D. Energy Changes and Rates of Reaction · Energy Changes and Rates of ReactionEnergy Technologies: Efficiency and Environmental Impact
D1.1 D2.3 D3.1 A1.9 A2.1About 70 minutes
Learning goals- I can calculate the efficiency of an energy technology.
- I can compare conventional and alternative energy technologies by efficiency and environmental impact.
- I can use enthalpies of combustion to compare the CO₂ released by different fuels per unit of energy.
- I can evaluate an energy choice, considering communities, including First Nations communities, as well as numbers.
Words to know
efficiency
The percentage of the input energy that is converted into useful output energy.
conventional energy technology
A long-established technology such as a fossil-fuel, nuclear or large hydroelectric power plant.
alternative energy technology
A newer or less common technology such as wind turbines, solar panels or fuel cells.
fuel cell
A device that converts the chemical energy of a fuel (such as hydrogen) and oxygen directly into electricity.
intermittent
Describes a source, like wind or solar, whose output varies with conditions it cannot control.
kilowatt-hour (kWh)
An energy unit used for electricity: 1 kWh = 3.6 MJ.
Where does your electricity come from?
Flip on a light in Ontario and the energy most likely came from a nuclear station on Lake Ontario or Lake Huron, or from water falling at Niagara or on a northern river. In 2025, nuclear and hydro together supplied about two thirds of Ontario's electricity, with natural gas, wind, solar and bioenergy making up the rest. Ontario stopped burning coal for electricity in 2014. Each of these technologies converts one form of energy into electricity, and each has a different efficiency and a different environmental impact. This lesson uses the chemistry of this unit to compare them (expectation D1.1).
Efficiency
Efficiency = (useful energy output ÷ total energy input) × 100%
No device is 100% efficient: some energy always ends up as heat that cannot be used. Thermal power plants (coal, natural gas, nuclear) boil water to drive a steam turbine, and the laws of thermodynamics limit how much of the heat can become electricity: typically about one third. A combined-cycle gas plant reuses the hot exhaust from a gas turbine to make steam for a second turbine, which raises its efficiency to around 50% to 60%. Hydro turbines convert the energy of falling water directly and are the most efficient. Solar panels convert sunlight to electricity directly, but only about one fifth of the light energy; a wind turbine can capture at most 59% of the wind's kinetic energy (the Betz limit), and real turbines capture somewhat less. Fuel cells convert chemical energy directly into electricity without burning, which avoids the thermal limit.
| Technology | Energy source | Main environmental impacts | Key advantages |
|---|
| Coal-fired plant | chemical (combustion of carbon) | highest CO₂ per kWh; SO₂ (acid rain), NOₓ, particulates, mercury; mining | reliable, any time of day |
| Natural gas plant | chemical (combustion of CH₄) | CO₂ (about half of coal's per kWh or less); NOₓ; methane leaks from wells and pipelines | can start quickly to meet peaks |
| Nuclear (fission) | nuclear | radioactive used fuel needing long-term storage; uranium mining; heated cooling water | no CO₂ during operation; large steady output |
| Hydroelectric | gravitational (falling water) | flooded land and changed river ecosystems; mercury released from flooded soils can build up in fish | very efficient; no fuel; long lifetime |
| Wind | kinetic energy of air | land use, effects on birds and bats, noise; materials for manufacture | no fuel or emissions during operation |
| Solar photovoltaic | light | mining and manufacturing of panels; recycling at end of life | no moving parts; no emissions during operation |
| Hydrogen fuel cell | chemical (H₂ + O₂ → H₂O) | depends on how H₂ is made: from natural gas it releases CO₂; from electrolysis it depends on the electricity source | only water at the point of use; quiet |
Using thermochemistry to compare fuels
The chemistry of this unit lets you compare fossil fuels quantitatively. Each mole of carbon burned makes one mole of CO₂, but different fuels release different amounts of energy per mole of CO₂:
- Carbon (a simple model for coal): C + O₂ → CO₂, 393.5 kJ per mol of CO₂.
- Octane (gasoline): 5470.1 kJ per 8 mol of CO₂ = 683.8 kJ per mol of CO₂.
- Methane (natural gas): 890.3 kJ per mol of CO₂.
Methane releases the most energy per mole of CO₂ because much of its energy comes from forming water (4 H atoms per C atom). That is the chemical reason natural gas is called the "cleanest" fossil fuel, though it is still a fossil fuel, and leaked unburned methane is itself a powerful greenhouse gas.
Worked exampleExample 1: CO₂ released per megajoule
Calculate the mass of CO₂ released per megajoule (1000 kJ) of heat for carbon, octane and methane. (M(CO₂) = 44.01 g/mol)
- Carbon: 1000 kJ ÷ 393.5 kJ/mol CO₂ = 2.541 mol CO₂ × 44.01 g/mol = 111.8 g.
- Octane: 1000 ÷ 683.8 = 1.462 mol CO₂ × 44.01 = 64.4 g.
- Methane: 1000 ÷ 890.3 = 1.123 mol CO₂ × 44.01 = 49.4 g.
Carbon 112 g/MJ, octane 64 g/MJ, methane 49 g/MJ. For the same heat, coal releases more than twice as much CO₂ as natural gas.
Worked exampleExample 2: CO₂ per kilowatt-hour of electricity
One kilowatt-hour (kWh) is 3.60 MJ. Estimate the CO₂ released per kWh of electricity from (a) a coal plant (35% efficient, coal modelled as carbon) and (b) a combined-cycle natural gas plant (55% efficient).
- (a) Heat needed = 3.60 MJ ÷ 0.35 = 10.29 MJ. CO₂ = 10.29 × 111.8 g = 1150 g.
- (b) Heat needed = 3.60 MJ ÷ 0.55 = 6.55 MJ. CO₂ = 6.55 × 49.4 g = 323 g.
(a) about 1.15 kg CO₂ per kWh; (b) about 0.32 kg CO₂ per kWh. Real coal contains hydrogen and other elements, and real plants vary, but the comparison shows why replacing coal with gas, and gas with non-emitting sources, cuts emissions.
Worked exampleExample 3: Solar panel efficiency
A solar panel with an area of 1.70 m² receives 1000 W/m² of sunlight at noon on a clear June day and delivers 340 W of electrical power. What is its efficiency?
- Input power = 1.70 m² × 1000 W/m² = 1700 W.
- Efficiency = 340 W ÷ 1700 W × 100% = 20.0%.
20.0%. The other 80% is reflected or becomes heat in the panel.
A hydrogen fuel cell. Hydrogen is oxidized at the anode, oxygen is reduced at the cathode, electrons flow through the external circuit (doing useful work), and the only product is water. You will study these half-reactions in the Electrochemistry unit.Worked exampleExample 4: Hydrogen for a fuel-cell vehicle
A fuel-cell car needs 180 MJ of electrical energy for a trip. Its fuel cell is 50.0% efficient (based on ΔH = −285.8 kJ per mol of H₂). What mass of hydrogen is used? (M(H₂) = 2.02 g/mol)
- Chemical energy needed = 180 MJ ÷ 0.500 = 360 MJ = 3.60 × 10⁵ kJ.
- n(H₂) = 3.60 × 10⁵ kJ ÷ 285.8 kJ/mol = 1260 mol.
- m = 1260 mol × 2.02 g/mol = 2.54 × 10³ g.
About 2.5 kg of hydrogen. Storing that much gas in a car requires tanks at very high pressure, one of the engineering challenges of hydrogen vehicles.
Energy, communities and Indigenous leadership
For decades, many remote First Nations communities in northwestern Ontario had no connection to the provincial grid and relied on diesel generators. Diesel is expensive to fly in or truck over winter roads, its combustion releases CO₂ and air pollutants, and fuel spills can damage land and water. The Wataynikaneyap Power transmission project, majority-owned (51%) by a partnership of 24 First Nations, has been connecting remote First Nations to the grid: by April 2026, 16 of the 17 communities it set out to connect were energized. Elsewhere in Canada, First Nations, Métis and Inuit communities lead or co-own many wind, solar and hydro projects. Decisions about energy technology are not only about efficiency numbers; they also involve who owns the project, who benefits, and whose land and water are affected.
Careers in energy
Power engineers operate the boilers and turbines in power plants and large buildings; chemical engineers design fuel-processing and hydrogen plants; energy auditors measure how efficiently homes and businesses use energy; environmental scientists assess the impacts of new projects. Many of these jobs start with Grade 12 chemistry and physics.
Hands-on activityYour home's natural gas, in chemistry
You need: a recent natural gas bill (ask a grown-up; many Ontario homes heat with natural gas), a calculator.
- Find the gas used in the billing period, in cubic metres (m³).
- Model natural gas as pure methane. At about 15 °C and 101.3 kPa, 1.00 m³ of gas contains n = PV ÷ RT = (101.3 kPa × 1000 L) ÷ (8.314 × 288 K) ≈ 42.3 mol (you used PV = nRT in SCH3U).
- Energy per m³ = 42.3 mol × 890.3 kJ/mol ≈ 37.7 MJ. CO₂ per m³ = 42.3 mol × 44.01 g/mol ≈ 1.86 kg.
- Calculate the energy and CO₂ for your bill's gas use. If your furnace label shows an efficiency (for example "AFUE 95%"), calculate how much of the energy actually heated the house.
What to look for: a typical Ontario home might use roughly 100 to 400 m³ in a winter month, which is about 3.8 to 15 GJ and 190 to 740 kg of CO₂. Your numbers will differ. Discuss as a family: which change would cut the most: a lower thermostat setting at night, better insulation, or a heat pump?
Just for funA wind turbine will work all day for nothing but a breeze. Unfortunately, it also takes the day off whenever the air is calm, which is why the grid needs storage and a mix of sources.
Your turn
1Define efficiency and write its formula.
The fraction of the input energy that becomes useful output: efficiency = (useful output ÷ input) × 100%.
2A gas plant burns fuel releasing 900 MJ of heat and produces 500 MJ of electricity. Find its efficiency.
500 ÷ 900 × 100% = 55.6% (a combined-cycle plant).
3Give one advantage and one environmental disadvantage of nuclear power.
Advantage: no CO₂ released during operation (or large, steady output). Disadvantage: radioactive used fuel that must be stored for a very long time (or uranium mining impacts).
4Why is methane said to release less CO₂ per unit of energy than coal? Use numbers.
Methane releases 890.3 kJ per mol of CO₂ (49 g CO₂/MJ) while carbon releases only 393.5 kJ per mol of CO₂ (112 g CO₂/MJ), because much of methane's energy comes from forming water.
5What does "intermittent" mean, and which two technologies in the table are intermittent?
Their output depends on conditions that come and go: wind and solar.
6Write the overall reaction in a hydrogen fuel cell and its ΔH.
H₂(g) + ½O₂(g) → H₂O(l) ΔH = −285.8 kJ
More practice: warm-up
1Which source supplied the largest share of Ontario's electricity in 2025?
2In what year did Ontario stop using coal to generate electricity?
3Which technology in the efficiency chart is usually the most efficient?
Hydroelectric turbine-generators.
4What is the only product of a hydrogen fuel cell?
5How many megajoules are in one kilowatt-hour?
More practice: on your own
1A coal plant is 38% efficient. How much heat (in MJ) must coal (modelled as carbon) release to produce 1.00 MWh (3600 MJ) of electricity, and how many tonnes of CO₂ are released?
Heat = 3600 ÷ 0.38 = 9474 MJ. CO₂ = 9474 × 111.8 g = 1.059 × 10⁶ g = 1.06 t (heat 9.47 × 10³ MJ).
2Hydrogen made from natural gas: overall, about 1 mol of CO₂ is released for every 4 mol of H₂ (CH₄ + 2H₂O → CO₂ + 4H₂). How much CO₂ is released per MJ of heat from burning that hydrogen (285.8 kJ/mol)? Compare with methane burned directly.
1 MJ ÷ 285.8 kJ/mol = 3.499 mol H₂ → 0.875 mol CO₂ × 44.01 = 38.5 g/MJ, compared with 49.4 g/MJ for burning methane directly. (In reality, making the hydrogen also needs energy, which adds more CO₂; only hydrogen made with clean electricity avoids most emissions.)
3Explain why a combined-cycle gas plant is more efficient than a simple steam plant.
It uses the fuel's heat twice: the hot exhaust gas from the gas turbine, which would otherwise be wasted, makes steam to drive a second turbine, so more of the fuel's energy becomes electricity.
4Evaluate this claim: "Electric cars produce zero emissions."
Partly true: they have no tailpipe emissions. But the electricity has to be generated, and the battery and car must be manufactured. In a grid like Ontario's, which is mostly nuclear and hydro, the emissions per kilometre are low; in a coal-heavy grid they can be much higher. The claim needs qualifying.
5A 3.0 MW wind turbine produces, on average, 30% of its maximum (its "capacity factor"). How much electricity does it produce in a year (8760 h), in MWh? How much CO₂ would a 55% gas plant release making the same electricity (323 g/kWh)?
Energy = 3.0 MW × 0.30 × 8760 h = 7884 MWh (about 7.9 × 10³ MWh). CO₂ avoided = 7.884 × 10⁶ kWh × 323 g = 2.55 × 10⁹ g ≈ 2.5 × 10³ t of CO₂.
Challenge
1Using the definition of efficiency and thermochemical data, explain why a car engine burning octane (efficiency about 25 to 30%) wastes more of the fuel's energy than an electric motor (about 90%) running from a hydrogen fuel cell (about 50%). Which system turns the larger fraction of its fuel energy into motion?
Engine: about 25 to 30% of the fuel's energy becomes motion. Fuel cell plus motor: 0.50 × 0.90 = 0.45, so about 45%. The fuel-cell system converts a larger fraction, because it avoids the thermal (heat-engine) limit; but the total also depends on how the hydrogen was produced and stored.
2Some people propose capturing CO₂ from power-plant exhaust and storing it underground. Capturing it uses about 20% of the plant's output. Recalculate the CO₂ per kWh delivered from a 35% coal plant (1150 g/kWh) if 90% of the CO₂ is captured.
Only 80% of the electricity is delivered, so per delivered kWh the plant makes 1150 ÷ 0.80 = 1438 g of CO₂ and releases 10% of it: about 144 g/kWh. Capture cuts emissions sharply but uses more coal per kWh delivered.
3Write a short (5 to 7 sentence) evaluation of whether a remote northern community should replace diesel generators with a grid connection or with local solar plus battery storage. Use at least one efficiency or environmental idea from this lesson.
A strong answer weighs: diesel's emissions, spill risks and high fuel transport costs; a grid connection gives reliable power from Ontario's mostly low-emission grid but needs long transmission lines through forest and wetlands; solar is intermittent (little sunlight in winter at high latitudes), so it needs batteries or backup; community ownership and decision-making matter (for example, the First Nations ownership of Wataynikaneyap Power). It should reach a justified conclusion, such as a grid connection for large communities and solar-diesel hybrids where lines are not practical.
Quiz
1. A power plant releases 1000 MJ of heat from its fuel and produces 350 MJ of electricity. Its efficiency is
2. Per megajoule of heat, which fuel releases the least CO₂?
3. Which technology produces no CO₂ during operation but creates long-lived radioactive waste?
4. Why do wind and solar power need storage or backup?
5. What is the main reason a hydrogen fuel cell can be more efficient than a gasoline engine?
6. About what fraction of Ontario's electricity came from nuclear and hydro together in 2025?
Big ideas (D1.1): energy technologies can be compared by efficiency (useful output ÷ input) and by environmental impact. Thermochemistry lets us calculate how much CO₂ different fuels release per unit of energy (natural gas less than oil, oil less than coal), and why fuel cells can beat heat engines. Real decisions also involve reliability, cost, land, and the communities affected, including First Nations communities that have relied on diesel.
How to help: Look at a natural gas or electricity bill together and do the activity calculation. Ask your teen to argue both sides of a local energy question (a wind farm, a new gas plant, a heat pump for your home) using numbers from the lesson.
Common difficulties:
- Treating efficiency as a number above 100%, or dividing input by output.
- Converting between kWh and MJ (1 kWh = 3.6 MJ).
- Thinking any technology has "no environmental impact". Every option has some; the task is to compare.
- Forgetting that hydrogen and electricity are energy carriers: their impact depends on how they are made.
A note on sources: supply-mix percentages come from the Ontario Energy Board's 2025 data; efficiency ranges are typical values and vary by plant. Encourage your teen to check and cite current sources when writing about energy, because numbers change from year to year.
Activity safety: reading a bill only.