A. Synthesis or composition reaction:
• In this type of reaction, two or more elements or compounds may mingle to form a more complex compound.
The basic formula for this type of reaction is : A + X → AX
Some instances of synthesis reactions are:
- Metal + oxygen → metal oxide
EX. 2Mg(s) + O2 (g) → 2MgO(s)
- Non-metal + oxygen → nonmetallic oxide
EX. C(s) + O2(g) → CO2(g)
- Metal oxide + water → metallic hydroxide
EX. MgO(s) + H2O(l) → Mg(OH)2(s)
- Non-metallic oxide + water → acid
EX. CO2(g) + H2O(l) → ; H2CO3(aq)
- Metal + non-metal → salt
EX. 2 Na(s) + Cl2(g) → 2NaCl(s)
- A few nonmetals combine with each other.
EX. 2P(s) + 3Cl2(g) → 2PCl3(g)
You ought to know these two reactions and try to remember them:
N2(g) + 3H2(g) → 2NH2(g)
- NH3(g) + H2O(l) → NH4OH(aq)
B. Decomposition reaction:
• In a decomposition reaction, one compound breaks down into its component parts or simpler compounds.
Basic equation formula for this type of reaction is AX → A + X
Some instances of decomposition reactions are
- Metallic carbonates, when heated, form metallic oxides and CO2(g).
EX. CaCO3(s) → CaO(s) + CO2(g)
- The majority metallic hydroxides, when heated, decompose to form metallic oxides and water.
EX. Ca(OH)2(s) → CaO(s) + H2O(g)
- Metallic chlorates, when heated, decompose to form metallic chlorides and oxygen.
EX. 2KClO3(s) → 2KCl(s) + 3O2(g)
- Some acids, when heated, decompose to form non metallic oxides and water.
EX. H2SO4 → H2O (l) + SO3(g)
C. Replacement reaction:
• A more reactive element takes the place of another element in a compound and frees the less active one.
• Basic form: A + BX → AX + B or AX + Y → AY + X
Examples of replacement reactions
- Replacement of a metal in a compound by a more active metal.
EX. Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
- Replacement of hydrogen in water by an active metal.
EX. 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
EX. Mg(s) + H2O(g) → MgO(s) + H2(g)
- Replacement of hydrogen in acids by active metals.
EX. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
- Replacement of nonmetals by more active nonmetals.
EX. Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(l)
• This takes place among ions in aqueous solution. The reaction will take place when one pair of ions approach together to create at least one among the following:
- a precipitate
Water or a number of other non-ionized substances.
Basic form of the equation: AX + BY → AY + BX
Some examples of ionic reactions:
- Formation of precipitate.
EX. NaCl (aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
EX. BaCl2(aq) + Na2SO4(aq) → 2NaCl(aq) + BaSO4(s)
- Formation of a gas.
EX. HCl(aq) + FeS(s) → FeCl2(aq) + H2S(g)
- Formation of water. When a reaction takes place between an acid and a base, the reaction is known as a neutralization reaction.)
EX. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)