Unit 3: Types of Chemical Reactions SCH3U/4C Ms. Richardson

Chemical Reactions A reaction occurs when one or more substance changes in chemical composition to form new substance(s). We can predict the type of reaction that occurs by looking at the reactants and products.

Synthesis Reactions Two or more reactants combine to form one product. General formula: A + B  C A + B  C

Synthesis Reactions Synthesis reactions can occur in different ways: Two or more reactants combine to form a binary compound S(g) + O2(g)  SO2(g) An element and a compound combine to form a new compound 2 SO2(g) + O2(g)  2 SO3(g) Two compounds combine to form a new compound SO3(g) + H2O(l)  H2SO4(aq)

Decomposition Reactions One compound breaks down into two or more products. General formula: C  A + B

Decomposition Reactions Decomposition reactions can occur in different ways: A binary compound decomposes into its elements 2 H2O(l)  2 H2(g) + O2(g) A metal nitrate decomposes into oxygen and a metal nitrite 2 KNO3(s)  2 KNO2(s) + O2(g)

Decomposition Reactions A metal carbonate decomposes into a metal oxide and carbon dioxide CaCO3 (l)  CaO(s) + CO2(g) A metal hydroxide decomposes into a metal oxide and water Ca(OH)2(s)  CaO (s) + H2O (g)

Single Displacement Reactions One element takes the place of another General formula: AX + B  BX + A

Single Displacement Reactions There are three main types of examples: A metal displaces another metal in an ionic compound Cu(s) + 2 AgNO3 (aq)  Cu(NO3)2(aq) + Ag(s) A non-metal displaces another non-metal in an ionic compound Cl2(g) + 2 NaBr (aq)  Br2(l) + 2 NaCl(aq)

Single Displacement Reactions A metal displaces hydrogen from an acid or water Mg(s) + 2 HCl(aq)  H2(g) + MgCl2(aq) 2 Na(s) + H2O(l)  H2(g) NaOH (aq)

Activity Series Single displacement reactions only occur when the displaced element is less reactive than the one replacing it. We can predict if a reaction will occur by comparing the involved elements on the activity series.

Activity Series of Metals

Example Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s) iron + copper (II) sulfate  iron (II) sulfate + copper This reaction occurs because iron donates its electrons more readily than copper (iron is more reactive). Fe(s) + AlCl3(aq)  No Reaction Iron is less reactive than aluminum.

Activity Series of Nonmetals

Example Cl2(g) + 2NaBr(aq)  2NaCl(aq) + Br2(g) Chlorine is more reactive than bromine, so this reaction occurs. Cl2(g) + 2NaFr(aq)  no reaction Chlorine is less reactive than fluorine, so no reaction occurs. We will show that no reaction occurs by simply writing “NR” on the products side of an equation.

Double Displacement Reactions Cations exchange places in aqueous solutions. General formula: AX + BY  AY + BX

Double Displacement Reactions A reaction may form a solid (precipitate) Cu(NO3)2(aq) + AgCl(aq)  CuCl2(s) + AgNO3(aq) We can predict if a precipitate will form by referring to a set of solubility guidelines that have been established based on experimental results

Anion + Cation  Solubility of Compound 1 Most Alkali metal ions (Li+, K+, Rb+, Cs+, Fr+) Soluble Hydrogen ion (H+) Ammonium ion (NH4+) 2 Nitrate (NO3-) Acetate (CH3COO-) Ag+ Low solubility Most others 3 Chloride (Cl-) Bromide (Br-), Iodide (I-) Ag+, Pb+, Hg2+, Cu+, Tl+ All others 4. Flouride (F-) Mg2+, Ca2+, Ba2+, Pb2+ Low solubiity 5. Sulfate (SO42-) Ca2+, Sr2+,Ba2+, Pb2+ 6. Sulfide (S2-) Alkali ions and H+, NH4+, Be2+, Mg2+, Ca2+, Sr2+, Ba2+ 7 Hydroxide (OH-) Alkali ions and H+, NH4+, Sr2+, Ba2+, Tl+ 8 Phosphate (PO43-) Alkali ions and H+, NH4+

Example Magnesium chloride and lead (II) acetate form a precipitate. Predict the products and write a balanced equation identifying the precipitate.

Solve Identify the ions that make up each reactant. Magnesium chloride: Mg2+, Cl- Lead acetate: Pb2+, CH3COO- Switch the pairs of ions to determine the products, and write the equation. MgCl2(aq) + Pb(CH3COO)2(aq)  Mg(CH3COO)2 + PbCl2

Use the solubility guidelines to determine the precipitate. Acetate is soluble with magnesium Lead (II) ions have low solubility with chloride ions. MgCl2(aq) + Pb(CH3COO)2(aq)  Mg(CH3COO)2(aq) + PbCl2(s)

Double Displacement Reactions A reaction may form a gas NaOH(aq) + NH4Cl (aq) HOH(l) + NaCl(aq) + NH3(g) A reaction may form water H2SO4(aq) + 2 NaOH(aq)  2 HOH(l) + Na2SO4(aq)

Summary of Double Displacement Reactions If one reactant… + the other reactant…  then the products include is an aqueous salt a precipitate is an acid has carbonate ions water and carbon dioxide has ammonium ions has hydroxide ions water and ammonia water

Specialized Chemical Reactions Neutralization Reactions: Specific type of double displacement reaction involving acids and bases: General Formula: acid + base  salt + water Example: HCl + NaOH  NaCl + HOH

Specialized Chemical Reactions Combustion Reactions: Oxygen combines with another substance to produce energy (heat and light) and one or more oxides

Combustion Reactions Hydrocarbon Combustion: Complete Combustion: A hydrocarbon reacts with an abundant amount of oxygen gas to produce water and carbon dioxide Incomplete Combustion: A hydrogen combustion reaction with a poor supply of oxygen which produces any combination of CO2, H2O, CO and C (soot)

Specialized Chemical Reactions Oxides and Water: Metal oxides combine with water to produce hydrogen gas and metallic hydroxides. The resulting metallic hydroxide solution is a basic solution. BaO(s) + 2 H2O (l)  H2 (g) + Ba(OH)2 (aq) Na2O(s) + 2 H2O (l)  H2 (g) + 2 NaOH (aq)

Specialized Chemical Reactions Non-metallic oxides combine with water to produce acidic solutions (oxyacids). CO2 (g) + H2O (l)  H2CO3 (aq) P4O10 (s) + 6 H2O (l)  4 H3PO4 (aq)