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Thermochemical equations. |
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Answer» Solution :A balanced chemical equation together with the value of its `Delta_(R) H` is called a thermochemical equation. It would be necessary to remember the FOLLOWING conventions regarding thermochemical equations. (1) The coefficients in a balanced thermochemical equation refer to the number of moles reaction. (2) The numerical value of `Delta_(r) H^( Theta)` refers to the number of moles of SUBSTANCES specified by an equation. Standard ENTHALPY change `Delta_(r ) H^( Theta)` will have units as `" kj mol"^(-1)`. e.g., `Fe_(2) O_(3(s)) + 3H_(2(g)) to 2Fe_(2(g)) to 2Fe_((s)) + 3H_(2) O _((l))` From standard enthalpy of formation, `Delta_(f) H^( Theta) (H_(2) O) = -285.83` kj/mol `Delta_(f) H^( Theta) (Fe_(2) O_(3) )=-824.2` kj/mol `Delta_(f) H^( Theta) (FE) and Delta_(f) H^( Theta) (H_(2) ) =0` Then, `Delta _(f) H^( Theta) = [3(-285.83)] - [1(-824.2)]` `= -33.3` kj/mol If we had balanced the equation differently, for example, `(1)/(2) Fe_(2) O_(3 (s)) + (3)/(2) H_(2(g)) to Fe_((s)) + (3)/(2) H_(2) O_((l))` `Delta_(r) H_(2) ^( Theta) = [(3)/(2) (-285.83)]-[(1)/(2) (-824.2)]` `=(-428.7 + 412.1) "kj mol"^(-1)` `=-16.2 "kj/mol"=(1)/(2) Delta_(r) Delta H_(1)^( Theta)` It shows that enthalpy is an extensive quantity. (3) When a chemical equation is reversed, the value of `Delta_(r) H^( Theta)` is reversed in sign. `N_(2(g) ) + 3H_(2(g)) to 2NH_(3(g)) , Delta_(r) H^( Theta) =-91.8 "kj/mol"` `2NH_(3(g)) to N_(2 (g)) + 3H_(2 (g)) , Delta_(r) H^( Theta) = +91.8 "kj/mol"` |
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