CHEMISTRY – Chapter 8
Chemical Equations and Reactions
Chapter 8 – Section 1
Objectives:
1. List three observations that suggest that
a chemical reaction has taken place.
2. List three requirements for a correctly
written chemical equation.
3. Write a word equation and a formula
equation for a given chemical reaction.
Chemical Reactions
 A process by which one or more
substances are changed into one or
more different substances
 Reactants – the original substances
 Products – the resulting substances
 Mass is always conserved
Chemical Equation
 Represents the identities and
relative amounts of reactants and
products in a chemical rx
Chemical Reactions
 A new substance must be produced
 The following changes indicate a
chemical reaction:
 Heat and light – release of E
 Production of gas – bubbles
 Formation of a precipitate – a solid
produced in a solution that settles
 Color change
Characteristics of Chemical
Equations
 Chemical Equations must do the
following:
 Represent known facts
 Contain the correct formulas for the
reactants and products
 Diatomic molecules, etc.
 Law of conservation of mass must be
satisfied
 Atoms are not created or destroyed, just
rearranged
Word Equations
 Equations where products and reactants
are represented by words
 Qualitative
 Does not give the whole story
 No quantities are given
 ex.
methane + oxygen → carbon dioxide + water
Formula equation
 Represents reactants and products of
a chemical rx with symbols or
formulas
 Use coefficients to balance
 Small whole number that appears in front
of a formula in an equation
 ex. CH4 + 2 O2 → CO2 + 2 H2O
Symbols in Chemical Equations
 →
- yields
 ↔
- reversible reaction
–
chem rx in which the products
reform the originals reactants






(s)
↓
(l)
(aq)
(g)
heat→
– solid state
- precipitate
– liquid
– aqueous solution (dissolved in water)
– gas
- reactants are heated


0°→
- temperature at which reaction is carried out
MnO →
2
- catalyst – substance used to alter the rate of rx
Review and Assignment
1. List three observations that suggest that
a chemical reaction has taken place.
2. List three requirements for a correctly
written chemical equation.
3. Write a word equation and a formula
equation for a given chemical reaction.
Assignment: HW 8-1 and HW 8-2
Chapter 8 - Section 2
Objectives:
1. Balance a chemical formulas by
inspection.
Chemical Equations

Give us quantitative information
 Coefficients indicate relative, not absolute amounts
of reactants and products

H2 + Cl2 → 2 HCl



Relative masses of the reactants and products of a
chemical rx can be determined from the coefficients

H2 + Cl2 → 2 HCl




1 mole of H2 and 1 mole of Cl2
Yields 2 moles HCl
1 mole H2 = 2 g H2
1 mole Cl2 = 71 g Cl2
Yields 2 moles HCl = 73 g HCl
Reverse reaction for a chemical equation has the
same relative amounts of substances as the forward
reaction
Balancing Chemical Reactions
 1. Identify the names of the reactants and the
products, and write a word equation.
 2. Write a formula equation by substituting correct
formulas for the names of reactants and products.

Don’t forget diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2)
 3. Balance the formula equation.




Balance the different types of atoms one at a time
1st balance atoms of elements that are combined and appear only
once on each side
Balance polyatomic ions that appear on both sides
Balance H and O atoms
 4. Count atoms to be sure that the equation is
balanced.
Example
 1. zinc + hydrochloric acid → zinc chloride + hydrogen
 2. Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)
 3. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
 4. 1 Zn + 2H and 2 Cl → 1 Zn and 2 Cl + 2 H
Demo.

Zinc + hydrochloric acid → zinc chloride + hydrogen gas

Aluminum sulfate + calcium chloride → aluminum chloride +
calcium sulfate

Balance the equations from the previous demonstrations

Zn + HCl → ZnCl2 + H2

Zn + 2 HCl → ZnCl2 + H2

Al2(SO4)3 + CaCl2 → AlCl3 + CaSO4

Al2(SO4)3 + 3 CaCl2 → 2AlCl3 + 3 CaSO4
Review and Assignment
1. Balance a formula equation by inspection.
Assignment: HW 8-3 and HW 8-4
Chapter 8 - Section 2
Objectives:
1. Define and give general equations for
synthesis, decomposition, singlereplacement, and double-replacement
reactions.
2. Classify a reaction as synthesis,
decomposition, single-replacement, and
double-replacement, or combustion.
Types of Chemical Reactions
1. Synthesis reaction
 A + X → AX
2. Decomposition
 AX → A + X
3. Single-Replacement
 A + BX → AX + B
 Y + BX → BY + X
4. Double-Replacement
 AX + BY → AY + BX
Synthesis
 2 or more substances combine to
produce a new compound
 Also known as composition reaction
 Forms oxides with oxygen and metals
Decomposition
 A single compound undergoes a
reaction that produces two or more
simpler substances
 Most are endothermic when energy is
supplied by electricity or heat
 ex. electrolysis – decomposition of a
substance by an electric current
Single-Replacement
 One element replaces a similar
element in a compound
Also called displacement
Many are in aqueous solutions
Less E required
The more active element replaces the
less active one
 Most active metals (group 1) react
w/water and produce metal hydroxides




Double-Replacement
 Ions of two compounds exchange
places in an aqueous solution to form
two new compounds
 One compound formed is usually a
precipitate, an insoluble gas, or a
molecular compound (usually water)
 Other compound remains dissolved
Combustion Reaction
 A substance combines with oxygen,
releasing a large amount of E in the
form of light and heat
 Always have oxygen as a reactant
Review and Assignment
1. Define and give general equations for
synthesis, decomposition, singlereplacement, and double-replacement
reactions.
2. Classify a reaction as synthesis,
decomposition, single-replacement, and
double-replacement, or combustion.
Assignment: WS, HW 8-5, HW 8-7
Chapter 8 - Section 3
Objectives:
1. Explain the significance of an activity
series.
2. Use the activity series to predict whether
a given reaction will occur and what the
products will be.
Chemical Activity
 The ability of an element to react
 Metals lose e- easily
 Non-metals gain e- easily
Activity Series
 A list of elements organized according to
ease with which the elements undergo
certain chemical reactions
 Metals – greater activity = greater ease to lose e Non-metals – greater activity = greater ease to
gain e Order is determined by single-replacement
reactions
 The metal at the top can replace anything
beneath it
 Predicts if a reaction will actually take place
 d/o what they are reacting with (water vs. acid)
Examples
 2 Al
(s)
+ 3 ZnCl2(ag) → 3 Zn(s) + 2AlCl3(aq)
 Al will replace Zn
 This reaction will occur
 Co(s) + 2 NaCl(s) → no reaction
 Cr(s) + H2O(l) → no reaction
(b/c water, not steam)
Examples
 Zn(s) + H2O(l)

50°C→
No reaction (b/c water is not steam)
 Sn(s) + O2(g) →

yes, Sn reacts w/oxygen
 Cd(s) + Pb(NO3)2(aq) →

yes, Pb is below Cd
 Cu(s) + HCl(aq) →

no, Cu is below H2
Review and Assignment
1. Explain the significance of an activity
series.
2. Use the activity series to predict whether
a given reaction will occur and what the
products will be.
Assignment: HW 8-8