WRITING & BALANCING CHEMICAL EQUATIONS

Law of Conservation of MassDefinition•matter is neither created nor destroyed

Application•each side of a chemical equation must have equal masses (what goes in, must come out)

Chemical EquationsChemical Equationsobey the law of conservation of mass when properly written

reactants on the left; products on the right

an arrow separates the reactants from the products

symbols used in complete chemical equations:

Symbol Meaningyields; produces

reversiblereactants are heated

catalyst added to reactants(l) liquid phase(s) solid phase(g) gas phase(aq) aqueous phase: dissolved in H2O

acids and bases are usually in the aqueous (aq) phase

metals are usually in the solid (s) phase

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Balanced Equations

Definition•have equal numbers of each type of atom on each side of the equation

•an unbalanced equation has unequal numbers of atoms on each side of the equation

Rules:

•you can never change a subscript

•you may only change coefficients

•you cannot place a coefficient in the middle of a formula

•coefficients are only allowed in the front of a formula

•ensure that the final coefficients are reduced

Example:

Balance this equation: H2 + Cl2 → HCl

Now try the PRACTICE…

Chemical

Reactions

Synthesis

Reactions

Definition• two or more reactants combine to form one

product

• A + B → AB

Example• 2Mg + O2 → 2MgO

Decomposition

Reactions

Definition• a single reactant breaks down into two or

more products

• AB → A + B

Example• FeS → Fe + S

Single Replacement

Reactions

Definition• a single element or diatomic molecule reacts with a compound to produce a new compoundand a single element or diatomic molecule

• metals (A) and (B) swap positions:A + BC → B + AC

• nonmetals (A) and (C) swap positions:A + BC → C + BA

Example• metals swap:

Cu + 2AgNO3 → Ag + Cu(NO3)2

• nonmetals swap:

Cl2 + 2NaBr → Br2 + 2NaCl

Activity Series

a metal will only replace another metal if it is more reactiveif the metal is not more reactive, then the reaction will not occur.

ACTIVITY SERIESfor single replacement reactions

MOST REACTIVECesium (Cs)

Rubidium (Rb)Potassium (K)Sodium (Na)Lithium (Li)

Strontium (Sr)Calcium (Ca)

Magnesium (Mg)Aluminum (Al)

Zinc (Zn)Chromium (Cr)

Iron (Fe)Cadmium (Cd)

Cobalt (Co)Nickel (Ni)

Tin (Sn)Lead (Pb)

Hydrogen (H)Copper (Cu)Silver (Ag)

Mercury (Hg)Gold (Au)

Platinum (Pt)LEAST REACTIVE

AB + CD AD + CB

Compound + compound yields compound + compound

DOUBLE

Replacement

Reactions

LiF + KOH LiOH + KF

Notice that the positive and negative ions switched positions

DOUBLE

Replacement

Reactions

CxHy + O2 CO2 + H2O

Hydrocarbon + Oxygen yields Carbon Dioxide + Water

COMBUSTION

Reactions

• C2H4 + O2 ____ + ____

• C3H8 + ____ CO2 + _____

COMBUSTION

Reactions

HCl + NaOH HOH + NaCl

Strong Acid + Strong Base produces Water + SaltWater can be written as: HOH or H2O

ACID-BASE

Reactions

Acid-Base Rxn:

• HF + KOH ____ + ____

• HNO3 + Ca(OH)2 ____ + ____

• ____ + ____ HOH + Li3PO4