Chemical Thermodynamics — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryChemical Thermodynamics3 Marks
Question
Explain the sign convention used for $\Delta H$.
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Answer
The change in enthalpy or heat of reaction $\Delta H$ is given by, $\begin{aligned} \Delta H & =\left[\begin{array}{c} \text { Sum of enthalpies } \\ \text { of products } \end{array}\right]-\left[\begin{array}{c} \text { Sum of enthalpies } \\ \text { of reactants } \end{array}\right] \\ & =\Sigma H_{\text {products }}=\Sigma H_{\text {reactants }} \text {} \end{aligned}$
(i) If the sum of enthalpies of products, $\Sigma H$ and reactants, $\Sigma_R H$ are equal then $\Delta H$ for the reaction is $\begin{aligned} & \text { zero, }(\Delta H=0) \text {. } \\ & \text { i. e. } \Sigma_p H=\Sigma_R H \\ & \therefore \Delta H=\Sigma_p H-\Sigma_R H=0 \end{aligned}$
(ii) If the sum of enthalpies of products $\Sigma_{ P } H$ is greater than the sum of enthalpies of reactants $\Sigma_{ R } H$, then $\Delta H$ is positive, $(\Delta H>0)$. Since such reactions take place with the absorption of heat from surroundings, they are called endothermic reactions. $\begin{aligned} & \therefore \Sigma H_{\text {products }}>\Sigma H _{\text {reactants }} \\ & \therefore \Delta H>0 \end{aligned}$
(iii) If the sum of enthalpies of products $\Sigma_P H$ is less than the sum of enthalpies of reactants, $\Sigma_R H$ then $\Delta H$ is negative, $(\Delta H<0)$. Since in such reactions heat is given out to the surroundings, they are called exothermic reactions. $\begin{aligned} & \therefore \Sigma_P H<\Sigma_R H \\ & \therefore \Delta H<0 \end{aligned}$
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