ReactionsMaking sense of chemical equations

Types of Reactions

• There are millions of reactions, and we cannot remember them all. Luckily they fall into several categories.• By looking at the reactants, we will:• learn the 5 major types• predict the products• predict if they happen at all

Types of Reactions

Type 1: Combination

• Combination reactions happen when 2 elements combine to make a compound (also called synthesis).

• Examples:• Ca + O2 CaO

• SO3 + H2O H2SO4

Type 2: Decomposition

• Decomposition reactions occur when a compound breaks apart into two or more elements or compounds.

• Examples:• NaCl Na + Cl2

• CaCO3 CaO + CO2

• **Note that energy is usually required.

Type 3: Single Replacement

• In a single replacement reaction, one element replaces another.• Reactants must be one element and one

compound.• Products will be a different element and a

different compound.

Type 3: Single Replacement

•Examples:

•Na + KCl K + NaCl• cation replaces cation

•F2 + LiCl LiF + Cl2

• anion replaces anion

Type 4: Double Replacement

• Double replacement reactions occur when two elements replace each other in two different compounds.• Reactants must be ionic compounds in aqueous

solution.

• NaOH + FeCl3 Fe(OH)3 + NaCl• The positive ions change places.

How to recognize each type:

•Look at the reactants:

E + E = CombinationC = DecompositionE + C = Single ReplacementC + C = Double Replacement

E = element; C = compound

Type 5: Combustion

• Combustion means “add oxygen”. • Combustion reacts occur when

compounds containing C, H, and O react with oxygen – usually called “burning”.• If the combustion reaction is complete,

the products will be CO2 and H2O.

• If the combustion reaction is incomplete, the products will be CO (possibly just C) and H2O.

Predicting Products

Type 1: Combination

•We can predict the products, especially if the reactants are two elements:•Mg + N2

• Al + Cl2

Mg3N2

AlCl3

Type 1: Combination

• Practice:• Ca + Cl2

• Fe + O2 (assume Fe (II) in the

product)

• Al + O2

Type 1: Combination

• Watch out for:• Some nonmetal oxides react with water to

form acids:• SO2 + H2O H2SO4

• **This is what happens to make “acid rain”

• Some metallic oxides react with water to for bases:• CaO + H2O Ca(OH)2

• **The hydroxide polyatomic ion indicates that the compound is a base.

Type 2: Decomposition

• We can predict the products if it is binary compound (made up of two elements) – it breaks apart into the elements.

• Examples:• H2O

• HgO

Type 2: Decomposition

• If the compound has more than two elements, you must be given at least one of the products. The other product will be made from the missing pieces.

• Examples:• NiCO3 CO2 + _______

• H2CO3(aq) CO2 + _______

Type 3: Single Replacement

• Metals will also replace other metals (and also hydrogen)• Examples:• K + AlN • Zn + HCl

• Think of water at HOH:• Metals replace the first H and then combine

with OH-. • Na + HOH

Type 3: Single Replacement

• We can even tell whether or not a single replacement reaction will happen:• Because some metals are more “active” than

others.• More active replaces less active.

• There is a list on page 217 in the textbook.• Called the Activity Series of Metals.• Higher on the list replaces lower on the list.

Type 3: Single Replacement• Rules:• 1) Metals can replace

other metals provided they are above the metal they are trying to replace. For example, zinc will replace lead.• 2) Metals above hydrogen

will replace hydrogen in acids.• 3) Metals from sodium

upward can replace hydrogen in water (HOH).

Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead Hydrogen Bismuth Copper Mercury Silver Platinum Gold

Higher activity

Lower activity

Type 3: Single Replacement

• Practice:

• Fe + CuSO4

• Pb + KCl

• Al + HCl

Type 3: Single Replacement

• Halogens have a similar activity series. • Halogens can replace other

halogens in compounds if they are above the halogen they are trying to replace.

• Examples:• 2NaCl + F2

• MgCl2 + Br2

Fluorine Chlorine Bromine Iodine

Higher Activity

Lower Activity

Type 4: Double Replacement

• Double replacement reactions occur because of certain driving forces (or reasons). The reaction will only happen if one of the products:• doesn’t dissolve in water and forms a solid (a

precipitate), or• is a gas that bubbles out, or• is a molecular compound (which will usually

be water).

Complete and Balance

• assume all of the reactions take place

•CaCl2 + NaOH

•CuCl2 + K2S

•KOH + Fe(NO3)3

•(NH4)2SO4 + BaF2

How to recognize each type:

•Look at the reactants:

E + E = CombinationC = DecompositionE + C = Single ReplacementC + C = Double Replacement

E = element; C = compound

Practice

•H2 + O2

•H2O

•Zn + H2SO4

•HgO •KBr + Cl2

•AgNO3 + NaCl

•Mg(OH)2 + H2SO3

Type 5: Combustion

•C4H10 + O2 (assume complete)

•C4H10 + O2 (incomplete)

•C6H12O6 + O2 (complete)

•C8H8 + O2 (incomplete)

SUMMARY

• Reactions come in 5 types.• We can tell what type of reaction by looking at

the reactants.• Single replacement reactions occur based on

activity series.• Double replacement reactions occur if one

product is: 1) a precipitate, 2) a gas, or 3) water (a molecular compound).