Chapter 8 Section 1 Describing Chemical Reactions
Objectives
• List evidence that suggests that a chemical reaction has occurred and evidence that proves that a chemical
reaction has occurred.
• Describe a chemical reaction by using a word equation and a formula equation.
• Interpret notations in formula equations, such as those relating to states of matter or reaction conditions.
Chemical Reaction
• A chemical reaction is the process by which one or more substances change into one or more new
substances.
• Reactants are the original substances in a chemical reaction.
• Products are the substances that are created in a chemical reaction.
Evidence of a Chemical Reaction
• solution color changes
• solution bubbles
• copper is used up
Chemical Reaction Versus Physical Change
• chemical change – new substance forms with properties that differ from original substance
• density
• boiling point
• melting point
• physical change - changes of state
• evaporation
• condensation
• melting
• freezing
Reactions and Energy Changes
• Energy can be released in a chemical reaction.
methane + oxygen  carbon dioxide + water + energy
Energy is a product.
• Energy can be absorbed in a chemical reaction.
dinitrogen tetroxide + energy  nitrogen dioxide
Energy is a reactant.
Constructing a Chemical Equation
• A chemical equation shows the chemical formulas and relative amounts of all reactants and products.
• A word equation contains the names of the reactants and products.
• Equations must be balanced.
Writing a Word Equation or Formula Equation
methane + oxygen  carbon dioxide + water
?CH4 + ?O2 
?CO2
+ ?H2O
Equations and Reaction Information
• Physical States
NaHCO3(s) + HC2H3O2(aq)  NaC2H3O2(aq) + CO2(aq) + H2O(l)
solid
liquid
• Reaction Conditions
350°C, 25 000 kPa
Section 2 Balancing Chemical Equations
Objectives
• Relate the conservation of mass to the rearrangement of atoms in a chemical reaction.
• Write and interpret a balanced chemical equation for a reaction, and relate conservation of mass to the
balanced equation.
Reactions Conserve Mass
• Mass cannot be created or destroyed by a chemical or physical change
• Equations must be balanced.
?Na + ?H2O  ?NaOH + ?H2
Balancing Equations
• The number of atoms for each element must be the same on the reactants’ side and on the products’ side.
• A coefficient multiplies the number of atoms of each element in the formula that follows.
H2O: 2 hydrogen atoms, 1 oxygen atom
2H2O: 4 hydrogen atoms, 2 oxygen atoms
Reading a Chemical Equation
Balancing a Chemical Equation by Inspection
Sample Problem A
Balance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water.
Sample Problem A Solution
1. Identify reactants and products.
reactants
products
Fe2O3 + H2  Fe + H2O
2. Count atoms
Reactants Products Balanced?
Unbalanced Fe2O3 + Fe +
formula
H2
H2O
equation
Iron atoms 2
1
No
Oxygen
3
1
No
atoms
Hydrogen 2
2
Yes
atoms
3. Insert coefficients.
Reactions Conserve Mass
• Balanced equations show mass conservation
?Na + ?H2O  ?NaOH + ?H2
2Na + 2H2O  2NaOH + H2
• Never change subscripts to balance equations
Unbalanced: H2 + O2  H2O
Incorrect:
H2 + O2  H2O2
H2O  H2O2
Correct:
2H2 + O2  2H2O
Reactions Conserve Mass
Polyatomic Ions can be balanced as a group
Balancing Equations
Sample Problem C
Aluminum reacts with arsenic acid, HAsO3, to form H2 and aluminum arsenate. Write a balanced equation for this
reaction.
Sample Problem C Solution
1. Identify reactants and products.
reactants
products
Al + HAsO3  H2 + Al(AsO3)3
2. Count Atoms
Reactants Products Balanced?
Unbalanced
Al +
H2 +
formula equation HAsO3 Al(AsO3)3
Iron atoms
Oxygen atoms
Hydrogen atoms
1
1
1
1
2
3
yes
no
no
Section 3 Classifying Chemical Reactions
Objectives
• Identify combustion reactions, and write chemical equations that predict the products.
• Identify synthesis reactions, and write chemical equations that predict the products.
• Identify decomposition reactions, and write chemical equations that predict the products.
• Identify displacement reactions, and use the activity series to write chemical equations that predict the
products.
• Identify double-displacement reactions, and write chemical equations that predict the products.
Combustion Reactions
• A combustion reaction is a reaction of a carbon-based compound with oxygen.
Combustion of propane:
C3H8 + 5O2  3CO2 + 4H2O
Combustion of ethanol:
CH3CH2OH + 3O2  2CO2 + 3H2O
Synthesis Reactions
• In a synthesis reaction a single compound forms from two or more reactants.
• Two elements form a binary compound
C + O2  CO2
2C + O2 2CO
• Two compounds form a ternary compound
CaO(s) + H2O(l)  Ca(OH)2(s)
CO2(g) + H2O(l)  H2CO3(aq)
Decomposition Reactions
• In a decomposition reaction a single compound breaks down, often with the input of energy, into two or
more elements or simpler compounds.
• Decomposition of water
• A metal carbonate decomposes to form a metal oxide and carbon dioxide.
Sample Problem D
Predicting Products
Predict the product(s) and write a balanced equation for the reaction of potassium with chlorine.
Sample Problem D Solution
• Reaction is most likely a synthesis reaction, so the product will be binary
• Potassium will lose one electron to become a 1+ ion.
• Chlorine will gain one electron to become a 1– ion.
K + Cl2  KCl
• Balance the equation.
2K + Cl2  2KCl
Displacement Reactions
• In a displacement reaction a single element reacts with a compound and displaces another element from the
compound.
2Al(s) + 3CuCl2(aq)  2AlCl3(aq) + 3Cu(s)
Aluminum displaces copper.
• The activity series ranks the reactivity of elements
Sample Problem E
Determining Products by Using the Activity Series Magnesium is added to a solution of lead(II) nitrate. Will a
reaction happen? If so, write the equation and balance it.
Sample Problem E Solution
1. Identify the reactants.
Magnesium will attempt to displace lead from lead(II) nitrate.
2. Check the activity series.
Magnesium is more active than lead and displaces it.
3. Write the balanced equation.
Mg + Pb(NO3)2  Pb + Mg(NO3)2
Double-Displacement Reactions
• In a double-displacement reaction two compounds in aqueous solution appear to exchange ions and form
two new compounds.
• One of the products must be a solid precipitate, a gas, or a molecular compound, such as water.
HCl(aq) + NaOH(aq)  HOH(l) + NaCl(aq)
Section 4 Writing Net Ionic Equations
Objectives
• Write total ionic equations for reactions in aqueous solutions.
• Identify spectator ions and write net ionic equations for reactions in aqueous solutions.
• Ionic compounds dissolve in water
KI(aq) = K+(aq) + I–(aq)
Pb(NO3)2(aq) = Pb2+(aq) +
(aq)
• Reaction between KI and Pb(NO3)2
2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
• Total Ionic Equation
2K+(aq) + 2I–(aq) + Pb2+(aq) +
(aq)
 PbI2(s) + 2K+(aq) +
(aq)
• Spectator ions remain unchanged in the solution as aqueous ions. They do not react.
2K+(aq) + 2I–(aq) + Pb2+(aq) +
(aq)
 PbI2(s) + 2K+(aq) +
(aq)
• The net ionic equation is the chemical equation that shows only the net change.
2I–(aq) + Pb2+(aq)  PbI2(s)
• Net Ionic equations can be used for displacement reactions.
Zn(s) + Cu2+(aq) +
(aq)
 Cu(s) + Zn2+(aq) +
(aq)
• net ionic equation
• Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq)
• Check atoms and charge to balance net ionic equations.
(aq) + Ba2+(aq)  BaSO4(s)
Charge:
(2–) + (2+) = 0
0
Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq)
Charge:
2+
2+