15.1 Energy Cycles

Standard Enthalpy of Formation Revision

QUESTION: What is the definition of Standard Enthalpy of Formation?

______________________________________________________________________

______________________________________________________________________

QUESTION: Write an equation to represent the Standard Enthalpy of Formation of Sodium Chloride (NaCl) - include state symbols

______________________________________________________________________

Born-Haber cycles map out the formation of Ionic compounds in several steps.

The obvious step is the formation of the ionic compound, eg NaCl

But a Born-Haber cycle provides an alternative route, which we can apply Hess's Law to.

QUESTION: What is Hess's Law?

_________________________________________________________________________________

_________________________________________________________________________________

∆H_at or ∆H_a is the Enthalpy change of Atomisation of Sodium

Na_(s) --> Na_ag)

QUESTION: What is the definition of Standard Enthalpy of Atomisation of Sodium?

______________________________________________________________________

______________________________________________________________________

∆H_i or ∆H_{ie is the First Ionisation Energy of Sodium}

Na_(g) -> Na_(g) + e^-

QUESTION: What is the definition of First Ionisation Energy of Sodium?

______________________________________________________________________

______________________________________________________________________

We also need to atomise the Chlorine.

QUESTION: What is the relationship between ∆H_a Chlorine and the Bond enthalpy of Chlorine?

______________________________________________________________________

______________________________________________________________________

Next we need to form Chloride ions from Chlorine atoms.

∆H_EA is the Electron Affinity of Chlorine

Cl_(g) + e^- -> Cl^-_(g)

QUESTION: What is the definition of the electron affinity of Chlorine?

______________________________________________________________________

______________________________________________________________________

QUESTION: How is the electron affinity of Chlorine different to the ionisation energy of Chlorine?

______________________________________________________________________

______________________________________________________________________

Now we can finish the cycles.

∆HL_E is the Lattice Formation Enthalpy of NaCl

Cl^-_(g) + Na⁺_(g)-> NaCl_(s)

QUESTION: What is the definition of Lattice Formation Enthalpy?

______________________________________________________________________

______________________________________________________________________

QUESTION: Apply Hess's Law to calculate Lattice Formation Enthalpy?

______________________________________________________________________

______________________________________________________________________

Construct a Born Haber Cycle for Lithium Fluoride

Construct a Born Haber Cycle for Sodium Sulphide

Not all Lattices are the same.

QUESTION: Explain why Lattice Energies for LiF, LiCl & LiI decline as we go down Group 7 Lithium salts.

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

QUESTION: Explain why Lattice Energies for LiCl, NaCl, NaBr &NaI decline as we go down Group 1 Halides.

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

QUESTION: Explain the variation in Lattice Energies for Period 3 Chloride, NaCl and MgCl₂

__________________________________________________________________________________________

__________________________________________________________________________________________

__________________________________________________________________________________________

We can construct energy cycles to represent dissolving too.

QUESTION: What is different about the Lattice Enthalpy used in this cycle?

____________________________________________________________________________________

_____________________________________________________________________________________

QUESTION: What is the definition ∆H (hydration) of Chloride ions?

____________________________________________________________________________________

_____________________________________________________________________________________

QUESTION: Write an equation for ∆H (hydration) of Chloride ions, including state symbols

____________________________________________________________________________________

QUESTION.

Construct an energy Cycle for the solution (dissolving) of Sodium Hydroxide.

∆H(LE) NaOH = +900 kJ/mol, ∆H(Hyd)Na⁺ = -424 kJ/mol, ∆H(Hyd)OH^- = -519 kJ/mol

Calculate ∆H(solution) NaOH =