Question

A scientist measures the standard enthalpy change for the following reaction to be -25.2 kJ :

I2(g) + Cl2(g) 2ICl(g)

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of ICl(g) is______ kJ/mol

Answer 1

The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 degrees Celsius). Its symbol isΔH_f^o

All elements in their standard states (oxygen gas, solid carbon in the form of graphite,Iodine, Chlorine....etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.

Therefore, ΔH_f^o (I2) = 0 and ΔH_f^o (Cl2) =0

For any reaction, ΔH = Moles x ΔH_f^o _(products) - Moles x ΔH_f^o(reactants)

Given reaction is I2(g) + Cl2(g) 2ICl(g)

So, for above reaction ΔH = 2 ΔH_f^o _(ICl) -[ΔH_f^o (I2) +ΔH_f^o (Cl2)]
Given that ΔH = -25.2 kJ

So, -25.2 kJ= 2 ΔH_f^o (ICl) - [ 0+ 0]
2 ΔH_f^o (ICl) = -25.2 kJ
Then, ΔH_f^o (ICl) = -25.2 kJ/ 2 = -12.6 kJ

Therefore, ΔH_f^o (ICl)= -12.6 kJ

i.e The standard enthalpy of formation of ICl(g) is_-12.6 kJ/mol