Entropy Flashcards
What is electron affinity?
The enthalpy change when 1 mole of electrons is added to 1 mole of atoms in the gaseous phase to form 1 mole of 1- ions
What is enthalpy of atomisation?
The enthalpy change that takes place when 1 mole of gaseous atoms form from the element in its standard state
What is lattice enthalpy?
The enthalpy change when 1 mole of an ionic substance is formed from its gaseous ions under STD conditions
What is the strength of an ionic bond related to?
Lattice enthalpy
If a compound has greater ionic bonding, will it be more EXO or ENDO?
More exo
Why can’t lattice enthalpy be measured directly?
As cannot form 1 mole of ionic lattice from gaseous ions
What factors affect lattice enthalpy?
Charge
Size
Why does charge affect lattice enthalpy?
Bigger ion = stronger attraction = more exo
Why does size affect lattice enthalpy?
Smaller = ions more tightly packed together = greater attraction = more exo
What happens to decomposition temperatures down a group?
Increase
Why do decomposition temperatures increase down a group?
Mg2+ = smaller than Ba2+
So charge occupies smaller vol
= Mg2+ has higher “charge density”
This distorts electron cloud within negative ion
Weakening ionic bonding = reduces decomp temp
What can Born-Haber cycles be used to calculate?
Ionic bond strength based on experimental data
What equation do you use to calculate a missing enthalpy change on a Born-Haber cycle?
Lattice enthalpy = formation - sum of rest
What is the cycle of a Born-Haber cycle?
Formation Atomisation of metal Atomisation of non-metal 1st IE of metal 1st electron affinity of non-metal Lattice enthalpy of compound
What should you do to the enthalpy change when there is two moles of an element?
Multiply it by 2
Which ions have a more negative lattice enthalpy?
Smaller sized ions as they can get closer together = stronger attraction
What is enthalpy of solution?
The enthalpy change when 1 mole of ionic compound is completely dissolved in water under STD conditions
What happens when you dissolve an ionic compound?
Break up ionic lattice into gaseous ions
What is enthalpy of solution the reverse of?
Lattice enthalpy
What is enthalpy of hydration?
The enthalpy change when 1 mole of gaseous ions are dissolved in water forming 1 mole of aqueous ions under STD conditions
What should all hydration values be?
Exo
Why is enthalpy of hydration exo?
Because it is bond making
What equation must you use to calculate enthalpy of solution from a cycle?
L.E + enthalpy of solution = sum of enthalpies of hydration
What factors affect enthalpy of hydration?
Charge
Size
Why does charge affect enthalpy of hydration?
Higher charge on ion = greater attraction for H2O molecule = more exo
Why does size affect enthalpy of hydration?
Smaller sized ions have greater charge density compared to large ions = greater attraction for H2O = more exo
Why do exothermic chemical reactions happen?
A chemical reaction will proceed if products are energetically more stable than reactants
What is entropy?
The measure of the dispersal of energy in a system
What is more likely exist disordered or ordered states?
Disordered
How is entropy related to spontaneous direction of change?
Spontaneous direction of change is from a less probable to more probable
Meaning entropy always INCEASES
What is the symbol for entropy?
S
ΔS
What number will entropy produce?
Always a positive number
What are the units for entropy?
J K-1 mol-1
K = kelvin
Out of a solid, liquid and gas which has the lowest entropy?
Solid = more order but still some entropy as particles vibrate in fixed position
Out of a solid, liquid and gas which has the highest entropy?
Gas = least order
What happens to entropy as temperature increases?
Entropy increases
What happens to ΔS if a change makes the system more random?
+ΔS
What happens to ΔS if a change makes the system less random (more order)?
-ΔS
For chemical reactions we can predict entropy change to be +/-, by using what?
State symbols
CaCO3 (s) —-> CaO (s) + CO2 (g)
+ΔS
N2O4 (g) —-> 2NO2 (g)
+ΔS
As more moles on right side = more chance for disorder
2CH3OH (l) + 3O2 (g) —-> 2CO2 (g) + 4H2O (l)
-ΔS
As less moles of gas on right side
What is the definition for calculating entropy?
The entropy change that accompanies a reaction in molar quantities expressed in the equation, under STD conditions
What is the equation for calculating entropy?
ΔS = sum of S products - sum of S reactants
What are the steps for calculating entropy?
Decide whether +/-ΔS from symbol equation
Put values given in question in equation
What type of reaction must it be for it to be spontaneous?
Exo
Why must a reaction be exo to be spontaneous?
As the heat given out by the recations it increases the disorder (entropy) of the surroundings so in the universe disorder increases
What is the equation of entropy of surroundings?
ΔS surroundings = - ΔH / Temp
In the entropy of surroundings equation what must the ΔS be?
Positive
Why must the ΔS be positive in the equation for entropy of surroundings?
For spontaneous change
What is the Gibbs Free Energy equation?
ΔG = ΔH -TΔS <0
What is the Gibbs Free Energy equation dictated by?
Temperature
What mus you always do if you get an equation on the Gibbs Free Energy?
Write out the equation
What must ΔG be for a reaction to proceed?
Negative
If ΔG is positive what will happen?
Th reaction won’t be feasible
What does ΔG= at the point of feasibility?
0
What is the assumption made for the point of feasibility?
ΔH and ΔS do not vary with temperature
What does temperature adjust in the GIbbs Free Energy equation?
The significance of ΔS system
At low temperature what happens in Gibbs Free Energy equation?
ΔG= ΔH (-TΔS = negible)
So only reations with -ΔH can proceed so ΔG<0
At high temperatures what happens in Gibbs Free Energy equation?
ΔG = -TΔS (ΔH = negible)
So any reactions with +ΔH can proceed as -TΔS<0
What are the limitations of ΔG?
Even if ΔG is negative the reaction may not be feasible
Because AE might be too high
Or rate of reaction is very slow
How do you calculate ΔG (feasibility)?
Calculate ΔS
Calculate ΔH
Divide ΔS by 1000 (for kJ)
Then put into ΔG= ΔH -TΔS
What units must the temperature be in for Gibbs Free Energy equation?
Kelvin
If you calculate the value of ΔG, and it is positive what does this mean?
The reaction is not feasible as it must be <0
How would you then use the Gibbs Free Energy equation to calculate the temperature?
Assume feasibility = ΔG = 0
So ΔH -TΔS = 0
Rearrange to give ΔH/ ΔS = T
The convert answer to degrees C
How do you calculate ΔS?
Products - reactants
How do you calculate Δf?
Products - reactants
