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Model Paper 9 — Chemistry STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 ScienceChemistryModel Paper 93 Marks
Question
1. The fact that the enthalpy is a state function forms the basis of a very useful law. Name the law.
2. If the combustion of 1 g of graphite produces 20.7 kJ of heat, what will be molar enthalpy change? Give the significance of the sign also.
3. Consider the same expansion, but this time against a constant external pressure of 1 atm.
✓

Answer

1. The name of the law is Hess's law of heat summation.
2. According to the question, the combustion of 1 g of graphite produces 20.7 kJ of heat.
The molar enthalpy change for the combustion of graphite, $\Delta H=$ enthalpy of combustion of 1 g graphite $\times$ molar mass
$
\begin{aligned}
& \Delta H=-20.7 kJ g^{-1} \times 12 g mol^{-1} \\
& =-2.48 \times 10^2 kJ mol^{-1}
\end{aligned}
$
Here, a negative sign indicates that the reaction is exothermic.
3. We have $q =- w = p _{ ex }(8)=8$ litre-atm

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