly any randomness in movement. So, have low entropy

Question 39

how does dissolving affect entropy

Answer 39

dissolving a solid increases the entropy, as the dissolved particles can move more freely and are not held in a fixed position. So, more disorder

Question 40

how do more particles affect entropy

Answer 40

the more particles there are, the more ways they and their energy can be arranged. So, entropy increases