Hess's Law & Types of Enthalpy
Decode the heat of reactions. Master the standard states of formation, Hess's Law of Constant Heat Summation, the tricky Bond Enthalpy formula, and Kirchhoff's Law.
Module Focus
Thermochemistry applies the First Law of Thermodynamics to chemical reactions. While calculating $\Delta H$ seems like simple addition and subtraction, examiners exploit specific traps: identifying the correct Standard Reference State (e.g., Graphite vs. Diamond), flipping the equation correctly in Hess's Law, and remembering that Bond Enthalpy is calculated as Reactants minus Products.
1. Standard Enthalpy of Formation ($\Delta H_f^\circ$)
The enthalpy change that occurs when exactly 1 mole of a compound is formed from its constituent elements in their most stable state of aggregation (standard state) at 1 bar pressure and specified temperature (usually 298 K).
By convention, the standard enthalpy of formation ($\Delta H_f^\circ$) of an element in its most stable, naturally occurring reference state is taken as ZERO. You MUST memorize these specific states:
- Carbon: Graphite (NOT Diamond)
- Sulphur: Rhombic (NOT Monoclinic)
- Phosphorus: White (NOT Red or Black)
- Oxygen: $\mathbf{O_2(g)}$ (NOT $O_3$ or $O(g)$)
- Bromine: $\mathbf{Br_2(l)}$ (NOT $Br_2(g)$)
- Iodine: $\mathbf{I_2(s)}$ (NOT $I_2(g)$)
- Metals (except Hg): Solid state (e.g., $Na(s), Fe(s)$)
Calculating Enthalpy of Reaction ($\Delta H_{rxn}^\circ$)
The standard enthalpy of a reaction can be calculated from the standard enthalpies of formation of products and reactants.
2. Hess's Law of Constant Heat Summation
Because enthalpy ($H$) is a state function, the total enthalpy change for a chemical reaction is the same regardless of whether the reaction takes place in one step or in multiple steps.
- Reversing the reaction: The sign of $\Delta H$ is reversed.
Example: If $A \rightarrow B$ has $\Delta H = +50 \text{ kJ}$, then $B \rightarrow A$ has $\Delta H = -50 \text{ kJ}$. - Multiplying the reaction: If the coefficients are multiplied by a factor $x$, the $\Delta H$ must also be multiplied by $x$. (Because Enthalpy is an extensive property).
- Adding reactions: When you add two or more chemical equations to get a net equation, you simply add their corresponding $\Delta H$ values.
3. Important Types of Enthalpy
The enthalpy change when 1 mole of a substance is completely burnt in excess oxygen.
The enthalpy change when 1 mole of a given substance is completely broken down into its constituent gaseous atoms.
For a diatomic molecule like $Cl_2(g)$, the enthalpy of atomization is exactly equal to its Bond Dissociation Enthalpy.
C. Enthalpy of Neutralization ($\Delta H_{neut}^\circ$)
The heat released when 1 gram equivalent of an acid is completely neutralized by 1 gram equivalent of a base in dilute aqueous solution.
- For a Strong Acid + Strong Base, the value is always constant: $\mathbf{-57.1 \text{ kJ/eq}}$ (or $-13.7 \text{ kcal/eq}$). This is because the net reaction is always just $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$.
- If either the acid or the base is Weak, the magnitude of heat released will be less than 57.1 kJ.
Reason: A weak acid/base is not fully ionized. Some of the released heat is consumed to completely dissociate the weak component (Enthalpy of Ionization).
4. Bond Enthalpy (Bond Energy)
The average energy required to break 1 mole of bonds of a particular type between two gaseous atoms.
Usually, $\Delta H$ = Products - Reactants. However, when calculating the Enthalpy of Reaction using Bond Energies, the formula is reversed because breaking bonds (reactants) requires energy (+), and forming bonds (products) releases energy (-).
5. Kirchhoff's Law (Temperature Dependence)
The enthalpy of a reaction ($\Delta H$) changes with temperature. Kirchhoff's Law relates this change to the difference in heat capacities ($\Delta C_p$) of products and reactants.
Where $\Delta C_p = \sum C_p(\text{Products}) - \sum C_p(\text{Reactants})$
Similarly, at constant volume: $\Delta U_{T_2} = \Delta U_{T_1} + \Delta C_v (T_2 - T_1)$.
NEET Grand Test: Thermochemistry
15 High-Order Thinking Questions testing reference states, Hess's law, and Bond Enthalpy traps.
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