Aqueous Solutions and Chemical Reactions

Reaction Types

Combination Reactions

Also known as synthesis reactions.
Begin with two or more simpler reactants.
Reactants combine to form a single, more complex product.
Example: Rust formation on iron.
- Iron combines with oxygen to form rust (iron oxide).
- $4 \text{Fe(s)} + 3 \text{O}<em>2\text{(g)} \rightarrow 2 \text{Fe}</em>2\text{O}_3\text{(s)}$

Decomposition Reactions

Occur when a single reactant breaks down.
Produces two or more less complex products.
Example: Electrolysis of water.
- Water breaks down into hydrogen and oxygen gas.
- $2 \text{H}<em>2\text{O(l)} \rightarrow 2 \text{H}</em>2\text{(g)} + \text{O}_2\text{(g)}$

Single-Replacement Reactions

Also known as single-displacement or displacement reactions.
An element reacts with a compound.
The element displaces one of the elements in the compound.
Results in a new compound and a new element.
Example: Zinc metal reacting with hydrochloric acid.
- Zinc displaces hydrogen from hydrochloric acid to form zinc chloride and hydrogen gas.
- $\text{Zn(s)} + 2 \text{HCl(aq)} \rightarrow \text{ZnCl}<em>2\text{(aq)} + \text{H}</em>2\text{(g)}$

Double-Replacement Reactions

Also known as double-displacement or metathesis reactions.
Two ionic compounds exchange ions.
Forms two entirely new compounds.
Example: Reaction of potassium iodide (KI) with lead(II) nitrate ($\text{Pb(NO}3)2$).
- Potassium ions exchange with lead(II) ions, and iodide ions exchange with nitrate ions.
- $2 \text{KI(aq)} + \text{Pb}(\text{NO}<em>3)</em>2\text{(aq)} \rightarrow \text{PbI}<em>2\text{(s)} + 2 \text{KNO}</em>3\text{(aq)}$

Acid-Base Reactions

A specific type of double-replacement reaction.
Acid: A compound typically written with H at the beginning of its formula (e.g., HCl, HBr).
Base: A compound often written with OH at the end of its formula, as many bases are hydroxide compounds (e.g., NaOH, KOH).
Example: Reaction of hydrobromic acid (HBr) with potassium hydroxide (KOH).
- $\text{HBr(aq)} + \text{KOH(aq)} \rightarrow \text{KBr(aq)} + \text{H}_2\text{O(l)}$

Combustion Reactions

The rapid combination of a substance with oxygen.
Often involves a hydrocarbon fuel reacting with oxygen to produce carbon dioxide and water.
Example: Combustion of propane ($\text{C}3\text{H}8).
- $\text{C}<em>3\text{H}</em>8\text{(g)} + 5 \text{O}<em>2\text{(g)} \rightarrow 3 \text{CO}</em>2\text{(g)} + 4 \text{H}_2\text{O(l)}$

Summary of Reaction Types

Reaction Type	Generic Formula	Examples
Combination	$\text{A} + \text{B} \rightarrow \text{AB}$	$4 \text{Fe(s)} + 3 \text{O}<em>2\text{(g)} \rightarrow 2 \text{Fe}</em>2\text{O}_3\text{(s)}$
Decomposition	$\text{AB} \rightarrow \text{A} + \text{B}$	$2 \text{H}<em>2\text{O(l)} \rightarrow 2 \text{H}</em>2\text{(g)} + \text{O}<em>2\text{(g)}$ ; $2 \text{KClO}</em>3\text{(s)} \rightarrow 2 \text{KCl(s)} + 3 \text{O}_2\text{(g)}$
Single-Replacement	$\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$	$\text{Zn(s)} + 2 \text{HCl(aq)} \rightarrow \text{ZnCl}<em>2\text{(aq)} + \text{H}</em>2\text{(g)}$
Double-Replacement	$\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$	$2 \text{KI(aq)} + \text{Pb(NO}<em>3)</em>2\text{(aq)} \rightarrow \text{PbI}<em>2\text{(s)} + 2 \text{KNO}</em>3\text{(aq)}$ ; $\text{HBr(aq)} + \text{KOH(aq)} \rightarrow \text{KBr(aq)} + \text{H}_2\text{O(l)}$
Combustion	$\text{C}<em>x\text{H}</em>y + \text{O}<em>2 \rightarrow \text{CO}</em>2 + \text{H}_2\text{O}$	$\text{C}<em>3\text{H}</em>8\text{(g)} + 5 \text{O}<em>2\text{(g)} \rightarrow 3 \text{CO}</em>2\text{(g)} + 4 \text{H}_2\text{O(l)}$

Driving Forces for Reactions in Aqueous Solutions

Reactions occur spontaneously due to a combination of:
- Changes in heat energy (enthalpy).
- Changes in randomness (entropy) – discussed in future chapters.
Spontaneous reactions follow typical patterns classified by their