chem energetics Flashcards
standard enthalpy change of combustion
energy evolved when one mole of the substance is completely burnt in oxygen under standard conditions
define standard enthalpy change of hydration
energy evolved when one mole of gaseous ions is hydrated under standard conditions
define standard enthalpy change of solution
energy change when one mole of the substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions
standard enthalpy change of neutralisation
energy evolved when one mole of water is formed from the neutralisation between acid and base under standard conditions
define standard enthalpy change of atomisation
energy absorbed when one mole of gaseous atoms is formed from the element under standard conditoins
define bond energy
average energy adsorbed to break one mole of covalent bonds between 2 atoms in the gaseous state to form gaseous atoms under standard conditions
define lattice energy
energy evolved when one mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions
effect of ionic charge and of ionic radius on the numerical magnitude of a
lattice energy
|△Hlatt|∝ (q+)(q-) /(r+)+(r-)
define entropy
measures the degree of disorder/randomness in a system
effects on the entropy of a chemical system
(i) change in temperature
- more energy to distribute the particles
(ii) change in phase (solid < liquid < gas)
-move more freely and with greater speeds, more ways..
(iii) change in the number of particles (especially for gaseous systems)
- arrangement of molecules more disordered, more gasesous particles moving randomly, more ways..)
(iv) mixing of particles(expansion of volume for gaseous system)
- more ways @ larger volume!!!
more ways to distribute the particles and energy
limitations in the use of ΔG⦵
to predict the spontaneity of a reaction
a) only valid for STANDARD CONDITIONS
b) kinetic feasibility not considered
define standard enthalpy change of formation
energy change when one mole of the substance is formed from its constituent elements under standard conditions
define 1st ionisation energy
energy absorbed when one mole of gaseous atoms loses one mole of electrons to form one mole of singly positively charged gaseous ions
define 2nd ionisation energy
energy absorbed when one mole of singly positively charged gaseous ions loses one mole of electrons to form one mole of doubly positively charged gaseous ions
define first electron affinity
energy evolved when on mole of gaseous atoms acquires one mole of electrons to give one mole of singly negatively change gaseous ions
ΔHr =
ΔHf products - ΔHf reactants
ΔHc reactants - ΔHc products
BE reactants - BE products
why ΔHn (weak acid/base) < 57.0?
- weak acids/bases are only sightly dissociated in aq solution. ionisation, which involves bond breaking, is an endo process.
- some energy evolved from neutralisation process is used to further dissociate the weak acid/base completely
-thus lest exo
why 2d E.A is endo while 1st E.A is exo?
2nd EA also endo since e is added to -vely charged ion.
energy has to be supplied to overcome repulsive forces between 2 -vely charged species
ΔHf of an element at 25C/298K is always
zero
manyy enthalpy changes cannot be determined directly by experiments due to :
- reaction does not take place at standard conditions upon mixing reactants
- ΔHrxn is just a theoretical value as reaction does not take place in practice
assumptions of conducting experiment in calorimeter: (2)
- negligible heat lose to or gained from the surroundings due to insulation -> 100% efficiency
- specific heat capacity of solution = specific heat capacity of water = 4.18
density of solution = desity of water = 1.00gcm3
theoretical vs experimental lattice energies differs due to
partial covalent character in the ionic bond has not been accounted for by the theoretical ionic compound model.
this results in discrepancy between ..
more exo experimental lattice energies = stronger ionic bonding
salt is likely to be soluble when
ΔHsoln < 0. |ΔHhyd|(enough to compensate for latt energy) > |L.E.|
and vice versa
At ΔG =0,
reaction is at equilibrium (PHASE CHANGES (melting, freezing, vapourisation, condensation, sublimation) OCCURS AT EQUILIBRIUM!!!)
why there is a diff in value for theoretical and experimental values for bond energy?
bond energy values taken from data booklet for C-H and O-H bonds are only average values obtained from polyatomic molecules
ΔHsoln =
+|ΔHlatt| - |ΔHhyd|
Energetic stability of a substance arises when
its enthalpy content is relatively lower than that of the product of a chemical reaction
Kinetic stability of a substance arises when
the rate of chemical reaction is extremely slow, or negligible. this occurs when Ea is extremely high
spontaneous reaction
can occur without any external assistance. it is irreversible as it cannot be brought back to its original state again under the same conditions
