Chapter 8.1

Chapter 8:

Describing Chemical Reactions

Chemical Equation – uses symbols and formulas to represent the identity and

relative amounts of reactants and products in a chemical reaction.

Chapter 8.1

Indications of a Chemical Reaction

Evolution of heat and light

Production of a gas

Formation of a precipitate

Color change

Word equation – an equation in which the reactants and

products in a chemical reaction are represented by

words.

hydrogen + oxygen water

Magnesium + oxygen

Chapter 8.1

formula equation – represents the reactants and products in a chemical reaction by

their symbols or formulas.

H2 + O2 H2O

A formula equation is not necessarily balanced.

Chapter 8.1

For an equation to be balanced, the same number of each type of atom must be

represented on both sides of the equation.

2 H2 + O2 2 H2O

H=2x2=4 H=2x2=4

O=2x1=2 O=2x1=2

Balance the following chemical Equation:

HCl + Mg MgCl2 + H2

Balancing Chemical Equations

1. Identify the names of reactants and products and write the word equation.

2. Write a formula equation for the reaction by substituting the correct formulas for the reactants and products.

Chapter 8.2

3. Balance the formula equation according to the Law of Conservation of Mass.

4. Count Atoms to be sure that the equation is balanced.

Chapter 8.2

Guidelines for Balancing Equations

Balance the different types of atoms one at a time.

First balance the atoms of elements that are combined and that appear only once on each side of the equation.

Chapter 8.2

Balance polyatomic ions that appear on both sides of the equation as single units.

Balance hydrogen and oxygen atoms after atoms of all other elements have been balanced.

Chapter 8.2

Common Mistakes

Writing incorrect formulas for reactants or products.

Trying to balance the equation by changing the subscripts.

Chapter 8.2

Types of Chemical Reactions:

There are trillions of possible combinations for mixing reactants together to get products.

However, each of those trillions of possible combinations fall into 5 categories or types of chemical reactions:

1. Synthesis

2. Decomposition

3. Single Replacement

4. Double Replacement

5. Combustion

Types of Chemical Reactions

Synthesis Reactions

Also known as a composition reaction

Two or more substances combine to form a new compound.

A + X AX

Types of Reactions Examples:

The combination of an element with oxygen to produce the oxide of the element.

Magnesium + oxygen Magnesium Oxide

Calcium + oxygen Calcium Oxide

Potassium + oxygen

Most metals react with the halogens to form either ionic or covalent compounds.

Berylium + Chlorine Berylium Chloride

Aluminum + Bromine Aluminum Bromide

Strontium + Fluorine

Decomposition Reactions

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

compounds.

AX A + X

A and X can be elements or compounds.

Types of Reactions Examples:

Decomposition of Binary Compounds.

Simplest kind of decomposition reaction2 H2O 2H2 + O2

Electrolysis – The decomposition of a substance by electric current

Types of Reactions

Decomposition of Metal Carbonates

CaCO3 CaO + CO2

Decomposition of Metal Hydroxides

Ca(OH)2 CaO + H2O

** Water and Carbon Dioxide are very stable, form them when you can!!!**

Decomposition of Metal Chlorates

2KClO3 2KCl + 3O2

Decomposition of Acids

H2CO3 (aq) CO2 (g) + H2O (l)

Single Replacement Reactions

One element replaces a similar element in a compound. (aka displacement)

A + BX AX+ B

Y + BX BY+ X

Replacement of a metal in a compound by another metal

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

Types of Reactions Continued

Replacement of hydrogen in water by a metal

Active metals:

2Na (s) + 2H2O(l) 2NaOH(aq) + H2(g)

Less active metals:

3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

Types of Reactions Continued

Replacement of Halogens

Br2(g) + 2KI(aq) 2KBr(aq) + I2(l)

Types of Reactions Continued

Double Replacement Reactions

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

new compounds.

AX + BY AY + BX

Formation of a precipitate

2KI(aq) + Pb(NO3)2(aq)PbI2(s) + 2KNO3(aq)

Formation of a gas

FeS(s) + 2HCl(aq) H2S (g) + FeCl2(aq)

Formation of water Acid and Base Reaction

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

Types of Reactions Continued

Combustion Reactions

A substance combines with oxygen, releasing a large amount of energy in the

form of heat and light

2C2H6(g) + 7 O2(g) 4CO2(g) +6H2O(g)

Types of Reactions Continued