Chemical Equations

and Reactions

Some Definitions Chemical reaction = The process by

which one or more substances are changed into one or more different substances

Chemical equation = Represents a reaction (with symbols and formulas) the identities and relative molecular or molar amounts of the reactants and products in a reaction

Indications of a Chemical Reaction 1.1. Evolution of energy as heat and Evolution of energy as heat and

lightlight

2.2. Production of a gasProduction of a gas

Indications of a Chemical Reaction 3.3. Formation of a precipitateFormation of a precipitate

A solid that is produced as a result of a chemical reaction in solution and that

separates from the solution

4. Color Change4. Color Change

Characteristics of Chemical Equations

1. The equation must represent known facts: all reactants and products must be identified

2. The equation must contain the correct formulas for the reactants and products: use knowledge of writing formulas with oxidation states

Assigning unknown oxidation numbers to multi atom elements

For example: Cr2O72- is Cr = +6 O = -2

Characteristics of Chemical Equations

3. The law of the conservation of mass must be satisfied Balancing equations --- YAY!! Same # of atoms of each element must appear

on each side of a correct chemical equation

*Memorize

symbols on

page 266

Word EquationAn equation which the reactants and

products in a chemical reaction are represented by words

ONLY Qualitative Information Given

Ethane + Oxygen Carbon dioxide + Water

Time to practice writing word equations from a word problem!

Formula EquationAn equation representing the reactants

and products of a chemical reaction by their symbols or formulas (also

including state symbols)ONLY Qualitative Information Given

(Not usually balanced!!)

C2H6 (g) + O2(g)   CO2(g)   +  H2O(l)

WordWord and Formula

Word equationWord equation:               

Aluminum + Hydrochloric acid Aluminum chloride + Hydrogen

Formula equation Formula equation (not balanced):        

Al(s)  +  HCl(aq)    AlCl3(s) +  H2(g)

Correct Chemical EquationBegin with formula equation

Balance using coefficientsNow it is correctly written!!

WrongWrong:Al(s)  +  HCl(aq)    AlCl3(s) +  H2(g)

RightRight:2Al(s)  +  6HCl(aq)    2AlCl3(s) +  3H2(g)

Balancing Chemical Balancing Chemical EquationsEquations

1. Balance the different types of atoms 1 at a time

2. 1st balance the atoms of elements that are combined and that appear only once on each side of the equation

3. Balance polyatomic ions that appear on both sides of the equation as single units

4. Balance H and O atoms after atoms of all other elements have been balanced

5. Check by final count

Balancing from Word Balancing from Word ProblemProblem

1. 1st figure out the word equation

2. Then write the formula equation from the word equation

3. Balance the equation following the rules from previous slide

PracticePractice: Nitrogen dioxide gas reacts with water to form aqueous nitric acid and

nitrogen monoxide gas.

Significance of a Chemical Significance of a Chemical EquationEquation

1. The coefficient of a chemical reaction indicate relative, not absolute, amount of reactants and products Smallest possible amounts

2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients Figure out grams from moles to get total of product

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

5 Types of Chemical Reactions

1.Synthesis (or Composition)2.Decomposition3.Single-displacement (or

Replacement)4.Double-displacement5.Combustion

Synthesis ReactionsSynthesis Reactions2 or more substances combine

AA(element or compound) + XX (element or compound) AXAX

(Compound) Elements with oxygen and sulfur (forms oxides

and sulfides)

CaO(s) + HH22OO(l) Ca(OH)Ca(OH)22(s)

8Ba(s) + SS88(s) 8BaS8BaS(s)

Metals with Halogens (usually forming ionic compounds)

2K(s) + Cl2(g) 2KCl(s)

Reactions with OxidesOxides of active metals + water =

metal hydroxidesOxides of nonmetals + water =

oxyacids

SynthesisSynthesis ReactionsReactions

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

SynthesisSynthesis ReactionsReactions

Decomposition Decomposition ReactionsReactions

Single compound produces 2 or more simpler

substances

AXAX (Compound) A A(element or compound) + XX (element or

compound)

Decomposition Decomposition ReactionsReactions

Binary Compounds Electrolysis: decomposition by

electric current 2H2O(l) 2H2(g) + O2(g) = LAB!

Carbonates Forming a metal oxide and CO2

Hydroxides Pb(OH)2(s) ----> PbO(s) + H2O(g)

Single-Displacement Reactions

In the case of a positive ion being replaced: 

A + BC B + ACOR

In the case of a negative ion being replaced: 

A + BC C + BA

For either case we have:element + compound element + compound

Double-Displacement Double-Displacement Reactions Reactions

AB + CD CB + AD

Exchange Partners!Exchange Partners!

Basically:Compound + Compound Compound + Compound

Double-Displacement Double-Displacement Reactions Reactions

Formation of a PrecipitateFormation of a Precipitate

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Formation of a GasFormation of a Gas

FeS(s) + 2HCl(aq) H2S(g) + FeCl2(aq)

Formation of WaterFormation of Water

H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

Double-Displacement Double-Displacement Reactions Reactions

Formation of a PrecipitateFormation of a Precipitate

 

 AgNO3(aq) + KCl(aq) � AgCl� (s) + KNO3(aq)

Combustion Combustion ReactionsReactions

Usually:hydrocarbon + oxygen carbon dioxide and water

Example:Example:

CHCH4(g)4(g) + 2O + 2O2(g)2(g) 2H 2H22OO(g)(g) + + COCO2(g)2(g)

Activity SeriesWhether or not a chemical Whether or not a chemical reaction will really occur!reaction will really occur!

See Handout and/or Page 286 in See Handout and/or Page 286 in your textbook (same thing!)your textbook (same thing!)

Activity Series Practice

11stst can they occur, 2 can they occur, 2ndnd if so write if so write product, then balance:product, then balance:

MgCl2(aq) + Zn(s) NO

Al(s) + H2O(g) Yes (Al2O3(s) + H2(g))