Oceans Flashcards
Define: ‘Lattice Enthalpy’
The enthalpy change when 1 mole of a solid ionic compound is formed from seperate, GASEOUS ions
Define: ‘Enthalpy of Hydration’
The enthalpy change when 1 mole of gaseous ions are added to water to give 1 mole of aqueous ions
What two factors affect Lattice Enthalpy and Enthalpy of Hydration? Explain how they do this
• 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)
Which ion is smaller: Na+ or Mg2+? Explain why
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
Define: ‘Enthalpy Change of Solution’
The enthalpy change when 1 mole of ionic solid dissolves in enough water to make an ‘infinitely’ dilute solution
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
• 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
How do you convert from Ka to pKa, and vice versa?
Ka > pKa: -log(Ka)
pKa > Ka: 10^-Ka
Write the equilibrium for the ionisation of an acid, HA, in aqueous solution, labelling the conjugate acid and conjugate base
• HA ⇌ H+ + A-
• HA is the conjugate acid, A- is the conjugate base
Describe what a buffer solution does
• It minimises the change in pH
• When a small volume of Acid or Alkali is added
Why is the pH of a buffer not affected by dilution?
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
Define ‘Entropy’
A measure of disorder; the number of ways that molecules and their associated energy quanta can be arranged
Give the equation for ΔtotalS, including the units and describe the sign required for a reaction/process to occur
• ΔsystemS + ΔsurroundingsS (surroundings can be represented as (-ΔH/T)) = ΔtotalS (in JK-1mol-1)
• Sign must be + for a process to be fesable
Give the equation for the solubility product of AgCl, and state what solubility product signifies
• Ksp = [Ag+]×[Cl-]
• Ksp is the MAX concentration of ions that can be in solution before a ppt forms
What is solubility?
The maximum quantity of a solute that can dissolve in a given volume of solvent
Write the equation for the pH of a weak acid, given [HA] and Ka
-log(sqrt(Ka×[HA]))
Write the equation for the Ka of a weak acid, given pH and [HA]
(10^-2pH)÷[HA]
Write the equation for the pH of a buffer solution, given Ka, [Acid] and [Salt]
-log(Ka×(Acid÷Salt))
Write the equation for the Ka of a buffer solution, given pH, [Acid] and [Salt]
(10^-pH×[Salt])÷[Acid]
Explain why, using the acid dissociation equation, why a weak acid does NOT act as a buffer
• HA ⇌ A- + H+
• Addition of a weak acid would shift the equilibrium position to the left to minimise the change in [H+]
• However, there is very low [A-], so the A- will run out almost immediately, meaning any more H+ added cannot be taken up and the pH will change
When drawing a diagram for a hydrated anion, what features must be included?
• H2O with BOTH H pointing to the anion
• δ- on O and δ+ on H
• O facing away from the anion
