Ontario switches on lights, furnaces and electric cars using three very different kinds of energy change. Natural gas releases chemical energy when its bonds rearrange during combustion. Hydroelectric dams use falling water, and steam turbines everywhere rely on the physical change of boiling water. About half of the province's electricity comes from a third kind: the splitting of uranium nuclei. Choosing among them means comparing numbers: how much energy each source releases per gram, how much carbon dioxide comes with each kilojoule, and how much of the released energy actually reaches the grid. This lesson puts physical, chemical and nuclear changes on one scale and uses your enthalpy skills to compare fuels.
Three scales of energy change
| Change | Example | Energy per mole | Energy per gram |
|---|---|---|---|
| Physical | Boiling water, | 40.7 kJ/mol absorbed | 2.26 kJ/g |
| Chemical | Burning methane, | 890.3 kJ/mol released | 55.5 kJ/g |
| Nuclear fission | Uranium-235 split by a neutron (about 200 MeV per nucleus) | about kJ/mol | about kJ/g |
| Nuclear fusion | Deuterium + tritium → helium-4 + neutron (17.6 MeV) | about kJ/mol | about kJ/g of fuel |
Where nuclear energy comes from: E = mc²
Protons and neutrons in a nucleus are held together by the strong nuclear force, and the energy needed to pull a nucleus apart is its binding energy. A nucleus has slightly less mass than its separate protons and neutrons; this mass defect is the binding energy, related by Einstein's . Medium nuclei near iron-56 are the most tightly bound per nucleon, so energy is released by splitting a heavy nucleus (fission) or joining light ones (fusion). When uranium-235 absorbs a slow neutron it splits, for example into barium-141, krypton-92 and three neutrons. The products have about 0.2 u less mass than the reactants, and that missing mass appears as about 200 MeV of kinetic energy. The released neutrons can split more nuclei, making a chain reaction. Fusion powers the Sun, but on Earth the nuclei must be held at over 100 million degrees, so no fusion power plant yet produces net electricity. In a chemical reaction mass also changes by , but the change is so tiny (far less than a billionth of the mass) that it cannot be measured, which is why the law of conservation of mass works in chemistry.
CANDU reactors in Ontario
Ontario's reactors are CANDU (CANada Deuterium Uranium) designs, developed in Canada. Most reactors elsewhere need uranium enriched in uranium-235, but CANDU burns natural uranium (only about 0.7% uranium-235), pressed into ceramic pellets and sealed in zirconium alloy tubes bundled into fuel bundles about 50 cm long. Fission neutrons are too fast to split more uranium-235 efficiently, so they are slowed by a moderator of heavy water, , in which hydrogen is the isotope deuterium. Heavy water absorbs far fewer neutrons than ordinary water, which is what lets natural uranium sustain a chain reaction. Pressurized heavy water also carries heat from the fuel channels to steam generators, where ordinary water boils and drives turbines. The horizontal fuel channels allow refuelling while the reactor runs. Ontario's three stations are Pickering (east of Toronto, operating since 1971), Bruce (on Lake Huron near Kincardine, among the largest nuclear stations in the world) and Darlington (in Clarington). Nuclear power helped Ontario close its last coal-fired plant in 2014. The trade-offs are high construction and refurbishment costs and used fuel that stays dangerously radioactive for thousands of years. Used fuel is stored on site, and in 2024 the Nuclear Waste Management Organization chose an area near Ignace in northwestern Ontario for a deep geological repository.
Comparing fuels: energy, CO₂ and efficiency
| Fuel | Heat of combustion (kJ/mol) | Energy per gram (kJ/g) | CO₂ per kJ (g/kJ) |
|---|---|---|---|
| Hydrogen, | 285.8 | 141.8 | 0 (but making from natural gas releases ) |
| Methane (natural gas), | 890.3 | 55.5 | 0.0494 |
| Octane (gasoline), | 5470 | 47.9 | 0.0644 |
| Ethanol, | 1367 | 29.7 | 0.0644 (partly offset when crops regrow) |
| Carbon (coal, approximated as graphite) | 393.5 | 32.8 | 0.1118 |
To find CO₂ per kilojoule, divide the mass of produced per mole of fuel by the heat released per mole. Methane makes 1 mol (44.01 g) of for 890.3 kJ, so g/kJ; carbon makes the same 44.01 g for only 393.5 kJ, more than twice as much per kilojoule. Released heat is not the end of the story, though. A thermal power plant, whether it burns gas or splits uranium, turns heat into electricity with a heat engine, and the second law of thermodynamics forces much of the heat to be rejected to a lake or the air. Efficiency is the useful energy out divided by the energy in. Coal and nuclear steam plants run at about 30 to 40%; a combined-cycle gas plant, which reuses its hot exhaust to raise steam, reaches about 55 to 60%.