Question

Standard heat of formation of `CH_(4),CO_(2)` and `H_(2)O_((g))` are `-76.2,-394.8` and `-241.6kJ mol^(-1)` respectively. Calculate the amount of heat evolved by burning `1m^(3)` of `CH_(4)` measured under normal conditions.

Answer 1

`C+2H_(2)rarrCH_(4), DeltaH=-7.62kJ …(1)`
`C+O_(2) rarr CO_(2), DeltaH=-394.8kJ …(2)`
`H_(2)+_(1)/(2)O_(2)rarr H_(2)O_((g)), DeltaH=-241.6kJ ....(3)`
Multiply `eq. (3)` and 2 and add in `eq.(2)`
`C+2H_(2)+2O_(2) rarr CO_(2)+2H_(2)O_((g)), DeltaH=-878kJ ..(4)`
Subtract `eq.(1)` from `eq. (4)`
`ul ({:(C,+2H_(2),rarr,CH_(4),DeltaH=,-76.2kJ, ....(5)),(-,-,,-,-,):})`
`CH_(4)+2O_(2)rarr CO_(2)+2H_(2)O_((g)), DeltaH=-801.8kJ`
`:. 22.4 `litre `CH_(4)` on burning gives energy `=801.8kJ` or `22.4xx10^(-3)m^(3)CH_(4)` on burning gives energy `=801.8kJ`
`:. 1m^(3)CH_(4)` on burning gives energy `=(801.8)/(22.4xx10^(-3))`
`=35.795xx10^(3)kJ`
`:. `Heat evolved by burning `1m^(3) CH_(4)` ltbr. `=35.795xx10^(3)kJ`