Thermodynamics Flashcards
Define mean bond enthalpy
Is this endothermic, exothermic, or does it depend?
Enthalpy change for the breaking of 1 mole of covalent bonds, averaged over a range of compounds
Endothermic
Define Bond Dissociation
Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms
This is endothermic
Define standard molar enthalpy of atomisation (🔺H at)
Enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
This is endothermic
Define first electron affinity (EA)/(🔺H ea)
The enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous negative ions
This is exothermic
Define second electron affinity (🔺H ea-)
The enthalpy change when a mole of electrons is added to a mole of gaseous negative ions, to form ions each with a doubly negative charge
Why is first electron affinity exothermic but second is endothermic
1st - attraction is created between electron and the nucleus
2nd- energy is required to overcome repulsion between negative electron and negative ion
Define lattice formation enthalpy
The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
Define lattice dissociation energy
Enthalpy change to separate one mole of an ionic substance into its gaseous ions
Always endothermic (positive value)
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions become aqueous ions
Why is enthalpy of Hydration exothermic
Attraction is created between the ion and water
Define enthalpy of solution
When one mole of solid dissolves in water to form aqueous ions
Breaking attractions is ______
Making attractions is ______
Endothermic (🔺H >0)
Exothermic (🔺H<0)
What are the 4 stages in forming an ionic lattice?
Stage 1: atomisation
2: ionisation
3: electron affinity
4: lattice formation
For lattice formation, what does enthalpy of formation equal?
All enthalpy changes added together
What is lattice enthalpy
The energy released when gaseous ions combine to form an ionic lattice (formation is exothermic) or the energy required to break apart the lattice to form gaseous ions (dissociation is endothermic)
What are the 2 main things that affect lattice enthalpy?
Why?
Charge of the ion
Size of the ion
Higher the charge the smaller its radius, meaning the ions will be closer together in the lattice, increasing the attractions’ strength
If the attraction is stronger is more or less energy released on the formation of the lattice
More energy
Why do lattice dissociation enthalpies decrease as the size of ions increase?
Larger ions have a weaker electrostatic attraction between oppositely charged ions
What is the perfect ionic model?
Why may this value be different to the experimental value?
It assumes all ions are perfect spheres with purely ionic attraction
Covalent Character
What does covalent character do?
What does this mean when comparing theoretical and experimental values?
Why?
Puts extra energy into the ionic bond, making the bond stronger
Experimental value will be greater
Covalent character makes the bond stronger so more energy is released/ required
Define enthalpy of solution
Enthalpy change when one mole of a solid dissolves in water to form aqueous ions
What must a solvent be for ionic solids to dissolve in it
Polar eg water
If NaCl was dissolved in water, what would happen?
The Na+ would be attracted to the delta-negative Oxygen of the water and the Cl- would be attracted to the delta-positive hydrogen
What does 🔺H sol =
🔺H (L. D.) + 🔺H(hyd)
What is entropy
A measure of disorder in a system
What does an increase in disorder mean?
The reaction is ‘feasible’ or ‘spontaneous’
NAme the states of matter from most ordered to most disordered
Solid
Liquid
Gas
What affects entropy?
Number of moles/ particles
State
If a solid becomes liquid, or liquid becomes gas what does this mean for the entropy?
Entropy increases so 🔺S is positive
If the number of moles decreases, what does this mean for the entropy
Disorder decreases so 🔺S is negative
If a question asks about entropy what must you mention
Disorder
The more positive the 🔺H…
The more disordered a system is and the more feasible the reaction
Equation for entropy change
🔺S = S(products) - S (reactants)
[SliPeR]
What units are used to measure 🔺S
J/K/mol
Why is entropy 0 at 0K?
There is no movement at absolute zero so there is no disorder
When is the greatest 🔺S as temperature increases?
The boiling point
Substance is becoming a gas so there is a large increase in disorder
(use question’s terminology)
What is the equation for Gibbs Free Energy?
What are the units for this?
🔺G=🔺H -T🔺S
kJ/mol
When is a reaction feasible?
🔺G<0
So if the value is negative or 0 the reaction IS feasible. The more negative the more feasible
If the question asks when a reaction: becomes feasible Is no longer feasible At equilibrium What do you do
Set 🔺G=0
What is the 🔺G value at equilibrium?
0
Why does 🔺G=0 at equilibrium?
Forwards reaction is equally feasible as the backward reaction
Relate y=mx+c to
🔺G=🔺H-T🔺S
y= 🔺G m= -🔺S x= T c= 🔺H
If 🔺S is positive and 🔺H is positive, when is the reaction feasible?
At high temperatures, when T🔺S>🔺H
When 🔺S is negative and 🔺H is positive, when is the reaction feasible
Never
When 🔺S is positive and 🔺H is negative when is the reaction feasible?
Always
If 🔺S is negative and 🔺 is negative when is the reaction feasible
At low temperatures, when T🔺S>🔺H
Define bond dissociation enthalpy
Give example
Enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms
Eg Cl2 —> 2Cl
Define standard molar enthalpy of atomisation
Enthalpy change for he formation of one mole of gaseous atoms from an element in its standard state under standard conditions
1/2 Cl2 (g) —> Cl(g)
Difference between bond dissociation and atomisation
BD: breaking 1 mole of bonds
Atom: forming one gaseous mole from standard state
Define first ionisation enthalpy
Minimum amount of energy required to remove one mole of e- from one mole of gaseous atoms
Why is second ionisation energy more endothermic
More difficult to remove an e- from a more positive ion
First electron affinity
🔺H when a mole of gaseous atoms is converted to gaseous negative ions
Second e- affinity
🔺H when a mole of electrons is added to a mole of gaseous negative ions to form ions each with a doubly negative charge
Why is 2nd electron affinity endothermic
Energy is required to overcome repulsion between negative e- and negative ion
Lattice formation enthalpy
🔺H when one mole of a solid ionic compound is formed from its gaseous ions
Lattice dissociation enthalpy
🔺H to separate one mole of an ionic substance into its gaseous ions
🔺H hydration
Enthalpy change when one mole of gaseous IONS become aqueous ions
🔺H solution definition
🔺H when one mole of solid dissolves in water to form aqueous ions
Define enthalpy change
The heat energy change at constant pressure
