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How to calculate molar enthalpy in AP Chemistry?

Answer :

Molar enthalpy is a concept in chemistry that represents the enthalpy change per mole of a substance when it undergoes a chemical reaction or a change of state. Here's how you can calculate molar enthalpy step-by-step:

  1. Understand Enthalpy (H):
    Enthalpy is a measurement of energy in a thermodynamic system and is expressed in units of energy, usually joules (J) or kilojoules (kJ). It accounts for the heat content of a system.

  2. Use the Reaction Equation:
    When a chemical reaction occurs, the change in enthalpy (∆H) can be calculated using the equation:

    [tex]\Delta H = H_{ ext{products}} - H_{ ext{reactants}}[/tex]

    Here, [tex]H_{\text{products}}[/tex] is the enthalpy of the products, and [tex]H_{\text{reactants}}[/tex] is the enthalpy of the reactants.

  3. Determine Moles of the Substance:
    Identify the substance for which you want to find the molar enthalpy. You'll need the number of moles. This can typically be found using the balanced chemical equation or by dividing the sample mass by the molar mass of the substance:

    [tex]\text{Moles} = \frac{\text{Mass of substance (g)}}{\text{Molar mass (g/mol)}}[/tex]

  4. Calculate Molar Enthalpy (∆Hₘ):
    Once you have the enthalpy change (∆H) for the reaction and the number of moles, you can calculate the molar enthalpy, which is the enthalpy change per mole:

    [tex]\Delta H_m = \frac{\Delta H}{\text{Moles of substance}}[/tex]

  5. Consider the Sign of ∆H:

    • If [tex]\Delta H[/tex] is negative, the reaction is exothermic, meaning it releases heat.
    • If [tex]\Delta H[/tex] is positive, the reaction is endothermic, meaning it absorbs heat.

By following these steps, you should be able to calculate the molar enthalpy for any given reaction or process accurately. Understanding these calculations is key for predicting reaction behavior and energy changes in chemical processes.

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