Topic 13- Further energetics

Question 1

what does Hess’ law state?

Answer 1

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

Question 2

what is standard enthalpy of atomisation?

Answer 2

The enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions

Question 3

define first ionisation energy

Answer 3

The enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 4

define second ionisation energy

Answer 4

The enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions

Question 5

define first electron affinity

Answer 5

The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions

Question 6

define second electron affinity

Answer 6

The enthalpy change when one mole of gaseous -1 ions gains one mole of electrons to form one mole of gaseous 2- ions

Question 7

define lattice enthalpy of formation

Answer 7

The enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions

Question 8

define enthalpy of hydration

Answer 8

The enthalpy change when one mole of gaseous ions becomes hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)

Question 9

define enthalpy of solution

Answer 9

The enthalpy change when one mole of a solute dissolves completely in a solvent to infinite dilution

Question 10

define mean bond dissociation enthalpy

Answer 10

The enthalpy change when one mole of ( a certain type of) covalent bonds is broken, with all species in the gaseous state

Question 11

what factors affect the lattice energy of an ionic compound?

Answer 11

• size of ions

- charge of ions

Question 12

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

Answer 12

• 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 the charge of the anion has a much smaller effect than increasing the charge on the cation, since increasing anion charge also has the effect of increasing ionic size

Question 13

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

Answer 13

use known data to predict certain values of theoretical compounds, and then see if these compounds would be thermodynamically stable. (was used to predict the existence of the first noble gas containing compound)

Question 14

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

Answer 14

• break lattice —> gaseous ions, dissolve each gaseous ion in water.
• the aqueous ions are surrounded by water molecules *which has a permanent dipole due to O-H polar bond)

Question 15

what is the perfect ionic model?

Answer 15

• assumes that the ions are perfectly spherical

- even charge distribution (100% polar bonds)

Question 16

why is the perfect ionic model often not accurate?

Answer 16

• ions aren’t perfectly spherical
• polarisation often occurs when small cations or large anions are involved (so the ionic bond gain covalent character)
• some lattices are not regular and the crystal structure can differ

Question 17

which kind of bonds will be the most ionic? why?

Answer 17

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

Question 18

define the terms spontaneous and feasible

Answer 18

if a reaction is spontaneous and feasible, it will take place on its own accord, does not take into account rate of reaction

Question 19

what is a spontaneous process?

Answer 19

a spontaneous process is one that takes place without continuous intervention from us

Question 20

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

Answer 20

negative (exothermic)

Question 21

what is entropy?

Answer 21

entropy is a measure of the dispersal of energy in a system which is greater when the system is more disordered

Question 22

what is the symbol of entropy?

Answer 22

S

Question 23

which is more disordered- solid or gas?

Answer 23

gas

Question 24

what is the unit of standard entropy?

Answer 24

J K-1 mol-1

Question 25

how does temperature affect entropy?

Answer 25

• increased temperature increases entropy
• the particles have more energy so they move more. therefore particle arrangement becomes more random- so higher entropy

Question 26

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

Answer 26

entropy increases because the ions are more disordered

Question 27

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

Answer 27

increase in number of gas molecules= increase in entropy

decrease in number of gas molecules= decrease in entropy

Question 28

what is the equation to calculate entropy change?

Answer 28

entropy of reaction= sum of entropy of products - sum of entropy of reactants

Question 29

write the gibb’s free energy equation

Answer 29

gibbs energy= enthalpy change - temperature (K) x entropy of reaction

Question 30

what is the second law of thermodynamics?

Answer 30

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

Question 31

what does the value for Gibb’s free energy of a reaction show?

Answer 31

• if G < 0, reaction is feasible
• if G=0, reaction is just feasible
• if G > 0, reaction is not feasible

Question 32

what is significant of the temperature at which G=0?

Answer 32

this is the temperature in kelvin at which the reaction becomes feasible

Question 33

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

Answer 33

T= enthalpy change/ entropy

Question 34

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

Answer 34

Gibbs free energy only tells you whether a reaction is feasible. It does not take in the rate of reaction (the kinetics). Many reactions that are feasible at a certain temperature have such a slow rate that almost no reaction is occurring at all.

Question 35

what is another equation for Gibbs free energy?

Answer 35

Gibbs energy= -RT ln K (gas + equilibrium constant, log and temperature)

Question 36

why is entropy zero at 0K?

Answer 36

no disorder- molecules/ atoms aren’t moving or vibrating and cannot be arranged in any other way. Maximum possible state of order

Question 37

what are the 2 key things to look out for to decide if entropy increases/decreases/stays relatively constant?

Answer 37

• number of moles (more moles made the higher the entropy)

- going from solid to liquid/ gas or liquid to gas

Question 38

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

Answer 38

the heat that is given out escapes to the surroundings

Question 39

Define lattice energy

Answer 39

the energy change when one mole of an ionic compound/solid is formed from its gaseous ions