ReactionsReactions

Chapter 8Chapter 8

Chemical Reaction EquationsChemical Reaction EquationsA reaction equation must…A reaction equation must…

Represent all known factsRepresent all known factsContain correct formulas for reactants and Contain correct formulas for reactants and

productsproductsFollow the law of conservation of mass.Follow the law of conservation of mass.

Reactants/ProductsReactants/ProductsReactants are the substances present Reactants are the substances present

when the reaction starts.when the reaction starts.Products are the substances formed after Products are the substances formed after

the reaction takes place.the reaction takes place.

Writing EquationsWriting EquationsGiven a word equation you can write a Given a word equation you can write a

formula equation. formula equation. This is done by…This is done by…

Determining the reactants and productsDetermining the reactants and productsWriting the correct formula for themWriting the correct formula for themBalancing the reaction.Balancing the reaction.

Equation SymbolsEquation Symbols An arrow is used in a formula equation to An arrow is used in a formula equation to

represent the change from reactants to products. represent the change from reactants to products. Arrows always point to the products. Arrows always point to the products.

Any conditions under which the reaction takes Any conditions under which the reaction takes place are written above the arrow.place are written above the arrow.

A reversible reaction that can proceed in both A reversible reaction that can proceed in both directions is shown using a double arrows.directions is shown using a double arrows.

The phases of the products can be shown in The phases of the products can be shown in parenthesis as a subscript after the substance. parenthesis as a subscript after the substance. Common phases are (s) solid, (l) liquid, (g) gas, Common phases are (s) solid, (l) liquid, (g) gas, and (aq) aqueous.and (aq) aqueous.

Types of ReactionsTypes of ReactionsThere are many ways to classify chemical There are many ways to classify chemical

reactions.reactions.One way breaks the reactions down into One way breaks the reactions down into

five basic types:five basic types:SynthesisSynthesisDecompositionDecompositionSingle displacementSingle displacementDouble displacementDouble displacementCombustionCombustion

Synthesis Synthesis In a synthesis reaction, two or more In a synthesis reaction, two or more

substances combine to form a new substances combine to form a new compound.compound.

A + X A + X AX AX

Iron metal reacts with oxygen to form iron III oxide (rust).4Fe + 3O2   -->    2Fe2O3

The copper Statue of Liberty reacting with oxygen to form the green copper oxide.2Cu + O2   -->    2CuO

Sodium metal reacts with chlorine gas to form sodium chloride (table salt).2Na + Cl2   -->     2NaCl

Decomposition Decomposition In a decomposition reaction, a single In a decomposition reaction, a single

compound undergoes a reaction that compound undergoes a reaction that produces two or more simpler substances.produces two or more simpler substances.

AX AX A + X A + X

Carbonic acid decomposes to form water and carbon dioxide.

H2CO3   -->    H2O + CO2

Single DisplacementSingle Displacement In a single displacement reaction, one lone In a single displacement reaction, one lone

cation replaces a cation within a cation replaces a cation within a compound.compound.

A + BX A + BX B + AX B + AX

Copper wire reacts with silver nitrate solution to form silver and copper nitrate solution

Cu + 2AgNO3  -->    Cu(NO3)2 + 2Ag2Na + 2HOH  -->    2NaOH + H2

*Can form precipitates

Activity SeriesActivity Series To determine whether a single cation has the To determine whether a single cation has the

ability to replace another in a compound ability to replace another in a compound depends on the activity series.depends on the activity series.

This shows the ease with which the elements This shows the ease with which the elements undergo change in a reaction.undergo change in a reaction.

An element can replace any element that falls An element can replace any element that falls below it on the activity series.below it on the activity series.

If the element it is trying to replace is above it on If the element it is trying to replace is above it on the chart, it will not be able to replace it and no the chart, it will not be able to replace it and no reaction will occur.reaction will occur.

Double DisplacementDouble Displacement In a double displacement reaction, the In a double displacement reaction, the

cations of two compounds exchange cations of two compounds exchange places to form two new compounds.places to form two new compounds.

AX + BY AX + BY AY + BX AY + BX

Magnesium carbonate (an antacid) reacts with hydrochloric acid (found in the stomach) to form

magnesium chloride and carbonic acidMgCO3 + 2HCl   -->    MgCl2 + H2CO3

*Can form precipitates

CombustionCombustion In a combustion reaction, a substance In a combustion reaction, a substance

combines with oxygen gas to produce a combines with oxygen gas to produce a large amount of energy in the form of heat large amount of energy in the form of heat and light.and light.

Hydrocarbons burn completely in oxygen Hydrocarbons burn completely in oxygen to produce carbon dioxide and water to produce carbon dioxide and water vapor.vapor.

Predicting ProductsPredicting ProductsGiven the reactants and the type of Given the reactants and the type of

reaction, the products of the reaction can reaction, the products of the reaction can be predicted.be predicted.

To do so, the elements must rearrange in To do so, the elements must rearrange in the appropriate way and the correct the appropriate way and the correct formulas for the new compounds must be formulas for the new compounds must be written.written.

Balancing Balancing To balance you must begin by counting To balance you must begin by counting

the number of atoms of each element the number of atoms of each element present on the reactant side and product present on the reactant side and product side.side.

Coefficients must be added that balance Coefficients must be added that balance the number of atoms of the element.the number of atoms of the element.

Coefficients not only change the one Coefficients not only change the one element by also any other element within element by also any other element within the compound.the compound.

Balancing HintsBalancing Hints Write a list of the elements below the reaction Write a list of the elements below the reaction

with the number that exist.with the number that exist. Cross out these numbers and change them Cross out these numbers and change them

accordingly as you go.accordingly as you go. Start with the elements that only appear once on Start with the elements that only appear once on

both sides of the equation.both sides of the equation. Polyatomic ions are balanced as single units, not Polyatomic ions are balanced as single units, not

as separate elements.as separate elements. Save hydrogen and oxygen to be balanced last.Save hydrogen and oxygen to be balanced last. Count atoms one last time to ensure everything Count atoms one last time to ensure everything

is equal.is equal.