Chemical Reactions

Notes

1. Chemical Reactions

A. Evidence of Chemical Reactions

Four indicators a chemical reaction has occurred:– Color change– Becomes cloudy = Precipitate forms– Bubbles = Gas forms– Heat is produced (hot/light) or absorbed (cold)

B. Chemical Equations

A chemical equation represents a chemical reaction – – Chemicals present before the reaction are

shown to the left of the arrow and are called the REACTANTS.

– Chemical formed by the reaction are shown to the right of the arrow and are called the PRODUCTS.

REACTANTS * PRODUCTS

* arrow – indicates direction of change and is read as “yields” or “produces”

Example - CH4 + 2O2 CO2 + 2H2O

Reactants Yield Products

C. Reaction Rates

• Reaction rate depends on the collisions between reacting particles.

• Successful collisions occur if the particles...

– with each other

– have the correct

– have enough to break bondsbreak bonds

collide

orientation

kinetic energy

• To speed up the rate of the reaction:– Increase (smaller particles or

dissolve in water)– Increase (add more reactant)– Increase (add heat source)– Add

surface area

concentration

temperaturecatalyze/enzyme

• Exothermic reactions release heat– Heat is a product– Feels hot

• Endothermic reactions absorb heat– Heat is a reactant– Feels cold

D. Heat in Reactions

2. Types of Reactions

Double Displacement:

Single Displacement:

Decomposition:

Synthesis:

Combustion:

AB A + B

AB +YZ AZ + YB

A +YB Y + AB

A + B AB

CxHx + O2 CO2 + H2O

Reactants are: 2 compounds

Reactants are: 1 element & 1 Compound

Reactant is: 1 compound

Reactants are: 2 elements or 2 oxides

Reactants are: hydrocarbon and oxygen

Double Displacement reactions

Precipitation Reaction: Double Double Displacement reactions where the driving Displacement reactions where the driving force is formation of a force is formation of a solidsolid. .

Acid-Base/ Neutralization Reaction: Double Displacement reactions where the Double Displacement reactions where the driving force is formation of a driving force is formation of a liquidliquid. (water). (water)

Oxidation Reduction reactions: (redox)

• Driving Force = Transfer of electron• Always in single displacement reactions• Sometimes in Synthesis and Decomposition• Never in Double Displacement

 Example:

Anytime a single element is present a reaction is also classified as redox

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

NaCl(aq)+Na(s) Cl2(g) 22

Zn(NO3)2(aq) + 2 Na(s) 2NaNO3(aq) + Zn(s)

decomposition & redox

synthesis & redox

single displacement & redox

Learning CheckClassify the following reactions:

2 KNO3 2 KNO2 + O2  

2 C2H2 + 5 O2 4 CO2 + 2 H2O 

CaO + H2SO4 CaSO4 + H2O

3 CaCl2 + 2 Na3PO4 Ca3(PO4)2(s) + 6 NaCl

2 Fe + 6 HC2H3O2 2 Fe(C2H3O2)3 + 3 H2

2 KCl + 3 O2 2 KClO3

• The reactants and the products contain the same atoms, but the chemical reaction has changed the way they are grouped.

• In a chemical reaction, atoms are neither created nor destroyed, all atoms present in the reactants must be present in some form in the products.

• Balancing an equation makes sure that there is the same number and type of atom on both sides of the equation.

Law of Conservation of Mass

3. Balancing Chemical Equations

The chemical equation for a reaction provides us with three important pieces of information:

1. Identities of the reactants and products

2. Relative numbers of each atom

3. State of each reactant and product

States of matter are shown with the following symbols:

(s) solid (l) liquid (g) gas (aq) aqueous, dissolved in water

HINTS : – Don’t forget the diatomics – BrINClHOF if they appear

by themselves they must be written as:

Br2, I2, N2, Cl2, H2, O2, F2,– Balance oxygen and hydrogen last.– If you have a split element, make the odd number

even– If you have a polyatomic ion that does not change,

balance it as the ion

Examples –

1. Liquid ethanol, C2H5OH, reacts with oxygen gas to produce carbon dioxide gas and water vapor.

2. Solid potassium reacts with liquid water to form hydrogen gas and potassium hydroxide that dissolves in water.

Examples:1) C3H8(g) + O2(g) CO2(g) + H2O(g)

2) NH4NO2(s) N2(g) + H2O(g)

3) NO(g) N2O(g) + NO2(g)

LEARNING CHECK:

HNO3(aq) NO2(g) + H2O(l) + O2(g)

Mg(s) + H2O(l) Mg(OH)2(s) + H2(g)

1. Soluble solid – readily dissolves in water

2. Insoluble and slightly soluble solid – a solid where such a tiny amount dissolves in water that it is undetectable to the naked eye

3. Solubility is temperature dependent

State is aqueous - (aq)

State is solid - (s)

4. States of Substances

States of reactants can be manipulated

Solubility Rules (on snoopy sheet)

Ex. Predict whether the following substances are soluble or insoluble.AgNO3

Al(OH)3

Cu3PO4

Li2CO3

ZnSO4

(aq)

(aq)

(aq)

(s)

(s)

Solubility RulesMainly water soluble (aq)All nitrates are soluble.All acetates are soluble.All chlorates are soluble.All chlorides are soluble except AgCl, Hg2Cl2, and

PbCl2All bromides are soluble except AgBr, Hg2Br2, PbBr2,

and HgBr2

All iodides are soluble except AgI, Hg2I2, PbI2, and HgI2

All sulfates are soluble except CaSO4, SrSO4, BaSO4, PbSO4, Hg2SO4, and Ag2SO4

Mainly water insoluble (s)All sulfides are insoluble except those of 1A and 2A

elements and (NH4)2SAll carbonates are insoluble except those of 1A and

(NH4)2CO3

All phosphates are insoluble except those of 1A and (NH4)3PO4

All hydroxides are insoluble except those of 1A, Ba(OH)2, Sr(OH)2 and Ca(OH)2

Learning Check

• Try these: Determine if the following are AQUEOUS or SOLID

1. lead (II) nitrate

2. potassium sulfide

3. barium hydroxide

4. ammonium carbonate

• Turn to your neighbor and compare answers

When a soluble ionic salt dissolves in water the ions separate and a hydration shell is formed around each ion (Dissociation)

Ex. Ba(NO3)2 in water –

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

-1(aq)

Al2(CO3)3 (s) Al2(CO3)3 (s) or No RXN

Aluminum carbonate

Draw a beaker of dissociated sodium chloride; a beaker of dissociated Aluminum nitrate;

and a beaker of silver chloride.

Na+1

Na+1

Na+1

Na+1

Cl-1

Cl-1

Cl-1

Cl-1

Cl-1Na+1

Cl-1

Na+1

= sodium ion

= chloride ion

Al+3

Al+3

NO3-1

NO3-1

NO3-1

NO3-1

NO3-1

NO3-1

Al+3

NO3-1

= aluminum ion

= nitrate ion

Cl-1

Ag+1 Cl-1Cl-1

Cl-1Ag+1

Ag+1

Ag+1

Cl-1

Ag+1 = silver ion

= chloride ion

Special Rules

1. Acids are aqueous

2. Most metal oxides are solids

3. Most non-metal oxides are gases

Learning Check

• Determine the state:1. lead(II) phosphate

2. magnesium oxide

3. nickel

4. dinitrogen monoxide

5. chlorine

6. sulfuric acid

7. sodium sulfide

5. Predicting Whether a Reaction Will Occur

A. Four Driving Forces1. Formation of a solid (precipitate)

2. Formation of water

3. Transfer of electrons

4. Formation of a gas

B. If a driving force occurs the reaction will take place.

Steps for Predicting Products

1.Formulas for reactants

2.Type of Reaction

3.Predict Product formulas (using ions)

4.States (using solubility rules)

5.Balance Equation or NO REACTION (if missing driving force)

6. Predicting Products

A. Double Displacement reactions: two compounds combine to produce two different compounds - Acid-Base

and Precipitation Reactions.

* Use solubility rules.

General Equation:

Example:

AB + YZ AZ + YB

Lead (II) acetate + sodium chloridePb(C2H3O2)2 (aq)

+ NaCl(aq)

PbCl2 (s)+ NaC2H3O2 (aq)

2 2

Driving force = liquid

Driving force = solid

Pb+2 C2H3O2-1 Na+1 Cl-1

B. Single Replacement Reactions: Activity Series

Single Displacement reactions: an element and a compound combine to form a new element and compound.

**Use the activity series**

General Equation:

 

Example:

 

A + YB Y + AB (Cation) B + AZ Z + AB (Anion)

Sodium + Lead (II) acetateNa+1 Pb+2 C2H3O2

-1

Na(s) + Pb(C2H3O2)2(aq) Pb(s) + NaC2H3O2 (aq)2 2

*If you don’t know the charge use +2

Activity Series

• Active metal elements can replace less active metals, active nonmetal elements can replace less active nonmetals.

• Use the Activity series (snoopy sheet) to determine whether or not the reaction will occur.

• Driving force is the transfer of electrons.

1.___ I2 + ___ NaCl

2. ___ H2SO4 + ___ NaOH

Learning Check

7. Rules for Oxidation Numbers

1. The oxidation number of any uncombined element is 0.

2. The oxidation number of a monatomic ion equals the charge on the ion.

3. The more-electronegative element in a binary compound is assigned the number equal to the charge it would have if it were an ion.

4. The oxidation number of fluorine in a compound is always -1.

5. Oxygen has an oxidation number of -2 unless it is combined with F (when it is +2), or it is in a peroxide (such as H2O2 or Na2O2), when it is -1.

6. The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1.

Oxidation vs. Reduction

• The sum of the oxidation states in a neutral compound must equal zero and must be equal to the overall charge in an ionic compound.

Assign oxidation states to each of the atoms in the following compounds:

a. FeF2 b. H2O c. KMnO4

d. C2H6 e. ICl5 f. SO42-

2(-1)1(+2) 1(-2)2(+1) 4(-2)1(+1)+7

6(+1)2(-3) 5(-1)+5 4(-2) +2+6

Fe = +2, F = -1

C = -3, H = +1

H = +1, O = -2 K = +1, Mn = +7, O = -2

I = +5, Cl = -1 S = +6, O = -2

Assign oxidation states to each atom in the equation.Fe2O3 + 2 Al Al2O3 + 2 Fe

0 0-2+3 -2+3Fe gains electrons. It has been REDUCED

Al lost electrons. It has been OXIDIZED

Remember LEO GER: LOSE ELECTRONS OXIDATIONGAIN ELECTRONS REDUCTION

Something that is reduced is called an oxidizing agent.Something that is oxidized is called a reducing agent.

For each reaction, identify the atoms that undergo reduction, or oxidation.

a. 2 H2 (g) + O2 (g) 2 H2O (g)

b. Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

c. 2 AgCl (s) + H2 (g) 2 H+ (aq) + 2 Ag (s) + 2 Cl- (aq)

d. 2 MnO4-(aq) + 16 H+

(aq) + 5 C2O42-

(aq) 2 Mn2+(aq) + 10 CO2(g) + 8 H2O(l)

0 0 -2+1

0 +2 +2 0

-1+1 0 +1 0 -1

-2+7 +1 -2+3 +2 -2+4 -2+1

oxidized: hydrogen (0 to +1)reduced: oxygen (0 to -2)

oxidized: zinc (0 to +2)reduced: copper (+2 to 0)

oxidized: hydrogen (0 to +1)reduced: silver (+1 to 0)

oxidized: carbon (+3 to +4)reduced: manganese (+7 to +2)

Learning Check

Assign oxidation states to each of the atoms in the following compounds:

a. SO2 b. S c. SO3

Assign oxidation states to each atom in the equation. Determine which element was oxidized and which was reduced.

2 Al + 3 CuCl2 2 AlCl3 + 3 Cu