Chemical Equations and Reactions

Describing Chemical Reactions

Indications of a chemical reaction

A chemical reaction is the process by which one or more substances are changed into one or more different substances.

A chemical equation represents with symbols and formulas (or with words), the identities and the relative amounts of the reactants and products.

The original substances are called reactants and are shown on the left side of the equation.

The substances formed are called products and are shown on the right side of the equation.

Chemical symbols seen in a chemical equation

Indications of a chemical reaction There are several ways to tell if a chemical reaction has

occurred.

• Evolution of heat and/or light

• Production of a gas

• Formation of a precipitate(a solid that settles to the bottom of the test tube or a cloudiness that occurs)

• Color change (may or may not indicate a chemical change)

• Evolution of sound (may or may not indicate a chemical change)

• Formation of a new substance

• Products cannot be easily changed back into reactants.

Characteristics of a chemical equation The equation must represent the facts. All

reactants and products must be identified by chemical analysis.

The equation must contain the correct formulas for all substances involved. • Remember to use oxidation states when writing

formulas. • Remember that some elements are diatomic and

must have a 2 as a subscript when written in an equation.

Diatomic elements

Characteristics of a chemical equation

The law of conservation of mass must be observed at all times. Atoms may not be created or destroyed but may be rearranged to make new substances. • To equalize the number of atoms on both sides

of an equation, coefficients are used. It is placed in front of the compound.

• NEVER change the subscripts in a formula.

Characteristics of a chemical equation

Writing a word equation is helpful to organize the facts that are known.

EX. Methane gas reacts with oxygen in the presence of a spark to make carbon dioxide and water.

The reactants are known and the products are known. The condition required for this reaction is also known.

Characteristics of a chemical equation Next a formula equation can be written. CH4 + O2 --> CO2 + H2O

Note that there are 4 H atoms on the left but only 2 H atoms on the right. A coefficient can be placed in front of the water to equal them out. That will make 2 O atoms on the left but 4 O atoms on the right. Place a 2 in front of the O2 and now check for equal numbers of atoms.

CH4 + 2 O2 --> CO2 + 2 H2O

Significance of a chemical equation

The coefficients indicate the relative amounts of reactants and products. The lowest whole number ratio is shown.

The relative masses can be determined from the coefficients. Once done, the law of conservation of mass can be shown to be true.

Looking at a balanced equation

Types of chemical reactions There are five basic types of reactions. Not

all reactions fall into these five categories but these are the most common kinds. • Synthesis reactions• Decomposition reactions• Single replacement reactions• Double replacement reactions• Combustion reactions

• complete

• incomplete

Types of chemical reactions

Synthesis reactions

Are also known as composition reactions or as direct combination reactions

Occur when 2 or more elements or small compounds combine to form 1 larger compound.

A + X --> AX (may or may not have subscripts)

Examples of synthesis reactions reactions with sulfur to form sulfides

• Fe + S --> Fe2S3

reactions with oxygen to form oxides• S + O2 --> SO2

reactions of metals with halogens to form salts• 2Na + Cl2 --> 2NaCl

reactions of oxides of active metals with water to form hydroxides• CaO + H2O --> Ca(OH)2

Decomposition reactions

Occur when a single compound breaks down into two or more simpler substances.

Are the opposite of synthesis reactions. Usually energy must be added to cause

these to occur. AX --> A + X(may or may not have

subscripts)

Examples of decomposition reactions decomposition of binary compounds

• 2H2O electricity > 2H2 + O2 (electrolysis)

decomposition of metal carbonates• CaCO3 --> CaO + CO2

decomposition of metal hydroxides• Ca(OH)2 --> CaO + H2O

decomposition of metal chlorates• 2KClO3 --> 2KCl + 3O2

decomposition of acids• H2CO3 --> H2O + CO2

Single replacement reactions Occur when an active element replaces a less

active element. The activity series table (p. 266) is required to

predict whether a reaction will occur or not. An element high on the table will replace an

element in a compound that is lower than it is, i.e. lithium will replace lead in a compound.

Metals will replace metals; nonmetals will replace nonmetals.

Have a net ionic equation which does not include spectator ions.

Examples of single replacement reactions Li + MgCl2 LiCl + Mg

Mg + ZnSO4 MgSO4 + Zn

Co + H2O N.R.

Zn + HCl ZnCl2 + H2

Ag + ZnBr2 N.R.

Double replacement reactions Occur when the electropositive elements in two

compounds switch places. Usually occur when the reactants are in aqueous

solution. Require a driving force to determine whether or

not they occur.• Formation of a precipitate (solubility table)

• Formation of a gas (CO2, SO2, NO2, SO3)

• Formation of water

Have a net ionic equation which does not include spectator ions.

Examples of double replacement reactions

MgCO3(aq) + 2HCl(aq) MgCl2(aq) + H2O(l) + CO2(g)

2KCl(aq) + Pb(NO3)2(aq) 2KNO3(aq) + PbCl2(s)

NaNO3 + KCl N.R. (no driving force observed)

Combustion reactions

Occur in the presence of oxygen. Produce carbon dioxide and water as

products if one of the reactants is an organic compound and the combustion is complete.

Produce carbon monoxide and water if the combustion is incomplete. You would need to be told if this was the case.

Examples of combustion reactions

C7H16 + 11O2 7CO2 + 8H2O CH4 + 2O2 CO2 + 2H2O C6H12O6(aq) + 6O2(g) → 6CO2(aq)

6H2O(l ) C2H5OH(aq) + 2O2(g) incomplete→

2CO(g) + 3H2O(l )

More types of reactions

Acid-base reactions are also called neutralization reactions because they produce a salt and water.

Precipitation reactions are those which produce a precipitate as a product (double replacement reactions).

Oxidation-reduction reactions are those in which two of the reacting atoms have their charges changed. One is oxidized and one is reduced.

Examples of acid-base reactions

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H2SO4(aq) + KOH(aq) K2SO4(aq) + H2O(l)

HC2H3O2(aq) + Ba(OH)2(aq) Ba(C2H3O2)2(aq) + H2O(l)

Examples of precipitation reactions

Ba(NO3)2(aq) + Na2SO4(aq) 2NaNO3(aq) + BaSO4(s)

2KCl(aq) + Pb(NO3)2(aq) 2KNO3(aq) + PbCl2(s)

Examples of redox reactions Li + MgCl2 LiCl + Mg

• Li goes from 0 to +1 and is oxidized and Mg goes from +2 to 0 and is reduced

Mg + ZnSO4 MgSO4 + Zn

• Mg goes from 0 to +2 and is oxidized and Zn goes from +2 to 0 and is reduced

Zn + HCl ZnCl2 + H2

• Zn goes from 0 to +2 and is oxidized and H goes from +1 to 0 and is reduced