Thermodynamics (P1+P3) Flashcards
Define standard enthalpy of formation
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states.
Na(s) + 1/2Cl2(g) –> NaCl(s)
Define 1st ionisation enthalpy
Standard enthalpy change when 1 mole of electrons is removed from 1 mole of atoms to give 1 mole of gaseous ions with a single positive charge.
Ca(g) –> Ca+(g) + e-
Define standard enthalpy of atomisation
Enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state.
1/2Br2(l) –> Br(g)
Define mean bond enthalpy
Enthalpy change when 1 mole of gaseous molecules each break a covalent bond to form 2 free radicals over a range of compounds.
Br2(l) –> 2Br(g)
Define 1st electron affinity
Standard enthalpy change when 1 mole of gaseous atoms is converted into 1 mole of gaseous ions each with a single negative charge under standard conditions.
O(g) + e- –> O-(g)
Define lattice formation enthalpy
Standard enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions.
Mg2+(g) + 2Br(g) –> MgBr2(s)
Define lattice dissociation enthalpy
Enthalpy change when 1 mole of solid ionic compound dissociates into its gaseous ions.
MgBr2(s) –> Mg2+(g) +2Br-(g)
Define standard enthalpy of hydration
Standard enthalpy change when 1 mole of gaseous ions in converted into 1 mole of aqueous ions.
Cl-(g) + aq –> Cl-(aq)
Define standard enthalpy of solution
Standard enthalpy change when 1 mole of a solute dissolves to form its aqueous ions.
NaCl(s) + aq –> Na+(aq) + Cl-(aq)
How does charge and ionic radius determine how exothermic a lattice will be?
The bigger the charge and the smaller the ion, the greater the charge density.
The more negative the lattice formation means…
More stronger ionic bonds
How to determine if a compound will have covalent character
- A positive ion which is small and highly charged.
- A negative ion which is large and negatively charged.
The perfect ionic (theoretical) model suggests that…
Ions have point charges and are perfectly ionic.
The born haber cycle (experimental) suggests that…
Ions are polarisable and have SOME covalent character.
If an ionic compound has NO covalent character…
∆HLf value for perfect ionic model and experimental model would be very similar
If an ionic compound HAS covalent character…
∆HLf value calculated by the perfect ionic model should be less exothermic than the
∆HLf calculated by the experimental born haber cycle
Define entropy
A measure of disorder in a system.
What happens to the entropy from a solid to a liquid to a gas?
Increases
What happens to the entropy from a gas to a liquid to a solid?
Decreases
What happens to the entropy from a small number of moles to a large number of moles?
Increases
What happens to the entropy from a large number of moles to a small number of moles?
Decreases
Equation for entropy (∆S)
Products - Reactants
Equation for Gibbs free energy (∆G)
∆G = ∆H - T∆S
∆H = enthalpy change Kj mol-1
T = temp in K
∆S = entropy change KjK-1mol-1
key point = divide ∆S by 1000
Equation to calculate the temperature at which the reaction becomes feasible?
T = ∆H / ∆S
When does a reaction become feasible?
When ∆G is LESS than 0
Equation for Gibbs free energy in terms of y = mx + c
∆G = ∆S T + ∆H
If positive gradient - ∆S is negative
If negative gradient - ∆S is positive
If ∆H is negative and ∆S is positive, ∆G will always be… and feasible at…
- negative
- any temp
If ∆H is positive and ∆S is negative, ∆G will always be… and NOT feasible at…
- positive
- at any temp
If ∆H is negative and ∆S is negative, ∆G will always be… and feasible at…
- negative
- at low temp
If ∆H is positive and ∆S is positive, ∆G will always be… and feasible at…
- negative
- at high temp
