Chapter 9 – Chemical Reactions NOTES

Chemical Reactions
o
Chemical reaction:

o
Chemical change
 4 Indicators of Chemical Change:
o (1)
o
(2)
o
(3)
o
(4)
 Consist of reactants (starting materials) and products (substances formed)
Represented by a chemical equation
 Uses symbols and formulas to show the identities and relative amounts of the reactants
and products

Symbols:
Symbol
+
→
Purpose
Separates two or more reactants or products
Separates reactants from products
Separates reactants from products and indicates a reversible reaction
(s)
(l)
(g)
(aq)
Identifies a solid state
Identifies a liquid state
Identifies a gaseous state
Identifies a water (aqueous) solution
Shows heat added
H2SO4
Shows a catalyst added (Catalyst - something that speeds up a chemical reaction, but is
not used up in the reaction)


Example of a Chemical Equation
 Combustion Reaction: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
o Needs to be balanced (balance using coefficients – 1:2:1:2):
 Reactants: 1 C; 4 H; 4 O
 Products: 1 C; 4 H; 4 O
o Word equation:
 methane(g) + oxygen(g) → carbon dioxide(g) + water(l)
Balancing Chemical Equations
o
IMPORTANT: When hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine are not
bonded to anything else in a chemical reaction they need to be written as diatomics (two
atoms)!
 Diatomics: H2, N2, O2, F2, Cl2, Br2, and I2.
o
Balancing chemical equations shows that ____________________________________________
has been obeyed (mass of reactants = mass of products)
o
Steps to balance a chemical equation:
 (1) Write the skeleton equation for the reaction
 Ex) H2(g) + Cl2(g) → HCl(g)
 (2) Count the atoms of the elements in the reactants
 Ex) H2 +Cl2 →
 2 atoms H, 2 atoms Cl
 (3) Count the atoms of the elements in the products
 Ex) → HCl
 1 atom H, 1 atom Cl
 (4) Change the coefficients to make the number of atoms of each element equal on both
sides of the equation (trial and error)
 Ex) H2 + Cl2 → 2HCl
 2 atoms H + 2 atoms Cl → 2 atoms H, 2 atoms Cl
 (5) Write the coefficients in their lowest possible ratio (reduce coefficients to the
smallest whole numbers possible)
 Ex) H2(g) + Cl2(g) → 2HCl(g)
 Ratio: 1:1:2 (1 H2 to 1 Cl2 to 2 HCl)
 (6) Check your work
 Ex) H2(g) + Cl2(g) → 2HCl(g)
 2 atoms H + 2 atoms Cl → 2 atoms H, 2 Cl atoms
 Law of conservation of mass is obeyed!

o
Balancing Chemical Equations Practice Problem #1
 Write and balance the reaction between solid calcium and solid sulfur to produce solid
calcium sulfide.
o
Balancing Chemical Equations Practice Problem #2
 Write and balance the reaction between methane and oxygen to yield carbon dioxide
and water.
o
Balancing Chemical Equations Practice Problem #3
Types of Chemical Reactions
o
5 Types of Chemical Reactions
 (1) Synthesis
 (2) Combustion
 (3) Decomposition
 (4) Single Replacement/Displacement (SR)
 (5) Double Replacement/Displacement (DR)
TYPES OF CHEMICAL REACTIONS INFORMATION SCAVENGER HUNT
Walk around the classroom and lab area to find the missing information needed to fill in the table
below.
Type of
Chemical
Reaction
SYNTHESIS
COMBUSTION
DECOMPOSITION
SINGLE
REPLACEMENT
(SR)
DOUBLE
REPLACEMENT
(DR)
Description
Generic
Equation
Example #1
Example #2
o
Synthesis
 Synthesis Reaction Practice
o
Combustion
 Combustion Reaction Practice
o
Decomposition
 Decomposition Reaction Practice
o
Single-replacement (SR)
 According to the activity series, which helps to determine whether or not a reaction will
occur, some reactions might not happen
 Ex) Ag(s) + Cu(NO3)2(aq) → NR (NR = no reaction)
 Since Copper (Cu) is more reactive than Silver (Ag) the Copper will stay reacted
with nitrate (NO3-) – the more reactive element will end as a compound

Single-replacement (SR) Reaction Practice
o
Double-replacement (DR)
 All double-replacement reactions will produce either water, a precipitate (a solid), or a
gas
 YOU MUST KNOW YOUR POLYATOMIC IONS!
 Generic Equation: AX + BY → AY + BX
 Steps for Writing Double-Replacement Reactions:
 (1) Write the components of the reactants in a skeleton equation.
o Ex) Al(NO3)3 + H2SO4
 (2) Identify the cations and anions in each compound.
o Ex) Al3+ and NO3-; H+ and SO42 (3) Pair up each cation with the anion from the other compound.
o Ex) Al3+ pairs with SO42-; H+ pairs with NO3 (4) Write the formulas for the products using the pairs from Step 3.
o Ex) Al2(SO4)3 and HNO3
 (5) Write the complete equation for the double-replacement reaction.
o Ex) Al(NO3)3 + H2SO4 → Al2(SO4)3 + HNO3
 (6) Balance the equation.
o Ex) 2Al(NO3)3 + 3H2SO4 → Al2(SO4)3 + 6HNO3
 Double-replacement (DR) Reaction Practice
Predicting Reaction Types Flow Chart
1) Is there a single reactant?
YES
NO
Decomposition Reaction
2) Are the reactants O2
and a hydrocarbon?
YES
NO
Combustion Reaction
3) Are the reactants
two ionic compounds?
YES
NO
Double Replacement Reaction
4) Are the reactants an
element and compound?
YES
Single Replacement Reaction
NO
Synthesis Reaction

Net Ionic Equations
o
o
o
o
o
Definitions:
 Net ionic equation – chemical equation which includes only those compounds left after
the spectator ions are eliminated
 Spectator ions – ions that show up on both sides of the reaction
 Aqueous solution – solution which contains one or more substances dissolved in water
 Solute – what is dissolved
 Solvent – what does the dissolving (often water)
Steps to Writing Net Ionic Equations:
 (1) Make sure everything that is aqueous (aq) is written as its ions
 Table R-8 “Solubility Guidelines” on page 974 in textbook will help you
determine if something is soluble (aq) or insoluble (s)
 Ex) 2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)
o 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq) → 2Na+(aq) + 2Cl-(aq) +
Cu(OH)2(s)
 This is the complete ionic equation
 (2) Cancel spectator ions (spectator ions don’t participate in the reaction and appear on
both sides of the equation)
 Ex) 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq) → 2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s)
 (3) Write net ionic equation
 Ex) 2OH-(aq) + Cu2+(aq) → Cu(OH)2(s)
Ex #1: Water is formed) HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
 Net ionic equation: H+(aq) + OH-(aq) → H2O(l)
Ex #2: Gas is formed) 2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)
 Net ionic equation: 2H+(aq) + S2-(aq) → H2S(g)
Net Ionic Equation Practice