Chapter 9

Chemical Reactions

Reactions and Equations 9.1Reactions and Equations 9.1

A A chemical reactionchemical reaction is another name for chemical is another name for chemical change.change.

How to tell if a chemical reaction has taken placeHow to tell if a chemical reaction has taken place– Is there evidence? (Temperature change, color change, Is there evidence? (Temperature change, color change,

odor, gas bubbles, general appearance)odor, gas bubbles, general appearance)

Equations are used to represent chemical reactionsEquations are used to represent chemical reactions– Show reactants: the starting substances Show reactants: the starting substances – Show Products: resulting or ending substancesShow Products: resulting or ending substances– Do not show numerical values so arrow is used instead of Do not show numerical values so arrow is used instead of

an equals signan equals sign– Show indicator of chemical stateShow indicator of chemical state

Types of EquationsTypes of Equations Three types of equationsThree types of equations

– WordWord– SkeletonSkeleton– ChemicalChemical

Word equationsWord equations– Used to indicate the reactants and products of chemical reactionsUsed to indicate the reactants and products of chemical reactions– Uses the actual names of the substances involvedUses the actual names of the substances involved– Setup: Reactant 1 + Reactant 2 Setup: Reactant 1 + Reactant 2 Product 1 Product 1 Ex: Iron(s) + Chlorine(g) Ex: Iron(s) + Chlorine(g) Iron(III) Chloride(s) Iron(III) Chloride(s)

Skeleton EquationsSkeleton Equations– Uses chemical formulas to represent the substances unlike the word Uses chemical formulas to represent the substances unlike the word

equationequation– Chemical formulas take the place of the wordsChemical formulas take the place of the words– Iron(s) + Chlorine(g) Iron(s) + Chlorine(g) Iron(III) Chloride(s) would become: Iron(III) Chloride(s) would become:

Fe(s) + ClFe(s) + Cl22(g) (g) Fe Cl Fe Cl33(s)(s)

Chemical EquationsChemical Equations– Uses chemical formulas like the skeleton equationUses chemical formulas like the skeleton equation– Shows matter is conserved during a reaction (Law of Shows matter is conserved during a reaction (Law of

Conservation of Mass)Conservation of Mass)– Chemical eqation: Uses identities and relative amounts Chemical eqation: Uses identities and relative amounts

of the substances involved in a chemical reactionof the substances involved in a chemical reaction– Everything is balanced in a chemical equationEverything is balanced in a chemical equation– Fe(s) + ClFe(s) + Cl22(g) (g) Fe Cl Fe Cl33(s) would become:(s) would become:

22Fe(s) + 3ClFe(s) + 3Cl22(g) (g) 2Fe Cl 2Fe Cl33(s)(s)

Balancing Chemical EquationsBalancing Chemical Equations– Finding the correct coefficients for chemical formulas in Finding the correct coefficients for chemical formulas in

the skeleton equation is very importantthe skeleton equation is very important Coefficient is usually a whole number and 1 is understoodCoefficient is usually a whole number and 1 is understood

Steps to Balance a Chemical Steps to Balance a Chemical EquationEquation

1: Write the skeleton equation1: Write the skeleton equation

2: Count the atoms of elements in the reactants2: Count the atoms of elements in the reactants

3: Count the atoms of elements in the products3: Count the atoms of elements in the products

4:Change the coefficients to make both sides of the equation equal4:Change the coefficients to make both sides of the equation equal

5:Write the coefficient(s) in the lowest ratio possible5:Write the coefficient(s) in the lowest ratio possible

6:Check your work6:Check your work

– Ex: Ex: H H2 2 (g) + O(g) + O22(g) (g) H H22O(g)O(g)

4H4H2 2 (g) + 2O(g) + 2O22(g) (g) 4H 4H22O(g)O(g)

2H2H2 2 (g) + O(g) + O22(g) (g) 2H 2H22O(g)O(g)

Classifying Chemical ReactionsClassifying Chemical Reactions9.29.2

There are five types of chemical reactions.There are five types of chemical reactions.– SynthesisSynthesis– CombustionCombustion– DecompositionDecomposition– Single-ReplacementSingle-Replacement– Double-ReplacementDouble-Replacement

Synthesis ReactionSynthesis Reaction– Chemical reaction in which two or more Chemical reaction in which two or more

substances combine to form one single product. substances combine to form one single product. – A + B A + B AB AB

Ex: 2Na(s) + ClEx: 2Na(s) + Cl22(g) (g) 2NaCl(s) 2NaCl(s)

Combustion ReactionCombustion Reaction– Oxygen combined with a substance releasing energy in Oxygen combined with a substance releasing energy in

the form of light and heat the form of light and heat – Many combustion reactions are also synthesis reactionsMany combustion reactions are also synthesis reactions

Ex: 2HEx: 2H22(g) + O(g) + O2 2 2H 2H22O(g)O(g)

Decomposition reactionsDecomposition reactions– Only one compoundOnly one compound– Breaks down into 2 or more compounds or elementsBreaks down into 2 or more compounds or elements– Setup: AB Setup: AB A + BA + B– Often requires an energy source (i.e. heat, light, Often requires an energy source (i.e. heat, light,

electricity)electricity) Ex: NHEx: NH44NONO33(s) (s) N N22O(g) + 2HO(g) + 2H22O(g)O(g) The above reaction occurs when the reactant is heatedThe above reaction occurs when the reactant is heated

Replacement ReactionsReplacement Reactions– Unlike the other reactions, replacement reactions Unlike the other reactions, replacement reactions

involve the replacement of an element in a a compoundinvolve the replacement of an element in a a compound– 2 kinds of replacement reactions:2 kinds of replacement reactions:

Single Single DoubleDouble

Single Replacement reactions Single Replacement reactions – When atoms of one element replace the atoms of When atoms of one element replace the atoms of

another element in a compoundanother element in a compound– Setup: A+ BX Setup: A+ BX AX + B AX + B– Whether or not a metal will replace another metal or any Whether or not a metal will replace another metal or any

other substance depends on the reactivity it hasother substance depends on the reactivity it has Double Replacement reactionsDouble Replacement reactions

– Invovles the exchange of ions between 2 compoundsInvovles the exchange of ions between 2 compounds– Setup: AX + BY Setup: AX + BY AY + BX AY + BX– If a solid is produced during the reaction it is called a If a solid is produced during the reaction it is called a

precipitateprecipitate

Reactions in Aqueous Solutions Reactions in Aqueous Solutions 9.39.3

VocabVocab Aqueous solution- Solution in which the solvent is waterAqueous solution- Solution in which the solvent is water Solvent- The water in the solutionSolvent- The water in the solution Solute- The compound dissolved in the waterSolute- The compound dissolved in the water Precipitate- Solid formed in a chemical reactionPrecipitate- Solid formed in a chemical reaction Spectator ion- Ions that do not participateSpectator ion- Ions that do not participate Complete ionic equation- Shows all particles as they really Complete ionic equation- Shows all particles as they really

existexist Net ionic equation- Only includes particles that participate Net ionic equation- Only includes particles that participate

(No spectator ions)(No spectator ions)

Aqueous SolutionsAqueous Solutions

When two aqueous When two aqueous solutions react it is always solutions react it is always a double replacement a double replacement reaction.reaction.

The water doesn’t reactThe water doesn’t react There are three possible There are three possible

products when aqueous products when aqueous solutions react, they are; solutions react, they are; precipitates, water, or precipitates, water, or gases.gases.

Spectator ions do not Spectator ions do not participate in the reaction participate in the reaction and are usually not and are usually not included in the ionic included in the ionic equation.equation.

Writing Ionic EquationsWriting Ionic Equations

Ionic and chemical equations differ in that Ionic and chemical equations differ in that substances that are ions in the solution are substances that are ions in the solution are written as ions in the equation.written as ions in the equation.

To write an ionic equation you must show the To write an ionic equation you must show the reactants and the product as ions.reactants and the product as ions.

A complete ionic equation shows all of the A complete ionic equation shows all of the particles in the solution as they realistically exist.particles in the solution as they realistically exist.

A net ionic equation is a simplified version of the A net ionic equation is a simplified version of the complete ionic equation. It only shows particles complete ionic equation. It only shows particles that participate in the reaction.that participate in the reaction.

To write a net ionic equation from a complete To write a net ionic equation from a complete ionic equation cross out all spectators.ionic equation cross out all spectators.

Ionic equationsIonic equations

AgNOAgNO33(aq) + NaCl(aq) (aq) + NaCl(aq) AgCl(s) + NaNO AgCl(s) + NaNO33(aq)(aq)

What type of reaction is this?What type of reaction is this? A A complete ionic equationcomplete ionic equation shows shows

dissolved ionic compounds as their free dissolved ionic compounds as their free ions.ions.

Ag+(aq) + NOAg+(aq) + NO33-(aq) + Na+(aq) + -(aq) + Na+(aq) + Cl-(aq) Cl-(aq) AgCl(s) + Na+(aq) + NO AgCl(s) + Na+(aq) + NO33-(aq) -(aq)

Ionic EquationsIonic Equations

Ag+(aq) + NOAg+(aq) + NO33-(aq) + Na+(aq) + -(aq) + Na+(aq) + Cl-(aq) Cl-(aq) AgCl(s) + Na+(aq) + NO AgCl(s) + Na+(aq) + NO33-(aq)-(aq)

Ions that are not directly involved in a Ions that are not directly involved in a reaction are called reaction are called spectator ionsspectator ions..

Rewriting the equation, leaving out the Rewriting the equation, leaving out the spectator ions, gives us the spectator ions, gives us the net ionic net ionic equationequation……

Ag+(aq) + Cl-(aq) Ag+(aq) + Cl-(aq) AgCl(s) AgCl(s)

Ionic EquationsIonic Equations

You also have to make sure that the You also have to make sure that the net charge is balanced. For net charge is balanced. For example…example…

Pb(s) + Pb(s) + 2Cl2Cl--(aq) (aq) PbClPbCl2(s)2(s)

ExamplesExamples

HCl(aq) + ZnS(aq) HCl(aq) + ZnS(aq) H H22S(g) + S(g) + ZnClZnCl22(aq)(aq)

ClCl22(g) + NaBr(aq) (g) + NaBr(aq) Br Br22(l) + NaCl(aq)(l) + NaCl(aq)