Chemical ReactionsChemical ReactionsChapter 8Chapter 8

Chemical Chemical reactions…reactions…

involve a rearrangement of involve a rearrangement of the ways atoms are the ways atoms are grouped together. grouped together.

Ex:Ex:

Remember, in a chemical Remember, in a chemical change, a new substance is change, a new substance is formed!formed!

Chemical ReactionChemical Reaction – –

2H2 + O2 → 2H2O

Signs a Chemical Signs a Chemical Reaction has occuredReaction has occured

1.1. Color ChangeColor Change

2.2. Solid formsSolid forms

3.3. Bubbles formBubbles form

4.4. Heat and/or flame produced Heat and/or flame produced or heat is absorbedor heat is absorbed

Parts of a chemical reaction:

• Reactants – substances that are reacting; the starting substances• Products – new substances that are made; substances formed during reaction

2H2 + O2 → 2H2O

Reactants ProductsYieldSign

Physical states of compounds are often given in Physical states of compounds are often given in a chemical equation. a chemical equation.

Conservation of Mass- During a chemical reaction, mass is not created or destroyed.

– Mass is conserved!-All atoms present in the reactants must -All atoms present in the reactants must be present in the products. be present in the products.

-Same number of each -Same number of each type atom on both sides of arrowtype atom on both sides of arrow

reactants products

Balancing Chemical Equations

- The number of atoms of each element must be the same on both sides of the yield sign.

2H2 + O2 → 2H2O

4 Hydrogen2 Oxygen

4 Hydrogen2 Oxygen

-When balancing an equation, you can only change the coefficient before the compound. Never change the subscript!

2Ca(NO3)2

coefficient subscript

- Multiply the coefficient by any subscripts to determine the number of atoms of each element.

2Ca(NO3)2

coefficient subscript2 x 1 = 2

For Calcium:2 calcium atoms4 nitrogen atoms12 oxygen atoms

For Nitrogen:For Oxygen:

2 x 2 = 4 distribute -

distribute -

just like

just like

math cla

ss!

math cla

ss!

22x___ 6

2 x 6 = 12

Balancing equations:Balancing equations:

Step 1Step 1: Determine the : Determine the formulasformulas for for reactants and productsreactants and products

Step 2Step 2: Write reactants on the : Write reactants on the leftleft and and products on the products on the rightright

Example:Example:

Aluminum + oxygen Aluminum + oxygen aluminum aluminum oxideoxide

Al + OAl + O22 Al Al22OO33

Step 3Step 3: : CountCount the number of each the number of each element in the reactants and element in the reactants and products.products.

Al + OAl + O22 Al Al22OO33

1 Al, 2 O1 Al, 2 O 2 Al, 3 O 2 Al, 3 O

Step 4Step 4: : BalanceBalance the elements one at the elements one at a time by using coefficients (a time by using coefficients (you you should not write 1sshould not write 1s). ). Do not change Do not change the formulas as you balance!the formulas as you balance!

4Al + 3O4Al + 3O22 2Al 2Al22OO33

4 Al, 6 O4 Al, 6 O 4 Al, 6 O 4 Al, 6 O

Finally,Finally, Step 5Step 5: Check to make sure each : Check to make sure each

atom is balanced, and make sure all atom is balanced, and make sure all the balanced coefficients are in the the balanced coefficients are in the lowest possible ratio.lowest possible ratio.

CH4 + 2O2 → CO2 + 2H2O

Reactants

1 carbon atom

4 hydrogen atoms

2 oxygen atoms

Products

1 carbon atom

2 hydrogen atoms

3 oxygen atoms

4 hydrogen atoms

4 oxygen atoms4 oxygen atoms

What went in came out…the reaction is balanced!

CH4 + 2O2 → CO2 + 2H2O

What went in came out…the reaction is balanced!

2Al + 6HCl → 2AlCl3 + 3H2

Reactants

1 aluminum atom

1 hydrogen atom

1 chlorine atom

Products

1 aluminum atom

2 hydrogen atoms

3 chlorine atoms

6 hydrogen atoms

6 chlorine atoms6 chlorine atom

6 hydrogen atoms

2 aluminum atoms2 aluminum atoms

Congratulations…it’s balanced!

Practice 1. The reaction below shows the process of oxidizing iron. Balance the

equation by calculating the coefficients.

a. 2, 3, 1b. 1, 1, 1c. 4, 3, 2d. 4, 2, 2

34 2

Practice 2. Which chemical equation supports the law of conservation of mass?

a. 2H2O → H2 + O2

b. Zn + HCl → ZnCl2 + H2

c. Al4C3 + H2O → CH4 + Al(OH)3

d. CH4 + 2O2 → CO2 + 2H2O

Practice 3. What are the coefficients that will balance the chemical reaction above?

a. 2, 1, 1b. 3, 4, 2c. 2, 2, 1d. 4, 3, 2

2 2

Practice 4. What is the coefficient for H2O when the equation above is balanced?

a. 1b. 2c. 3d. 4

2K + 2H2O → 2KOH + H2

Ways to Classify ReactionsWays to Classify Reactions

There are 5 general types of There are 5 general types of chemical reactions:chemical reactions:

CombinationCombination DecompositionDecomposition Single-replacementSingle-replacement Double replacementDouble replacement CombustionCombustion

Types of ReactionsTypes of Reactions

Combination or synthesisCombination or synthesis reactions- reactions- occur when two elements combine to occur when two elements combine to form a single compound as a product.form a single compound as a product.

X + Y X + Y XY XY A synthesis reaction may also involve A synthesis reaction may also involve

the combination of an element plus a the combination of an element plus a compound or the combination of two compound or the combination of two compounds, always forming a single compounds, always forming a single compound as a product.compound as a product.

ExamplesExamples::Ca + ClCa + Cl22 CaCl CaCl22PClPCl33 + Cl + Cl22 PCl PCl55NaNa22O + HO + H22O O 2NaOH 2NaOHRules to remember:Rules to remember:Any metallic oxide plus water yields a metallic Any metallic oxide plus water yields a metallic

hydroxide, as inhydroxide, as inKK22O + HO + H22O O 2KOH 2KOHAny nonmetallic oxide plus water yields an acidAny nonmetallic oxide plus water yields an acidNN22OO55 + H + H22O O 2HNO 2HNO33

Decomposition reaction-Decomposition reaction- occurs where one occurs where one compound breaks down into two or more compound breaks down into two or more elements or compounds. Heat, electricity, elements or compounds. Heat, electricity, or a catalyst is usually necessary.or a catalyst is usually necessary.

XY XY X + Y X + Y

ExamplesExamples::

2NaCl 2NaCl 2Na + Cl 2Na + Cl22HH22SOSO33 H H22O + SOO + SO22

Rules to remember:Rules to remember: When heated, When heated, metal carbonatesmetal carbonates break break

down to yield down to yield metal oxides and carbon metal oxides and carbon dioxidedioxide

ExampleExample: : MgCOMgCO33 MgO + CO MgO + CO22

Hydrogen peroxideHydrogen peroxide breaks down into breaks down into water water and oxygen gasand oxygen gas

HH22OO22 H H22O + OO + O22

Carbonic acidCarbonic acid decomposes into decomposes into water and water and carbon dioxidecarbon dioxide

HH22COCO33 H H22O + COO + CO22

Single replacement reaction

(or single displacement)…• occurs when one element displaces another element in a compound.

• A new element and a new compound are formed.

A + BC B + AC

Details about single Details about single replacement reactions… replacement reactions…

Active metals displace less active Active metals displace less active metals or hydrogen from their metals or hydrogen from their compounds in aqueous solution. compounds in aqueous solution. Consult an Consult an activity series of metalsactivity series of metals. .

A + BY A + BY AY + B AY + B

In general, Group I and Group II metals and In general, Group I and Group II metals and aluminum are most active, transition metals are aluminum are most active, transition metals are more active than hydrogen, and the jewelry more active than hydrogen, and the jewelry metals (Cu, Ag, Au) and Hg are the least active. metals (Cu, Ag, Au) and Hg are the least active. You do not need to memorize the activity series.You do not need to memorize the activity series.

ExamplesExamples::

Mg + NiClMg + NiCl22

Zn + CuSOZn + CuSO44

2HNO2HNO33 + Pb + Pb

More single replacement…More single replacement…

Some nonmetals can also displace Some nonmetals can also displace other nonmetals: active other nonmetals: active halogenshalogens replace less active halogens from replace less active halogens from their compounds in aqueous solution. their compounds in aqueous solution.

Fluorine at the top of group 7A is the Fluorine at the top of group 7A is the most active, with activity decreasing most active, with activity decreasing as you move down the group.as you move down the group.

MX + Y MX + Y MY + X MY + X

b) Some nonmetals can also react in single b) Some nonmetals can also react in single replacement reactions: active halogens replace replacement reactions: active halogens replace less active halogens from their compounds in less active halogens from their compounds in aqueous solutions. Fluorine at the top of the aqueous solutions. Fluorine at the top of the group is the most active, with the activity group is the most active, with the activity decreasing as you move down the group.decreasing as you move down the group.

Examples:Examples:

2NaCl + F2NaCl + F22 NaF + Cl NaF + Cl22ClCl22 + 2KI + 2KI 2KCl + I 2KCl + I22

Double replacement Double replacement reactionsreactions

Two compounds (in aqueous Two compounds (in aqueous solution) react to form two new solution) react to form two new compounds. These are also compounds. These are also called exchange reactions.called exchange reactions.

AB + CD AB + CD AD + CB AD + CB

Double replacement reactions require one of Double replacement reactions require one of three kinds of chemical change: three kinds of chemical change:

Formation of a precipitate (solid)Formation of a precipitate (solid) Formation of a gasFormation of a gas Formation of waterFormation of water

Types of double Types of double replacement…replacement…

Formation of a Formation of a precipitateprecipitate (a solid product)(a solid product)

BaNOBaNO3(aq)3(aq) + K + K22CrOCrO4(aq)4(aq) 2KNO 2KNO3(aq)3(aq) + BaCrO + BaCrO4(s)4(s)

Types of double Types of double replacement…replacement…

Formation of a Formation of a gasgas

Since Since HH22COCO33 and and HH22SOSO33 are unstable, if are unstable, if produced in a reaction, they will produced in a reaction, they will decompose.decompose.

HH22COCO33 decomposes into decomposes into HH22O and COO and CO22.. HH22SOSO3 3 decomposes into decomposes into HH22O and SOO and SO22..

Types of double Types of double replacement…replacement…

Formation of Formation of waterwater

Includes acid-base reactions…Includes acid-base reactions…

acid + base acid + base water and salt water and salt

HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO

Involve combining with Involve combining with oxygen oxygen These reactions produce energy so rapidly that a These reactions produce energy so rapidly that a

flame resultsflame results CHCH44(g)(g) + 2O + 2O22(g) (g) CO CO22(g)(g) + 2H + 2H22OO(g)(g)

Combustion Reactions