Chemical Reactions: Safety and Environmental Issues

Chemistry with consequences

Chemical reactions can keep people safe or put them in danger. The same reaction that lets a mine recover gold can poison a river if something goes wrong. The reaction that rusts a bridge's steel happens silently for years before anyone sees a crack. The chlorine that keeps a pool safe can make swimmers cough if it is not managed well.

Expectation C1.1 asks you to analyse, on the basis of research, safety and environmental issues linked to chemical reactions and their reactants and products. In this lesson you will study three cases from your curriculum, and you will practise the research skills that make an analysis trustworthy.

Case 1: Corrosion of metal supports on bridges

The chemistry. Corrosion is the slow reaction of a metal with substances in its environment. For iron and steel, corrosion is rusting: iron reacts with oxygen and water to form rust, a hydrated iron(III) oxide. A simplified equation is:

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

Salt speeds it up. In Canadian winters, tonnes of road salt are spread on roads and bridges. Dissolved salt makes water a better conductor of electricity (it contains ions), which speeds up the reactions of rusting. Salty meltwater also seeps into cracks.

Why it is dangerous. Many bridges are made of reinforced concrete: concrete with steel bars (rebar) inside. When water, oxygen and salt reach the rebar, rust forms. Rust takes up more space than the original steel, so it pushes on the concrete from inside until pieces crack and break off. The steel also gets thinner and weaker. In Ontario, provincial and municipal bridges must be inspected every two years under the direction of a professional engineer, to find this damage early.

Solutions using chemistry:

• Coatings such as paint and epoxy keep water and oxygen away from the steel.
• Galvanizing coats steel with zinc. Zinc is more reactive than iron (higher in the activity series), so it corrodes first, even where the coating is scratched. This is called sacrificial protection.
• Stainless steel or other corrosion-resistant rebar, better concrete and good drainage.
• Using less road salt, or alternatives, which also protects streams and lakes from salt pollution.

Case 2: Cyanide in gold mining

The chemistry. Ontario has many gold mines, in places such as Timmins and Red Lake. Gold is often spread through rock in tiny amounts. To recover it, mines crush the ore and treat it with a dilute solution of sodium cyanide. With oxygen from the air, the cyanide dissolves the gold:

4Au(s) + 8NaCN(aq) + O₂(g) + 2H₂O(l) → 4Na[Au(CN)₂](aq) + 4NaOH(aq)

The gold is then recovered from the solution. (Notice that this equation is balanced: count the atoms!)

The risks. Cyanide is highly toxic to fish, wildlife and people. If cyanide solution reacts with an acid, it can release poisonous hydrogen cyanide gas, so the solutions are kept basic. Leftover solution and crushed rock (tailings) are stored in ponds held back by dams. A leak or dam failure can harm rivers and lakes for long distances downstream.

How risks are reduced. Mines use further chemical reactions to destroy cyanide, changing it into much less toxic compounds before water is released. They monitor water quality, follow provincial and federal regulations, and many follow an international code for managing cyanide. Many mines are on or near the lands of First Nations communities, who live with the effects on water, fish and wildlife. Today, these communities are involved in decisions about many projects through agreements with mining companies and governments, and many run their own environmental monitoring of the lands and waters they rely on.

Case 3: Antibacterial agents in recreational pools

The chemistry. Pools and hot tubs must be disinfected to kill bacteria and other germs. Most pools use chlorine, added as chlorine gas, sodium hypochlorite (liquid bleach for pools) or calcium hypochlorite (tablets). In the water, these form hypochlorous acid, HClO, the chemical that actually kills germs. For example:

Cl₂(g) + H₂O(l) → HClO(aq) + HCl(aq)

The pH matters. Hypochlorous acid works best when the pool's pH is kept around 7.2 to 7.8. Pool operators test the water often and add acids or bases to adjust it.

The issues. Chlorine also reacts with substances swimmers bring into the pool, such as sweat, urine and sunscreen, to form chloramines. Chloramines cause the strong "pool smell", red and itchy eyes, and coughing, especially in indoor pools. Pool chemicals are also dangerous to store: calcium hypochlorite must never be mixed with acids or other pool chemicals, because the reactions can release toxic chlorine gas or start fires.

Bromine is an alternative, used especially in hot tubs, because bromine-based disinfectants stay effective at higher temperatures. Every choice has trade-offs in cost, effectiveness, safety and comfort.

Doing research well (your inquiry skills)

To analyse an issue, you need good information, organized clearly (A1.3, A1.7, A1.9):

1. Find a range of sources: government agencies (Health Canada, Environment and Climate Change Canada, the Government of Ontario), universities, scientific organizations, industry, environmental groups and community organizations, including First Nations, Métis and Inuit organizations where an issue affects their lands and communities.
2. Check reliability: Who wrote it? What are their qualifications? Is it recent? Does it give evidence and cite sources? Do other reliable sources agree?
3. Check for bias: Does the author gain something (money, votes, support) from one answer? A mining company and an environmental group may describe the same spill very differently. Both can be useful, but you must notice whose view you are reading.
4. Record your sources with author, title, date and URL, using the citation style your teacher asks for.
5. Analyse: identify the stakeholders, the benefits and risks (to health, the environment, the economy), and possible solutions. Then make a recommendation and justify it with evidence.