Energetics Definitions Flashcards
Standard Enthalpy Change of Formation
of a substance is the enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states
Bond Enthalpy
is the energy needed to break one mole of bonds in gaseous molecules under standard conditions.
First Ionisation Energy
is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms
X(g) —> X+(g) + e
(endothermic) +
First Electron Affinity
is the enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms.
X(g) + e —> X-(g)
(exothermic) -
Lattice Enthalpy
is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions.
XY(s) —> X+(g) + Y-(g)
(endothermic)
Enthalpy Change of Atomisation
is the enthalpy change that occurs when one mole of gaseous atoms is formed from the element in its standard state
X(s) —> X(g)
Enthalpy Change of Solution
is the enthalpy change when one mole of a solute is dissolved in a solvent to infinite dilution under standard conditions of temperature
related to the lattice enthalpy and the hydration enthalpies of the constituent ions
Enthalpy of Hydration of an Ion
is the enthalpy change that occurs when one mole of gaseous ions is dissolved to form an infinitely dilute solution of one mole of aqueous ions
(exothermic)
Entropy
refers to the distribution of available energy among the particles, the more ways energy can be distributed the higher the entropy
or
a measure of the disorder or randomness of the particles
Gibbs Free Energy
relates to the energy that can be obtained from a chemical reaction to the change in enthalpy, change in entropy and absolute temperature
measure of the driving force of a reaction, its tendency to proceed spontaneously
Enthalpy
is the heat content of a system
Endothermic
heat is added to a system
enthalpy change is positive
graph shows products have higher energy than reactants
Exothermic
heat is released to the surroundings
enthalpy change is negative
graph shows products of lower energy than reactants
Standard Enthalpy Change of Combustion
the enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions
Why do experimental values of standard enthalpy of combustion differ from data booklet ones?
- not all the heat produced by the combustion reaction is transferred to the water, some is needed to heat the calorimeter and some has passed to the surroundings
- the combustion of the ethanol is unlikely to be complete owing to the limited oxygen available, as assumed by the literature value
- some experiments may not be carried out under standard conditions
Hess’s Law
the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy
Why are bonds in oxygen and ozone broken by UV of different wavelengths?
O2 has a higher bond enthalpy because it has a double bond (bond order 2) and therefore needs a shorter wavelength of higher energy.
O3 has a lower bond enthalpy (bond order 1.5) due to delocalisation. Therefore it needs UV light of a longer wavelength (less energy)
Steps of the natural formation and depletion of Ozone
step 1:
O2 –> 2O.
endothermic
high energy needed
step 2:
O. + O2 (in eq. with) O3
forward is exothermic
lower energy
step 3:
O3 + O. –> 2O2
exothermic
Born Haber Cycle
is an energy cycle based on Hess’s Law but shows the formation of an ionic compound
What do lattice enthalpies depend on?
depends on the product of the ionic charges and the sum of the ionic radii
- an increase in the ionic radius of one of the ions decreases the attraction between the ions
- an increase in the ionic charge increases the ionic attraction between the ions
G and Spontaneity
if G < 0 it is a spontaneous reaction
if G > 0 it is not a spontaneous reaction
if G = 0 the reaction is at equilibrium
Determining G and spontaneity based on H and S
H = + and S = + : G depends on T and reaction is only spontaneous at high T
H = + and S = - : G always + so reaction is never spontaneous
H = - and S = + : G is always negative so the reaction is always spontaneous
H = - and S = - : G depends on T and the reaction is only spontaneous at low T
What do values of S mean?
if deltaS is + there has been an increase in the disorder of the system
if deltas is - there has been a decrease in disorder (become more ordered)
Formula for enthalpy for calorimetry (experimental data)
H = q/n
enthalpy change = heat energy / number of moles
Formula to use when directed to enthalpy of formations
H = sum of Hf products - sum of Hf reactants
* remember Hf of elements = 0
Formula to use when directed to bond enthalpies
H = H bonds broken - H bonds formed
Formula to use when finding change in entropy
S = sum S products - sum of S reactants
Formula to use when finding G not using H or T or S
G = G products - G reactants
