Words to know
double displacement reaction
A reaction in which two ionic compounds exchange ions: AB + CD → AD + CB.
soluble / insoluble
Able / not able to dissolve to a significant extent in water.
combustion
A rapid reaction of a substance with oxygen that releases heat and light.
hydrocarbon
A compound made of only hydrogen and carbon, such as methane or propane.
complete combustion
Burning with plenty of oxygen; a hydrocarbon produces only carbon dioxide and water.
incomplete combustion
Burning with limited oxygen; produces carbon monoxide and/or soot as well.
carbon monoxide
CO, a colourless, odourless, toxic gas produced by incomplete combustion.
Swapping partners and burning fuels
In this lesson you meet two more kinds of reaction. In a double displacement reaction, two compounds swap partners, often producing a solid that suddenly appears in a clear liquid. In a combustion reaction, a fuel burns in oxygen. Combustion heats most Canadian homes in winter, moves our cars and buses, and produces the carbon dioxide you will study in the Climate Change unit. Your curriculum lists "reactions occurring when fossil fuels burn" as an example to describe (C3.5).
Double displacement: AB + CD → AD + CB
In a double displacement reaction, two ionic compounds, usually dissolved in water, exchange ions. The positive ion of each compound pairs up with the negative ion of the other.
The reaction only "counts" (something actually changes) when one of the products is:
- a precipitate (an insoluble solid),
- a gas, or
- water (this is neutralization, the topic of a later lesson).
Example: silver nitrate solution + sodium chloride solution:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Silver ions and chloride ions meet and form silver chloride, which does not dissolve, so a white precipitate appears. The sodium and nitrate ions stay dissolved.
Other examples:
- Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq): a striking yellow precipitate of lead(II) iodide.
- CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq): a white precipitate of calcium carbonate (chalk).
Combustion: burning fuels
A combustion reaction is a fast reaction of a substance with oxygen that releases energy as heat and light. Many fuels are hydrocarbons: compounds of only hydrogen and carbon, such as methane (CH₄, natural gas), propane (C₃H₈, barbecues) and octane (C₈H₁₈, in gasoline). Fossil fuels are mostly hydrocarbons.
Complete combustion happens when there is plenty of oxygen:
hydrocarbon + oxygen → carbon dioxide + water (+ energy)
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
The flame is blue and very hot, as in a well-adjusted gas stove.
Incomplete combustion happens when oxygen is limited. Some of the carbon ends up as carbon monoxide (CO) or solid carbon (soot):
2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)
The flame is yellow and smoky. Incomplete combustion wastes fuel and is dangerous: carbon monoxide is a colourless, odourless, toxic gas. This is why furnaces and fuel-burning appliances must be maintained and vented properly, why you never run a car or a generator in a closed garage, and why Ontario homes with fuel-burning appliances, fireplaces or attached garages are required to have carbon monoxide alarms.
Hands-on activityTwo reactions in your kitchen
Part A: make a precipitate. You need: eye protection, dish gloves, Epsom salts (magnesium sulfate, from a pharmacy or grocery store), washing soda (sodium carbonate, sold in the laundry aisle; this is not baking soda), 2 clear glasses, warm water, a spoon.
Safety: washing soda is an irritant to skin and eyes: wear gloves and eye protection, and keep it away from young children and pets. Do not taste anything.
- Dissolve 1 tablespoon of Epsom salts in 1/2 cup of warm water in one glass. Dissolve 1 tablespoon of washing soda in 1/2 cup of warm water in the other. Both should be clear.
- Pour one solution into the other. Observe for 5 minutes, then let it settle.
- Write the word equation and balanced equation. (Hint: magnesium carbonate is insoluble.)
Part B: products of combustion (grown-up only). You need: a grown-up, a candle in a stable holder on a heat-proof plate, a lighter, a clean dry drinking glass, oven mitts.
- The grown-up lights the candle.
- Wearing an oven mitt, the grown-up holds the dry glass upside down about 10 cm above the flame for 3 to 5 seconds only, then removes it.
- Look at the inside of the glass. What do you see?
- Blow out the candle and let everything cool.
What to look for: in Part A a cloudy white precipitate forms immediately: MgSO₄(aq) + Na₂CO₃(aq) → MgCO₃(s) + Na₂SO₄(aq). In Part B, a mist of water droplets briefly appears inside the cool glass: water is a product of the combustion of candle wax (a hydrocarbon). If the glass is held too low in the flame, a black smudge of soot appears, which shows incomplete combustion. Clean-up: pour Part A down the sink with lots of water; wipe the glass.