Thermodynamics Flashcards
Physical
Standard enthalpy of solution
Enthalpy change when 1 mol of an ionic compound is dissolved in solvent/ H20 to make aq ions.
Enthalpy of hydration
Enthalpy change when 1 mol of gaseousions is converted into aqueous ions
What are the units for entropy?
JK-1mol-1
Definition for enthalpy change
Heat change at a constant pressure
Gases have high entropy due to random arrangement of particles
How is disorder measured?
JK-1mol-1
Symbol (S)
From a solid to aqueous ions= entropy is positive as there is more disorder
From gas to liquid= entropy is negative as less disorder
What does it mean when a reaction is feasible?
Reaction can take place. Overall if Gibbs free is less than or equal to to 0 then reaction is feasible.
Why might a reaction that is feasible not take place?
May have a high Ea that’s too high to overcome.
Favourable conditions for Gibbs free energy reaction
1) Enthalpy change= Negative
2) Entropy value= Positive
What has zero entropy?
Only perfect crystals at absolute zero (T = 0 K) will have zero ∆S entropy.
Normally substances don’t have zero entropy.
How does the size and charge of the ion affect the size of the lattice enthalpy of an ionic solid?
The smaller the ion and the higher its charge, the stronger the lattice
How will a significant entropy change occur?
1) If there is a change of state from solid or liquid to gas
2) There is a significant increase in number of molecules between products and reactants.
What happens to the feasiblity of a reaction when increasing entropy?
If the reaction involves an increase in entropy (∆S is +ve) then increasing temperature will make it more likely that ∆G is negative and more likely that the reaction occurs e.g. NaCl + aq Na+ (aq) + Cl - (aq)
Example Q
State why there is a difference between the theoretical and experimental values.
because the compound may have some covalent character
Example Q
Explain why the enthalpy of hydration becomes less exothermic from Li+ to K+
because the size of the ionic radius increases so the attraction between the outer electron and the nucleus decreases
so less energy is required
Define enthalpy of formation
Define enthalpy of formation
The enthalpy change when 1 mole of a substance is formed from its constituent elements when all products and reactants are in their standard states under standard conditions
Define standard enthalpy of combustion
The enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all products and reactants in their standard states under standard conditions
Define enthalpy of atomisation
The enthalpy change when one mole of gaseous atoms are formed from the element in its standard state
Define enthalpy of lattice formation
The enthalpy change when 1 mole of an ionic compound is formed from its component gaseous ions
Define enthalpy of lattice dissociation
The enthalpy change when 1 mole of ionic compound is (infinitely) separeted to form its constituent gaseous ions
Why is the phrase ‘infinitely separated’ used of enthalpy of lattice dissociation ?
All bonding in the ions must be broken
Which enthalpy changes are endothermic ?
5
- Enthalpy of lattice dissociation
- Enthalpy of atomisation
- 1st/2nd Ionisation enthalpy
- 2nd electron affinity
- Bond dissociation enthalpy
Which enthalpy changes are exothermic ?
2
- 2nd electron affinity
- Enthalpy of formation (mostly)
What is the relationship between enthalpy of atomisation and bond dissociation enthalpy?
Enthalpy of atomisation produces 1 mole of gaseous atoms
Bond dissociation enthalpy produces 2 moles of gaseous atoms
BDE = EA X 2
Define first electron affinity
The enthalpy change when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of 1- ions
Why is the lattice enthalpy greater for smaller ions ?
Smaller ions are more closely packed in the lattice
There is a stronger attraction
Why is the lattice enthalpy greater for ions with a high charge?
There is a stronger attraction
What does the perfect ionic model assume?
The bonding in the compound if 100% ionic
The ions are perfect spheres/point charges and not distorted
Why do some compounds have covalent character?
3
One of the ions is small and has a large charge
This distorts the other ion
The ions are not spherical
What happens when an ionic compound dissolves in water?
The ionic lattice breaks up (enthalpy of lattice dissociation)
The polar water molecules form bonds with the ions (enthalpy of hydration)
Define enthalpy of hydration
The enthalpy change when 1 mole of gaseous ions is converted to 1 mole of aqueous ions
Why is the enthalpy of hydration exothermic ?
Energy is released when ions are attracted to the polar water molecules
