CLASSIFYING
CHEMICAL REACTIONS
Advanced Chemistry
Driving Force 3 – Formation of a Gas
• Double replacement reactions may also form a gaseous product
• Physically leaves and prevents reformation of reactants
• Very strong driving force
Compounds that Tend to Form Gases
• If you see the following substances in a reaction, realize that they
will break up into gases and leave
• These compounds are formed from acids with carbonates, sulfites,
sulfides, nitrites, and bases with ammonium compounds
It Turns Into…
Watch For…
H2CO3
→ CO2 (g) + H2O
H2SO3
→ SO2 (g) + H2O
H2S
→ H2S (g)
[rotten egg smell]
NH4OH
→ NH3 + H2O
2HNO2
→ NO (g) + NO2 (g) + H2O
Driving Force 4 – Transfer of Electrons
• Occurs between a metal and a non-metal
• Forms an ionic compound
• Metal transfers electrons to the non-metal
• Metal becomes a cation and non-metal becomes an anion
• Called an oxidation-reduction reaction (REDOX)
• Oxidation
• Loss of electrons
• Reduction
• Gain of electrons
LEO GER
OIL RIG
Oxidation
• Losing Electrons is Oxidation (LEO)
Al → Al3+ + 3eMg → Mg2+ + 2e-
Reduction
• Gaining Electrons is Reduction (GER)
Fe3+ + 3e- → Fe
Cl + e- → Cl-
Oxidation-Reduction Mneumonic
Summary of Oxidation-Reduction Reaction
Practice, Practice, Practice
2Cs(s) + F2(g) → 2CsF(s)
• Ions present in CsF?
• Which element is oxidized?
• Which element is reduced?
2Na(s) + Br2(l) → 2NaBr(s)
• Ions Present:
• Oxidation:
• Reduction:
2Ca(s) + O2(g) → 2CaO(s)
• Ions Present:
• Oxidation:
• Reduction:
Transfer of Electrons Visual
Driving Forces of Chemical Reactions
• Remember:
• Chemical changes are a result of chemical reactions
• Chemical reactions involve a change in energy
• Two compounds will react if there is at least one driving force present:
• Formation of a solid (precipitate)
• Formation of water
• Transfer of electrons
• Formation of a gas
• But there’s so many compounds and so many reactions! How can I
possibly know them all??
Why Classify Chemical Reactions?
• There are MILLIONS of chemical reactions that occur in the world
• By classifying the type of chemical reaction, reactions become
easier to understand
• One popular classification scheme breaks up the types of chemical
reactions into six different categories:
•
•
•
•
•
•
Double Replacement
Acid-Base Neutralization
Synthesis
Decomposition
Combustion
Single Replacement
Double Displacement Reactions
• Anions and cations of two different molecules switch places
• Form entirely different compounds
• Driving forces observed:
• Formation of a solid
• Formation of a gas
• General form:
AB + CD→AD + CB
Double Replacement Reactions
• Assume each reactant is aqueous
• Need to use solubility rules to determine if a solid forms
• If a solid does form, reaction can be written as a net ionic equation
eliminating the spectator ions that remain unchanged during the
reaction
• Example:
Reaction of lead (II) nitrate with potassium iodide to form lead (II)
iodide and potassium nitrate
𝑃𝑏 𝑁𝑂3
2
+2 𝐾𝐼 → 𝑃𝑏𝐼2 + 2𝐾𝑁𝑂3
Acid-Base Neutralization Reactions
• Special kind of double displacement reaction
• Takes place when a strong acid and strong base react with each
other
• Driving forces observed:
• Formation of water
• H+ ion in the acid reacts with the OH- ion in the base
• Products formed are water and some ionic salt
• General form:
HA + BOH → BA + HOH (H2O)
• Example:
Reaction of hydrobromic acid with sodium hydroxide
𝐻𝐵𝑟 + 𝑁𝑎𝑂𝐻 → 𝑁𝑎𝐵𝑟 + 𝐻2 𝑂
Synthesis Reactions
• Two or more simple compounds combine to form a more complicated one
• Driving forces observed:
• Transfer of electrons
• General form
A + B→AB
or
AB + C → ABC
• Example:
Reaction of iron and sulfur to form iron (II) sulfide
8 𝐹𝑒 + 𝑆8 → 8 𝐹𝑒𝑆
Decomposition Reactions
•
•
•
Opposite of a synthesis reaction
A complex molecule breaks down to make simpler ones
Driving forces observed:
•
•
•
Transfer of electrons
Formation of a gas
General form:
AB → A + B
•
or
ABC → AB + C
Example:
Electrolysis of water to make oxygen and hydrogen gas
(Electricity is applied to break up the water into its components)
2𝐻2 𝑂 → 2𝐻2 + 𝑂2
Some Decomposition Rules
Compound
Decomposition Products
Carbonates
(HCO3-, CO3 2-)
CO2 + Metal Oxide
[Metal oxide can be H2O]
Hydroxides
(OH-)
H2O and Metal Oxide
Chlorates
(ClO4-, ClO3-, ClO2-, ClO-)
O2 and Metal Chloride
Oxides
O2 and Metal
Combustion Reactions
• Oxygen combines with a hydrocarbon (compound that contains
only carbon and hydrogen) to form water and carbon dioxide
• Reactions produce heat
• Exothermic
• Driving forces observed:
• Formation of a gas
• Formation of water
• General form:
CxHy + O2 → CO2 (g) + H2O (g)
• Example
Burning of napthalene
𝐶10 𝐻8 + 12 𝑂2 → 10 𝐶𝑂2 + 4 𝐻2 𝑂
Single Displacement Reactions
• One element trades places with another element in a compound
• Driving forces observed:
• Transfer of electrons
• General form:
A + BC → AC + B
or
A + BC → BA + C
• Example
Magnesium replaces hydrogen in water to make magnesium
hydroxide and hydrogen gas
𝑀𝑔 + 2𝐻2 𝑂 → 𝑀𝑔(𝑂𝐻)2 + 𝐻2
Metals and Non-metals Reactivity
• Not all elements are equal in their ability to replace other
elements
• Metals, such as lithium, sodium, potassium, strontium and
calcium are referred to as “very active” metals due to the fact
that they give up electrons very easily
• Other metals are “less active”
• Non-metals also vary in their reactivities
• In order to predict products of single replacement reactions
or to predict if they will occur at all, an activity series is used
• New pink sheet!
A Handy Checklist for Classifying Type of
Reactions
• Follow this series of questions
• When you answer “yes” to a question, stop!
• Does your reaction have oxygen as one of its reactants and carbon dioxide and
water as products?
• If yes, then it’s a combustion reaction!
• Does your reaction have two (or more) chemicals combining to form one chemical?
• If yes, then it’s a synthesis reaction!
• Does your reaction have one large molecule falling apart to make several small
ones?
• If yes, then it’s a decomposition reaction!
• Does your reaction have any molecules that contain only one element?
• If yes, then it’s a single displacement reaction
• Does your reaction have water as one of the products?
• If yes, then it’s an acid-base reaction!
• If you haven’t answered “yes” to any of the questions above, then you’ve got a
double displacement reaction!