Question

Derive a relation between ΔH and ΔU for a chemical reaction. Draw neat labelled diagram of calomel electrode. Resistance and conductivity of a cell containing 0.001 M KCI solution at 298K are 1500Ω and 1.46x10^-4 S.cm^-1 respectively.

Answer 1

Relation between ΔH and ΔU

The heat of reaction is given by enthalpy change

ΔH=H₂ - H₁

by defination H= U+PV

H₁=U₁+P₁V₁

H₂=U₂+P₂V₂

ΔH = (U₁+P1V₁) - (U₂+P₂V₂)

=(U₂-U₁)+(P₂V₂-P₁V₁)

=ΔU+(P₂V₂-P₁V₁)(∵ΔU=U₂-U₁)

since PV=nRT

for initial state P₁V₁=n₁RT

for final state P₂V₂=n₂RT

P₂V₂-P₁V₁=n₂RT-n₁RT

=(n2-n)RT

=ΔnRT

where Δn = number of moles of gaseous product - number of moles of gaseous reactant.

ΔH=ΔU+ΔnRT

Diagram : Standard calomel electrode

Data: R 1500Ω

C=0.001M

k= 1.46x10^-4Scm^-1

To find: b = ?

Solution : `k=b/R`

b=kxR

=1.46x10^-4x1500

b=2190x10^-4 cm^-1

b = 0.219cm-1