physical - thermodynamics Flashcards
what is the enthalpy of lattice dissociation
-standard enthalpy change when a mole of solid ionic compound dissociates into its gaseous ions
what is the enthalpy of lattice formation
-standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions
what is the enthalpy of electron affinity
-first electron affintiy is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge under standard conditions
O(g) + e- = O-(g)
-second electron affinity - the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions under standard conditions
O-(g) + e- = O2-(g)
what is the enthalpy of atomisation
enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard states under standard conditions
Mg(s) = Mg(g)
what is the enthalpy of ionisation
the first ionisation energy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
Mg(g) = Mg+(g) + e-
the second ionisation energy is the the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions.
Mg+(g) Mg 2+ (g) + e-
what is the enthalpy of solution
standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules/ions are far enough apart to not interact with each other
NaCl (s) + aq = Na+ (aq) + Cl-(aq)
what is enthalpy of hydration
standard enthalpy change when one mole of gaseous ions become aqueous ions .
Na+(aq) + aq = Na+(g)
what is enthalpy change of formation
enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states
what are the two factors thsnt govern the size of lattice enthalpy
-ionic size
-ionic charge
what does a large exothermic value indicate in lattice enthalpies
-large electrostatic force of attraction between oppositely charged ions and that the ionic bonds are strong
how do small ions pack tg
very closely and attract each other strongly
how do large ions arrange themselves
far apart and attract weaker
what happens as the ionic radius increases
attraction bwt ions decrease
lattice enthalpy becomes less -ve/ less exothermic
how does ionic charge affect lattice enthalpy
-most -ve lattice enthalpy are those molecules which have small, highly charged ions
-as +ve charge increases it produces a greater attraction bwt +ve and -ve ions
-ionic radius decreases and ions in lattice become closely packed producing more attraction
-become more exothermic
what happens when two opposite charges are present in a lattice
attract strongly and a very exothermic reaction will take place
what factors affect enthalpy of hydration
-ionic charge
-ionic size
how does ionic size affect enthalpy of hydration
-becomes smaller the value of enthalpy becomes more -ve
-hyrdation depends don the ability of an ion to attract a water molecule
-small ions exert more attraction on water molecules and more energy is released
-as ionic radius increases
-enthalpy of hydration becomes less exo
how does ionic charge affect enthalpy of hydration
-as charge increases on an ion it has a greater attraction for water molecules and the enthalpy of hydration becomes more -ve
-across period 3 there is a decrease in size as charge increases - get more exo
how are lattice enthalpies measured
using a born-haber cycle
what is important to remeber for born-haber cycles
-draw separate step for every enthalpy change - eg each molecule gets own step
-second and third electron affinities are endothermic and shown by drawing arrow going up not down
-write numerical values with each step
what are the common pathways for born-haber cycle
1 mole of ionic compound on the bottom
then arrow going to the ionic compound = formation
arrow up from formation = atomisation - need to do separate steps for each element
arrow up from atomisation = ionisation - again separate steps for each ionisation
then metal ions at top
arrow down from metal ions to show electron affinity of the non-metal
then arrow down from electron affinity to the ionic compound showing lattice formation
what is the triangleHformation
sum of all the other triangle H / enthalpy changes
how are experimental lattice enthalpies calculated
using a born-haber cycle
-these are the true values
how are theoretical lattice enthalpies calculated
-representative of the perfect ionic model
-just by calculating
-assuming the size,charge and arrangement of ions in the lattice is perfectly ionic
when are ionic compounds said to have some covalent character
when there is distortion in the lattice
what types of ions are good at distortion
+ve/cations
-small and can polarise -ve ions
what types of ions are bad at distortion
-ve/anions
-large and highly charged - so easier to be distorted then distort themselves
do ionic componds that have covalent character tend to be soluble or insoluble
-insoluble
-and mp and electrical conductivity are not as high as expected
what does a high percentage difference from the experimental and theoretical values indicate
-significant covalent character
what does a low percentage difference bwt experimental and theoretical values indicate
-stronger ionic bonding
what does the magnitude of lattice enthalpy indicate
the overall strength of the ionic bonding
what does the difference bwt the experimental and theoretical value indicate
the amount of covalent character
what is entropy
measure of disorder/order in a substance
-the more disorder the greater the entropy
which state has the highest entropy
gases- particles are moving more rapidly and randomly
liquid
solid - least - particles are in fixed positions
what is the 2nd law of thermodynamics
over time entropy will increase
-more disorder
-more favourable
what are the units of entropy
J mol-1 K-1
what is the the 3rd law of thermodynamics
entropy of a substance is 0 at absolute 0 and increases with temperature
-the higher the temp the faster the particles vibrate and so the greater the entropy/disorder
how does temperature and the state of a substance vary due to its entropy
-entropy increases with temp
-there is a big increase in entropy on state changes
-the entropy increase from liquid to gas is greater than that from solid to liquid due to the large amount of disorder in gases compared to solids and liquids
how does the entropy of different substances compare
the more ordered the structure the lower the entropy
-structures like diamond and graphite are very highly structured and so have very low entropy
how do you calculate entropy
sum of the products - sum of the reactants
-when there is an increase in entropy - positive value
-when there is a decrease in entropy - negative value
how can you predict if the entropy will be positive or negative
-if there is a change of state eg solid - liquid, liquid - gas
then entropy will increase
-aqueous is bwt solid and liquid
-change of state gas - liquid, liquid - solid
entropy will decrease
-if there is more mol on the products than reactants - increase
-if there are more mol on reactants - decrease
what is gibbs free energy
ΔG° = ΔH° - TΔS°
-combines enthalpy change and entropy change
what conditions are favourable for a reaction to be feasible
-decrease in enthalpy
-increase in entropy
-ΔG° is less than or equal to 0 then the reaction is feasible
what does feasible mean in terms of reactions
- if it is feasible then the reaction can take place
-if greater than 0 then wont happen
what is another way a reaction can be termed other than feasible
spontaneous
how to work out weather a reaction is feasible if ΔH is negative and ΔS is positive
In the equation ΔG = ΔH - TΔS:
ΔH is negative. TΔS is positive, and so -TΔS is negative.
Both terms are negative irrespective of the temperature, and so ΔG is also bound to be negative. The reaction will be feasible at all temperatures.
how to work out weather a reaction is feasible if ΔH is positive and ΔS is negative
In the equation ΔG = ΔH - TΔS:
ΔH is positive. TΔS is negative, and so -TΔS is positive.
Both terms are positive irrespective of the temperature, and so ΔG is also bound to be positive. The reaction will not be feasible at any temperature
how to work out weather a reaction is feasible if ΔH is positive and ΔS is positive
In the equation ΔG = ΔH - TΔS:
ΔH is positive. TΔS is positive, and so -TΔS is negative.
Now increasing the temperature will change things. At higher temperatures, -TΔS will become more and more negative, and will eventually outweigh the effect of ΔH.
The reaction won’t be feasible at low temperatures, but if you heat it, there will be a temperature at which it becomes feasible, because ΔG becomes negative.
how to work out weather a reaction is feasible if ΔH is negative and ΔS is negative
In the equation ΔG = ΔH - TΔS:
ΔH is negative. TΔS is negative, and so -TΔS is positive.
Again there will be a temperature effect. As temperature increases, -TΔS will become more and more positive, and will eventually outweigh the effect of ΔH.
At low temperatures, ΔG will be negative because of the effect of the negative ΔH, but as you increase the temperature, the effect of the positive -TΔS will eventually outweigh that. The value of ΔG will then become positive, and the reaction will no longer be feasible.
how are changes in state controlled by entropy and gibbs
-below melt point the reaction is not feasible as gibbs is positive but at the melt point gibbs = 0 and so the reaction is feasible
–below the boiling point the reaction is not feasible as gibbs is positive but at the boiling point gibbs = 0 so the reaction is feasible
what to remember in gibbs calc
-divide the entropy by 1000 or the temp
-but remember to times the temp by the entropy
which way does the sign < go if the reaction if endothermic
T > number
which way does the sign < go if the reaction is exothermic
T < number
when do molecules have the least covalent character
-Cations present have a high charge density and polarising power
-anions present are large and polarisable
-SO largest cation and lowest charge
what is the trend for the enthalpy of hydration going down group one
-the attraction bwt the ion and the delta negative oxygen atoms in water decreases down the group
-hydration enthalpies are always -ve
-the bonds are formed btw ions and polar water molecules when the lattice is dissolved in water
-the meta l ions/cations are +ve so they will be attracted to the delta -ve O in the water not the delta H
how does bond length affect the exothermic values of enthalpy change
-shorter bond = more exothermic - smaller ionic radius
-longer bond = less exothermic - larger ionic radius
what does a positive value on a born-haber cycle indicate
endothermic reaction
what does a negative value on a born-haber cycle indicate
exothermic reaction
why are the arrows in a born-haber diagram going opposite ways for electron affinity
the repulsion bwt the electron and the negative ion needs to be overcome and this requires a large amount of energy to do so
why are some electron affinities exothermic
-the overall net attraction bwt the atoms (sub in atom in the question) nucleus and the extra electron
-results in more energy being lost than gained
why is hydration an exothermic process
-water is polar/ water has delta H
-negative ion/anion attracts delta H in water molecules
