Chemical Thermodynamics Chapter 4 Chemistry Class 12 Textbook Solution

    4. Answer the following questions.

    iii. State Hess’s law of constant heat summation. Illustrate with an example.State its applications.

    Answer:-

    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

    Answer:-

    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,
    1. \[2H_{2(g)} + N_{2(g)} \rightarrow N_2H_{4(g)}, \Delta_{r}H^{\circ}_{1} = +95.4 \text{ kJ}\]
    2. \[N_2H_{4(g)} + H_{2(g)} \rightarrow 2NH_{3(g)}, \Delta_{r}H^{\circ}_{2} = -187.6 \text{ kJ}\]
    3. \[H_{2(g)} + N_{2(g)} \rightarrow 2NH_{3(g)}, \Delta_{r}H^{\circ} = -92.2 \text{ kJ}\]

    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 reactions when their enthalpies are not known experimentally.

    Q 4.2
    Q 4.3
    Q 4.4

    Chemical Thermodynamics Chapter 4 Chemistry Class 12 Textbook Solution