Thermodynamics - examples

Enthalpy of formation

For most substances: Exothermic

E.g. Na₂O_(s)

2 Na_(s) + ½ O_2(g) → Na₂O_(s)

Enthalpy of combustion

Exothermic

E.g. H_2(g)

H_2(g) + ½ O_2(g) → H₂O_(s)

Enthalpy of neutralization

Exothermic

E.g. H₂SO_4(aq)+NaOH_(aq)

½ H₂SO_4(aq)+NaOH_(aq) → Na₂SO_4(aq) + H₂O_(l)

1st Ionization enthalpy

 Endothermic

E.g. Mg_(g)

Mg_(g) → Mg⁺_(g) + e^-

2nd Ionization energy

 Endothermic

E.g. Mg⁺_(g)

Mg⁺_(g) → Mg²⁺_(g) + e^-

1st Electron affinity

Exothermic for many non-metals

E.g. O_2(g)

O_(g) + e^-  → O^-_(g) 

2nd electron affinity

Endothermic as adding -ve electron to -ve ion

E.g. O^-_(g)

O^-_(g) + e^-  → O^2-_(g) 

Enthalpy of atomization

Endothermic

E.g. ^I_2(s)

^{½ I}_2(s) → I_(g)

Hydration enthalpy

Exothermic 

E.g. Mg²⁺_(g)

Mg²⁺_(g)  + aq  → Mg²⁺_(aq)

Enthalpy of solutions

varies

E.g. MgCl_2(s)

MgCl_2(s) + aq → Mg²⁺_(aq) + 2 Cl^-_(aq)

Bond dissociation enthalpy 

Endothermic 

e.g. I—I bond 

I_2(g) → 2 I_(g)

Lattice enthalpy of formation

Exothermic

e.g. MgCl_2(s)

Mg²⁺_(g) + 2 Cl^-_(g) → MgCl_2(s)

Lattice enthalpy of dissociation 

Endothermic

e.g. MgCl_2(s)

MgCl_2(s) →  Mg²⁺_(g) + 2 Cl^-

Enthalpy of vaporization

Endothermic

e.g. H₂O_(l)

H₂O_(l) → H₂O_(g)

Enthalpy of fusion

Endothermic

e.g. Mg_(s)

Mg_(s) → Mg_(l)