Enthalpy Change in Chemical Reactions
When a reaction happens, heat is either absorbed or released. We call this heat change at constant pressure the reaction enthalpy or \(\Delta_rH\).
Key Rules 🔥
- Exothermic reaction: Heat released → \(\Delta_rH\) is negative (e.g., burning graphite).
- Endothermic reaction: Heat absorbed → \(\Delta_rH\) is positive (e.g., melting ice).
Measuring Heat Change (Calorimetry)
We use a calorimeter (foam cup with thermometer!) to measure heat at constant pressure:
- \(\Delta H = q_p\) (heat at constant pressure).
- For a bomb calorimeter (constant volume): \[ q = -C_v \times \Delta T \] Example: Burning 1g graphite raises temperature from 298K → 299K. Calorimeter heat capacity \(C_v = 20.7\ \text{kJ/K}\): \[ q = -20.7 \times (299-298) = -20.7\ \text{kJ} \] (Negative = exothermic!).
Standard Reaction Enthalpy (\(\Delta_r H^\ominus\))
This is the heat change when all substances are in their standard states (1 bar pressure, 25°C/298K):
\[ \Delta_r H^\ominus = \sum a_i H_{\text{products}} – \sum b_i H_{\text{reactants}} \]Example for \(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\):
\[ \Delta_r H^\ominus = \left[H_m^\ominus (\text{CO}_2) + 2H_m^\ominus (\text{H}_2\text{O})\right] – \left[H_m^\ominus (\text{CH}_4) + 2H_m^\ominus (\text{O}_2)\right] \]Phase Changes & Enthalpy 🌊→💨
Phase transitions absorb/release heat at constant temperature:
| Process | Symbol | Example |
|---|---|---|
| Melting (Fusion) | \(\Delta_{\text{fus}} H^\ominus\) | \(\text{H}_2\text{O}(s) \rightarrow \text{H}_2\text{O}(l);\) \( \Delta_{\text{fus}} H^\ominus = 6.01\ \text{kJ/mol}\) |
| Vaporization | \(\Delta_{\text{vap}} H^\ominus\) | \(\text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{O}(g);\) \( \Delta_{\text{vap}} H^\ominus = 40.79\ \text{kJ/mol}\) |
| Sublimation | \(\Delta_{\text{sub}} H^\ominus\) | \(\text{CO}_2(s) \rightarrow \text{CO}_2(g);\) \( \Delta_{\text{sub}} H^\ominus = 25.2\ \text{kJ/mol}\) |
Fun fact: Water needs more heat to vaporize than acetone because of strong hydrogen bonds! 💧
Standard Enthalpy of Formation (\(\Delta_f H^\ominus\))
Heat change when 1 mole of compound is formed from its elements in their most stable states (e.g., \(O_2\) gas, \(C_{\text{graphite}}\)).
- \(\Delta_f H^\ominus\) for elements = 0 (by convention).
- Examples:
- \(\text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l);\) \(\Delta_f H^\ominus = -285.8\ \text{kJ/mol}\)
- \(\text{C (graphite)} + 2\text{H}_2(g) \rightarrow \text{CH}_4(g);\) \(\Delta_f H^\ominus = -74.81\ \text{kJ/mol}\)
Calculate \(\Delta_r H^\ominus\) using \(\Delta_f H^\ominus\):
\[ \Delta_r H^\ominus = \sum a_i \Delta_f H^\ominus (\text{products}) – \sum b_i \Delta_f H^\ominus (\text{reactants}) \]Example: Decomposing \(\text{CaCO}_3\):
\[ \Delta_r H^\ominus = \left[\Delta_f H^\ominus (\text{CaO}) + \Delta_f H^\ominus (\text{CO}_2)\right] – \left[\Delta_f H^\ominus (\text{CaCO}_3)\right] = 178.3\ \text{kJ/mol} \]Thermochemical Equations 📝
A balanced equation + \(\Delta_r H\) value. Rules:
- Coefficients = moles (not molecules).
- \(\Delta_r H^\ominus\) units = kJ/mol (per mole of reaction).
- Reverse reaction → Reverse \(\Delta_r H^\ominus\) sign!
E.g., \(\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3;\) \(\Delta H = -91.8\ \text{kJ/mol}\)
Reverse: \(2\text{NH}_3 \rightarrow \text{N}_2 + 3\text{H}_2;\) \(\Delta H = +91.8\ \text{kJ/mol}\)
Hess’s Law ⚖️
“Total heat change is path-independent!” Add up steps:
Example: Find \(\Delta_r H^\ominus\) for \(\text{C(graphite)} + \frac{1}{2}\text{O}_2 \rightarrow \text{CO}\):
- \(\text{C} + \text{O}_2 \rightarrow \text{CO}_2;\) \(\Delta H_1 = -393.5\ \text{kJ/mol}\)
- \(\text{CO} + \frac{1}{2}\text{O}_2 \rightarrow \text{CO}_2;\) \(\Delta H_2 = -283.0\ \text{kJ/mol}\)
Flip equation (2) → \(\Delta H = +283.0\ \text{kJ/mol}\), then add to (1):
\[ \Delta_r H^\ominus = -393.5 + 283.0 = \mathbf{-110.5\ \text{kJ/mol}} \]Enthalpy of Combustion (\(\Delta_c H^\ominus\)) 🔥
Heat released when 1 mole of substance burns completely in oxygen:
- \(\text{C}_4\text{H}_{10}(g) + \frac{13}{2}\text{O}_2 \rightarrow 4\text{CO}_2 + 5\text{H}_2\text{O};\) \(\Delta_c H^\ominus = -2658.0\ \text{kJ/mol}\)
- \(\text{C}_6\text{H}_{12}\text{O}_6(s) + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O};\) \(\Delta_c H^\ominus = -2802.0\ \text{kJ/mol}\)
NEET High-Yield Concepts 🎯
Memorize these!
- \(\Delta_r H\) sign: Negative (exothermic), Positive (endothermic).
- \(\Delta_f H^\ominus\): Formation of 1 mole from elements (elements = 0).
- Hess’s Law: Total \(\Delta H\) = Sum of steps (path independent).
- Phase change enthalpies: \(\Delta_{\text{fus}} H^\ominus\) (solid→liquid), \(\Delta_{\text{vap}} H^\ominus\) (liquid→gas).
- Combustion: \(\Delta_c H^\ominus\) is always negative (heat released!).
Practice calculations with formation/combustion data – they love these in exams! 💯
