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chemistry > O abc: Enthalpies of ionic lattices & solutions > Flashcards

O abc: Enthalpies of ionic lattices & solutions Flashcards

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1
Q

How is lattice enthalpy represented symbolically?

A

ΔLEH

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2
Q

Define lattice enthalpy.

A

The enthalpy change when one mole of ionic solid is formed by the coming together of gaseous anions and cations.

Usually 1 mole of each ion, except in cases like Mg(OH)2

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3
Q

ΔLEH:

  • Exothermic or endothermic?
    • or -?
  • Which way does the arrow point?
A
  • Exothermic (ionic bonds formed)
  • Negative
  • Gaseous ions to ionic solid (down)

The stronger the lattice, the more exothermic ΔLEH is

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4
Q

What factors increase lattice enthalpy?

A

Stronger ionic forces due to increased charge density:

  • Increased charge
  • Decreased ionic radii
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5
Q

State and explain which of MgO and Na2O has the more exothermic lattice enthalpy.

A

MgO

  • Mg2+ has the same ionic radius as, but has a higher charge than, Na+
  • So Mg2+ has a higher charge density
  • So attracts O2- more strongly
  • So MgO has the more exothermic lattice enthalpy
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6
Q

State the bonds broken and made when an ionic substance dissolves in water.

A

Broken: ionic + hydrogen bonds between water molecules

Made: ion-dipole

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7
Q

Draw diagrams of the hydrated ions in NaCl(aq)

A

Draw 4 water molecules around each ion

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8
Q

What factor increases the size of a hydrated ion?

A

Increased charge density:

  • Increased charge
  • Decreased ionic radii

The higher an ion’s charge density, the more water molecules it attracts.

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9
Q

How is the enthalpy change of hydration represented symbolically?

A

ΔhydH

ΔhydrationH

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10
Q

Define the enthalpy change of hydration.

A

The enthalpy change when one mole of gaseous ions form a very dilute solution in water.

Values quoted depend on concentration of solution

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11
Q

ΔhydH:

  • Exothermic or endothermic?
    • or -?
A
  • Exothermic (gaseous ions form bonds with water)
  • Negative
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12
Q

What factors make the value of ΔhydH more exothermic?

A

Higher charge density:

  • Smaller ionic radii
  • Greater charge
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13
Q

What is the equivalent of ΔhydH when considering solvents other than water?

A

Enthalpy of solvation: ΔsolvH

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14
Q

How is enthalpy change of solution represented symbolically?

A

ΔsolutionH

ΔsolnH

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15
Q

Define the enthalpy change of solution.

A

The enthalpy change when one mole of solute dissolves in water to form a very dilute solution.

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16
Q

Write an expression which could be used to find the value of ΔsolnH experimentally.

A

ΔsolnH = ΔhydH (cation) + ΔhydH (anion) - ΔLEH

Break ionic lattice into gaseous ions, then form solution

17
Q

Draw an enthalpy level diagram in which the value of ΔsolnH is negative.

A
18
Q

Draw an enthalpy level diagram in which the value of ΔsolnH is very positive.

A
19
Q

An endothermic value of ΔsolnH means that dissolving appears to be energetically unfavourable. If the value is small enough, the reaction can still happen. Why?

A

If the entropy of the system increases, the solute dissolves despite the change being endothermic.

20
Q

An ionic solid was placed in a non-polar solvent. Draw an enthalpy level diagram to represent this.

A
21
Q

Explain why the solubility of group 2 sulphates and carbonates decreases down the group.

A
  • Solubility is determined by ΔsolnH = ΔhydH - ΔLEH
  • Down group, ions remain same size but become larger, so charge density decreases
    • ΔLEH + ΔhydH both become less exothermic
  • ΔLEH decreases at lower rate because:
    • Cations become larger, approaching size of anions; lattice becomes more tightly packed
    • Electrostatic attraction increases
  • Overall effect is that ΔsolnH becomes less exothermic, so solubility decreases
22
Q

Explain why the solubility of group 2 hydroxides increases down the group.

A
  • Solubility is determined by ΔsolnH = ΔhydH - ΔLEH
  • Down group, ions remain same size but become larger, so charge density decreases
    • So ΔLEH + ΔhydH both become less exothermic
  • ΔLEH decreases at higher rate because:
    • Cations become increasingly larger than OH-; lattice becomes more loosely packed
    • Electrostatic attraction decreases
  • Overall effect is that ΔsolnH becomes more exothermic, so solubility increases
23
Q

Suggest which of Na+ and Ca2+ has the most exothermic enthalpy of hydration.

A
  • Ca2+ is larger than Na+, which would decrease Ca2+’s charge density, but it has double the charge, which would increase its charge density
  • Charge has dominant effect since size doesn’t change by same degree
  • So Ca2+ has higher charge density
  • Attracts water more strongly / stronger ion-dipole interactions
  • More exothermic enthalpy of hydration

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