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