Balancing Chemical Balancing Chemical EquationsEquations

Balancing Chemical EquationsBalancing Chemical Equations

– – an equation that usesan equation that uses chemical formulas and symbols to showchemical formulas and symbols to show the reactants and products in athe reactants and products in a chemical reactionchemical reaction

– – the smallest relativethe smallest relative number of moles of the number of moles of the

substances involved in asubstances involved in a reactionreaction

5.3

Parts of an equationParts of an equation

Reactants – yield sign - productsReactants – yield sign - products The left side of an equation- chemicals The left side of an equation- chemicals

that react with each other – the that react with each other – the reactantsreactants

The arrow separating the beginning and The arrow separating the beginning and end result – the yield signend result – the yield sign

The right side of an equation – the NEW The right side of an equation – the NEW substances that are formed as a result substances that are formed as a result of a chemical reaction - productsof a chemical reaction - products

Balancing equations – Balancing equations – balancing atomsbalancing atoms

Remember the Law of Conservation Remember the Law of Conservation of mass: …..of mass: …..

Mass cannot be created or Mass cannot be created or destroyed…destroyed…

# of atoms of each element on # of atoms of each element on reactants side reactants side mustmust equal # of equal # of atoms of each element on the atoms of each element on the products side!products side!

Is this equation balanced?Is this equation balanced?

Count the atoms……Count the atoms…… HCl + NaOHHCl + NaOH NaCl+ H NaCl+ H22OO left side has 2-H, 1-Cl, 1-Na, and 1-Oleft side has 2-H, 1-Cl, 1-Na, and 1-O right side has 1-Na, 1-Cl, right side has 1-Na, 1-Cl, 2-H, and 1-O2-H, and 1-O Yes!!!!!! Yes!!!!!!

Is this equation balanced?Is this equation balanced?

NaCl + HNaCl + H22O O NaOH + Cl NaOH + Cl22 left side has 1-Na atom, 1-Cl, 2-H, and 1-Oleft side has 1-Na atom, 1-Cl, 2-H, and 1-O right side has 1-Na, 1-O, right side has 1-Na, 1-O, 1-H, and 2-Cl1-H, and 2-Cl NO!!!!!!NO!!!!!! How can we make it balance????How can we make it balance????

Use coefficients….Use coefficients….

List all elements and # of atomsList all elements and # of atoms Add more molecules of a substance Add more molecules of a substance

by adding coefficients…… to balanceby adding coefficients…… to balance Ca + HCa + H22O O Ca(OH) Ca(OH)22 + H + H22

Ca 1 atom Ca 1 atomCa 1 atom Ca 1 atom

H 2 atoms H 2 atoms H 4 atomsH 4 atoms O 1 atom O 2 atomsO 1 atom O 2 atoms

Continued……Continued……

Ca + HCa + H22O O Ca(OH) Ca(OH)22 + H + H22

Ca 1Ca 1 Ca 1Ca 1 H 2H 2 H 4H 4 O 1 O 1 O 2O 2

Ca + Ca + 22HH22O O Ca(OH) Ca(OH)22 + H + H22 Ca = 1 Ca = 1Ca = 1 Ca = 1 H = H = 22 x 2 = 4 H = 4 x 2 = 4 H = 4 O = O = 22 x 1 = 2 O = 2 x 1 = 2 O = 2

How to balance equationsHow to balance equations

1.1. Identify the individual elements in Identify the individual elements in reactants and productsreactants and products

2.2. Count atoms on each sideCount atoms on each side3.3. ““Try” different coefficients to Try” different coefficients to

balance the numbers of atoms.balance the numbers of atoms.4.4. Balance the equation one element Balance the equation one element

at a time.*at a time.*

*In general – balance the metals (and any other non-metals) first and save hydrogen and oxygen for last

1 1 Ca + Ca + 2 2 HH22O O 1 1 Ca(OH)Ca(OH)22 + + 1 1 H H22

Just like in in formula writing the Just like in in formula writing the “1’s” are understood. This equation “1’s” are understood. This equation should be written in a simpler way:should be written in a simpler way:

Ca + Ca + 2 2 HH22O O Ca(OH)Ca(OH)22 + H + H22

Practice Balancing Chemical Practice Balancing Chemical EquationsEquations

1.1. Copper (II) sulfate, CuSOCopper (II) sulfate, CuSO44, and aluminum , and aluminum react to form aluminum sulfate, Alreact to form aluminum sulfate, Al22(SO(SO44) ) and copper. Write the balanced equation and copper. Write the balanced equation for this single-displacement reaction.for this single-displacement reaction.

1.1. In a double-displacement reaction, sodium In a double-displacement reaction, sodium sulfide, Nasulfide, Na22S, reacts with silver nitrate, S, reacts with silver nitrate, AgNOAgNO33 to form sodium nitrate, NaNO to form sodium nitrate, NaNO33 and and silver sulfide, Agsilver sulfide, Ag22S. Balance this equation.S. Balance this equation.

CuSO4 + 2 Al Al2(SO4) + Cu

Na2S + 2AgNO3 2NaNO3 + Ag2S

Balancing EquationsBalancing Equations

1.1. Hydrogen peroxide, HHydrogen peroxide, H22OO22, is sometimes , is sometimes used as a bleach or as a disinfectant. used as a bleach or as a disinfectant. Hydrogen peroxide decomposes to give Hydrogen peroxide decomposes to give water and molecular oxygen. Write a water and molecular oxygen. Write a balanced equation for the decomposition balanced equation for the decomposition reaction.reaction.

Hydrogen sulfide, HHydrogen sulfide, H22S, is a gas that smells S, is a gas that smells like rotten eggs. Write and balance an like rotten eggs. Write and balance an equation for the oxidation by molecular equation for the oxidation by molecular oxygen of hydrogen sulfide to make sulfuric oxygen of hydrogen sulfide to make sulfuric acid, Hacid, H22SOSO44..

2H2H22OO22 -> 2H -> 2H22O + OO + O22

HH22S + 2OS + 2O22 -> H -> H22SOSO44

Determining Mole RatiosDetermining Mole Ratios

Balanced equations indicate particles and Balanced equations indicate particles and molesmoles

Balanced equations show the conservation Balanced equations show the conservation of massof mass

The Law of Definite Proportions:The Law of Definite Proportions:““A compound always contains the same A compound always contains the same elements in the same proportions, elements in the same proportions, regardless of how the compound is made regardless of how the compound is made or how much of the compound is formed.”or how much of the compound is formed.”

Mole RatiosMole Ratios Look at the reaction of magnesium with oxygen to Look at the reaction of magnesium with oxygen to

form magnesium oxide:form magnesium oxide:magnesium + oxygen magnesium + oxygen magnesium oxidemagnesium oxide

2Mg + O2Mg + O22 2MgO 2MgOThe single molecule of oxygen in the equation might The single molecule of oxygen in the equation might

be shown as 1Obe shown as 1O22. However, a coefficient of 1 is . However, a coefficient of 1 is never written.never written.

The equation can also be read as describing mole The equation can also be read as describing mole quantities – 2 mol of magnesium can react with 1 quantities – 2 mol of magnesium can react with 1 mol of oxygen to produce 2 mol of magnesium mol of oxygen to produce 2 mol of magnesium oxide.oxide.

Mole ratios can Mole ratios can onlyonly be derived from balanced be derived from balanced equations.equations.

Information from the balanced Information from the balanced equation 2Mg + Oequation 2Mg + O22 2MgO 2MgO

Equation 2 Mg + OEquation 2 Mg + O22 2MgO 2MgO Amount (Mol) 2 1 2Amount (Mol) 2 1 2 Molecules Mg = (6.02 x 10Molecules Mg = (6.02 x 102323) x 2) x 2 O = (6.02 x 10O = (6.02 x 102323) x 1) x 1 MgO = (6.02 x 10MgO = (6.02 x 102323) x 2) x 2 Mass (g) Mg = 24.3 g/mol x 2 molMass (g) Mg = 24.3 g/mol x 2 mol O = 32.0 g/mol x 1 molO = 32.0 g/mol x 1 mol

MgO = 40.3 g/mol x 2 molMgO = 40.3 g/mol x 2 mol Total Mass (g) 48.6 + 32.0 + 80.6Total Mass (g) 48.6 + 32.0 + 80.6

Mole Ratio is 2:1:2

AssignmentAssignment Balancing Equations - worksheetBalancing Equations - worksheet Word Problems - worksheetWord Problems - worksheet

1 - write word equation1 - write word equation2 – write skeletal equation2 – write skeletal equation3 – balance3 – balance

EX: When a match is lit, sulfur (SEX: When a match is lit, sulfur (S88) ) reacts withreacts with oxygen to release energy and oxygen to release energy and formform sulfur dioxide, sulfur dioxide, SOSO22..

sulfur + oxygen sulfur + oxygen sulfur dioxide sulfur dioxide

SS88 + O + O22 SO SO22

SS88 + 8O + 8O22 8SO 8SO22