Thermodynamics Flashcards
What is enthalpy of formation?
- when one mole of a substance is formed from its constituent elements with all substances in their standard states
What is enthalpy of combustion
- enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states
What is first ionisation enthalpy
The enthalpy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions
What is second ionisation enthalpy
When one mole of gaseous 2+ ions is produced from one mole of 1+ ions
What is second ionisation enthalpy
The enthalpy change when one mole of gaseous 2+ ions is produced from one mole of 1+ ions
What is electron affinity
The enthalpy change when one mole of gaseous atoms gain one electron per atom to produce gaseous 1- ions
What is the second electron affinity?
- the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions
What is enthalpy of atomisation
When one mole of gaseous atoms is produced from an element in its normal state
What is hydration enthalpy?
- when one mole of gaseous ions becomes hydrated
What is enthalpy of solution?
- when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other
What is bond dissociation enthalpy?
- enthalpy change when one mole of covalent bonds is broken in the gaseous state
I2 (g) -> 2I (g)
What is the lattice enthalpy of formation/disassociation?
Enthalpy change when one mole of a solid ionic compound is formed/broken up into its constituent ions in the gas phase
What factors affect lattice enthalpy?
- ionic radius
- ionic charge
- covalent character
What is lattice enthalpy an indicator of?
- ionic radius
- ionic charge
- covalent character
When a reaction takes place, the main compound that will form will be…
The most energetically stable (at lower energy)
The more exothermic the enthalpy of formation..
The more stable the compound is relative to its elements
Why is CaF2 more likely to form than CaF and CaF3
- CaF3 is at a higher energy than its substituent elements
- CaF has a less exothermic enthalpy of formation
What would happen if CaF is formed?
CaF will decompose into Ca and CaF2
What assumptions does the perfect ionic model make?
- perfectly spherical ions
- no electron density between the ions
What is covalent character?
- when electron density is found between the two nuclei within a covalent bond
Describe a polarising cation
- small ionic radius
- highly charged
Describe a polarisable anion
- diffuse electron cloud
The greater the different between the experimental and theoretical values of lattice enthalpy…
The further the bonding is away from the perfect ionic model and the more covalent character present in the bond
What is enthalpy change of solution equal to?
Lattice enthalpy + enthalpy of hydration
Ionic solid -> gaseous ions -> dissolved ions
What affects lattice enthalpy?
The strength of the interactions between cations and anions
- ionic radius
- ionic charge
What affects the size of hydration enthalpies
Charge density
Explain why larger charge density leads to a more exothermic enthalpy of hydration
- More charge dense ions attract water molecules more strongly
- more energy is released when ion-dipole attractions formed
How can we tell if a solid would dissolve by enthalpy changes?
- a solid will always dissolve if hydration of the anions and cations is sufficient to provide enough energy to dissociate the lattice.
- some may dissolve if Hyd(cat+ani) provides almost enough energy
- enthalpy of solution must be very slightly endothermic
What is entropy?
- a measure of disorder in a chemical system
- measured in JK mol
Phase changes always cause…
A sudden change in entropy
Do more elections mean higher or lower entropy
higher
Does diamond or graphite have higher entropy
Graphite - has delocalised electrons q
What four processes have positive entropy change?
- change in state
- dissolving (ions in a solution are delocalised)
- solid -> gas
- increase in number of moles
How do you calculate entropy change?
- entropy of products - entropy of reactant
What does feasibility refer to
- when a reaction is able to take place
What does the word spontaneous refer to?
- reactions that happen of their own accord
Are exothermic reactions spontaneous?
Yes
Why are some endothermic reaction spontaneous?
- increase in entropy
What does it mean when a reaction has entropy change
Reactions that bring about an increase in disorder also tend to be feasible
For a reaction to be feasible we ideally want…
A exothermic enthalpy change and a positive entropy change
What is Gibbs’s free energy change
ΔG = ΔH – TΔS
Why might a reaction still not happen even if gibbs is smaller than 0
Kinetics
- may have a very high activation energy
