Chapter 8:

Chemical Equations and Reactions

8.1 Describing Chemical Reactions

• Chemical reaction- a process in a process in which 1 or more substances are which 1 or more substances are converted into a NEW substance converted into a NEW substance with with differentdifferent chemical and chemical and physical propertiesphysical properties.

• Is an uncooked egg and a cooked egg exactly the same??

• In a chemical reaction there are 2 types of substances: reactants and products.– The substance that enters into the reaction is the

REACTANT.– The new substance(s) produced by the reactions

is/are the PRODUCT(s).

Ex. HH22 + Cl + Cl22 → 2 HCl2 HCl

Reactants Products

• Chemical equation- describes what is describes what is happening in a chemical reaction. IDs happening in a chemical reaction. IDs the reactants and products.the reactants and products.

Indications of a Chemical Reaction

1. Evolution of Light

2. Production of a Gas

3. Formation of a Precipitate

4. Color Change

Characteristics of Chemical Equations

1. The equation must represent known facts

2. The equation must contain the correct formulas for the reactants and products.

3. The law of Conservation of mass must be satisfied.– In order to balance the equation… you must

use COEFFICIENTSCOEFFICIENTS- whole #s written before whole #s written before the formula of reactant or products.the formula of reactant or products.

Word Equations• Simplest type of equation; give names of

reactants and products.

Aluminum ++ Hydrochloric acid →→ Aluminum chloride ++

Hydrogen

• On the REACTANTs side the ++ reads “reacts withreacts with”• On the PRODUCTs side the ++ reads “andand”

• The →→ reads “yieldsyields”, “formsforms” or “producesproduces”

Formula Equations• More common than word equations; uses

formulas instead of names. AKA- chemical equation.

– Al  + HCl  →  AlCl3  +  H2

• You would still “read” the equation the same as the word equation

• Ex Reactants Products 2CH4 ++ 4O2 →→ 2CO2 ++ 4H2O

You read the equation as Methane reacts withreacts with oxygen to formto form carbon dioxide andand water.

Writing Complete Chemical Equations

• Indicate the states of matter of the reactants and products. – Use (g) for gaseous substances. – Use (s) for solids. – Use (l) for liquids. – Use (aq) for species in solution in water. Write the state of matter immediately

following the formula of the substance it describes.

Additional symbols table p.246

Significance of a Chemical Equation

1. The coefficients for a chemical rxn indicate relative, not absolute, amounts of reactants and products.

HH22 + Cl + Cl22 → 2 HCl2 HCl

Shows the smallest whole # ratio of Shows the smallest whole # ratio of the molecules:the molecules:

11moleculemolecule HH22, 1 , 1 moleculemolecule ClCl22, 2 , 2 moleculesmolecules HClHCl

• 2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction's coefficients.

• 3. The reverse reaction for a chemical equation has the same relative amounts of substance as the forward reaction.

Balancing Chemical Reactions1. ID names of reactants & products, write word

equation.2. Convert word equation to formula equation.3. Balance formula equation according to the law

of conservation of mass. Balance the different types of atoms one at a time. 1st balance atoms of elements that appear once on each side of

equation Balance polyatomic ions that appear on both sides of equation as

a unit Balance H & O atoms last.

4. Count atoms to be sure the equation is balanced.

Practice• Ca + O2 → CaO

– 22Ca + O2 → 22CaO

• Mg + N2 → Mg3N2

– 33Mg + N2 → Mg3N2

• CH4 + O2 → CO2 + H2O

– CH4 + 22O2 → CO2 + 22H2O

• FeCl2 + Na3PO4 → NaCl + Fe3(PO4)2

– 33FeCl2 + 22Na3PO4 → 66NaCl + Fe3(PO4)2

8.2 Types of Chemical Rxns

• There are 4 general types of Chemical rxns: Direct Combination, Decomposition, Single-replacement and Double-replacement.

• Each type is distinguished by how the products are formed.

Direct Combinationaka Synthesis Rxn

– Synthesis rxns. combine substances.

– General Synthesis (combination) rxn formula• A + B → ABex. 2Na + Cl2 → 2NaCl

2Mg+O2 → 2MgO2C(s) +2O2(g) → 2CO2 (g)CaO+H2O → Ca(OH)2

It’s always 2 substances that react to form 1 new substance.

Decomposition Rxn

– Decomposition rxns - Break substances apart.

– General rxn formula:• AB → A + BEx. 2H2O → 2H2 + O2

H2CO3 → H2O + CO2

2KClO3 → 2KCl + 3O2

• It’s always 1 substance breaks down to form 2 new substances.

Single- Replacement rxns

• Single Replacement rxn - a rxn in which one element takes the place of another element in a compound– General equation: • AX + B → BX + A

Ex. 3CuCl2 + 2Al → 2AlCl3 + 3Cu2AgNO3(aq) + Zn(s) → 2Ag(s) +

Zn(NO3)2(aq)

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Double-Replacement rxns

• Double-Replacement rxns - a rxn in which a substance is formed from the exchange of 2 atoms or ions between 2 compounds.– General formula• AX + BY → AY + BX

Ex. Pb(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3

Combustion Rxn– Combustion Rxns use O2 as a

reactant and always produce CO2 & H2O. In combustion rxns, the products depend on the amount of O2.

– General Combustion rxn formula:

• CnH2n+2 + O2 → CO2 + H2O

Ex. 2CH4 + 4O2 → 2CO2 + 4H2O

8.3 Activity Series of the Elements

• Activity series- a list of elements organized according to the ease with which the elements undergo certain chemical reactions.

• For metals: greater activity = easy to lose an electron.

• For non-metals: greater activity = easy to gain an electron.

Predict the products of these Single Replacement rxns.

– zinc + lead (II) nitrate– iron + zinc sulphate– lead + copper (II) nitrate– magnesium + zinc chloride– copper + sodium chloride– zinc + iron (II) sulphate– gold + silver nitrate– magnesium + calcium nitrate