Enthalpy change, ΔH:- heat change of chemical reaction under const pressure (kJ/mol) Standard conditions: temp: 25°C, pressure: 1 atm or 101325 Pa Standard enthalpy change, ΔH:- enthalpy change measured under standard conditions Endothermic reaction -heat absorbed > ΔH +ve
-heat content of products higher than reactants (heat absorbed)
Ea: activation energy
ΔH: enthalpy change
Exothermic reaction
-heat given off > ΔH -ve
-heat content of products lower than reactants (heat released)
Ea: activation energy
ΔH: enthalpy change
Standard enthalpy change of reaction, ΔHr: heat change when molar quantities of reactants(specified by balance equation) react to for products under standard conditions Standard enthalpy change of formation, ΔHf : heat change when 1 mole of product formed from elements in the most stable state under standard conditions Standard enthalpy change of combustion, ΔHc: heat change (exothermic only) when 1 mole of element / cpd is completely burnt in excess oxygen under standard conditions
Hess's Law (conservation of heat energy) -states that total enthalpy change of a chemical reaction is the same regardless of intermediate steps provided initial and final conditions are constant
A + B >(ΔH1)> C >(ΔH2)> D >(ΔH3)> E
A + B >(ΔH4)> E
By Hess's Law: ΔH1 + ΔH2 + ΔH3 = ΔH4
Determination of enthalpy change of reaction Heat evolved = mass × specific heat capacity × Δtemp = mcΔT = VρcΔT (ΔT = initial - final temp)
enthalpy change of reaction = heat evolved / mol of substance M = VρcΔT/n kJ/mol (of M)
assumptions;
(1) -density of solution = density of water = 1g/cm3
(2) -csolution = cwater = 4.2J/(g°C)
standard enthalpy change of neutralization, ΔHn: heat evolved when 1 mole of H+(aq) from acid reacts with 1 mole of OH-(aq) from base to form 1 mole of water under standard conditions of 1 atmosphere & 25°C
assumptions;
(1)dilute sol so that
-density of solution = density of water = 1g/cm3
-csolution = cwater = 4.2J/(g°C)
(2)heat loss by conduction is negligible standard enthalpy change of neutralisation with: strong acid & strong base
-always constant [-57.3kJ/mol (of H2O formed)] as molecules are 100% dissociated to ions > heat change only from formation of H2O molecules (no energy used to dissociate molecules) weak acid / weak base -less than -57.3kJ/mol (of water formed) as molecules not fully ionised > heat energy absorbed to dissociate molecules
Born-Haber cycle for formation of ionic solid cpd ΔHsub, enthalpy change of sublimation of element: heat absorbed to convert solid element into 1 mole of gaseous atom
M(s) >(ΔHsub)> M(g)
[M(s) >(ΔHsub)> M(g) = M(s) >(ΔHfusion)> M(l) >(ΔHvap)> M(g)
ΔHsub = ΔHfusion + ΔHvap = enthalphy change of (fusion of solid M+ vaporization liquid M)
ΔHsub can be also called enthalpy change of atomization,ΔHatomisation: energy absorbed to change a subs from solid, liquid or gaseous state into 1 mole of gaseous atoms; M(s/l/g) > M(g)
for gaseous diatomic molecules, X2: ΔHatomisation = ½ bond energy]
ΔHIE, 1st ionisation energy: min energy absorbed to remove 1 mole of most loosely bound valence electrons in ground state from 1 mole of isolated, gaseous atoms to form 1 mole of isolated gaseous singly-charged positive ions
M(g) - e- >(ΔHIE)> M+(g) Electron affinity, ΔHEA: energy evolved when 1 mole of valence electrons is added to 1 mole of isolated gaseous atoms to from 1 mole of isolated, gaseous singly-charge negative ions
X(g) + e- > X-(g)
1st EA- form univalent anion from neutral atom: exothermic (valence electrons added to form anion)
2nd EA- form divalent anion from univalent anion : endothermic (valence electrons added to anion > energy absorbed to overcome repulsion) Lattice energy, ΔHL: energy evolved when isolated, gaseous anions & cations combine to form 1 mole of ionic solid
M+(g) + X-(g) >(ΔHL)> M+X-(s) Factors affecting lattice energy Charges of ions: greater # of charges > stronger electrostatic force of attraction > greater LE Ionic radii of ions: smaller radius > stronger electrostatic force of attraction > greater LE
Enthalpy changes of dissolution of ionic solid Enthalpy change of solution, ΔHsolution: energy change when 1 mole of ionic solid dissolved in such an amt of water that further dilution causes no more heat change (solution of infinite dilution)
M+X-(s) + aq >(ΔHsolution)> M+(aq) + X-(aq) Enthalpy change of hydration, ΔHhyd: energy evolve when 1 mole of gaseous ions is hydrated in water to form aqueous ions
M+(g) + aq >(ΔHhyd)> M+(aq)
Factors affecting enthalpy change of hydration Charges of ions: greater # of charges > stronger electrostatic force of attraction > greater HE Ionic radii of ions: smaller radius > stronger electrostatic force of attraction > greater HE
Bond energy, ΔHBE (bond enthalpy / bond dissociation energy)
(1)for a diatomic molecule:- energy absorbed to dissociate 1 mole of the X-Y covalent bonds of gaseous XY to form gaseous X & Y atoms
XY(g) >(ΔHBE)> X(g) + Y(g)
(2)for a polyatomic molecule:- average energy absorbed to dissociate 1 mole of the X-Y covalent bonds in gaseous XYn to form gaseous X & Y atoms
XYn(g) >(ΔH1)> XYn-1(g) + Y(g)
XYn-1(g) >(ΔH2)> XYn-2(g) + Y(g)
........
XY(g) >(ΔHn)> X(g) + Y(g)
bond energy = (ΔH1 + ΔH2 + ... + ΔHn)/n
ΔHBE increases w/:
-decrease in bond length
-polarity of bond (more polar bond > greater ΔHBE)
The more exothermic the reaction > the more readily it occurs
