Making Reactions More Efficient: Conditions, Catalysts and Sustainability

Same reaction, less waste

Industries and households run the same reactions over and over: burning fuels, decomposing limestone, neutralizing acids. Small improvements in how a reaction is run, multiplied by millions of furnaces or thousands of tonnes of product, add up to large savings in energy, money and emissions. In this lesson you will apply what you know about energy changes and rates to judge how conditions such as temperature, pressure, surface area and catalysts can make reactions more efficient (expectation D1.2), and how that connects to sustainability.

Combustion: getting more heat from each mole of fuel

Natural gas (mostly methane) heats most Ontario homes. Look at two thermochemical equations:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890.3 kJ

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) ΔH = −802.3 kJ

The difference, 88.0 kJ, is the energy released when 2 mol of water vapour condense (Hess's law). An older furnace sends hot water vapour up the chimney, wasting that energy. A condensing furnace has a second heat exchanger that cools the exhaust enough for the water to condense, capturing the extra energy. Most new natural-gas furnaces sold in Canada are condensing models, typically rated at about 90% to 98% efficiency.

Other ways to improve combustion efficiency:

• Correct air-to-fuel ratio: too little oxygen gives incomplete combustion (carbon monoxide and soot, less energy); far too much air carries heat away up the chimney.
• Surface area: power plants and cement kilns that burn solid fuels grind them to fine powder so they burn quickly and completely.
• Catalysts: a car's catalytic converter uses metals such as platinum, palladium and rhodium to convert carbon monoxide, unburned hydrocarbons and nitrogen oxides into carbon dioxide, water and nitrogen at exhaust temperatures.

Decomposition: making lime

Lime (calcium oxide) is used to make cement, steel and paper, and to treat water. It is made by heating limestone in a kiln:

CaCO₃(s) → CaO(s) + CO₂(g) ΔH = +178.3 kJ

This reaction is endothermic and very slow at room temperature, so kilns run at about 900 °C or higher. Ways to make it more efficient:

• Temperature: high enough for a fast rate; much hotter only wastes fuel.
• Surface area: crushed limestone of a suitable size decomposes faster than large blocks, but powder that is too fine can block the gas flow through the kiln.
• Removing CO₂: carrying the carbon dioxide away helps the reaction go to completion (you will study why in the Chemical Equilibrium unit).
• Heat recovery: using the hot exhaust gases to preheat the incoming limestone.

Note that the CO₂ released by the reaction itself is a large source of emissions from cement and lime plants, in addition to the CO₂ from the fuel. Researchers are developing ways to capture it.

Neutralization: restoring acidic land and water

Acid precipitation and mining emissions acidified many lakes and soils in Northern Ontario. Around Sudbury, the regreening program has spread crushed limestone over acidic, metal-contaminated soils since the late 1970s. The limestone neutralizes the acid:

CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g)

which allows grass and trees to grow again. Choosing ground limestone (large surface area) makes it react faster, and it is much safer to handle than quicklime or strong bases. In industry, acidic wastewater is neutralized before it is released, and choosing the right base, concentration and mixing rate keeps the process safe and controlled, because neutralization is exothermic.

Catalysts and sustainability

Catalysts lower activation energy, so reactions can run at lower temperatures and pressures, saving energy:

• The Haber process for ammonia (N₂ + 3H₂ ⇌ 2NH₃) uses an iron-based catalyst; the choice of temperature and pressure is a compromise you will study in the Chemical Equilibrium unit. Fertilizer made from this ammonia feeds a large share of the world's population.
• Enzymes in laundry detergents (proteases, lipases and amylases) break down protein, fat and starch stains in cold water. Heating water accounts for much of the energy used in washing clothes, so washing in cold water saves energy.
• Contact-lens cleaning systems use a small platinum-coated disc to catalyse the decomposition of hydrogen peroxide into water and oxygen, so the solution becomes safe to use on the eye within hours.

Green chemistry (a set of twelve principles published by Paul Anastas and John Warner in 1998) encourages chemists to design reactions that use catalysts, run at moderate temperatures and pressures, use safer substances and waste fewer atoms. One measure is atom economy:

atom economy = (mass of desired product ÷ total mass of all products) × 100%

Careers

Chemical engineers design and optimize industrial processes such as kilns and ammonia plants. Process technologists, often trained at Ontario colleges, monitor and adjust reaction conditions in plants. Energy advisors evaluate homes for efficiency upgrades such as condensing furnaces and heat pumps. Environmental scientists plan and monitor projects such as the Sudbury regreening.