1.8 Thermodynamics
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What is a Born-Haber cycle?
An application of Hess’ law Where there are multiple steps in one leg of the cycle
How are ionic solids produced?
Between ions in the gas phase
What is exothermic and endothermic in terms of bonds? Arrow representation?
Exothermic - bond forming (down arrow) Endothermic - bond breaking (up arrow)
What is significant about the ionic solid?
The ions in an ionic lattice are electrostatically bonded by ionic bonds The standard lattice enthalpy of formation is a measure of the strength of an ionic bond
What is standard enthalpy of formation? ΔHθf
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with all reactants and products in their standard states
What is standard enthalpy of combustion? ΔHθc
The enthalpy change when one mole of a substance reacts completely in excess oxygen under standard conditions with all reactants and products in their standard states
What is ionisation enthalpy? ΔHθi
The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS with a single POSITIVE charge
What is bond dissociation enthalpy? ΔHθdiss
The enthalpy change when one mole of GASEOUS molecules each breaks a covalent bond to form 2 free radicals, averaged over a range of compounds
What is enthalpy of atomisation? ΔHθat
The standard enthalpy change that accompanies the formation of one mole of GASEOUS ATOMS from the element in its standard state under standard conditions
What is electron affinity? ΔHθea
The standard enthalpy change when one mole of GASEOUS ATOMS is converted into one mole of GASEOUS IONS, each with a single NEGATIVE charge
What is the lattice enthalpy of formation? ΔLHθ
The standard enthalpy change that accompanies the formation of one mole of a solid ionic compound from its gaseous ions Exothermic
What is the enthalpy of lattice dissociation? ΔLHθ
The standard enthalpy change that accompanies the separation of one mole of a solid ionic compound into its gaseous ions Endothermic
How can we tell the difference between ΔLHθ with the formation and dissociation of lattices?
We can only tell from the positive or negative values due to bond forming/making therefore exo/endo
What is the enthalpy of hydration? ΔhydHθ
The standard enthalpy change for the process where one mole of gaseous ions are surrounded by a maximum of water molecules to produce one mole of hydrated ions
What is enthalpy of solution? ΔsolHθ
The standard enthalpy change that occurs when one mole of an ionic solid dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other
In ionisation enthalpy what must you specify?
Whether it is first or second Both are endothermic
In electron affinity what must you specify?
Whether it is first or second electron affinity First - exothermic Second - endothermic
Why is the first and second electron affinity exothermic and then endothermic?
First is exothermic as an attraction is formed between a neutral atom and an electron Second is endothermic as energy needs to be put in to over come the repulsion of two negative ions
What are the intermediates in a simple hess cycle for Enthalpy of lattice formation/dissociation?
Elements The ionic solid At the top the gaseous ions of the elements
What are lattice enthalpy values based on?
The Born-Haber cycles They are experimental values
What is the perfect ionic model?
They are theoretical values
What does the perfect ionic model presume?
Assumes there is 100% ionic bonding The ions are perfect spheres with a uniform distribution of ion charge
Why are experimental values and theoretical values of lattice enthalpy different?
In reality ions are perfectly spherical due to different charge densities and electrons aren’t always completely donated Therefore there can be a degree of covalency = not perfect ionic bonding Mor discrepancy = greater degree of covalency
What happens to a positive ion in water? (Enthalpy of solution)
The δ- oxygen of the polar water molecule is electrostatically attracted to the positive metal ion