ALEV CHEM BORN-HABER CYCLE

Define the term electron affinity for chlorine

The enthalpy change / heat energy change / ΔH for the formation of one mole of (chloride)

ions from (chlorine) atoms

Explain why the lattice enthalpy of calcium chloride is different from the lattice enthalpy of

dissociation for magnesium chloride.

Magnesium ion is smaller than the calcium ion

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Therefore, it attracts the chloride ion more strongly / stronger ionic bonding

In terms of electrostatic forces, suggest why the electron affinity of fluorine has a negative value.

There is an attraction between the nucleus / protons and (the added)

electron(s)

Define the term enthalpy of hydration of an ion.

Enthalpy change / ΔH when 1 mol of a gaseous ion forms aqueous ions

Suggest why hydration of the chloride ion is an exothermic process.

Water is polar / water has Hδ+

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(Chloride ion) attracts (the H in) water molecules

Define the term lattice enthalpy of dissociation.

Enthalpy change (to separate) 1 mol of an (ionic) substance into its ions forms ions in the gaseous state

Lattice enthalpy can be calculated theoretically using a perfect ionic model.

Explain the meaning of the term perfect ionic model.

Any one of:

• Ions are point charges

• Ions are perfect spheres

• Only electrostatic attraction / bonds (between ions)

• No covalent interaction / character

• Only ionic bonding / no polarisation of ions

Suggest two properties of ions that influence the value of a lattice enthalpy

calculated using a perfect ionic model.

(Ionic) radius / distance between ions / size

Allow in any order.

Do not allow charge / mass or mass / charge.

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(Ionic) charge / charge density

Explain why the lattice dissociation enthalpy of magnesium chloride is greater than

that of calcium chloride.

The magnesium ion is smaller / has a smaller radius / greater charge density

(than the calcium ion)

Attraction between ions / to the chloride ion stronger

Explain why the lattice dissociation enthalpy of magnesium oxide is greater than

that of magnesium chloride.

The oxide ion has a greater charge / charge density than the chloride ion

So it attracts the magnesium ion more strongly

Suggest why a value for the enthalpy of solution of magnesium oxide is not found

in any data books.

Magnesium oxide reacts with water / forms Mg(OH)

State the meaning of the symbol ϴ

in ∆Hϴ

Standard pressure (100 kPa) (and a stated temperature)

The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 °C when it

freezes.

The heat given out escapes

Explain the meaning of the term electron affinity.

Enthalpy change/heat energy change when one mole of gaseous atoms form one mole of gaseous negative ions

Explain why the electronegativity of fluorine is greater than the electronegativity of

chlorine.

Fluorine (atom) is smaller than chlorine/shielding is less/ outer electrons closer to

nucleus

Explain why the hydration enthalpy of the fluoride ion is more negative than the

hydration enthalpy of the chloride ion.

Fluoride (ions) smaller (than chloride) / have larger charge density

So (negative charge) attracts (δ+ hydrogen on) water more strongly