Thermodynamics

Question 1

What is the definition of enthalpy change?

Answer 1

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

Question 2

What is the definition of enthalpy of formation?

Answer 2

• enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (298K + 100kPa)

Question 3

What is the definition of ionisation enthalpy/energy?

Answer 3

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

Question 4

What is the definition of first ionisation energy?

Answer 4

• energy required to remove 1 mole of electrons from 1 mole of gaseous atoms of an element to form 1 mole of gaseous 1+ ions

Question 5

What is the definition of enthalpy change of atomisation?

Answer 5

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

Question 6

What is the definition of bond (dissociation) energy/enthalpy?

Answer 6

• energy required to break 1 mole of a specific covalent bond in the gas phase

Question 7

How do you write the bond enthalpy?

Answer 7

• E (H-H)
• bond energy of a mole of single bonds between 2 H atoms

Question 8

What is the definition of lattice enthalpy?

Answer 8

• enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions, under standard conditions

Question 9

What is the definition of enthalpy of solution?

Answer 9

• enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

Question 10

What is the definition of enthalpy of hydration?

Answer 10

• enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution

Question 11

What are hydration enthalpies?

Answer 11

• measure of energy released when an attraction forms between ions + water molecules
• O (δ-) atom in H2O is attracted to positive ions + H (δ+) atoms are attracted to negative ions

Question 12

What are the 2 key factors which affect lattice enthalpy?

Answer 12

• ionic charge + radius of ions making up lattice

Question 13

How does ionic radius affect the lattice enthalpy?

Answer 13

• as the ionic radius inc, the LE becomes less exo.
• bc the charge density dec. as the ions are further apart from each other
• so the electrostatic forces of attraction between the oppositely charged ions are weaker
• e.g. the LE for NaF is more exo. than for KF

Question 14

How does ionic charge affect the lattice enthalpy?

Answer 14

• as ionic charge inc, the LE gets more exo
• bc the charge density inc.
• so the electrostatic forces of attraction between the oppositely charged ions are stronger
• e.g. the LE of CaO is more exo than that of KCl

Question 15

What are the factors affecting enthalpy of hydration?

Answer 15

• ionic charge + radius which affects the amount the ions are attracted to the water molecules

Question 16

How does ionic radius affect the enthalpy of hydration?

Answer 16

• as ionic radius dec, the ΔHhyd becomes more exo.
• bc smaller ions have a greater charge density
• so the ion-dipole attractions between the water molecules + ions in solution are stronger
• so more energy is released when they become hydrated
• e.g. the ΔHhyd of MgSO4 is more exo than that of BaSO4

Question 17

How does ionic charge affect the enthalpy of hydration?

Answer 17

• as ionic charge inc, the ΔHhyd is more exo.
• bc ions w a large ionic charge have a greater charge density
• so the ion-dipole attractions between the water molecules + ions in solution are stronger
• so more energy is released when they become hydrated
• e.g. the ΔHhyd of CaO is more exo than that of KCl

Question 18

What is entropy?

Answer 18

• measure of how disordered or chaotic a system is
• more disordered = inc in entropy = system more energetically stable

Question 19

Why will a system w a higher entropy by energetically favourable?

Answer 19

• bc the energy of the system is more spread out when.it’s in a disordered state

Question 20

What is the unit for entropy (S)?

Answer 20

• JK^-1mol^-1

Question 21

What is the equation to calc. the standard entropy change (ΔS) of a system?

Answer 21

ΔS = S of products - S of reactants

Question 22

What can the Gibbs equation be used to calc?

Answer 22

• whether a reaction is feasible or not
• neg. ΔG = feasible reaction + likely to occur
• pos. ΔS = non-feasible reaction + unlikely to occur

Question 23

What is the Gibbs equation?

Answer 23

ΔG [kJ mol^-1] = ΔH [kJ mol^-1] - TΔS [K][J K^-1mol^-1 (must be converted to kJ K^-1mol^-1) ]

Question 24

What can the feasibility of a reaction be affected by?

Answer 24

• the T°C

Question 25

What will affect the feasibility of an exothermic reaction?

Answer 25

- exo. reaction = neg. ΔH - if **ΔS is pos**, the 1st + 2nd term will be neg. = **neg. ΔG** = feasible reaction - if **ΔS is neg**, the 1st term is neg. + the 2nd term is pos. so **at v high T°Cs** -TΔS is v large + pos. + will overcome ΔH = **pos. ΔG** = non-feasible reaction

Question 26

What will affect the feasibility of an endothermic reaction?

Answer 26

- endo. reaction = pos. ΔH - if **ΔS is neg**, the 1st + 2nd term will be pos. = **pos. ΔG** = non-feasible reaction - if **ΔS is pos**, the 1st term is pos. + the 2nd term is neg. so **at low T°Cs** -TΔS is small + neg. + will not overcome ΔH = **pos. ΔG** = non-feasible reaction - at **high T°Cs, reaction is more feasible** bc 2nd term becomes neg. enough to overcome the ΔH = neg ΔG