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CHAPTER 8
Chemical
Reactions!!!
WHAT ARE CHEMICAL REACTIONS?
Chemical Reaction—A process by which one or more substances are
changed into a different substance.
Indicators of a chemical reaction:
1.) Evolution of energy as heat and light.
 Ex. Mg solid + Oxygen
2.) Production of gas
 Ex. NaHCO3 and CH3COOH
3.) Formation of a precipitate (solid)
 Ex. Mg(OH)2 from NaOH and MgSO4
4.) Color change
 Ex. Phenolphthalein in basic solutions
5.) Temperature Change
6.) Odor
Chemical Equations
Represents the identities and relative molecular or molar amounts of the reactants and products
Reactants
Products
Potassium Iodide(l) + Lead (II) Nitrate(l)  Lead (II) Iodide(s) + Potassium Nitrate(l)
State of Matter
“Yields”
Reactants: Substances that react in a chemical change (left side of arrow)
Products: Substances formed in a chemical change (right side of arrow)
http://www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/chemrxn/7perform.htm
2HgO(s) → 2Hg(l) + O2(g)
• Coefficients represent the lowest whole number ratio of
substances that will react
Coefficient is a small whole number that appears in front of a formula in a
chemical equation
• Subscripts represent the lowest whole number ratio of
elements in a compound.
• Both are MOLE RATIOS !!!!!!!!!!!!!!!!
• Can get mass ratios by multiplying mole ratios by the molar mass.
Other Symbols in Equations
Δ
2H2(g) + O2 (g) →
2H2O (ℓ)
(g) - gas state
(s) - solid state
(l) - liquid state
(aq) - aqueous (dissolved in H2O)
Δ - heated
photon - light
elec - electricity
More extensive list on pg. 266
ON A BLANK PIECE OF PAPER…
Write the following questions in a word equation, then convert the word equation to a formula
equation(unbalanced). Include symbols for physical states in the formula equation.
1.) Solid Sodium oxide is added to water at room temperature and forms sodium hydroxide
(dissolved in water).
2.) Solid calcium reacts with solid sulfur to produce solid calcium sulfide (Hint: Solid sulfur’s
formula is S8.
3.) Hydrogen gas reacts with fluorine gas to produce hydrogen fluoride gas. (Remember
diatomic molecules).
Chemical equations ALWAYS obey 2 laws…
Law of Conservation of Matter
•
•
matter can neither be created nor destroyed, but
it can change forms
chemical equations must show that matter
was conserved
Law of Conservation of Mass
• mass can neither be created nor destroyed, but
it can change forms
• chemical equations must show that mass was
conserved
BALANCED EQUATIONS
Balanced equations are equations for a chemical reaction in which
the number of atoms for each element in the reaction and the total
charge are the same for both the reactants and the products .
In other words…the mass and the charge are balanced on both
sides of the reaction.
BALANCING EQUATIONS
1.) Balance different types of atoms one at a time.
2.) Start with most complex molecule first.
3.) Balance H and O atoms last.
4.) NEVER change subscripts.
5.) Change coefficients to balance the number of each atom on both sides of
the equation.
6.) Check to make sure it is the LOWEST possible ratio.
PRACTICE
Balance the following equations:
1.) ___H2O(g) → ____ H2(g) + ____ O2(g)
2.) ___ Zn(s) + ____HCl(aq) → ____ZnCl2(aq) + H2(g)
3.) ___ Fe(s) + ___H2O(g) → ___Fe3O4(s) + ___H2(g)
Reasons Chemical Rxns Take Place
• A more reactive element will displace a less reactive
element if it can become more stable.
•Based on electron affinity
• If the element with a greater electron affinity can:
a) lower its overall energy state, or
b) have less molecular stress
it will “displace” the element with a lower activity
• Activity Series - a list of elements based on their
ability to undergo a chemical change
PRACTICE
1.) Based on the activity series of metals and halogens, which element is more likely to
replace the other element in a compound?
a. K or Na
b. Al or Ni
c. Bi or Cr
d. Cl or F
e. Au or Ag
f. Cl or I
g. Fe or Sr
h. I or F
TYPES OF REACTIONS
Chapter 8, Section 2
http://youtu.be/A5H6DVe5FAI
SYNTHESIS
AKA composition reaction. Two or more substances combine to
form a new compound.
Fe(s) + Cl2 (g)
→
FeCl3 (s)
CaO(s) + H2O(l)
→
Ca(OH)2 (s)
DECOMPOSITION
A single compound undergoes a reaction that produces two or more substances.
Think about it as breaking up a compound into its separate elements.
AX  A + X
2H2O(l)  2H2(g) + O2(g)
Electrolysis—decomposition of a substance by an electric current.
NH4NO3 (s)
→ N2O(g) +
H2O(g)
SINGLE-DISPLACEMENT
One element replaces a similar element in a compound.
A + BX  AX + B
or
Y + BX  BY + X
Remember activity series. If A is not more reactive than B, the reaction
will not take place.
Li(s) +
F2(g) +
H2O(l)
→
NaBr(aq)
LiOH(aq) +
→
H2(g)
NaF(aq) + Br2(l)
COMBUSTION
http://youtu.be/LzagxjtZrJY
A substance combines with oxygen, releasing a large amount of energy in the form of
light or heat.
Hydrocarbons, when combined with oxygen in a combustion reaction, release carbon
dioxide and water.
C2H6 (g) + O2 (g)
→ CO2 (g) + H2O (l)
DOUBLE-DISPLACEMENT
The ions of two compounds exchange places in an aqueous solution to form two new
compounds.
Forms either a precipitate, a gas, or water.
AX + BY  BX + AY
NaOH(aq) +
CuCl2(aq) →
NaCl(aq) + Cu(OH)2(s)
FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)
Ca(OH)2(aq) +
HCl(aq) → CaCl2(aq) +
H2O(l)
Predict the products of the following reactions and
balance the equation (must include physical states).
1.) MgSO4(aq) + NaOH(aq) 
2.) Na2S(aq) + HCl(aq) 
3.) Li2SO4(aq) + BaCl2(aq) 
Rxns in Aqueous Solutions
solution – a homogeneous mixture of 2 or
more substances
solvent – the substance that does the dissolving
(usually more present)
solute – the substance that is dissolved
(usually less present)
aqueous solution – water is the solvent
2. The Dissolving Process
• Many rxns are carried out in aqueous solutions
a) Dissociation – process of ionic compounds forming
ions in solution
NaOH(s) + H2O(l) → Na+(aq) + OH-(aq) + H2O(l)
• Bases produce OH- ions in solution
• Many covalently bonded substances will exist as
molecules in solution (sugars, alcohols, gases)
C12H22O11 (aq), CH3OH (aq), O2 (g), CO2 (g)
• Some will form ions (acids) through a process called:
b) Ionization – process of polar molecules forming
ions in solution
HCl(g) + H2O(l) →
H+(aq) +
Cl-(aq) + H2O(l)
• Acids produce H+ ions when in solution
• Both processes produce electrolytic solutions
3. Net Ionic Equations
• Are used to show what is actually “driving” the rxn
•3 steps:
a) Write a balanced chemical equation
b) Write a complete ionic equation
this breaks down the components of the
balanced equation into their aqueous ions
c) Cancel “spectator ions” and write the
net equation
PRACTICE—NET IONIC EQUATIONS
Write the net ionic equations for the following reactions:
1.) Na3PO4(aq) + CaCl2(aq) -->
2.) (NH4)2CO3 (aq) + Al(NO3)3 (aq) --> NH4NO3 (aq) + Al2(CO3)(s)
3.) Na2CO3(aq) + HNO3(aq) --> NaNO3 (aq) + H2O (l) + CO2 (g)