Categories of Chemical Reactions
Senior 2 · Chemistry
Categories of Chemical Reactions
Lesson 2 of 4
25 min
Potato growers, including many in Rwanda's Northern Province, must protect their crops from blight, a fungal disease. One of the oldest fungicides is Bordeaux mixture, made from copper(II) sulfate and lime. Anyone preparing it is advised to use plastic containers, not iron buckets. The reason is a chemical reaction: iron pushes copper out of copper(II) sulfate, so the bucket is eaten away and the spray loses its copper.
That is a displacement reaction. In this lesson you will also meet double decomposition, in which two compounds swap partners, including the precipitation and neutralisation reactions that you will use to make and identify salts later in the year.
In a single displacement reaction, an element takes the place of another element in a compound: A + BC → AC + B. A more reactive metal displaces a less reactive metal from a solution of its salt.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
An iron nail in blue copper(II) sulfate solution becomes coated with pink-brown copper, and the blue colour fades to the pale green of iron(II) sulfate.
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
A copper wire in colourless silver nitrate solution grows grey crystals of silver, and the solution turns blue as copper(II) ions form.
If the free metal is less reactive than the metal in the compound, nothing happens. Copper placed in zinc sulfate solution stays unchanged, because copper is below zinc in the reactivity series.
The reaction of a metal with an acid is also a displacement: the metal displaces hydrogen. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Non-metals can displace each other too. A more reactive halogen displaces a less reactive one: Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq)
Using the reactivity series K, Na, Ca, Mg, Al, Zn, Fe, Pb, (H), Cu, Ag, Au, decide whether each mixture reacts. If it does, write the equation.
In displacement reactions, electrons move from one substance to another. In the iron and copper(II) sulfate reaction, each iron atom loses 2 electrons to become Fe2+, and each copper(II) ion gains 2 electrons to become a copper atom.
A reaction in which both happen is a redox reaction. All displacement and combustion reactions are redox reactions. Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
In a double decomposition (double displacement) reaction, two compounds exchange partners: AB + CD → AD + CB. No free element appears on either side. There are two important kinds.
(a) Precipitation. Two solutions of soluble salts are mixed and an insoluble solid forms. This solid is a precipitate.
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) (white precipitate)
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) (bright yellow precipitate)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq) (white precipitate)
(b) Neutralisation. An acid reacts with a base to form a salt and water.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)
Acids also react with carbonates to form a salt, water and carbon dioxide. This is how agricultural lime neutralises acid in soil:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Classify: (a) 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l); (b) KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l).
Solution. (a) An element (aluminium) takes the place of iron in its oxide: single displacement, and a redox reaction (aluminium is oxidised). It is used to weld railway lines. (b) Two compounds swap partners and water forms: double decomposition (neutralisation).
Tip
Spot the type from the equation. An element alone on the left, and a different element alone on the right? Single displacement. Two compounds on the left becoming two compounds on the right, with no free element? Double decomposition. Then look for a precipitate (s) or water to decide which kind.
Try each item before you read the solution.
Predict whether magnesium reacts with copper(II) sulfate solution. If so, write the equation and describe what you would see.
Solution: Yes; magnesium is above copper. Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s). The magnesium becomes coated with pink-brown copper and the blue solution becomes colourless.
Umutoni puts a piece of copper into iron(II) sulfate solution. Nothing happens. Explain.
Solution: Copper is less reactive than iron (it is below iron in the reactivity series), so it cannot displace iron from its salt.
Solutions of silver nitrate and potassium iodide are mixed and a yellow solid forms. Write the equation and name the type of reaction.
Solution: AgNO3(aq) + KI(aq) → AgI(s) + KNO3(aq). Silver iodide is the yellow precipitate. This is double decomposition (precipitation).
In Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s), which substance is oxidised and which is reduced?
Solution: Zinc atoms lose electrons to become Zn2+, so zinc is oxidised. Copper(II) ions gain electrons to become copper atoms, so copper(II) ions are reduced.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
