Enthalpy ($H$)
Heat Content & Heat of Reaction | Thermodynamics
1. Definition & Formula
Enthalpy ($H$): It is defined as the total heat content of a system. It is the sum of Internal Energy ($U$) and Pressure-Volume work energy ($PV$).
$$ H = U + PV $$
Since absolute values of $U$ cannot be determined, we measure the Change in Enthalpy ($\Delta H$).
$$ \Delta H = \Delta U + \Delta(PV) $$Relation to Heat ($q$)
- At constant volume: $q_v = \Delta U$
- At constant pressure: $q_p = \Delta H$
This is why most laboratory reactions (occurring in open beakers at atmospheric pressure) involve $\Delta H$.
2. Relation between $\Delta H$ and $\Delta U$
For chemical reactions involving gaseous species at constant temperature:
$$ \Delta H = \Delta U + \Delta n_g RT $$
Where:
- $\Delta n_g$ = (Moles of gaseous products) - (Moles of gaseous reactants).
- $R$ = Gas Constant ($8.314 J K^{-1} mol^{-1}$).
- $T$ = Temperature in Kelvin.
Note: Solids and Liquids are ignored in $\Delta n_g$ calculation.
3. Exothermic vs Endothermic
| Feature | Exothermic Process | Endothermic Process |
|---|---|---|
| Heat Flow | Released by system ($q < 0$) | Absorbed by system ($q > 0$) |
| Sign of $\Delta H$ | Negative ($\Delta H < 0$) | Positive ($\Delta H > 0$) |
| Energy Profile | $H_{products} < H_{reactants}$ | $H_{products} > H_{reactants}$ |
| Example | Combustion ($C + O_2 \to CO_2$) | Evaporation, Melting |
4. Characteristics of Enthalpy
- State Function: Depends only on initial and final states, not the path.
- Extensive Property: Depends on the quantity of matter. (If reaction coefficients are doubled, $\Delta H$ doubles).
- Unit: Joules ($J$) or kiloJoules per mole ($kJ/mol$).
5. Standard Enthalpy of Reaction ($\Delta_r H^\ominus$)
The enthalpy change when all reactants and products are in their Standard States (1 bar pressure, specific temperature usually 298 K).
- Standard Enthalpy of Formation ($\Delta_f H^\ominus$): Formation of 1 mole of a compound from its constituent elements in standard states.
- Note: $\Delta_f H^\ominus$ for an element in its standard state is Zero (e.g., $O_2(g), C(graphite), Na(s)$).
Practice Quiz
Test your knowledge on Enthalpy.
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