Biofuels and stored chemical energy
A diagram of biofuel energy represents how chemical potential energy changes as a biofuel reacts with oxygen to form carbon dioxide and water. Thermochemistry expresses this change primarily through the enthalpy change \(\Delta H\) and the activation energy barrier \(E_a\).
Biofuel combustion is exothermic under typical conditions: products are at lower enthalpy than reactants, so \(\Delta H < 0\), and heat is released.
Representative biofuel combustion
Ethanol is a common reference biofuel. Its complete combustion is:
\[ \mathrm{C_2H_5OH(l) + 3\,O_2(g) \rightarrow 2\,CO_2(g) + 3\,H_2O(l)} \]
Diagram of biofuel energy along the reaction coordinate
Thermochemical interpretation
Combustion breaks bonds in the biofuel and oxygen and forms strong bonds in \(\mathrm{CO_2}\) and \(\mathrm{H_2O}\). The product bond formation releases more energy than the bond breaking requires, leaving products at lower enthalpy than reactants. An activation barrier remains because bond breaking and electron-density reorganization require an initial energy input.
Connection to measured heat and sign conventions
Under constant external pressure (common laboratory conditions), the heat exchanged equals the enthalpy change: \[ q_p = \Delta H \]
For an exothermic combustion, \(\Delta H\) is negative and heat flows from the system to the surroundings. Calorimetry records this as a temperature increase of the surroundings (or calorimeter), consistent with energy conservation.
Compact mapping of diagram features to quantities
| Diagram feature | Thermochemistry quantity | Meaning for biofuel combustion |
|---|---|---|
| Vertical gap (reactants → products) | \(\Delta H\) | Net heat released at constant pressure; negative for exothermic combustion |
| Peak height above reactants | \(E_a\) | Barrier that controls ignition and reaction rate; lowered by catalysts but not removed |
| Overall downward profile | Exothermic reaction | Products are thermodynamically more stable (lower enthalpy) than reactants |
Common pitfalls
- Sign convention errors for combustion, with \(\Delta H\) incorrectly treated as positive despite heat release.
- Activation energy and enthalpy conflation, with the barrier \(E_a\) mistaken for the net energy change \(\Delta H\).
- Energy “stored in biofuel” interpreted as a single bond energy, despite combustion involving many bond-breaking and bond-forming contributions.
- Reaction-coordinate diagrams interpreted as time plots, despite the horizontal axis representing progress along a pathway rather than elapsed time.