Unit 8 Chemical Reactions

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Unit 8 Unit 8 Chemical ReactionsChemical Reactions

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Describing Chemical Reactions

• A chemical reaction is the process by which one or more substances are changed into one or more different substances.

•They are described by chemical equations

Describing Chemical Reactions

• In a chemical reaction:– bonds are broken in the reactants

–Bonds are formed in the products

Parts of a Chemical Parts of a Chemical EquationEquation

Reactants•original substances in a chemical reaction

•written on the left hand side of a chemical equation

Products•The resulting substances produced in a chemical reaction.

•written on the right hand side of the chemical equation

yields

Subscripts in (parenthesis)

•represents the physical states of the compounds (elements)

•Ex: (s)= solid, (l)=liquid, (g)= gas, (aq)= aqueous (dissolved in water)

•Words or symbols are placed over/under the arrow(s) to indicate certain conditions under which the reaction is carried out.

– : heat is applied– catalyst

• substance that speeds up a chemical reaction without becoming part of the reaction.

Law of conservation of mass

According to the law ofconservation of mass, •the total mass of reactants must equal the total mass of products for any given chemical reaction.

Law of conservation of mass

Mass must be equal50 g + 45 g

Balance atoms H2 (g) + O2 (g) H2O (g)22

Translate Chemical Translate Chemical Equations into WordsEquations into WordsTranslate Chemical Translate Chemical

Equations into WordsEquations into Words

Chemical Equations

•1) include all of the symbols appearing in the equation (with the exception of the

coefficients)

Chemical Equations•3. Diatomic Elements: Elements that combine with each other when found alone in nature

Diatomic Elements•7 diatomic elements (know)

H2 O2 F2 Br2 I2 N2 Cl2

Example #2Mg(s) + 2HCl(aq) MgCl2(aq) +

H2(g)Solid magnesium

reacts with

aqueous hydrochloric acid

to produceaqueous magnesium

chlorideand hydrogen gas

Check for Understanding

CaCO3(s) CaO(s) + CO2(g)

Solid calcium carbonate is heated and produces solid calcium oxide and gaseous carbon dioxide

Check for Understanding #2

3KOH(aq) + Fe(NO3)3(aq) Fe(OH)3(s) + 3KNO3(aq)

Aqueous potassium hydroxide reacts with aqueous iron (III) nitrate to produce solid iron (III) hydroxide and aqueous potassium nitrate

Writing Chemical Equations Writing Chemical Equations from a Written from a Written

DescriptionDescription

Writing Chemical Equations Writing Chemical Equations from a Written from a Written

DescriptionDescription

Example #1

Aluminum metal reacts with oxygen in the air to form solid aluminum oxide.

4 Al (s) + O2 (g)3 Al2O3 (s)2

Example #2

When solid mercury(II) sulfide is heated with oxygen gas, liquid mercury metal and gaseous sulfur dioxide are produced.

+ SO2 (g)HgS (s) + O2

Hg (l)

Oxygen gas can be produced by heating solid potassium chlorate in the presence of the catalyst manganese dioxide. Potassium chloride is a solid residue.

MnO2 + O2 (g)2 2 3KClO3 (s) KCl (s)

Aqueous potassium nitrate and a precipitate of barium chromate are formed when aqueous solutions of barium nitrate and potassium chromate are mixed.

Ba(NO3)2 (aq) + K2CrO4 (aq)

KNO3 (aq) + BaCrO4 (s)2

Types of Chemical Types of Chemical ReactionsReactions

Synthesis (Combination) Reactions

Two or more substances combine to form a new compound.

Formation of only ONE product.

Synthesis (Combination) Reactions

Examples:

2H2 + O2 2H2O

CaO + H2O Ca(OH)2

Decomposition Reactions

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

Decomposition Reactions

Decomposition Reactions Decomposition reactions are the opposite of synthesis reactions.

There is only one reactant.

Examples

•CaCO3 CaO + CO2

•Na2CO3 Na2O + CO2

Single-Replacement Reactions

• one element replaces a similar element in a compound.

• a single element reacts with a compound.

Single-Replacement Reactions

Distinguishing Characteristics

• Examples:

Pb + 2HNO3 Pb(NO3)2 + H2

F2 + 2NaI 2NaF + I2

Predict the Products

Zn + CuCl2 ZnCl2 + Cu

Double Replacement Reactions

The ions of two compounds exchange places in an aqueous solution to form two new compounds.

Two ionic compounds “swap” ions.

Double Replacement Reactions

Usually forms a precipitate

• precipitate: when two aqueous solutions are mixed and a solid product that is formed

AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)

Precipitate

Examples

• PbSO4 + 2KCl K2SO4 + PbCl2

• AgNO3(aq) + KCl(aq) ‑‑‑‑> AgCl(s) + KNO3(aq)

H2O 2KOH + H2SO4 K2SO4 +2HOH

Combustion Reactions

A hydrocarbon (CxHy) combines with oxygen, releasing a large amount of energy in the form of light and heat.

General Form

CxHy + O2 CO2 + H2O

Example

• C3H8 + 5O2 3 CO2 + 4 H2O

• CH4 + O2 CO2 + H2O

H2O C2H6 + O2 CO2+2 4 12101 5 2 6

1) 2 Li + 2 H2O 2 LiOH + H2

single replacement / displacement

2) NH4Cl NH3 + HCl

decomposition

3) AgNO3 + NaCl AgCl + NaNO3

double replacement / displacement

4) 2 C6H14 + 19 O2 12 CO2 + 14 H2O

combustion

5) N2 + 3 H2 2 NH3

combination / synthesis

Oxidation – Oxidation – ReductionReductionProcessesProcesses

Redox ReactionAny chemical reaction that involves the transfer of one or more electrons between atoms. One reactant gains electrons and the other reactant loses electrons.

Examples of Redox Reactions

•Combustion of gasoline •Burning of wood•Energy from food•Bleaching stains•Iron rusting

Oxidation and Reduction

• Opposing reactions

•Must occur together–Redox (Reduction – Oxidation)

Oxidation•Electrons are fully or partially

Mg0 (s) Mg 2+ + 2e-

Electrons lost: written as a product

Reduction• Electrons are fully or

partially gained

S0(s) + 2 e- S2-

electrons gained: written as a reactant

Mnemonic Device

• LEO the lion goes GER

LEO: Loss of Electrons is Oxidation

GER: Gain of Electrons is Reduction

Mnemonic Device

•OIL RIG

–Oxidation Is Loss of electrons

–Reduction Is Gain of electrons

Complete Redox Reaction

• Add together the reduction half-reaction with the oxidation half-reaction to get the complete redox reaction.

Complete Redox Reaction

Mg0 (s) Mg 2+ + 2e-

S0(s) + 2 e- S2-

Mg0 (s) + S0

(s) Mg 2+ + S2-

Mg + S Mg+2+ S -2

Assigning Oxidation Assigning Oxidation NumbersNumbers

Oxidation Numbers•Numbers assigned to all of

the elements involved in the reaction to determine if electrons have been transferred between atoms in a reaction.

Oxidation Numbers

• The oxidation number is usually equal to the charge on the ion if it was formed.

Rule #1•Free elements are assigned

an oxidation state of 0.

–Al = 0–Na = 0

–H2 = 0

Rule #2• The oxidation state for any

simple one-atom ion is equal to its charge.

–Na+ = +1–Be2+= +2–F- = -1

Rule #3•The alkali metals (Li, Na, K, Rb,

Cs and Fr) in compounds are always assigned an oxidation state of +1.

–LiOH: Li = +1–Na2SO4: Na= +1

Rule #4

•Fluorine in compounds is always assigned an oxidation state of -1.

–HF: F= -1

–MgF2: F= -1

Rule #5• The alkaline earth metals (Be,

Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2. Similarly, Al & Ga are always +3.

•MgF2: Mg = +2

•CaO: Ca= +2

•Al2O3: Al=+3

Rule #6• Hydrogen in compounds is assigned an

oxidation state of +1.

Exception - Hydrides, ex. LiH (H=-1).

•H2SO4: H=+1

•HCl: H=+1

Rule #7

• Oxygen in compounds is assigned an oxidation state of -2.

Exception - Peroxide, ex. H2O2 (O = -1).

– H3PO4: O=-2

– H2O: O=-2

Rule #8

•The sum of the oxidation numbers of all atoms in a neutral compound is 0.

–H2SO4: add up to 0–CO2: add up to 0

Rule #9

•The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge on the ion.

–SO42-: add up to -2

–NH4+: add up to +1

Examples•Al(s):

–Al = ____

Rule #1

Examples•CaCl2:

–Ca = ____–Cl=____

Rule #5

Ca+2 Rule #8

1 (+2) + 2 (Cl) = 0

Cl = -1

Examples•HNO3:

–H= ____–N= ____–O= ____

Rule # 6

Rule # 7

Rule # 8+1

-2 1(1) + 1(N) + 3(-2) = 0

N = +5

Examples• SO4

– S= ____– O= ____

Rule #7

Rule #9

1(S) + 4(-2) = -2+6

Examples• H2O:

– H= ____– O= ____

Rule #6

+1Rule #7

Examples•(NH4)2CO3

NH4+ CO3

N=____ C=___ H=____ O=___

Rule 6

Rule 7

Rule 9

1(N)+4(+1)=+1N= -3

1(C)+3(-2)=-2C=+4

• S8:

– S= ____

• AsO43-:

– As= ____ – O= ____

• Cr(OH)3:

– Cr= ____ – O= ____ – H= ____

• (NH4)3PO4:

– N= ____ – H=_____ – P= ____ – O= ____

NH4+ PO4

Oxidized Element

• Atoms that lose electrons in a chemical reaction.

– Elements that lose electrons are undergoing oxidation and are said to be oxidized.

Oxidized Element

• The substance that loses electrons is the oxidized element.

• Atoms that are oxidized will have an increase in their oxidation number

Oxidized Element

Mg0 (s) + S0

(s) Mg 2+ + S2-

Increase in oxidation number

Mg is oxidized

Reduced Element

• Atoms that gain electrons in a chemical reaction.

– Elements that gain electrons are undergoing reduction and are said to be reduced.

Reduced Element

• The substance that gains electrons is the reduced element.

• Atoms that are reduced will have an decrease in their oxidation number

Reduced Element

Mg0 (s) + S0

(s) Mg 2+ + S2-

Decrease in oxidation number

S is reduced

Oxidizing Agent•The element or compound that

is reduced.

•It is called the oxidizing agent because it oxidizes the other element or compound.

Oxidizing Agent

Mg0 (s) + S0

(s) Mg 2+ + S2-

Decrease in oxidation number

S is reduced

S is the oxidizing agent(it is oxidizing Mg)

Reducing Agent•The element or compound that

is oxidized.

• It is called the reducing agent because it reduces the other element or compound.

Reducing Agent

Mg0 (s) + S0

(s) Mg 2+ + S2-

Increase in oxidation number

Mg is oxidizedMg is the reducing agent

(it is reducing S)

Examples

• Are the following redox reactions?

• If yes:– What element is oxidized?– What element is reduced?– What is the oxidizing agent?– What is the reducing agent?

Examples

Mg(s) + S(s) MgS

Step #1: Assign Oxidation Numbers

0 0 +2-2

Step # 2: Do the oxidation numbers change?

- if no it is not a redox reaction

- if yes it is a redox reaction

Yes – It is a redoxWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?

Examples

Step #1: Assign Oxidation Numbers

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

NO3- NO3

Yes – It is a redox

Step # 2: Do the oxidation numbers change?

- if no it is not a redox reaction

- if yes it is a redox reaction

What element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?

AgNO3(aq)

• Are the following redox reactions?• If yes:

– What element is oxidized?– What element is reduced?– What is the oxidizing agent?– What is the reducing agent?

4Fe(s) + 3O2(g) 2Fe2O3(s)

0 0 -2+3

YesWhat element is oxidized?What element is reduced?What is the oxidizing agent?What is the reducing agent?

Ca(s) + H2O(l) Ca(OH)2(aq) + H2(g)

0 0+1 -2

+2 +1-2

HCl + NaOH NaCl + H2O +1-1 +1-2+1 +1-1 +1 -2

No – it is not a redox

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

-2 +2-2 0+3 +4-2

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

NO3- NO3

+1 0 +2 +0

AgNO3Cu

NaCl + AgNO3 NaNO3 + AgCl+1-1 +1

NO3- NO3

+1 +1-1

No – it is not a redox

2 H2(g) + O2(g) 2 H2O(l)

0 0 +1-1

CH4 + 2 O2 CO2 + 2 H2O -4+1 0 +4-2 +1-2

Unit 8 Chemical Reactions

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