Electrochemical and chemical energy
Chemical energy powers our devices and our bodies. This lesson connects thermodynamics to batteries, fuels, and energy choices.
Fuels and energy density
Fuels store chemical energy in bonds. Gasoline releases about 46 MJ per kilogram when burned, hydrogen about 142 MJ/kg, and a lithium-ion battery stores only around 0.5-0.9 MJ/kg. High energy density is why aircraft still use liquid fuels, while batteries suit cars that can recharge often.
Free energy and cell voltage
For an electrochemical cell, \(\Delta G = -nFE\), where \(n\) is moles of electrons transferred, \(F\) is Faraday's constant (96,485 C/mol), and \(E\) is the cell voltage. A positive voltage means negative \(\Delta G\), a spontaneous reaction that can do electrical work.
\Delta G = -nFEBatteries
Batteries are galvanic cells packaged for use. Alkaline batteries use zinc and manganese dioxide. Lithium-ion batteries shuttle lithium ions between a graphite anode and a metal-oxide cathode; recharging drives the reaction backward using external electricity. They power phones and electric cars.
Fuel cells
A hydrogen fuel cell combines hydrogen and oxygen to make water, producing electricity directly, with \(\Delta G^\circ \approx -237\) kJ per mole of water. It is more efficient than burning hydrogen in an engine because it avoids converting heat to work, which the second law limits.
Heat engines and efficiency limits
Engines that convert heat to work, like car engines and power stations, are limited by the second law: their maximum efficiency is \(1 - \frac{T_{\text{cold}}}{T_{\text{hot}}}\) (in kelvin). A power plant running between 800 K and 300 K can be at most 62.5% efficient; real plants achieve less.
Food as fuel
Food energy comes from oxidising carbohydrates, fats, and proteins. Fats provide about 9 kcal (38 kJ) per gram, carbohydrates and proteins about 4 kcal (17 kJ). A food 'Calorie' is a kilocalorie, 4.184 kJ. The body releases this energy gradually through many enzyme-controlled redox steps.
Energy and the environment
Burning fossil fuels releases \(\text{CO}_2\), driving climate change. Chemistry contributes solutions: better batteries, catalysts for fuel cells, solar cells, and carbon capture. Comparing energy options requires considering energy density, efficiency, cost, and emissions together.
Worked example
A cell with \(E = 1.10\) V transfers 2 mol electrons per mole of reaction. \(\Delta G = -2(96{,}485)(1.10) \approx -212\) kJ. That is the maximum electrical work available per mole of zinc consumed in a Daniell cell.
Key takeaways
Fuels differ in energy density, which shapes their uses. \(\Delta G = -nFE\) links cell voltage to free energy. Batteries and fuel cells convert chemical energy directly to electricity, avoiding heat-engine limits of \(1 - T_c/T_h\). Food energy is chemical energy released by controlled oxidation.
Flashcard
Energy stored per kilogram
Check your understanding
Why can a hydrogen fuel cell be more efficient than burning hydrogen in an engine?