Chemistry 20

Chemical Reactions

Changes

• Matter can be changed in two ways1. Physical Change2. Chemical Change

Physical Change

• Change in a substances state or shape.– The same combination of matter is still present.

(same properties)

Chemical Change

• When a brand new substance is formed.– The substance will have brand new properties.

Methane OxygenCarbon dioxide

Water

• Many chemical reactions take place in our world everyday that are both helpful and harmful.

Photosynthesis- Conversion of sunlight into sugar

CO2 + H2O → C6H12O6 + O2

?6 6 6

Cellular Respiration- Conversion of sugar into energy

C6H12O6 + O2 → CO2 + H2O66 6

SO3 + H2O ---> H2SO4

Formation of Acid Rain

Clues to Identify a Chemical Change

• There are 5 clues that can help identify a chemical change

1. ‘Unexpected’ colour change. 2. Heat or light is released.3. Gas, or bubbles are produced.4. Solid (precipitate) produced from mixing two

liquids.5. Difficult to reverse

Representing Chemical Changes

• There are several ways to represent what occurs during a chemical change.

• The change can be expressed through:1. Word equations.

Ex. Sodium + Chlorine → Sodium Chloride2. Skeleton equation.

Ex. Na + Cl2 → NaCl3. Balanced chemical equations.

Ex. 2Na + Cl2 → 2NaCl

• Every chemical change contains substance that are:–Reactants (starting materials that will

react)–Products (ending materials that were

made during the reaction)

Sodium + Chlorine → Sodium Chloride

Na + Cl2 → NaCl

Reactants Products

Word Equations

• The chemical change is expressed using the chemical names of the reactants and products.

• The chemical are separated by a ‘+’ sign.• The reaction is indicated by a ‘→’ sign.Ex.

Methane + Oxygen → Water + Carbon Dioxide

Ex. When Cu wire is put in a solution of AgNO3, we form a coat of Ag in a Cu(NO3)2 solution.

• Copper + silver nitrate → Silver + copper (II) nitrate

Skeleton Equations

• The chemical change is expressed using the chemical formulas of the reactants and products.

• The chemicals are separated by a ‘+’ sign.• The reaction is indicated by a ‘→’ sign.Ex.

CH4 + O2 → CO2 + H2O

Ex. When Copper wire is put in a solution of Silver nitrate, we form a coat of Silver in a Copper (II) nitrate solution.

• Cu + AgNO3 → Ag + Cu(NO3)2

Law of Conservation of Mass

• “In a chemical reaction, the total mass of the reactants is always equal to the total mass of the products.”

• Matter cannot be created or destroyed, it can only be moved around.

Methane OxygenCarbon dioxide

Water

• If 7 g of Methane was react with 12 g of oxygen, 11 g of water would be produced and 8 g of carbon dioxide.

• Mass of reactants: 7 g + 12 g= 19 g• Mass of products: 11 g + 8 g = 19 g

• Mass is conserved!

Balanced Chemical Equations

• An equation in which the reactants and products contain equal number of atoms of each element.

• A coefficient is added in front of a molecule or compound.– This indicates the number of molecules or

compounds needed in the equation to follow the law of conservation of mass.

• After the equation is balanced, add the state(phase) of each chemical(g)- gas(l)- liquid(s)- solid(aq)- aqueous (chemical is dissolved in water)

Methane OxygenCarbon dioxide

Water

CH4 + 2O2 2H2O + CO2

coefficients

(g) (g) (l) (g)

Balancing Chemical Equations

• There are 2 methods to balancing chemical equations.

1. Chart method2. Inspection method

Chart Method

• There are several steps to help balance chemical equations with the chart method.

1. Write the skeleton equation.2. Make a list of elements and polyatomic ions in the

reaction.3. Count the numbers of each type of element or

polyatomic ion the reaction.4. Multiply each of the formulas by the appropriate

coefficient to balance the number of atoms.(*Note- try to balance elements that appear only once on the reactant and

product sides first.)

Inspection method

• The inspection method balances atoms in the same chemical, which will lead to the next atom.– Steps1. Pick an atom on the reactant side and balance by

using coefficients with atoms on the product side.2. Balance the other atom in the same chemical on the

product side with the atom from the reactant side.3. Continue until all atoms are balanced.

Cu + AgNO3 → Ag + Cu(NO3)2

Cu

Ag

NO3

1 1

1 1

1 22( )

Multiply by 2 to get equal # of atoms.

2

Coefficients are added to the front of the compound

When one part of the chemical equation is changed every atom much be rechecked.

2 2( )

2

2

2

When all the atoms are equal, the chemical equation is balanced.

(s) (aq) (s) (aq)

Cu + AgNO3 → Ag + Cu(NO3)22 2(s) (aq) (s) (aq)

Start with an atom on the reactant side and balance with the same atom on the product side.

Balance the other atom from the chemical on the product side with the same atom on the reactant side.

Iron reacts with oxygen to form magnetic iron oxide

Fe + O2 → Fe3O4

Fe

O

1 3( )3

3

2 4

3

2( )

2(s) (g) (s)

Iron reacts with oxygen to form magnetic iron oxide

Fe + O2 → Fe3O43 2(s) (g) (s)

Energy in Chemical Changes

• Energy is involved in every chemical reaction.• Energy is needed to break bonds and energy is

needed to make bonds.• The amount of energy gained and lost in a

chemical change is never the same.

Endothermic reaction

• Energy goes into (endo-) the reaction. • More energy goes into the reaction than

comes out of the reaction.• Energy is considered part of the reactants.• The products may feel cooler.

Reactants + energy → Products (positive energy)

Exothermic Reactions

• Energy exits (exo-) the reaction. • More energy leaves the reaction than goes

into the reaction.• Energy is considered part of the products.• The products may feel warmer.

Reactants → Products + energy (negative energy)

Reactions with Catalysts

• A catalyst is a chemical added to a reaction to increase the rate of reaction.

• It is not part of the reaction.– Not a reactant or product.

• To indicate that a catalyst has been used, the catalyst is written above the arrow. (→)

2H2O2 2H2O + O2 MnO2

Catalyst

Types of Reactions

• Most reactions that we will see will fit into 4 major categories.1. Combination2. Decomposition3. Single replacement4. Double replacement

+ +

+ + + +

CombinationDecompositionSingle ReplacementDouble Replacement

Combination

• 2 chemicals combine to form 1 chemical.

+

A + B → ABElement or compound + Element or

compound Compound

A B A B

A

B

Decomposition

• 1 chemicals breaks apart to form 2 chemicals.

+

AB → A + BElement or compound+Element or

compoundCompound

A B BA

AB

Single Replacement

• 1 compound and 1 element rearrange to produce 1 different compound and 1 different element.

A + BC → B + ACCompound+ElementCompound

+ +

Element +

AB

C

AB C

A

BC

• In single replacement reaction:1. Metal elements will replace metal ions in the

compound.2. Diatomic halogen elements will replace halogen

ions in the compound.

• Not every single replacement reaction will occur.

• The Activity Series is used by chemists to determine if a single replacement reaction will occur.

Elements above will replace ions from the compound below it in a single replacement reaction.

Na + AgNO3 → Ag + NaNO3

Ag + NaNO3 → Na + AgNO3

or

I2 + 2NaCl → Cl2 + 2NaI

Cl2 + 2NaI → I2 + 2NaCl

or

Double Replacement

• 2 compounds rearrange to produce 2 different compounds.

AB + CD → AC + BDCompound+CompoundCompoundCompound +

+ +A

B B

AC

D

D

C

C

AB

D

• In a double replacement reaction:– Metal ions replace metal ions• Cations replace cations

– Nonmetal ions (polyatomic ions) replace nonmetal ions (polyatomic ions)• Anions replace anions

• Not every double replacement reaction will result in a successful reaction.

• A Solubility Chart is used to determine if a double replacement reaction will occur.

• For a double replacement to occur, at least one of the products has to be solid, liquid, or gas.– Products cannot all be soluble in water.

• Soluble means that they are dissolved in water and not actually bonded together.– Equations shows what could be produced from

the reaction.

NaCl (aq) + KF (aq) → NaF (aq) + KCl (aq)

Both products are still dissolved in water and have not actually bonded together.

No reaction- products would not be formed.

AlCl3 (aq) + 3KF (aq) → AlF3 (s) + 3KCl (aq)

AlF forms a bond and is no longer dissolved in water. (a precipitate)Reaction occurs- AlF forms a product, KCl remains dissolved in water.

Net Ionic Equations

• There are 2 types of chemicals in a double replacement reaction:– Spectator ions- ions that remain dissolved in water

over the entire reaction.– Reacting ions- ions that form bonds during the

reaction.

• When an ionic compound dissolves it water, it breaks up into its ions.

KBr(s) K+ (aq) + Br-(aq)

MgSO4(s) Mg+2 (aq) + SO4-2

(aq)

CaCl2(s) Ca+2 (aq) + 2Cl-(aq)

Dissolves in water

Dissolves in water

Dissolves in water

• A net ionic equation only includes the ions that form bonds as a product.– Spectator ions are not included in the equation.

• Rules for making net ionic equations:1. Write down the skeleton equation.2. Break apart all aqueous compounds into ions.3. Remove spectator ions. (all aqueous ions)4. Balance the reaction for charge and number of

atoms. (if needed)

Dissolved aluminum chloride is combined with aqueous potassium fluoride to produce a precipitate of aluminum fluoride and potassium chloride remains dissolved in water.

Al ClKF AlF K1.

2.

Cl (aq) (aq) (aq)(aq) (aq) (aq)(s)+++ +++3 -13 3

3

+1 -1 +1 -1

3.4.

Ex. A solution of sodium chloride is added to a solution of silver nitrate, a precipitate of silver chloride is formed.

NaCl(aq) + AgNO3(aq) → AgCl (s) + ?NaNO3(aq)

Na+1(aq) +Cl-1(aq)+ Ag+1(aq)+NO3-1(aq)→AgCl(s)+Na+1(aq)+NO3

-1(aq)

Cl-1(aq)+ Ag+1(aq)→AgCl(s)

Combustion

• Another type of reaction that occurs is a combustion reaction.

• We will look at the combustion of hydrocarbons.

• Combustion is a very rapid reaction between a hydrocarbon and oxygen to produce carbon dioxide and water as products.

Hydrocarbon + Oxygen → Carbon dioxide + Water + Energy*We don’t always include energy in the reaction because it is not

matter.*Ex.

CH4 + O2 → CO2 + H2O (Skeleton)

CH4 + 2O2 → CO2 + 2H2O (Balanced)

Combustion of Butane

C4H10 + O2→ CO2 + H2OC

H

O

4 14( )

44

10 25( )

5

10

2 8 5+ = 13

We cannot easily multiply 2 to equal 13. There are 2 ways to solve this:

1. Fraction

2. Multiplication

1. Fraction- Put the odd number over 2 to balance the oxygen. *(Only used for oxygen in a combustion reaction at this point.)*

( )13 2

13 2

Combustion of Butane

C4H10 + O2→ CO2 + H2O4 513 2

Combustion of Butane

C4H10 + O2→ CO2 + H2OC

H

O

4

44

10

5

10

2 8 5+ = 13

2. Multiplication- Multiply the odd coefficient by 2 and balance.

2 8 108 8

20 20

16 10

13

+ = 2626

Combustion of Butane

C4H10 + O2→ CO2 + H2O4 52 8 1013