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Wize High School Grade 11 Chemistry Textbook > Chemical Reactions

Chemical Reactions

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Formation and Decomposition Reactions

  • A formation or synthesis reaction is a reaction in which two or more reactants come together to form one larger compound. The general formula for a synthesis reaction is:
A+B→AB\boxed { \text {A} + \text {B} \to \text {AB} }
Example 1: Two elements combining to form a compound
N2(g)+3H2(g)→2NH3(g)N_2(g)+3H_2(g)\to 2NH_3(g)
Example 2: An element reacting with a compound to form a new. larger compound
2NaCℓ(aq)+3O2(g)→2NaCℓO3(aq)2NaC\ell(aq)+3O_2(g)\to 2NaC\ell O_3(aq)
Example 3: Two compounds combining to form an even more complex compound

CaO(s)+H2O(ℓ)→Ca(OH)2(aq)CaO(s)+H_2O(\ell)\to Ca(OH)_2(aq)
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  • A decomposition reaction is one in which a complex compound breaks down into simpler compounds. The general formula for a decomposition reaction is:
AB→A+B\boxed {\text{AB} \to \text{A+B}}
Example 1: Binary compounds decomposing into their elements
2H2O(ℓ)→2H2(g)+O2(g)2H_2O(\ell)\to2H_2(g)+O_2(g)
Example 2: A compound decomposing into a simpler compound and an element
2KCℓO3(s)→2KCℓ(s)+3O2(g)2KC\ell O_3(s)\to2KC\ell(s)+3O_2(g)
Example 3: A compound decomposing into simpler compounds

NH4Cℓ(s)→NH3(g)+HCl(aq)NH_4C\ell(s) \to NH_3(g)+HCl(aq)
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Replacement Reactions

  • In order for a displacement reaction to occur, we need to look at the activity series of metals
  • A more reactive element will displace a less reactive element in a displacement reaction

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  • In single replacement reactions, an element replaces a similar element in another reactant. The general formula for a single replacement reaction is:
AB+C→AC+B\boxed {\text{AB} + \text{C}\to \text{AC+B}}
Example 1: Hydrogen can be replaced by more reactive metals
2Na(s)+2H2O(ℓ)→H2(g)+2NaOH(aq)2Na(s)+2H_2O(\ell)\to H_2(g)+2NaOH(aq)
Example 2: Metals can replace other metals
Mg(s)+Cu(NO3)2(aq)→Mg(NO3)2(aq)+Cu(s)Mg(s)+Cu(NO_3)_2(aq)\to Mg(NO_3)_2(aq)+Cu(s)

Example 3: Halogens will replace other halogens. Activity increases from the bottom to the top in the group.
2NaBr(aq)+Cℓ2(g)→2NaCℓ(aq)+Br2(ℓ)2NaBr(aq)+C\ell _2(g)\to 2NaC\ell(aq)+Br_2(\ell)
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  • In double replacement reactions, the positive or negative ions of two ionic compounds switch places to form new compounds. The general formula for a double replacement reaction is:
AB+CD→AD+CB\boxed {\text{AB} + \text{CD}\to \text{AD+CB}}
Example 1: A double replacement reaction can be used to precipitate a compound
2HCℓ(aq)+Pb(NO3)2(sq)→2HNO3(aq)+PbCℓ2(s)2HC\ell(aq)+Pb(NO_3)_2(sq)\to 2HNO_3(aq)+PbC\ell_2(s)
Example 2: A double replacement reaction can be used to release a gas
FeS(s)+2HCℓ(aq)→H2S(g)+FeCℓ2(aq)FeS(s)+2HC\ell(aq)\to H_2S(g)+FeC\ell_2(aq)

Example 3: Neutralization reactions between an acid and a base are double replacement reactions
NaOH(aq)+HCℓ(aq)→H2O(ℓ)+NaCℓ(aq)NaOH(aq)+HC\ell(aq)\to H_2O(\ell)+NaC\ell(aq)

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Example: Using the Activity Series

Using the activity series, predict the products for each reaction. If there is no reaction, write NR (no reaction). If a reaction occurs, complete the equation and balance it.

  1. HCℓ(aq)+Mg(s)→HC\ell(aq)+Mg(s)\to
  2. BaCℓ2(aq)+Pb(s)→BaC\ell_2(aq)+Pb(s)\to
  3. NaCℓ(aq)+F2(g)→NaC\ell(aq)+F_2(g)\to
  4. PbCℓ2(aq)+Br2(ℓ)→PbC\ell_2(aq)+Br_2(\ell)\to

  1. 2HCℓ(aq)+Mg(s)→MgCℓ2(aq)+H2(g)2HC\ell(aq)+Mg(s)\to MgC\ell_2(aq)+H_2(g)
  2. BaCℓ2(aq)+Pb(s)→NRBaC\ell_2(aq)+Pb(s)\to NR
  3. 2NaCℓ(aq)+F2(g)→2NaF(aq)+Cℓ2(g)2NaC\ell(aq)+F_2(g)\to 2NaF(aq)+C\ell_2(g)
  4. PbCℓ2(aq)+Br2(ℓ)→NRPbC\ell_2(aq)+Br_2(\ell)\to NR
Classify the following reactions as synthesis (S), decomposition (D), single-replacement (SR) or double-replacement (DR)
  1. Al2(SO4)3+3BaCl2→3BaSO4+2AℓCℓ3Al_2(SO_4)_3+3BaCl_2\to3BaSO_4+2A\ell C\ell_3
  2. Al2S3→2Al+3SAl_2S_3\to2Al+3S
  3. 2NaOH+CuSO4→Na2SO4+Cu(OH)22NaOH+CuSO_4\to Na_2SO_4+Cu(OH)_2
  4. Fe+H2SO4→FeSO4+H2Fe+H_2SO_4\to FeSO_4+H_2

When a solution sodium bicarbonate is heated, water vapour, carbon dioxide gas and a precipitate are formed. What type of reaction is this?

Practice: Using the Activity Series

Based on the activity series of metals, which of the following reactions will not occur? Select all that apply.