Oceans

Question 1

Define: ‘Lattice Enthalpy’

Answer 1

The enthalpy change when 1 mole of a solid ionic compound is formed from seperate, GASEOUS ions

Question 2

Define: ‘Enthalpy of Hydration’

Answer 2

The enthalpy change when 1 mole of gaseous ions are added to water to give 1 mole of aqueous ions

Question 3

What two factors affect Lattice Enthalpy and Enthalpy of Hydration? Explain how they do this

Answer 3

• Charge of the ions > Ions with greater difference in charge attract together more strongly, so more energy is required to break the lattice (and more energy is released when forming the lattice)
• Size of the ions > Smaller ions will be closer to the opposite ions in the lattice, making them harder to break apart, so more energy is required to break the lattice (and more energy is released when forming the lattice)

Question 4

Which ion is smaller: Na+ or Mg2+? Explain why

Answer 4

Mg2+ is smaller, as both Mg2+ and Na+ have same no. of electrons, but magnesium has one more proton so pulls the electrons in more

Question 5

Define: ‘Enthalpy Change of Solution’

Answer 5

The enthalpy change when 1 mole of ionic solid dissolves in enough water to make an ‘infinitely’ dilute solution

Question 6

Given that the Lattice Enthalpy of Calcium Hydroxide is -2506 and the Hydration Enthalpy of Ca2+ and OH- are -1579 and -460 respectively, construct an Enthalpy Level Diagram to find the Enthalpy Change of Solution of Calcium Hydroxide

Answer 6

• Ca(OH)2 on line at bottom of diagram (L1)
• Ca2+(g) + 2OH-(g) on line at top of diagram (L2)
• Ca2+(aq) + 2OH-(aq) on line just above L1 (L3)
• Arrow pointing from L2 to L1 with -2506
• Arrow pointing from L2 to L3 with -2499 (-1579 +2(-460))
• Arrow pointing from L1 to L3 labelled ΔsolH
• Final result of 7KJmol-1

Question 7

Give the equation for Ka given pH and [HA]

Answer 7

Ka = (10^-2pH)/[HA]

Question 8

Give the equation for pH given Ka and [HA]

Answer 8

pH = -log(sqrt(Ka×[HA]))

Question 9

How do you convert from Ka to pKa, and vice versa?

Answer 9

Ka > pKa: -log(Ka)
pKa > Ka: 10^-Ka

Question 10

Write the equilibrium for the ionisation of an acid, HA, in aqueous solution, labelling the conjugate acid and conjugate base

Answer 10

• HA ⇌ H+ + A-
• HA is the conjugate acid, A- is the conjugate base

Question 11

Describe what a buffer solution does

Answer 11

• It minimises the change in pH
• When a small volume of Acid or Alkali is added

Question 12

Give the equation for finding [H+] in a buffer solution. What happens to this equation when two concentrations are equal?

Answer 12

• [H+] = (Ka × [Acid])/[Salt]
• When acid and salt concentrations are equal, the equation simplifies to [H+] = Ka

Question 13

Why is the pH of a buffer not affected by dilution?

Answer 13

Because adding water will dilute both the concentration of the acid and the salt equally, so both sides of the fraction will change by the same amount