Chemical Thermodynamics — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryChemical Thermodynamics4 Marks
Question
What are the guidelines followed for writing thermochemical equations?
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Answer
(1) Reaction is represented by balanced chemical equation for the number of moles of the reactants and the products. E.g. $\begin{aligned} & CH _{4( g )}+2 O _{2( g )}= CO _{2( g )}+2 H _2 O _{( l )} \\ & \Delta rH ^{\circ}=-890 kJ mol ^{-1} \end{aligned}$ (2) The physical states of all the substances in the reaction must be mentioned. E.g. (s) for solid, (I) for liquid and $( g )$ for gas. (3) Heat or enthalpy changes are measured at $298 K$ and 1 atmosphere (or 1 bar). (4) $\Delta H ^0$ is written at right hand side of thermochemical equation. (5) Proper sign must be indicated for $\Delta H^0$. For endothermic reaction $\Delta H^0$ is positive, $\left(+\Delta H^0\right)$ and for exothermic reaction $\Delta H$ is negative, $\left(-\Delta H^0\right)$. (6) The enthalpy of the elements in their standard states is taken as zero. $\left( H ^0\right.$ Element $=0 ; H ^0 C _{(5)}=0$, $H ^0 H _{2( g )}=0$ ) (7) When all the substances taking part in the reaction are in their standard states, the enthalpy change is written as $\Delta H^0$. (8) The enthalpy of any compound is equal to its heat of formation. (9) In case of elements, the allotropic form must be mentioned. E.g. $C _{\text {(graphite), }} S _{\text {(rhombic), }} Sn _{\text {(white) }}$ (10) For the reverse reaction, $\Delta H^0$ value has equal magnitude but opposite sign.
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