Chemical Reactions and Aqueous Solutions Notes

Develop the ability to recognize and describe the five basic types of chemical reactions.
Identify and describe the specific driving forces that allow reactions to proceed in aqueous solutions.

Definition: Synthesis reactions, also known as combination reactions, involve simple reactants that combine to form a single, more complex product.
Generic Formula: $A + B \rightarrow AB$
Named Example: The Rusting of Iron:
- Iron metal ( $Fe$ ) reacts with oxygen ( $O_{2}$ ) from the air.
- The resulting product is iron(III) oxide ( $Fe_{2}O_{3(s)}$ ), which is characterized as a reddish-brown solid known as rust.
- Chemical Equation: $4 Fe_{(s)} + 3 O_{2(g)} \rightarrow 2 Fe_{2}O_{3(s)}$

Definition: Decomposition reactions occur when a single complex reactant breaks down into two or more less complex products.
Generic Formula: $AB \rightarrow A + B$
Example 1: Electrolysis of Water:
- Water decomposes into its constituent elements by means of electrical energy (using a battery).
- Products: Hydrogen gas ( $H_{2}$ ) and oxygen gas ( $O_{2}$ ).
- Chemical Equation: $2 H_{2}O_{(l)} \rightarrow 2 H_{2(g)} + O_{2(g)}$
Example 2: Decomposition of Potassium Chlorate:
- Solid potassium chlorate ( $KClO_{3}$ ) decomposes into potassium chloride and oxygen.
- Chemical Equation: $2 KClO_{3(s)} \rightarrow 2 KCl_{(s)} + 3 O_{2(g)}$

Definition: A single-replacement reaction (also called a single-displacement or displacement reaction) occurs when a free element reacts with a compound and displaces one of the elements in that compound. This results in a new compound and a new free element.
Generic Formula: $A + BC \rightarrow AC + B$
Example: Reaction of Zinc with Hydrochloric Acid:
- Zinc metal ( $Zn$ ) replaces hydrogen ( $H$ ) in the hydrochloric acid ( $HCl$ ) compound.
- Products: Aqueous zinc chloride ( $ZnCl_{2}$ ) and hydrogen gas ( $H_{2}$ ).
- Chemical Equation: $Zn_{(s)} + 2 HCl_{(aq)} \rightarrow ZnCl_{2(aq)} + H_{2(g)}$

Definition: In a double-replacement reaction (also known as a double-displacement or metathesis reaction), Two ionic compounds exchange ions with each other to form two entirely new compounds.
Generic Formula: $AB + CD \rightarrow AD + CB$
Example: Lead(II) Nitrate and Potassium Iodide:
- Aqueous Lead(II) nitrate ( $Pb(NO_{3})_{2}$ ) reacts with aqueous potassium iodide ( $KI$ ).
- The ions involved are $Pb^{2+}$ , $NO_{3}^{-}$ , $K^{+}$ , and $I^{-}$ .
- A solid yellow precipitate of Lead(II) iodide ( $PbI_{2}$ ) is formed.
- Chemical Equation: $2 KI_{(aq)} + Pb(NO_{3})_{2(aq)} \rightarrow PbI_{2(s)} + 2 KNO_{3(aq)}$

Classification: Acid-base reactions are categorized as a specific type of double-replacement reaction.
Definitions:
- Acid: A compound usually written with hydrogen ( $H$ ) at the beginning of its chemical formula.
- Base: A compound often written with hydroxide ( $OH$ ) at the end of its formula, as many bases are hydroxide compounds.
Reaction Pattern: $\text{acid} + \text{base} \rightarrow \text{a salt} + \text{water}$
Examples:
- Reaction between hydrochloric acid and sodium hydroxide: $HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_{2}O_{(l)}$
- Reaction between sulfuric acid and potassium hydroxide: $H_{2}SO_{4(aq)} + 2 KOH_{(aq)} \rightarrow K_{2}SO_{4(aq)} + 2 H_{2}O_{(l)}$

Definition: Combustion is defined as the rapid combination of a substance with oxygen gas ( $O_{2}$ ).
Hydrocarbon Combustion: When hydrocarbon fuels undergo combustion, they react with oxygen to produce carbon dioxide ( $CO_{2}$ ) and water ( $H_{2}O$ ).
Generic Formula: $C_{x}H_{y} + O_{2} \rightarrow CO_{2} + H_{2}O$
Examples:
- Propane Combustion: $C_{3}H_{8(g)} + 5 O_{2(g)} \rightarrow 3 CO_{2(g)} + 4 H_{2}O_{(l)}$
- Octane Combustion: $2 C_{8}H_{18(l)} + 25 O_{2(g)} \rightarrow 16 CO_{2(g)} + 18 H_{2}O_{(l)}$

Reaction Type	Generic Formula	Example Equation
Synthesis	$A + B \rightarrow AB$	$4 Fe_{(s)} + 3 O_{2(g)} \rightarrow 2 Fe_{2}O_{3(s)}$
Decomposition	$AB \rightarrow A + B$	$2 H_{2}O_{(l)} \rightarrow 2 H_{2(g)} + O_{2(g)}$
Single-Replacement	$A + BC \rightarrow AC + B$	$Zn_{(s)} + 2 HCl_{(aq)} \rightarrow ZnCl_{2(aq)} + H_{2(g)}$
Double-Replacement	$AB + CD \rightarrow AD + CB$	$2 KI_{(aq)} + Pb(NO_{3})_{2(aq)} \rightarrow PbI_{2(s)} + 2 KNO_{3(aq)}$
Combustion	$C_{x}H_{y} + O_{2} \rightarrow CO_{2} + H_{2}O$	$C_{3}H_{8(g)} + 5 O_{2(g)} \rightarrow 3 CO_{2(g)} + 4 H_{2}O_{(l)}$

Spontaneity: Reactions occur spontaneously due to changes in heat energy (enthalpy) and randomness (entropy).
Product Stability: Driving forces are fundamentally associated with the formation of stable, lower-energy products.
Specific Driving Forces:
- Precipitation: This occurs in double-replacement reactions that result in the formation of lower-energy solid ionic compounds. The formation of the solid (precipitate) is the driving force.
- Neutralization: This is the driving force for acid-base reactions. It involves the production of a salt and, usually, water.
- Oxidation–Reduction (Redox): This is the driving force for synthesis, decomposition, and single-replacement reactions. These reactions involve the transfer of electrons to form lower-energy products.

$2 H_{2(g)} + O_{2(g)} \rightarrow 2 H_{2}O_{(l)}$ : Synthesis
$MgI_{2(aq)} + 2 AgNO_{3(aq)} \rightarrow 2 AgI_{(s)} + Mg(NO_{3})_{2(aq)}$ : Double-replacement
$CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)}$ : Decomposition
$2 C_{8}H_{18(l)} + 25 O_{2(g)} \rightarrow 16 CO_{2(g)} + 18 H_{2}O_{(l)}$ : Combustion

$2 H_{2(g)} + O_{2(g)} \rightarrow 2 H_{2}O_{(l)}$ : This is a synthesis reaction; the driving force is oxidation-reduction.
$MgI_{2(aq)} + 2 AgNO_{3(aq)} \rightarrow 2 AgI_{(s)} + Mg(NO_{3})_{2(aq)}$ : This is a double-replacement reaction; the driving force is precipitation.
$CaCO_{3(s)} \rightarrow CaO_{(s)} + CO_{2(g)}$ : This is a decomposition reaction; the driving force is oxidation-reduction.