Chapter 17 - Thermodynamics Year 2 Flashcards
Definition of STANDARD ENTHALPY OF FORMATION, ΔHf⍬
The enthalpy change when one mole of a compound is formed from its elements with all reactants and products in their standard states.
ΔH = positive OR negative
eg. Mg (s) + Cl2 (g) → MgCl2 (s)
What is the definition of STANDARD ENTHALPY OF COMBUSTION, ΔHc⍬
The enthalpy change when one mole of an element or compound is completely burned in oxygen, with all reactants and products in their standard states.
ΔH = negative
eg. C (s) + O2 (g) → CO2 (g)
What is the definition of STANDARD ENTHALPY OF ATOMISATION (Element), ΔHat⍬
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.
ΔH = positive
eg. ½ Cl2 (g) → 2 Cl (g)
What is the definition of STANDARD ENTHALPY OF ATOMISATION (Compound), ΔHat⍬
The enthalpy change when one mole of a compound in its standard state is converted into gaseous atoms.
ΔH = positive
eg. CH4 (g) → C(g) + 4H(g)
What is the definition of BOND DISSOCIATION ENTHALPY, ΔHdis⍬
The enthalpy change when one mole of gaseous molecules each breaks a particular covelant bond to form two radicals.
ΔH = positive
eg. CH4 (g) → CH3 (g) + H(g)
eg. Cl2 (g) → 2Cl (g)
What is the definition of IONISATION ENTHALPY, ΔHIE⍬
1st Ionisation Enthalpy
The enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous, singly charged, positive ions.
ΔH = positive
eg. Na (g) → Na+ (g) + e-
What is the definition of IONISATION ENTHALPY, ΔHIE⍬
2nd Ionisation Enthalpy
The enthalpy change when one mole of gaseous, singly charged, positive ions is converted into one mole of gaseous, doubly charged positive ions.
ΔH = positive
eg. Mg+(g) → Mg2+ (g) + e-
What is the definition of ELECTRON AFFINITY, ΔHEA⍬
1st Electron Affinity
The enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous, singly charged, negative ions
ΔH = negative
eg. O (g) + e- → O- (g)
What is the definition of ELECTRON AFFINITY, ΔHEA⍬
2nd Electron Affinity
The enthalpy change when one mole of gaseous, singly charged, negative ions is converted into one mole of gaseous, doubly charged negative ions.
ΔH = positive
eg. O- (g) + e- → O2- (g)
What is the definition of LATTICE ENTHALPY, ΔHlatt⍬
Lattice Dissociation Enthalpy
The enthalpy change when one mole of an ionic compound is separated into its gaseous ions.
ΔH = positive
eg. MgCl2 (s) → Mg2+ (g) + 2 Cl- (g)
What is the definition of LATTICE ENTHALPY, ΔHlatt⍬
Lattice Formation Enthalpy
The enthalpy change when one mole of an ionic compound is formed from its gaseous ions.
ΔH = negative
eg. Mg2+ (g) + 2 Cl- (g) → MgCl2 (s)
What is the definition of ENTHALPY OF HYDRATION, ΔHhyd⍬
The enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions.
ΔH = negative
eg. Na+ (g) + aq → Na+ (aq)
What is the definition of ENTHALPY OF SOLUTION, ΔHsol⍬
The enthalpy change when one mole of an ionic compound dissolves in water to give a solution of infinite dilution.
ΔH = positive OR negative
eg. NaCl (s) + aq → Na+ (aq) + Cl- (aq)
What is HESS’S LAW
The enthalpy change for a reaction depends only on the initial and final states of the reaction and is independent of the route by which the reaction may occur.
e.g. ΔH1 = ΔH2 + ΔH3 + ΔH4
What is the Born-Haber cycle
It is an energy cycle that includes the enthalpy of lattice formation of an ionic compound and shows every step within that cycle
What reactions are in a Born-Haber Cycle
Enthalpy of formation
Enthalpy of atomisation of element 1
Enthalpy of atomisation of element 2
1st ionisation energy
(2nd and 3rd ionisation energy)
Electron affinity
Lattice formation enthalpy
What do lattice enthalpies tell us
The strength of ionic bonding
Higher lattice enthalpy = stronger ionic bonding
What can ionic and covalent bonding be imagined
they are 2 extremes at either end of a continuum with some molecules have characters of both ionic and covalent
What causes covalent character in a ionic bond
It is caused by polarisation
How does polarisation work
When a small positive ion is near a large negative ion it distorts the shape of the electron cloud in the negative ion
This pulls electrons towards the positive ion causing to many electrons between the ions giving covalent characters
when is polarisation most likely to occur
small highly positive ions next to large highly negative ions
What are experimental lattice enthalpies
They are values using real data from experiments
What are theoretical lattice enthalpies
Where the values assume bonding is purely ionic and the ions are perfectly spherical
What is the difference between theoretical and experimental enthalpies
They show us how purely ionic bonding is in a reaction
What is the theoretical model of dissolving
Solid to gas is lattice dissociation enthalpy
Gas to hydrated ions is enthalpy of hydration
Equal to enthalpy of solution
What means a compound is soluble
enthalpy of solution is less than +30
The more negative, the more soluble
What is the equation for enthalpy of solution
lattice enthalpy + enthalpy of hydration
How is enthalpy of hydration measured
It isn’t possible to measure the enthalpy of hydration of a single ion so the value is the sum of 2 enthalpies of hydration
what factors affect enthalpy of hydration
Charge of ions
size of ions
How does charge of ions affect the enthalpy of hydration
higher charge is greater attraction and trhe more negative enthalpy
How does ion size affect enthalpy of hydration
The bigger it is, the smaller the attraction so bigger enthalpy
What is a spontaneous change
It is one which just happens without any extra energy needing to be supplied to it at a particular temperature
What is entropy
It is a measure of the degree of disorder in a system
What ways are entropy increased
melting
evaporation
solution from solid
gas from solids or liquids
when the number of molecules increases
What are the units of entropy change
J/K/mol
What is the equation for entropy change of reaction
entropy of products - entropy of reactants
What is the equation for total entropy change
entropy of reaction + entropy of surrondings
What is the equation of entropy of surrondings
-enthalpy change / temperature
What influences spontaneous reactions
enthalpy and entropy change
What is the equation for Gibbs free energy chnage
enthalpy change - temperature x Entropy change
What means a reaction is feasible
negative gibbs free energy change
What does gibbs free energy change equal for an equilibrium process
0
What is an equilibrium change
change of state
When is the standard entropy of an element 0
0 K
What is an enthalpy change
Heat (energy) change at constant pressure
