Energetics II Flashcards
Define Lattice Energy
This is the energy change when one mole of an ionic solid is formed from its gaseous ions
The more negative the lattice energy the ……….. the bonding
The more negative the lattice energy the ..Stronger.. the bonding
What is enthalpy change of formation
When 1 mole of the compound is formed from its elements under standard conditions (298K and
100kpa) with all reactants and products being in their standard states
Define Enthalpy of atomization
When 1 mole of gaseous atoms is formed from the element in its standard state
Na (s) —–> Na(g) [Delta at H = +148 kJ mol^-1]
What is first ionisation enthalpy
This 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 under standard conditions
Mg (g) —–> Mg^+ (g) + e
What is Lattice Enthalpy ?
It is the sum of the energy needed to break the bonds within the crystal lattice and the energy released when the ions come together to form the lattice.
Enthalpy of hydration
when 1 mole of GASEOUS ions becomes hydrated or dissolves in water
X^+(G) + aq —-> X^+ (aq)
Enthalpy of lattice formation
is the standard enthalpy change when
1 mole of an ionic crystal lattice is formed
from its constituent ions in gaseous form.
enthalpy change of solution
This is the enthalpy change when 1 mole of SOLUTE dissolves in water.
What is first electron Affinity
This 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
eg . O(g) + e^- —-> O^-(g)
What is Second Electron Affinity
This is the enthalpy change that occurs when 1 mole of gaseous ions with -1 charge forms a gaseous ion with a -2 charge
Why is first electron affinity exothermic (negative delta H ) but second and third electron affinity is endothermic?
This is because the nucleus initially attracts electrons hence energy is released when an electron is added to it, however after then it requires energy (endothermic) to overcome repulsive force between the negative ion formed and the electron
What is the difference between enthalpy change of hydration and solution
Hydration is when 1 mole of GASEOUS ions dissolve in water while
Solution is when 1 mole of SOLUTE dissolves in water
Why is hydration exothermic ?
Because energy is being given out as water molecules form bonds with the ions that is the delta negative oxygen with positive ions and delta positive hydrogen with negative ions
Formula for enthalpy change of solution
-lattice energy + hydration enthalpy
Why is the theoretical value of the perfect ionic model less that the actual calculated value
Because it assumes that that the compound is made of 100% ionic bonds therefore there no covalent bonding letting it have a smaller value and weaker bond but because most ionic compounds have a covalent property the bonds are stronger and hence larger
The longer the atomic radii
the lower the melting point
Look at a born haber cycle diagram and try to fill in the blanks
TREND IN LATTICE ENERGY BASED ON SIZE OF IONS
As you go down the group of a group 2 halide what do we notice in terms of covalency (they are ionic compounds)
As we go down the group it would become more covalent because :
- It is more highly distorted
- Electrons are further away from the nucleus hence there’s more shielding hence there’s more covalency in MgI2 compared to MgF2 which is more ionic
Trend in Lattice enthalpy based on ionic radius
- The smaller the ions
- the more negative the enthalpies of the lattice is (stronger lattice enthalpy)
- The smaller the distance between oppositely charged ions
- This increases electrostatic attraction and hence
- Increases Lattice energy
Trend in Lattice Energy is based on the charges on ions
- The smaller the charge of an ion
- The smaller the attraction between the ions
- The weaker the lattice enthalpy (less negative values)
What is entropy
This is the measure of the disorder of a system
The more disordered something is the
The higher the entropy
What is the formula of entropy of total
Stotal = Ssystem + Ssurroundings
What is the formula for delta s System
Sum of products - Sum of reactants
What is the formula of delta s Surroundings
-(enthalpy of reaction or delta h)/ Temperature in kelvin
What is the unit for deltaS surroundings
Jmol^-1k^-1
How do you convert from Celsius to kelvin ?
Celsius + 273 = Kelvin
Temperature in Kelvin can never be
Negative
How does size affect entropy
The bigger a molecule
The more disordered it is
This is because it bends more
How does state affect entropy
A molecule becomes more disorderly the more gaseous is becomes
Gas - The most disorderly
Liquid in the middle
Solid - The most orderly
Which attribute takes priority in entropy
State take priority over size and other attributes
What three things cause changes in entropy
- Change of state from solid to liquid to gas
- Dissolving a solid ionic lattice this converts it from solid to liquid or (aqueous)
- Change in number of moles from reactants to products (If the number of moles increases from reactants to products the entropy increases)
Entropy values are always…
positive
What is the formula for delta G
∆H - Temperature x ∆S (system)
How do you calculate the temperature in which a reaction is feasible
We rearrange the formula for delta G when it is 0 as that’s the value it’s just feasible and calculate with this formula
Temperature = delta H / delta S system
When lattice enthalpy is greater than hydration enthalpy then the reaction is
exothermic
When hydration enthalpy is greater than lattice enthalpy the rection is
endothermic
What is enthalpy change ?
Enthalpy change, ∆H (delta H), is the heat energy change in a reaction at constant pressure.
State Hess’ Law
The total enthalpy change of a reaction is always the same, no matter which route is taken.
An increase in entropy (positive entropy value eg + 40 J K^-1mol^-1) means the
Less ordered the system is
A decrease in entropy value means (negative entropy )
An increase in orderliness
A reaction is only feasible when delta G is
Less than or equal to 0
How do you calculate delta G (Gibbs Free energy) using gas constant and equilibrium constant
R = gas constant, 8.31 J K–1 mol–1
T = temperature (in K)
ln K = the natural log of the equilibrium constant
∆G = −RT ln K
What is equilibrium constant
the ratio of the concentration of products to reactants at equilibria for a reversible reaction at a specific temperature
Feasible reactions with negative ∆G have a ……. as their equilibrium constant value
large value greater than 1
Not Feasible reactions with positive ∆G have a ……. as their equilibrium constant value
smaller value less than 1
if the ∆G (free energy) of a reaction is less than the activation energy for a reaction then it is kinetically stable even though it is not feasible based on free energy hence the reaction will
a. Occur
b. Not Occur
occur
