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प्रश्न
Answer the following question.
State Hess’s law of constant heat summation. Illustrate with an example. State its applications.
उत्तर
1. Hess’s law of constant heat summation:
Hess’s law of constant heat summation states that “Overall the enthalpy change for a reaction is equal to the sum of enthalpy changes of individual steps in the reaction”.
2. Illustration:
- The enthalpy change for a chemical reaction is the same regardless of the path by which the reaction occurs. Hess’s law is a direct consequence of the fact that enthalpy is a state function. The enthalpy change of a reaction depends only on the initial and final states and not on the path by which the reaction occurs.
- To determine the overall equation of the reaction, reactants and products in the individual steps are added or subtracted like algebraic entities.
- Consider the synthesis of NH3,
- \[\ce{2H_{2(g)} + N_{2(g)} → N2H_{4(g)}, Δ_r H^0_1 = + 95.4 kJ}\]
- \[\ce{N2H_{4(g)} + H_{2(g)} → 2NH_{3(g)}, Δ_r H^0_2 = -187.6 kJ}\]
\[\ce{H_{2(g)} + N_{2(g)} → 2NH_{3(g)}, Δ_rH^0 = - 92.2 kJ}\]
The sum of the enthalpy changes for steps (1) and (2) is equal to the enthalpy change for the overall reaction.
3. Application of Hess’s law:
The Hess's law has been useful to calculate the enthalpy changes for the reactions with their enthalpies being not known experimentally. To calculate heat of formation, combustion, neutralization, ionization, etc.
संबंधित प्रश्न
Answer in brief.
How will you calculate reaction enthalpy from data on bond enthalpies?
Calculate the total heat required
a) to melt 180 g of ice at 0 °C
b) heat it to 100 °C and then
c) vapourise it at that temperature.
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Write an application of Hess’s law.
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For the reaction, aA + bB → cC + dD, write the expression for enthalpy change of reaction in terms of enthalpies of formation of reactants and products.
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The heat evolved in the combustion of 6.022 x 1021 carbon particles is 3.94 kJ. The heat of combustion of carbon is ______.
Heat of combustion of CH4(g) is -890 kJ/mole. What is the value of Δc H of 8gm of methane?
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