22) Enthalpy and entropy Flashcards
Define lattice enthalpy
the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
ΔLEH⦵ is an _ (exothermic / endothermic) change therefore, ΔH is _ (positive / negative)
exothermic
negative
What is lattice enthalpy a measure of?
the strength of ionic bonding
Lattice enthalpy cannot be measured directly therefore, what is required?
a Born-Haber cycle for indirect determination
Define standard enthalpy change of atomisation ΔatH⦵
the enthalpy change that takes place when one mole of gaseous atoms forms from the element in its standard state
ΔatH⦵ is always _ (exothermic / endothermic) ?
endothermic - bonds broken
Define first ionisation energy ΔIEH⦵
the energy required to remove 1 e- from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions
ΔIEH⦵ is _ (exothermic / endothermic) ?
endothermic - overcome nuclear attraction
What is first ionisation energy a measure of?
ability to lose e-
Define first electron affinity ΔEAH⦵
the enthalpy change that takes place when 1e- is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
What is first electron affinity a measure of?
ability to gain e-
ΔEAH⦵ is _ (exothermic / endothermic) ?
exothermic - e- added is attracted towards nucleus
What 2 things are important to remember about between each horizontal level of a Born-Haber cycle?
only one species has changed
all the species are balanced
Give the alternative path for lattice enthalpy on a Born-Haber cycle
(gaseous ions) - first electron affinity - first ionisation energy (gaseous atoms) - enthalpy change of atomisation (elements in standard states) + enthalpy change of formation (ionic lattice)
Define second electron affinity
the enthalpy change that takes place when 1e- is added to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
Second electron affinity is _ (exothermic / endothermic) ?
endothermic - negative ion repels e-
When are successive electron affinities required?
when an anion has a greater charge than 1-
What will you need to do if more than 1 atom of an element is involved throughout a Born-Haber cycle?
multiply anything involved by a factor of the no. of atoms
Define enthalpy change of solution
the enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions
How can enthalpy change of solution be determined experimentally?
using q = mcΔT
calculate moles dissolved
(if 1mol -> aqueous ions) 1 mole would gain energy q in kJ / mol
uses the mass of solution - not the mass of water alone!
Give two features of the dissolving process, when a solid ionic compound dissolves in water
ionic lattice breaks up (opposite of ΔLEH⦵)
water molecules are attracted to and surround the ions
Define enthalpy change of hydration
the enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions
Give the alternative path for hydration on a Born-Haber cycle
(gaseous ions) + lattice enthalpy (ionic lattice) + enthalpy change of solution (aqueous ions)
What can lattice enthalpy be a good indicator for?
melting point (other factors e.g. packing of ions, needs to be considered also)
How does increasing ionic radius affect lattice enthalpy and hydration, and therefore, melting point?
attraction between ions / ions and water molecules decreases
lattice enthalpy and hydration energy is less negative
melting point decreases
How does increasing ionic charge affect lattice enthalpy and hydration, and therefore, melting point?
attraction between ions / ions and water molecules increases
lattice enthalpy and hydration energy is more negative
melting point increases
What will happen if the sum of hydration enthalpies > magnitude of lattice enthalpy?
the overall energy change (enthalpy of solution) will be exothermic and the compound should dissolve
How are many compounds with endothermic enthalpy changes of solution still soluble?
solubility also depends on temperature and entropy
Define entropy S
term used for the dispersal of energy and disorder within the chemicals making up the chemical system
As entropy increases, the dispersal of energy _ and the disorder _
increases
increases
What happens if a system changes to be more random?
energy can be spread out more so ΔS is positive
What happens if a system changes to be less random?
energy becomes more concentrated so ΔS is negative
Give 2 changes that would result in a system becoming more random
solid -> liquid -> gas
increase in the number of gaseous molecules
Define standard entropy S⦵
the entropy of one mole of a substance, under standard conditions
(every substance has one, a positive value with the units JK-1 mol-1)
entropy change of a reaction ΔS⦵ = ?
∑S⦵(products) - ∑S⦵(reactants)
Define free energy change ΔG
the overall energy change during a chemical reaction; the balance between enthalpy, entropy and temperature
What is the Gibbs’ equation?
ΔG = ΔH - TΔS
free energy change = enthalpy change with surroundings - temp. in K x entropy change of system
What should you remember about ΔS in the Gibbs’ equation?
must be changed to kJ K-1 mol-1 by x 10^-3 to match units of ΔH
What must there be for a reaction to be feasible?
a decrease in free energy
ΔG < 0
ΔH has a _ than TΔS - however as temperature increases TΔS becomes _?
larger magnitude
more significant
Feasibility is supported when ΔH is _ and ΔS is _ - but as T increases the significance of _?
negative
positive
ΔS
Is a reaction feasible when ΔH is negative and ΔS is positive?
yes, ΔG is negative
Is a reaction feasible when ΔH is negative, ΔS is negative and temperature is low?
yes, ΔG is negative
Is a reaction feasible when ΔH is negative, ΔS is negative and temperature is high?
no, ΔG is positive
Is a reaction feasible when ΔH is positive and ΔS is negative?
no, ΔG is positive
Is a reaction feasible when ΔH is positive, ΔS is positive and temperature is low?
no, ΔG is positive
Is a reaction feasible when ΔH is positive, ΔS is positive and temperature is high?
yes, ΔG is negative
Give the two equations for minimum temperature for a reaction to be feasible
ΔG = ΔH - TΔS = 0 T = ΔH / ΔS
What are the limitations of predictions made for feasibility?
Why do many reactions have a negative ΔG and not seem to take place?
ΔG indicates thermodynamic feasibility but takes no account of kinetics or rate of reaction - so a reaction may not seem to take place if it has a very large Ea or very slow rate of reaction
