Chemical Reaction and Equations

Evidence of a ReactionChemical Equations

Balancing Chemical Equations

8.1 Writing and Balancing Chemical Reactions and Equations

• 1. List four observations that suggest that a chemical reaction has taken place.

• 2. List three requirements for a correctly written chemical equation.

• 3. Write a word equation and a formula equation for a given chemical reaction.

• 4. Write a chemical equation by balancing a formula equation by inspection.

Evidence of a Chemical ReactionEvidence of a Chemical Reaction

1.1. The release of The release of

lightlight

or or

heatheat . .

More Light and Heat EvidenceMore Light and Heat Evidence

Evidence of a Chemical ReactionEvidence of a Chemical Reaction

2.2. The production ofThe production of

a gas a gas

(bubbles) .(bubbles) .

More Gas Production EvidenceMore Gas Production Evidence

O2

2H2

O2O22 H2

H2O

Zn

H2HCl

Evidence of a Chemical ReactionEvidence of a Chemical Reaction

3.3. The formation of The formation of

a precipitatea precipitate

(solid) (solid)

when two aqueous when two aqueous solutions are solutions are

mixed together. mixed together.

Pb(NO3)2

KI

PbI2

When gases form a PrecipitateWhen gases form a Precipitate

NH4Cl(s)

Evidence of a Chemical ReactionEvidence of a Chemical Reaction

4.4. A A color changecolor change is often evidence of a is often evidence of a

chemical reaction.chemical reaction.

PbI2

Characteristics of Chemical Characteristics of Chemical Equations Equations

1.1. The equation represents known facts.The equation represents known facts.

2.2. The equation must contain correct formulas!The equation must contain correct formulas!

Reactant + Reactant Reactant + Reactant Product + Product Product + Product

3.3. The equation must obey The Law of The equation must obey The Law of Conservation of Mass.Conservation of Mass.

Mass of Reactants = Mass of ProductsMass of Reactants = Mass of Products

Word EquationsWord Equations

A chemical equation in which the A chemical equation in which the reactants and products are reactants and products are represented by words (names).represented by words (names).

Methane + Oxygen Methane + Oxygen Carbon dioxide + Water Carbon dioxide + Water

The The means yields or produces means yields or produces

The + means “and”The + means “and”

Formula EquationFormula Equation

• Reactants and products are Reactants and products are described by using their symbols described by using their symbols and/or formulas.and/or formulas.

CHCH44 + O + O22 CO CO22 + H + H22OO

• A formula equation A formula equation does not obeydoes not obey “The Law of Conservation of Mass.”“The Law of Conservation of Mass.”

Chemical EquationsChemical Equations

1. Use symbols and formulas to 1. Use symbols and formulas to represent the reactants and products.represent the reactants and products.

2. Adjust the coefficients in front of each 2. Adjust the coefficients in front of each reactant or product to identify the reactant or product to identify the number of each reactant or product.number of each reactant or product.

1 CH1 CH44 + 2 O + 2 O22 1 CO 1 CO22 + 2 H + 2 H22OO

Chemical EquationsChemical Equations

3. Use special symbols in parenthesis to indicate 3. Use special symbols in parenthesis to indicate the state of the reactants or products.the state of the reactants or products.

(s) = solid or (cr) Crystal(s) = solid or (cr) Crystal

(l) = liquid (l) = liquid

(g) = gas (g) = gas

(aq) = aqueous (dissolved in water)(aq) = aqueous (dissolved in water)

Chemical Equations Chemical Equations

4. Use symbols above the yield arrow to 4. Use symbols above the yield arrow to represent catalysts or special conditions.represent catalysts or special conditions.

heat heat ΔΔ atm pressure MnO atm pressure MnO22

Reversible Reactions and Reversible Reactions and EquationsEquations

Some chemical reactions occur in Some chemical reactions occur in both directions.both directions.

NN2 (g)2 (g) + 3 H + 3 H2 (g)2 (g) 2 NH 2 NH3 (g)3 (g)

Balanced Chemical EquationsBalanced Chemical Equations

The coefficients represent the relative The coefficients represent the relative amounts of reactants and products.amounts of reactants and products.

1 CH1 CH44 + 2 O + 2 O22 1 CO 1 CO22 + 2 H + 2 H22OO

1 molecule 2 molecules 1 molecule 2 molecules1 molecule 2 molecules 1 molecule 2 molecules

1 mole 2 moles 1 mole 2 moles1 mole 2 moles 1 mole 2 moles

The Importance of a Balanced The Importance of a Balanced Chemical Equation cont.Chemical Equation cont.

Coefficients can be used to determine the relative Coefficients can be used to determine the relative masses of reactants and productsmasses of reactants and products

1 CH1 CH44 + 2 O + 2 O22 1 CO 1 CO22 + 2 H + 2 H22OO

1 mole 2 moles 1 mole 2 moles1 mole 2 moles 1 mole 2 moles 1(16.05g) + 2(32.00g) = 1(44.01g) + 2(18.02g)1(16.05g) + 2(32.00g) = 1(44.01g) + 2(18.02g)

80.05 g = 80.05 g 80.05 g = 80.05 g Law of Conservation of Mass Law of Conservation of Mass

A Chemical ReactionA Chemical ReactionEvidence = ?Evidence = ?

CH4 2 O2

Reactants

Light from a Big Bang!Light from a Big Bang!

CH4 2 O2CO2 2 H2O

ENERGY

Reactants Products

Balancing Chemical EquationsBalancing Chemical Equations

1.Write the Word (Name) Equation.1.Write the Word (Name) Equation.

Methane + Oxygen Methane + Oxygen Carbon dioxide + Water Carbon dioxide + Water

2. Write the Formula Equation:2. Write the Formula Equation:

CHCH44 + O + O22 CO CO22 + H + H22OO

Balancing Chemical EquationsBalancing Chemical Equations

3. Change the Coefficients as needed.3. Change the Coefficients as needed.

4. NEVER change a subscript ! 4. NEVER change a subscript !

5. Balance different types of atoms one5. Balance different types of atoms one

at a time.at a time.

Balancing Chemical EquationsBalancing Chemical Equations

6. Balance elements that only appear once6. Balance elements that only appear once

on each side of the equation first.on each side of the equation first.

7. Balance polyatomic ions that appear on 7. Balance polyatomic ions that appear on both sides of the equation as one unit.both sides of the equation as one unit.

8. Balance “H” and “O” last.8. Balance “H” and “O” last.

9. Elements9. Elements

Use the symbol from the periodic table for an Use the symbol from the periodic table for an element. element.

Na Au S C Sn PNa Au S C Sn P

The following elements always appear as The following elements always appear as diatomic molecules in chemical equations.diatomic molecules in chemical equations.

HH22 N N22 O O22 F F22 Cl Cl22 Br Br22 I I22

Checking Your WorkChecking Your Work

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

Counting = Coefficient Counting = Coefficient xx subscript subscript

Example One

• Sodium metal (solid) combines with chlorine gas to produce solid sodium chloride.

Example One

• Sodium metal (solid) combines with chlorine gas to produce solid sodium chloride.

Na (s) + Cl2 (g) NaCl (s)

• Remember that chlorine is diatomic!

Example One

• Sodium metal (Solid) combines with chlorine gas to produce solid sodium chloride.

Na (s) + Cl2 (g) NaCl (s)

2 Na (s) + Cl2 (g) 2 NaCl (s)

Example Two

• When copper metal (solid) reacts with aqueous silver nitrate, the products formed are aqueous copper (II) nitrate and silver metal (solid).

Example Two

• When copper metal (solid) reacts with aqueous silver nitrate, the products formed are aqueous copper (II) nitrate and silver metal (solid).

Cu (s) + AgNO3 (aq) Cu(NO3)2 (aq) + Ag(s)

Example Two

• When copper metal (solid) reacts with aqueous silver nitrate, the products formed are aqueous copper (II) nitrate and silver metal (solid).

Cu (s) + AgNO3 (aq) Cu(NO3)2 (aq) + Ag(s)

Cu (s) + 2 AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag(s)

Example Three

• Solid Iron (III) oxide and carbon monoxide gas react to form solid iron and carbon dioxide gas.

Example Three

• Solid Iron (III) oxide and carbon monoxide gas react to form solid iron and carbon dioxide gas.

Fe2O3 (s) + CO(g) Fe(s) + CO2(g)

Example Three

• Solid Iron (III) oxide and carbon monoxide gas react to form solid iron and carbon dioxide gas.

Fe2O3 (s) + CO(g) Fe(s) + CO2(g)

Fe2O3 (s) + 3 CO(g) 2 Fe(s) + 3 CO2(g)

THE END