energetics 2 p1

Question 1

what is Hess’ Law?

Answer 1

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

Question 2

standard enthalpy of atomisation

Answer 2

enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
1/2I2(g) –> I(g)

Question 3

first ionisation energy

Answer 3

enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Li(g) –> Li+(g) + e-

Question 3

second ionisation energy

Answer 3

enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
Mg+(g) –> Mg2+(g) + e-

Question 4

first electron affinity

Answer 4

enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions
Cl(g) + e- –> Cl-(g)

Question 5

second electron affinity

Answer 5

enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions
O-(g) + e- –> O2-(g)

Question 6

lattice enthalpy of formation

Answer 6

enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
Na+(g) + Cl-(g) –> NaCl(s)

Question 7

enthalpy of hydration

Answer 7

enthalpy change when one mole of gaseous ions become hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)
Na+(g) –> Na+(aq)

Question 8

enthalpy of solution

Answer 8

enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution
NaCl(s) –> Na+(aq) + Cl-(aq)

Question 9

mean bond dissociation enthalpy

Answer 9

enthalpy change when one mole of (a certain type of) covalent bond is broken, with all species in the gaseous state
Br2(g) –> 2Br(g)

Question 10

what is a Born-Haber cycle?

Answer 10

thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in their standard states (enthalpy of 0)

Question 11

Born Haber cycle - up arrows and down arrows

Answer 11

up - atomisation, ionisation
down - formation, affinity, lattice formation

Question 12

what factors affect the lattice enthalpy of an ionic compound?

Answer 12

size of ions, charge on ions

Question 13

how can you increase the lattice enthalpy of a compound? why does this increase it?

Answer 13

smaller ions, since the charge centres will be closer together
increased charge, since there will be a greater electrostatic force of attraction between the oppositely charged ions

increasing charge on the anion has a smaller affect since increasing anion charge also increases ionic size

Question 14

how can Born-Haber cycles be used to see if compounds could theoretically exist?

Answer 14

use known data to predict certain values of theoretical compounds then see if these compounds would be thermodynamically stable
used to predict the first noble gas containing compound

Question 15

what happens when a solid is dissolved in terms of interactions of the ions with water molecules?

Answer 15

break lattice –> gaseous ions; dissolve each gaseous ion in water. the aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)

Question 16

what is the perfect ionic model?

Answer 16

assumes that ions are perfectly spherical and that there is an even charge distribution (100% polar bonds)
act as point charges

Question 17

why is the perfect ionic model often not accurate?

Answer 17

ions are not perfectly spherical
polarisation often occurs when small positive ions or large negative ions are involved, so the ionic bond gains covalent character
some lattices are not regular and the crystal structure can differ

Question 18

what kind of bonds are most ionic?

Answer 18

large positive ions and small negative ions e.g. CsF

Question 19

what does spontaneous and feasible mean in terms of a reaction?

Answer 19

it will take place of its own accord; does not take account of rate of reaction

Question 20

is a reaction with a positive or negative enthalpy more likely to be spontaneous?

Answer 20

negative - exothermic

Question 21

entropy

Answer 21

a measure of the dispersal of energy in a system which is greater when the system is more disordered
symbol = S

Question 22

what is the most disordered state?

Answer 22

gas

Question 23

unit of standard entropy

Answer 23

J/K/mol

Question 24

how does temperature affect entropy?

Answer 24

greater the temperature, particles have more energy and move more thus the arrangement of particles becomes more random more random arrangement = higher entropy

Question 25

when a solid ionic lattice is dissolved in solution what happens to entropy?

Answer 25

entropy increases because the ions are more disordered

Question 26

how does change in number of gas molecules in a reaction affect entropy?

Answer 26

increase in number of gas molecules = increase in entropy decrease in number of gas molecules = decrease in entropy

Question 27

equation to calculate entropy change

Answer 27

ΔSreaction = ∑Sproducts - ∑Sreactants

Question 28

Gibbs free energy equation

Answer 28

ΔG = ΔH - TΔS

Question 29

what does a positive Gibbs value mean for feasibility?

Answer 29

positive = not feasible =0 JUST feasible negative = feasible

Question 30

what is the second law of thermodynamics?

Answer 30

entropy (of an isolated system) always increases, as it is overwhelmingly ore likely for molecules to be disordered than ordered

Question 31

what is the significance of temperature when G=0?

Answer 31

temperature in K at which the reaction becomes feasible

Question 32

how would you calculate the temperature at which a reaction becomes feasible?

Answer 32

rearrange to T = ΔH/ΔS since G=0

Question 33

what are the limitations of using G as an indicator of whether a reaction will occur?

Answer 33

Gibbs free energy only indicates if a reaction is feasible it does not take into account the rate of reaction (the kinetics of the reaction) many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring

Question 34

another equation for Gibbs!

Answer 34

ΔG = -RTlnK

Question 35

why is entropy zero at 0K?

Answer 35

no disorder - molecules/atoms are not moving or vibrating and cannot be arranged in any other way maximum possible state of order

Question 36

what are the two key things to look out for to decide is entropy changes or stays constant?

Answer 36

number of moles - more moles made --> increase in entropy going from solid --> liquid/gas or liquid --> gas

Question 37

how is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant?

Answer 37

the heat that is given out escapes to the surroundings