Thermodynamics, Enthalpy and Entropy
Definitions
Spontaneous
Disordered state
Ordered state
Second Law of thermodynamics
Change in entropy
First law of thermodynamics
Non-Spontaneous
The reaction continuous after the activation energy has been supplied with no further input of energy needed.
The reaction needs energy to be constantly supplied for it to occur, you have to start the reaction and start the reaction and start the reaction over and over again, an example of this would be photosynthesis as this needs energy constantly supplied via respiration
When a process occurs the energy transferred equals the energy released/taken in by the reaction. Energy cannot be created or destroyed - the conservation of energy.
When a reaction occurs, the overall entropy of the surroundings to the reaction changes to oppose the entropy of the change in the reaction so that the overall entropy of the universe increases.
e.g. solid, ionic lattice, you
e.g. water/gas, dissolved ionic lattice in water, you after a car accident :
From order -> disorder is an increase - reaction will be spontaneous even if it is endothermic. From disorder -> order is a decrease, won't happen unless energy is donated
Boiling water experiment
Mass before
1000g
Mass after
955g
45g change
2200 W kettle so 2200x100
220,000J supplied
45/18 = 2.5 moles of water gone
220,000/2.5 = 88,000Jmol^-1
Entropy = 88000/373 = 235.9 -> 236 Jmol^-1K^-1
Enthalpy of formation
Enthalpy of combustion
Enthalpy of atomisation
First ionisation energy
Second ionisation energy
First electron affinity
Second electron affinity
The enthalpy change when one mole of a compound is formed from its constituent parts at standard conditions
The enthalpy change when one mole of a substance is completely burned in oxygen at standard conditions
The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state.
The enthalpy required to remove one mole of electrons from one mole of gaseous ions at standard conditions.
The enthalpy required to remove one mole of electrons from one mole of gaseous ions, which have already lost an electron, at standard conditions.
The enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, via the addition of one mole of electrons, so that each of the ions has a single negative charge. This is at standard conditions.
The enthalpy change when a mole of gaseous ions, each with a single negative charge, is converted to a mole of gaseous ions, each with a double negative charge, via the addition of one mole of electrons.This is at standard conditions.
Enthalpy of lattice association
Enthalpy of lattice dissociation
Enthalpy of hydration
Enthalpy of solution
The enthalpy change when one mole of solid ionic compound is formed from its gaseous ions at standard conditions.
The enthalpy change when one mole of solid ionic compound is dissociated into its gaseous ions at standard conditions.
The enthalpy change when water molecules surround one mole of gaseous ions.
The enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which molecules or ions are far enough apart to not interact with each other.
Mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds
Covalent character in an ionic bond
Increases polarisation
Due to
Small positive ion
Small size high charge - high charge density
Large negative ion
large size high charge - large charge density
This increases strength of lattice enthalpy
Large charges make covalent character more likely
This is why some experimental values of lattice enthalpy are greater than the theoretical values as theoretical values can assume that there is no covalent character and there just is perfect ionic bonding.
Entropy
Gibbs free energy change
This is the energy available to do useful work
A reaction will spontaneously occur if ΔG < 0
Exergonic reaction
A reaction will not spontaneously occur if ΔG > 0
Endergonic reaction
ΔG = ΔH - TΔS
When T is measured in K
Reaction becomes spontaneous when ΔG = 0
ΔS = ΣS(products) - ΣS(reactants)
DON'T FORGET THAT ENTROPY IS IN J MOL^-1 K^-1 AND ENTHALPY IS IN KJ MOL^-1 SO NEED TO CONVERT BETWEEN THEM
Measured in J mol^-1 K^-1
How disordered something is. Measured by how much energy is required to push a system towards disorder.