Chemical Thermodynamics Flashcards for NEET — 20 cards

First 8 of 20 Chemical Thermodynamics flashcards

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1. Card 1 of 20

   How do you calculate the Enthalpy of a Reaction ($\Delta_r H$) using Enthalpies of Formation vs using Bond Enthalpies?

   Hint: Watch the order! Bond energy calculation is Reactants - Products (Energy absorbed to break bonds - Energy released forming bonds).

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   Answer

   Using Formation: $\Delta_r H = \sum \Delta_f H^\circ(\text{Products}) - \sum \Delta_f H^\circ(\text{Reactants})$ Using Bond Energy: $\Delta_r H = \sum BE(\text{Reactants}) - \sum BE(\text{Products})$

2. Card 2 of 20

   What kind of property results from the ratio of two extensive properties?

   Hint: Extensive ÷ Extensive = Intensive

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   Answer

   An intensive property. Example: Mass (extensive) / Volume (extensive) = Density (intensive).

3. Card 3 of 20

   Identify which of the following are path functions and which are state functions: Internal Energy ($U$), Heat ($q$), Work ($w$), Enthalpy ($H$), Entropy ($S$).

   Hint: State functions only care about initial and final states, not how you got there.

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   Answer

   Path functions: Heat ($q$) and Work ($w$). State functions: Internal Energy ($U$), Enthalpy ($H$), Entropy ($S$). (Also $P, V, T, G$)

4. Card 4 of 20

   Write the mathematical expression for the First Law of Thermodynamics and define its terms.

   Hint: Law of conservation of energy.

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   Answer

   $\Delta U = q + w$ Where $\Delta U$ is the change in internal energy, $q$ is heat, and $w$ is work.

5. Card 5 of 20

   According to the IUPAC sign convention, what are the signs for heat absorbed by the system and work done BY the system (expansion)?

   Hint: Energy entering the system is positive (+). Energy leaving is negative (-).

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   Answer

6. Card 6 of 20

   What is the work done ($w$) during the free expansion of an ideal gas in a vacuum?

   Hint: No opposing force = no work done.

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   Answer

   $w = 0$. Since the external pressure ($P_{ext}$) is zero in a vacuum, work $w = -P_{ext}\Delta V = 0$.

7. Card 7 of 20

   State the formula for the maximum work done during the reversible isothermal expansion of an ideal gas.

   Hint: Reversible work is always the maximum magnitude of work.

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   Answer

   $w_{rev} = -2.303 nRT \log \left(\frac{V_2}{V_1}\right)$ OR $w_{rev} = -2.303 nRT \log \left(\frac{P_1}{P_2}\right)$

8. Card 8 of 20

   How is the change in Enthalpy ($\Delta H$) related to the change in Internal Energy ($\Delta U$) for a reaction involving gases?

   Hint: Only count the species in the (g) state!

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   Answer

   $\Delta H = \Delta U + \Delta n_g RT$ Where $\Delta n_g = (\text{moles of gaseous products}) - (\text{moles of gaseous reactants})$.