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Calculate enthalpy of formation of metha...

Calculate enthalpy of formation of methane `(CH_4)` from the following data :
(i) `C(s) + O_(2)(g) to CO_(2) (g) , Delta_rH^(@) = -393.5 KJ mol^(-1)`
(ii) `H_2(g) + 1/2 O_(2)(g) to H_(2)O(l) , Deta_r H^(@) = -285.5 kJ mol^(-1)`
(iii) `CH_(4)(g) + 2O_(2)(g) to CO_(2)(g) + 2H_(2)O(l), Delta_(r)H^(@) = -890.3 kJ mol^(-1)`.

Text Solution

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We need `C_((s))+2H_(2(g))rarrCH_(4(g))`,`Delta_(r)H^(0)=-890.3KJmol^(-1)` Multiplying equation (ii) with 2, adding to equation (i) and then substratcing equation (iii) from the sum, we get
`C+2H_(2)rarrCH_(4)`
`Delta_(f)H^(0)=2(-285.8)+(-393.5)-(-890.3)`
`=-74.8KJmol^(-1)`
Hence enthalpy of formation of methane is `Delta_(f)H^(0)=-74.8KJmol^(-1)`
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