The data given below are for vapour phase reactions at constant pressure.
`C_(2)H_(6)rarroverset(*)(C_(2))overset(*)(H_(5))+overset(*)(H), DeltaH=-4200kJmol^(-1)`
`overset(*)(C_(2))H_(6) rarr C_(2)H_(4)+overset(*)(H), DeltaH=168kJ mol^(-1)`
The enthalpy change for the reaction `,`
`2overset(*)(C_(2))H_(5) rarr C_(2)H_(6)+C_(2)H_(4)` is `:`

Options

A. `+250kJ mol^(-1)`
B. `+588kJ mol^(-1)`
C. `-252kJ mol^(-1)`
D. `-588kJ mol^(-1)`

Answer

C

Explanation

`overset(*)(C_(2))H_(5)+overset(*)(H)rarrC_(2)H_(6),DeltaH=-420kJ mol^(-1)`
`ul( overset(*)(C_(2))H_(5)rarr C_(2)H_(4)+overset(*)(H),DeltaH=168kJ mol^(-1) )`
`:. 2C_(2)H_(5) rarrC_(2)H_(6)+C_(2)H_(4),DeltaH=-252kJ mol^(-1)`