1.

What would be the standard free energy change forthe formation of methane at 298 K? The value of Delta H^@ for CH_4(g) is -74.81 kJ "mol"^(1) and S^@ values for C_("(grap)") H_2(g) and CH_4(g) are 5.70, 130.7 and 186.3 J K^(-1) "mol"^(-1)respectively.

Answer»

`-50.71 `
`47.56`
`60.45`
`-74.8`

Solution :For the reaction , ` C + 2H_2 to CH_4`
GIVEN `Delta H^@ = - 74.81 kJ "mol"^(-1) , Delta S_m^@` can be CALCULATED as
`Delta S_m^@ CH_4(g)- [S_m^@ C_("graphite") + 2S_m^@ H_2 (g) ]`
` =[ 186.3 - (5.70 + 2 xx 130.7)] JK^(-1) "mol"^(-1)`
`= -80.8 JK^(-1) "mol"^(-1)`
SINCE `Delta G^@ = Delta H^@ - T Delta S^@`
` = - 74.81 -[(298) (-80.8xx 10^(-3) ) ]`
`= -74.81 kJ "mol"^(-1) + 24.1 kJ"mol"^(-1) = -50.71 kJ "mol"^(-1)`


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