3.1.8 Thermodynamics
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What is the standard molar enthalpy of atomisation? (2)
• The enthalpy change for the formation of one mole of gaseous atoms • From the element in its standard state under standard conditions.
Example reaction for atomisation? (1)
½ Cl₂(g) → Cl(g)
What is the first ionisation energy? (1)
The minimum amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous state
Example reaction for first ionisation energy? (1)
Na(g) → Na⁺(g) + e⁻
Why is the second ionisation energy greater than the first? (1)
It is more difficult to remove an electron from a more positively charged ion.
What is the first electron affinity? (2)
The enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous negative ions
Example reaction for the first electron affinity? (1)
Cl(g) + e⁻ → Cl⁻(g)
Why is the first electron affinity exothermic? (2)
Due to the attraction between the electron and the nucleus
What is the second electron affinity? (2)
• The enthalpy change when a mole of electrons is added to a mole of gaseous negative ions • Forming ions with a doubly negative charge
Example reaction for the second electron affinity? (1)
O⁻(g) + e⁻ → O²⁻(g)
Why is the second electron affinity always endothermic? (2)
Energy is required to overcome the repulsion between the negative electron and the negative ion
What is lattice formation enthalpy? (1)
The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
Example reaction for lattice formation enthalpy? (1)
Na⁺(g) + Cl⁻(g) → NaCl(s)
What is lattice dissociation enthalpy? (2)
The enthalpy change to separate one mole of an ionic substance into its gaseous ions.
Example reaction for lattice dissociation enthalpy? (1)
NaCl(s) → Na⁺(g) + Cl⁻(g)
How are lattice formation and dissociation enthalpies related? (1)
They have the same value but opposite signs.
What is enthalpy of hydration? (2)
The enthalpy change when one mole of gaseous ions becomes aqueous ions
Example reactions for hydration enthalpy? (2)
• Na⁺(g) → Na⁺(aq) • Cl⁻(g) → Cl⁻(aq)
Why is enthalpy of hydration exothermic? (2)
Due to the attraction between: • Positive ions and δ⁻ O of water. (Na+) • Negative ions and δ⁺ H of water. (Cl-) e.g. in NaCl it would be Attraction between Na+ and O- and Cl- and H+
What is enthalpy of solution? (2)
The enthalpy change when one mole of solid dissolves in water to form aqueous ions.
Example reaction for enthalpy of solution? (1)
NaCl(s) → Na⁺(aq) + Cl⁻(aq)
Draw born-haber cycle for NaCl(s) (6)
22 got to quizlet ALSO YOU ALWAYS FORGET TO PUT THE ELECTRONS WHEN DOING THE IONISATION ENERGU DONT FORGET - include state symbols as well - Atomisation of the gas usually happens after the ionisation of teh metal
Draw born-haber cycle for Na2O(s)
go to quizlet 23
What two factors affect lattice enthalpy? (2) and why
• Charge on ions (larger = greater enthalpy) • The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values). • Ionic radius (Smaller = greater enthalpy) • The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them.