Chemical Reactions

Section 1: Objectives

–Identify the parts of a chemical equation

–Learn how to write a chemical equation

–Learn how to balance a chemical equation.

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

Chemical reactions are described by chemical equations.

A chemical equations represents, with symbols and formulas, the identities and relative amounts of the starting material and products.

Indications of a Chemical Reaction

1) Evolution of heat and light.

2) Production of a gas.

3) Formation of a precipitate (solid).

(a solid appears after two liquids are mixed)

4) Color change

All chemical reactions…

have two parts:1. Reactants = the substances you

start with (left side of arrow)2. Products = the substances you end

up with (right side of arrow)

The reactants will turn into the products.

Reactants Products

In a chemical reaction

Atoms aren’t created or destroyed (according to the Law of Conservation of Mass)

A reaction can be described several ways:

#1. In a sentence every item is a word:

Copper reacts with chlorine to form

copper (II) chloride.

Symbols in equations

the arrow (→) separates the reactants from the products (arrow points to products)

– Read as: “reacts to form” or yields

(s) after the formula = solid: Fe(s)

(g) after the formula = gas: CO2(g)

(l) after the formula = liquid: H2O(l)

(aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution

↑ used after a product indicates a gas has been produced: H2↑

used after a product indicates a solid has been produced: PbI2↓

Symbols in equations

■ shows that heat is supplied to the reaction

Symbols in equations

■ Additional symbols on page 246 of textbook.

Formula Equations:

Represents the reactants and products by symbols and formulas:

Fe(s) + O2(g) Fe2O3(s)

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

NO2(g) N2(g) + O2(g)

Balanced Chemical Equations Atoms can’t be created or destroyed in

an ordinary reaction:

–All the atoms we start with we must end up with (meaning: balanced!)

A balanced equation has the same number of each element on both sides of the equation.

Rules for balancing:1) Assemble the correct formulas for all the

reactants and products, using “+” and “→”2) Count the number of atoms of each type

appearing on both sides3) Balance the elements one at a time by

adding coefficients (the numbers in front) (order of balancing on next slide)

4) Double-Check to make sure it is balanced.

1) Metals

2) Non-metals

3) save balancing the H and O until LAST!

(hint: I prefer to save O until the very last)

Rules for balancing:

Never change a subscript to balance an equation (You can only change coefficients)

– If you change the subscript (formula) you are describing a different chemical.

– H2O is a different compound than H2O2

Never put a coefficient in the middle of a formula; they must go only in the front

2NaCl is okay, but Na2Cl is not.

Rules

Practice Balancing Examples

_AgNO3 + _Cu _Cu(NO3)2 + _Ag

_Mg + _N2 _Mg3N2

_P + _O2 _P4O10

_Na + _H2O _H2 + _NaOH

_CH4 + _O2 _CO2 + _H2O

2 2

3

4 5

2 22

2 2

Section 2Types of Chemical Reactions

OBJECTIVES:

–Describe the five general types of reactions.

–Predict the products of the five general types of reactions.

Type of Reactions

Chemical reactions are classified into five general types

Combination (synthesis)

Decomposition

Single Replacement

Double Replacement

Combustion

Combination (Synthesis)

Two or more elements or simple compounds combine to form (synthesize) one product

A + B AB

Combination (Synthesis)

1) Reactions of elements with oxygen and sulfur.

The groups 1 and 2 metals react with oxygen to form oxides.

2 Mg + O2 2 MgO (group 2)

4 Li + O2 2 Li2O (group 1)

Combination (Synthesis)

1) Reactions with oxygen and sulfur.

Sulfur which is right below oxygen reacts in a similar manner

8 Ba + S8 8 BaS

16 Rb + S8 8 Rb2S

Combination (Synthesis)

2) Reactions of metals with halogens

2 Na + Cl2 2 NaCl (Group 1)

2 K + I2 2 KI (Group 1)Mg + F2 MgF2 (Group 2)

Decomposition

One substance is broken down (split) into two or more elements.

These reactions usually require heat

AB A + B

Decomposition

1) Decomposition of binary compounds

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

electricity

2 HgO 2 Hg + O2

Decomposition

2) Decomposition of metal carbonates

CaCO3 CaO + CO2

K2CO3 K2O + CO2

3) Decomposition of metal hydroxides

Ca(OH)2 CaO + H2O

2NaOH Na2O + H2O

Learning Check

Classify the following reactions as

1) combination or 2) decomposition:

___A. H2 + Br2 2HBr

___B. Al2(CO3)3 Al2O3 + 3CO2

___C. 4 Al + 3C Al4C3

Solution

Classify the following reactions as

1) combination or 2) decomposition:

_1_A. H2 + Br2 2HBr

_2_B. Al2(CO3)3 Al2O3 + 3CO2

_1_C. 4 Al + 3C Al4C3

Single Replacement

One element replaces a similar element in a compound.

A + BX AX + B

Zn + 2HCl ZnCl2 + H2

Fe + CuSO4 FeSO4 + Cu

Many single replacement reactions take place in an aqueous solution.

Single Replacement

1) Replacement of a metal by another metal.

Refer to activity series table to determine if the reaction will occur.

2 Al + 3 Pb(NO3)2 3 Pb + 2 Al(NO3)3

Balanced!

Single Replacement

1) Replacement of a metal by another metal.

Refer to activity series table to determine if the reaction will occur.

Pb + Al(NO3)3 No Reaction

Pb is below Al on the activity series chart.

Single Replacement

2) Replacement of hydrogen in water by a metal.

Na + H2O NaOH + H2 (g)

Unbalanced!

Group 1 metals react vigorously with water.

Single Replacement

2) Replacement of hydrogen in water by a metal.

2 Na + 2 H2O 2 NaOH + H2 (g)

Balanced!

Single Replacement

3) Replacement of hydrogen in an acid by a metal.

Mg + 2 HCl MgCl2 + H2 (g)

balanced!

Group 1 and 2 metals can react with acids to produce hydrogen gas.

Double Replacement

Two elements in reactants exchange places.

AX + BY AY + BX

A and B are metals

X and Y are nonmetals

One new product is usually a precipitate or gas

Double Replacement

1) Formation of a precipitate.

MgI2 + Pb(NO3)2 Mg(NO3)2 + PbI2 (s)

balanced

Exchange of metals in the reaction.

PbI2 is a solid that precipitates out of the reaction.

Double Replacement

2) Formation of a gas.

CaS + 2 HCl H2S (gas) + CaCl2

balanced

Calcium sulfide reacts to form hydrogen sulfide gas.

Double Replacement

3) Formation of a water.

HCl + NaOH NaCl + H2O (liquid)

balanced

Usually an acid-base reaction

Learning Check

Classify the following reactions as

1) single replacement

2) double replacement

__A) 2Al + 3H2SO4 Al2(SO4)3 + 3H2

__B) Na2SO4 + 2AgNO3 Ag2SO4 + 2NaNO3

__C) 3C + Fe2O3 2Fe + 3CO

Solution

Classify the following reactions as

1) single replacement

2) double replacement

1_A) 2Al + 3H2SO4 Al2(SO4)3 + 3H2

2_B) Na2SO4 + 2AgNO3 Ag2SO4 + 2NaNO3

1_C) 3C + Fe2O3 2Fe + 3CO

Combustion

A reaction in which a compound (often carbon)

reacts with oxygen.

C + O2 CO2

CH4 + 2 O2 CO2 + 2H2O

C3H8 + 5 O2 3 CO2 + 4 H2O

C6H12O6 + 6 O2 6 CO2 + 6 H2O

Reaction releases energy in the form of heat or light.

Learning Check

Balance the combustion equation

___C5H12 + ___O2 ___CO2 + ___H2O

Solution

Balance the combustion equation

1 C5H12 + 8 O2 5 CO2 + 6 H2O

End

Oxidation and Reduction

Reactions that involve a loss or gain of electrons

Occurs in many of the 4 types of reactions and combustion

Important in food metabolism, batteries, rusting of metals

Requirements for Oxidization-Reduction

Electrons are transferred

Two processes occur

Oxidation = Loss of electrons (LEO)

Zn Zn2+ + 2e-

Reduction = Gain of electrons (GER)

Cu2+ + 2e- Cu

Balanced Red-Ox Equations

Combine the oxidation and reduction reactions to make

Loss of electrons = Gain of electrons

Zn + Cu2+ + 2e- Zn2+ + 2e- + Cu

Zn + Cu2+ Zn2+ + Cu

Gain/Loss of Hydrogen

In organic and biological reactions

oxidation = Loss of H

reduction = Gain of H

Learning Check R3

Identify the following as an 1) oxidation or a reduction process:

__A. Sn Sn4+ + 4e-

__B. Fe3+ + 1e- Fe2+

__C. Cl2 + 2e- 2Cl-

Solution R3

Identify the following as an 1) oxidation or a reduction process:

1_ A. Sn Sn4+ + 4e-

2_ B. Fe3+ + 1e- Fe2+

2_ C. Cl2 + 2e- 2Cl-

Learning Check R4

In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction

Ag+ + Cl- Ag + Cl

A. Which reactant is oxidized

1) Ag+ 2) Cl- 3) Ag

B. Which reactant is reduced?

1) Ag+ 2) Cl- 3) Cl

Solution R4

In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction

Ag+ + Cl- Ag + Cl

A. Which reactant is oxidized

2) Cl- Cl- Cl + e-

B. Which reactant is reduced?

1) Ag+ Ag+ + e- Ag