3.1.8: Thermodynamics

Question 1

What is enthalpy of formation (▲fH°)?

Answer 1

enthalpy change when one mole of substance is formed from its constituent elements, all substances in their standard states.

exothermic

Question 2

What is enthalpy of combustion (▲cH°)?

Answer 2

enthalpy change when one mole of substance undergoes complete combustion in oxygen, all substances in their standard states.

exothermic

Question 3

What is enthalpy of neutralisation (▲neutH°)?

Answer 3

enthalpy chaine when one mole of water is formed in a reaction between an acid and an alkali under standard condition.

exothermic

Question 4

What is first ionisation energy (▲ieH°)?

Answer 4

enthalpy change when each atom in one mole of gaseous atoms loses an electron to form one mole of gaseous 1+ ions

endothermic

Question 5

What is second ionisation energy (▲ieH°)?

Answer 5

enthalpy change when each ion in one mole of gaseous 1- ions gains an electron to form one mole of gaseous 2+ ions

endothermic

Question 6

What is first electron affinity (▲eaH°)?

Answer 6

enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions

exothermic

Question 7

What is second electron affinity (▲eaH°)?

Answer 7

enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions

endothermic

Question 8

Why is second electron affinity endothermic?

Answer 8

as it involves adding an electron to a negatively charged ion, which requires energy to overcome the repulsive forces between the negatively charged particles.

Question 9

What is enthalpy of atomisation (▲atH°)?

Answer 9

enthalpy change when one mole of gaseous atoms is produced from an element in its standard states (equal to 1/2 bond enthalpy)

endothermic

Question 10

What is enthalpy of hydration (▲hydH°)?

Answer 10

enthalpy change when one mole of gaseous ions becomes hydrated (dissolves in water)

exothermic

Question 11

What is enthalpy of solution (▲solH°)?

Answer 11

enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well seperated and don’t interact

varies

Question 12

What is bond dissociation enthalpy (▲disH°)?

Answer 12

enthalpy change when one mole of covalent bonds is broken in the gaseous state

endothermic

Question 13

What is lattice enthalpy of formation (▲LEFH°)?

Answer 13

enthalpy change when one mole of a solid ionic compound/ lattice is broken up from into its constituent ions in the gas phase

exothermic

Question 14

What is lattice enthalpy of dissociation (▲LEDH°)?

Answer 14

enthalpy change when one mole of solid ionic compound/ lattice is broken u[ from into its constituent ions in the gas phase

endothermic

Question 15

What is enthalpy of vapourisation (▲vapH°)?

Answer 15

enthalpy change when one mole of a liquid is turned into a gas

Question 16

What is enthalpy of fusion (▲fusH°)?

Answer 16

enthalpy change when one mole of a solid is turned into a liquid

Question 17

What is Born-Haber cycle used for?

Answer 17

• to find “experimental value”
• uses Hess’ law, includes all enthalpy changes in forming an ionic com[ound
• seperate step for each enthalpy change
• calculates (▲LEDH°) and (▲LEFH°)

Question 18

What is lattice enthalpy in Born-Haber cycles?

Answer 18

enthalpy change when ions in one mole of a solid ionic compound/ lattice are broken apart (▲LEDH°) or brought together (▲LEFH°)
- indication of strength of ionic bodning
- greater magnitude of lattice enthalpy, stronger bonding (compound with smaller ions or ions w/ higher charge)
- BHC to scale, arrows represent the amount of energy

Question 19

What are factors affecting lattice enthalpy?

Answer 19

• ionic radius- if radius increases, decreases lattice enthalpy
• charge- larger chage on metal, increase in lattice enthalpy

Question 20

What are perfect ions?

perfect ionic model

Answer 20

ions that are perfectly spherical, have an even distribution of charge and interactions are purely electrostatic (no covalent character)

Question 21

What is the difference between experimental and theoretical lattice enthalpy?

Answer 21

• experimental- real value (calculated by BHC)
• theoretical value- lattice enthalpy considering size, arrangement and charge in a lattice, assming structure is perfectly ionic

LARGER DIFFERENCE BETWEEN 2 VALUES, MORE COVALENT CHARACTER

Question 22

How are ions have covalent character?

Answer 22

• distortion occurs due to polarising
• nucleus of ion distorts another ions electron cloud
• ions are spherical
• still ions, just have covalent character

Question 23

What ions have most covalent charcter?

Answer 23

• cations (+ ions)- small and highly charged are very good at distorting as more charge to distort electron cloud with positive charge
• anions (- ions)- large and highly charges are easier to distort as further from nucelus, so easily polarised

Question 24

What are properties of ions with covalent character?

Answer 24

• low solubility in water
• melting points/ electrical conductivity not as high as expected
• larger difference between experimental and theoretical values

e.g. AlCl3
- should be ionic, have covalent charcter
- low m.p., sublimes under pressure
- poor conductor in molten state

Question 25

How do you calculate enthalpy of solution?

Answer 25

**▲H°sol= ▲H°latt + ▲H°hyd** BHC: - triangle (gas ions on top, ionic solid on left, dissolved ions on right) - arrows going down for ▲H°latt diss and ▲H°hyd (as both exothermic)

Question 26

What is entropy?

Answer 26

- a measure of disorder - more disordered, greater entropy - affected by temperature and no. of moles - tendency for entropy to increase **(2nd law of thermodynamics, entropy increases naturally over time)** - gases have most entropy, solids have least entropy

Question 27

How does temperature affect entropy?

Answer 27

- **3rd law of thermodynamics- entropy of a substance is zero at absolute 0 and increases w/ temperature** - higher temp, more kindetic energy, faster particles, greater disorder/ entropy - state changes have big increase in entropy - more moles, greater entropy

Question 28

How do you calculate entropy change?

Answer 28

▲S= (sum S products) - (sum S reactants) | in Jmol-1K-1

Question 29

What is gibbs free energy?

Answer 29

**▲G= ▲H- T▲S** - combines the 2 thermodynamic factors ▲H and ▲S - if ▲G≤0, reaction is feasible - divide T▲S /1000 to convert to kJ | in kJmol-1

Question 30

What does a feasible reaction mean?

Answer 30

- a reaction can take place - feasibility dependent on temperature, switched feasibility when ▲G=0 - feasible reaction may not take place due to high Ea - spontaneous= feasible, reaction is thermodynamically possible

Question 31

How does gibbs free affect changes of state?

Answer 31

- controlled in the same way - below m.p. of substance, melting isn't feasible if ▲G is positive - at m.p. ▲G=0 melting point becomes feasible, substance melts

Question 32

What is the chelate effect?

Answer 32

- bidentate and multidentate ligands will always displace monodentate ligands - due to more moles of product produced, results in increased entropy - increase T▲S so reaction is more feasible