C. Chemical Reactions · Chemical ReactionsChemical Reactions That Solve Problems
C1 C3 A1.9 A1.10 A1.11 A2.1 A2.2About 75 minutes
Learning goals- I can write and classify the reactions used in liming, scrubbers, catalytic converters, antacids and water treatment.
- I can assess how effective a reaction-based solution is, using criteria such as cost, side effects and whether it treats the cause.
- I can communicate a reasoned judgment about a chemical solution to a social or environmental problem.
- I can describe the contributions of scientists, including Canadians, to our understanding and use of chemical reactions.
Words to know
liming
Adding limestone (calcium carbonate) or lime to acidic lakes or soils to neutralize acid.
scrubber
Equipment that removes pollutants such as sulfur dioxide from smokestack gases, often by reacting them with limestone or lime.
catalytic converter
A device in a vehicle exhaust system that uses metal catalysts to convert CO, nitrogen oxides and unburned fuel into less harmful gases.
antacid
A base, such as calcium carbonate or magnesium hydroxide, that neutralizes excess stomach acid.
flocculation
Using a chemical (such as aluminium sulfate) to form a fluffy precipitate that traps tiny particles in water so they settle out.
criteria
Standards used to judge something, such as effectiveness, cost, safety and side effects.
Fighting chemistry with chemistry
In the last lesson you saw reactions that cause problems: acid rain from smelting, carbon monoxide from combustion, carbon dioxide from cement. The good news is that chemical reactions are also our main tools for solving problems like these. A farmer spreading lime on a field, a catalytic converter under a car, and a tablet for heartburn all use reactions from this unit.
Your curriculum asks you to assess the effectiveness of these solutions, not just describe them. That means asking: How well does it work? What does it cost? Does it create new problems? Does it fix the cause, or only the symptoms?
Neutralizing acid: liming lakes and soils
Acidic lakes and soils can be treated with limestone (calcium carbonate) or lime (calcium oxide or hydroxide). For example, with sulfuric acid from acid rain:
- CaCO₃(s) + H₂SO₄(aq) → CaSO₄(s) + H₂O(l) + CO₂(g) (double displacement, gas-forming)
- Ca(OH)₂(s) + H₂SO₄(aq) → CaSO₄(s) + 2H₂O(l) (neutralization)
Farmers in many parts of Canada lime acidic fields so crops grow well, and Sudbury's regreening program limed acidic soils before planting trees.
Effectiveness: liming works quickly and raises pH. But it treats the symptom: if acid keeps falling, liming must be repeated. It costs money and fuel to spread, and it is impractical for thousands of remote lakes. The lasting solution is to reduce emissions at the source.
Stopping sulfur dioxide at the source: scrubbers and acid plants
A scrubber sprays a slurry of limestone (or lime) and water through smokestack gas. Sulfur dioxide reacts and is trapped as a solid:
- CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)
- CaO(s) + SO₂(g) → CaSO₃(s) (synthesis: metal oxide + non-metal oxide → salt)
Often air is blown through the slurry to convert the calcium sulfite into calcium sulfate (gypsum), 2CaSO₃(s) + O₂(g) → 2CaSO₄(s), which can be used to make drywall. Smelters, as in Sudbury, can instead convert SO₂ into sulfuric acid to sell.
Effectiveness: scrubbers remove most of the sulfur dioxide and attack the cause of acid rain. But they are expensive to build and run, they need mined limestone, the limestone reaction releases some CO₂, and waste solids must be used or stored.
Cleaning car exhaust: the catalytic converter
Engine exhaust contains carbon monoxide, nitrogen oxides (formed when nitrogen and oxygen from air react in the hot engine, N₂ + O₂ → 2NO) and unburned fuel. A catalytic converter passes the exhaust over a honeycomb coated with tiny amounts of platinum, palladium and rhodium, which act as catalysts:
- 2CO(g) + O₂(g) → 2CO₂(g) (synthesis/combustion)
- 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)
- unburned fuel completes its combustion, for example 2C₈H₁₈(g) + 25O₂(g) → 16CO₂(g) + 18H₂O(g)
Effectiveness: converters greatly reduce carbon monoxide, nitrogen oxides and unburned fuel, and have made city air much cleaner. But they only work well once hot (so short trips pollute more), they do not reduce carbon dioxide (they turn CO into CO₂), and the precious metals must be mined. Electric vehicles avoid tailpipe emissions entirely, though the electricity and batteries have their own impacts.
Reactions in health and everyday life
- Antacids neutralize excess stomach acid (hydrochloric acid): CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g); Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l); Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l). They relieve symptoms quickly, but do not treat the cause of frequent heartburn, which is why packages advise talking to a pharmacist or doctor if symptoms continue.
- Drinking water treatment: aluminium sulfate and a base form a precipitate that traps particles (flocculation): Al₂(SO₄)₃(aq) + 3Ca(OH)₂(aq) → 2Al(OH)₃(s) + 3CaSO₄(s). Chlorine kills microbes: Cl₂(g) + H₂O(l) → HCl(aq) + HOCl(aq) (hypochlorous acid is the disinfectant). Very effective at preventing waterborne disease; the trade-off is that chlorine can form small amounts of by-products, which are monitored and limited by regulations.
- Hydrogen fuel cells in some buses and trucks combine hydrogen and oxygen: 2H₂(g) + O₂(g) → 2H₂O(l), producing electricity with only water at the tailpipe. Their overall benefit depends on how the hydrogen is made.
| Solution | Treats cause or symptom? | Main strengths | Main limitations |
|---|
| Liming lakes and soils | symptom | fast, simple, raises pH | must be repeated; costly for many lakes |
| Scrubbers / acid plants | cause | remove most SO₂; may make useful products | expensive; need limestone; some CO₂; waste solids |
| Catalytic converters | cause (for CO, NO) | big cut in CO, NO and unburned fuel | no cut in CO₂; need heat; mined metals |
| Antacids | symptom | fast relief | do not treat the underlying condition |
Scientists behind the reactions
- Antoine Lavoisier (France, late 1700s) showed that combustion is a reaction with oxygen and stated the law of conservation of mass.
- Fritz Haber and Carl Bosch (Germany, early 1900s) developed the industrial synthesis of ammonia that makes most of the world's nitrogen fertilizer.
- Thomas "Carbide" Willson, an Ontario-born inventor, found a practical way to make calcium carbide in 1892. Calcium carbide reacts with water to make acetylene gas, CaC₂(s) + 2H₂O(l) → C₂H₂(g) + Ca(OH)₂(aq), which was used for lighting and is still used for welding.
- John Polanyi of the University of Toronto shared the 1986 Nobel Prize in Chemistry for research on how chemical reactions happen at the level of individual molecules, measuring the faint infrared light given off by newly formed molecules.
Careers that use this lesson's chemistry include environmental engineer, water treatment operator, pharmacist, agronomist (soil and crop scientist) and automotive emissions technician.
Worked exampleAssessing a solution
A town proposes adding crushed limestone to a small acidic lake every year to protect a fish population. Assess the proposal.
- Chemistry: limestone neutralizes acid: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). It is a gas-forming reaction, and it raises the pH.
- Strengths: proven method, quick effect, limestone is cheap and not toxic, and fish may recover.
- Limitations: treats the symptom; must be repeated yearly; transport costs and fuel; too much at once can change the lake's chemistry suddenly.
- Better long-term: reduce acid-causing emissions (scrubbers, cleaner energy). Liming can protect the lake in the meantime.
Judgment: a reasonable short-term measure, effective for this one lake, but it should be paired with action on emissions to fix the cause.
Worked exampleChoosing the better antacid
Antacid A contains calcium carbonate; antacid B contains magnesium hydroxide. Both neutralize HCl. Write both equations and give one chemical reason a person might prefer B.
- A: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
- B: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)
- A releases carbon dioxide gas, which can cause bloating or burping. B is a neutralization that releases no gas.
Someone bothered by gas might prefer B. (Choices about medicine should be made with a pharmacist or doctor; this is just the chemistry.)
Hands-on activityModel and assess liming an "acid lake"
You need: red cabbage indicator (see the neutralization lesson), white vinegar, water, three clear jars, crushed clean eggshells (rinsed and dried; mostly calcium carbonate) or a stick of blackboard chalk crushed in a bag (check the label says calcium carbonate), a calcium carbonate antacid tablet (optional), a spoon, and a timer.
Safety: everything is food-safe, but do not drink the mixtures. Crush eggshells and chalk inside a sealed bag to avoid dust.
- In each jar, mix 150 mL water, 50 mL indicator and 2 teaspoons vinegar. All three "lakes" turn pink (acidic).
- Jar 1 is the control. Add 1 teaspoon of crushed eggshell to jar 2 and 1 teaspoon of crushed chalk (or a crushed antacid tablet) to jar 3. Stir once.
- Observe every 10 minutes for an hour. Note bubbles and colour changes.
- Acid rain returns: add 1 more teaspoon of vinegar to jar 2. What happens? Does the leftover eggshell keep working?
What to expect: jars 2 and 3 fizz (CO₂) and slowly move from pink toward purple as the acid is neutralized; finer powders work faster. Extra vinegar turns the water pink again until the remaining solid reacts.
Assess: using the four criteria in the table (cause or symptom, strengths, limitations, cost), write a short paragraph on whether liming is a good solution for acidified lakes.
Just for funCatalytic converters contain platinum, palladium and rhodium, metals also used in fine jewellery. So technically, most gasoline cars are wearing expensive jewellery underneath, right next to the muffler.
Your turn
1Write the reaction for limestone neutralizing sulfuric acid in a lake.
CaCO₃(s) + H₂SO₄(aq) → CaSO₄(s) + H₂O(l) + CO₂(g)
2Write the reaction in a lime (CaO) scrubber and classify it.
CaO(s) + SO₂(g) → CaSO₃(s): synthesis (metal oxide + non-metal oxide → salt).
3Write a reaction that happens in a catalytic converter.
2CO(g) + O₂(g) → 2CO₂(g) or 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g).
4Write the reaction of an aluminium hydroxide antacid with stomach acid.
Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l)
5Why is liming called a treatment of the symptom rather than the cause?
It neutralizes acid already in the lake but does not stop acid from being produced and falling; the cause is the emissions.
6Name one Canadian scientist and their contribution to understanding or using reactions.
John Polanyi (University of Toronto) shared the 1986 Nobel Prize in Chemistry for studying how reactions happen molecule by molecule; or Thomas Willson, who developed a practical process for making calcium carbide (used to make acetylene).
More practice: warm-up
1Which compound is the main ingredient in limestone and many antacids?
Calcium carbonate, CaCO₃.
2Name the three metals in a catalytic converter.
Platinum, palladium and rhodium.
3What gas forms when calcium carbonate neutralizes an acid?
4What useful product can scrubber waste be turned into?
Gypsum (calcium sulfate) for drywall.
5Which disinfectant is commonly added to drinking water?
Chlorine (or a chlorine compound).
More practice: on your own
1Explain why catalytic converters do not help with climate change, using an equation.
They convert CO to CO₂: 2CO + O₂ → 2CO₂. Carbon dioxide is a greenhouse gas, so the total CO₂ is not reduced (it may slightly increase).
2Write the two-step process by which a limestone scrubber can produce gypsum.
CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g), then 2CaSO₃(s) + O₂(g) → 2CaSO₄(s).
3Write the reaction that forms the floc in water treatment and classify it.
Al₂(SO₄)₃(aq) + 3Ca(OH)₂(aq) → 2Al(OH)₃(s) + 3CaSO₄(s): double displacement (precipitation).
4Give two criteria you would use to assess any reaction-based solution, and explain why each matters.
For example: whether it treats the cause (otherwise the problem returns); cost (it must be affordable to use widely); side effects or new wastes (it should not create a new problem); safety of the chemicals used.
5Calcium carbide reacts with water to form acetylene and calcium hydroxide. Write the balanced equation and classify it.
CaC₂(s) + 2H₂O(l) → C₂H₂(g) + Ca(OH)₂(aq). It is a double displacement pattern (calcium and hydrogen exchange partners: Ca²⁺ goes with hydroxide, and hydrogen goes with the carbide ion to form acetylene).
Challenge
1A coal-burning plant can either install a scrubber or switch to natural gas. Using chemistry, compare the two choices for sulfur dioxide and carbon dioxide emissions.
Scrubber: removes most SO₂ (CaCO₃ + SO₂ → CaSO₃ + CO₂), but the plant still burns coal, which releases a lot of CO₂, and the scrubber reaction adds a little more. Natural gas (mostly methane) contains very little sulfur, so SO₂ is greatly reduced, and burning methane (CH₄ + 2O₂ → CO₂ + 2H₂O) releases less CO₂ per unit of energy than coal because much of the energy comes from forming water. Switching fuel reduces both pollutants; the scrubber reduces mainly SO₂. (Methane leaks are a separate climate concern.)
2Write the overall equation for the catalytic converter removing CO and NO together. Explain why the same device is less effective during the first few minutes of a winter drive.
2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). The catalyst needs to be hot (several hundred degrees Celsius) to work well. On a cold winter start it takes longer to warm up, so more CO and unburned fuel leave the tailpipe in the first minutes.
3Write a short, reasoned recommendation (4 to 6 sentences) on whether a small northern community with acidic lakes should rely on liming. Include at least one equation and one alternative.
Sample: Liming uses the reaction CaCO₃(s) + H₂SO₄(aq) → CaSO₄(s) + H₂O(l) + CO₂(g) to neutralize acid, and it can quickly raise the pH of a lake and protect fish. However, it treats the symptom: acid keeps arriving with precipitation, so liming must be repeated, and transporting limestone to remote lakes is costly. It makes sense for a few important lakes, such as those used for fishing or drinking water, as a short-term measure. The long-term solution is to reduce sulfur and nitrogen oxide emissions upwind, through scrubbers, acid plants and cleaner energy. The community should be involved in choosing which lakes to treat and in monitoring the results.
Quiz
1. Which reaction occurs when limestone neutralizes acid in a lake?
2. A scrubber mainly removes which pollutant from smokestack gas?
3. Which statement about catalytic converters is TRUE?
4. Which solution treats the CAUSE of acid rain rather than a symptom?
5. Magnesium hydroxide antacid reacts with stomach acid to give:
6. John Polanyi of the University of Toronto is known for:
This lesson asks your teen to make and defend judgments, a skill used in every senior science course.
How to help: pick one solution from the lesson (for example catalytic converters) and ask your teen to argue for it, then against it. A strong answer includes the chemical equation, at least two criteria and a clear conclusion.
Common difficulties:
- Describing a solution without assessing it. Prompt: "How well does it work, and what are the downsides?"
- Thinking any "chemical" solution is bad, or that any solution is perfect. Most real solutions involve trade-offs.
Activity safety: the materials are food-safe, but the mixtures should not be eaten or drunk. Crush eggshells or chalk in a sealed bag to avoid breathing dust. Questions about antacid use for a real person belong with a pharmacist or doctor.