Given, C2H2(g)→C2H4(g):ΔH∘=−175kJ mol−1 ΔH∘f(C2H4,g)=50kJ mol−1;ΔH∘f(H2O,l)=−280kJ mol−1;ΔH∘f(CO2g)=−390kJ mol−1
If ΔH∘ is enthalpy of combustion (in kJ mole−1 of C2H2(g), then calculate the value of ∣∣∣ΔH∘257∣∣∣
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Solution
C2H2(g)+H2(g)→C2H4(g)
Given, ΔH∘f(C2H4,g)=50kj mol−1 ∴−175=−(ΔfH)C2H2+50 ⇒(ΔfH)C2H2=225kj mol−1
For enthalpy of combustion the corresponding chemical reaction is, C2H2+52O2→2CO2+H2O ΔH∘=(2×(−390)−280−225)kj mol−1=−1285kj mol−1 ∴∣∣∣ΔH∘257∣∣∣=∣∣∣−1285257∣∣∣=5kJmol−1