Choose the reaction in which ΔH is not equal to ΔU.
A
C(graphite)+O2(g)⟶CO2(g)
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B
C2H4(g)+H2(g)⟶C2H6(g)
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C
2C(graphite)+H2(g)⟶C2H2(g)
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D
H2(g)+I2(g)⟶2HI(g)
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E
N2(g)+O2(g)⟶2NO(g)
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Solution
The correct option is BC2H4(g)+H2(g)⟶C2H6(g) The reaction for which Δng≠0, ΔH≠ΔU because ΔH=ΔU+ΔngRT Δng=nP−nR (where, nP and nR represents the number of moles of gaseous products and reactants respectively.)
(a)
C(graphite)+O2(g)⟶CO2(g);Δng=nP−nR=1−1=0
(b) C2H4(g)+H2(g)⟶C2H6(g);Δng=1−2=−1
(c) 2C(graphite)+H2(g)⟶C2H2(g);Δng=1−1=0
(d) H2(g)+I2(g)⟶2HI(g);Δng=2−2=0
(e) N2(g)+O2(g)⟶2NO(g);Δng=2−2=0
Thus, for the reaction given in option (b), ΔH≠ΔU.