Thermodynamics
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What is enthalpy
Heat energy
What is entropy
Measure of chaos/disorder in substances and reactions.
Enthalpy of formation
The enthalpy to produce 1 mol of a compound from its constituent elements under standard conditions
Enthalpy of combustion
The enthalpy to burn 1 mol of a compound under standard conditions.
Enthalpy of atomisation
The enthalpy to produce 1 mol of gaseous atoms from an element in it’s standard state.
Enthalpy of dissociation
The enthalpy when all the bonds of the same type in 1 mole of gaseous molecules are broken. e.g. Cl2(g) -> 2CL(g)
Enthalpy of ionisation
The enthalpy to remove 1 mole of electrons from 1 mole of gaseous atoms, producing 1 mole of gaseous 1+ ions.
Enthalpy of electron affinity
The enthalpy required to add 1 mole of electrons to 1 mole of gaseous atoms, producing 1 mole of gaseous 1- ions.
Enthalpy of lattice dissociation
The enthalpy required to completely dissociate an 1 mole of an ionic solid compound to produce its gaseous ions under standard conditions.
Enthalpy of lattice formation
The enthalpy required to form 1 mole of a solid ionic compound from its gaseous ions under standard conditions.
Enthalpy of solution
Enthalpy required to dissolve 1 mole of solute completely in solvent so that no further enthalpy change occurs on further dilution.
Enthalpy of hydration
Enthalpy required to turn 1 mole of gaseous ions, into 1 mole of aqueous ions.
Which enthalpies are exothermic. Why? What arrow shows this on BH cycles?
• Formation, Electron affinity 1, Lattice Formation. • Loses energy to surroundings whilst undergoing reaction. • Downward arrows
Which enthalpies are endothermic? Why? What arrows show this?
• Atomisation, Ionisation, Dissociation, Hydration, Electon Affinity 2, Lattice Dissociation. • Absorbs energy from surroundings so goes up in enthalpy • Upward arrows
What two factors affect the enthalpy of something?
• Atomic radius - Atomic charge
Explain the trend down periodic groups.
As we go down, atomic radius increases. G1 - smallest radii G7 - largest radii
Explain the importance of atomic radius in terms of enthalpy.
Positively ions suited to smaller radii; stronger attraction (electrostatic) between nuclei - outer electron - easier to attract. Negative ions suited to larger radii; weaker attraction (electrostatic) to outer electron - easy to lose. Small positive ion with large negative ion creates a strong lattice (requires lot of enthalpy to dissociate/form)
Explain the trend across periods.
As we go across, nuclear charge increases; number of protons and electrons.
Explain the importance of nuclear charge.
Higher charge (on +ion), higher electrostatic force to -ion, greater bond strength; more enthalpy to form/dissociate. Lower charge (on -ion), higher electrostatic force to +ion, greater bond strength; more enthalpy to form/dissociate.
What is the perfect ionic model?
PIM - theoretical calculation of enthalpy; counts all ions as perfect spheres - perfect bond.
How is a BH value different to a PIM value?
BM value counts covalent characters; when ions have a strong electrostatic force to each other, becomes partly covalent due to strength; PIM value suggests perfect spherical structure w/o covalent character. Cov. Character increases lattice enthalpy.
What factors influence covalent characters?
• Large difference in atomic radii - Greater nuclear charges.
What is the formation enthalpy equation?
H = H(prod) - H(reac)
Enthalpy of vaporisation
Enthalpy required to produce 1 mole of gaseous ions from 1 mole of aqueous ions.