Chapter 7: Chemical Reactions – Structured Study Notes

Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions are processes in which substances (reactants) transform into new substances (products) with different properties. These reactions are fundamental to understanding changes in matter and energy in chemistry.

• Gas-evolution reactions: Occur when a reaction in a liquid produces a gas, such as the classic volcano experiment with baking soda and vinegar.

• Combustion reactions: Involve hydrocarbons reacting with oxygen to produce carbon dioxide, water, and heat. These are a type of oxidation–reduction reaction.

• Precipitation reactions: Occur when two solutions react to form a solid (precipitate), such as soap scum formation in hard water.

Evidence of a Chemical Reaction

While molecular changes are not directly visible, chemical reactions often produce observable macroscopic changes.

• Color change

• Formation of a solid (precipitate) in a clear solution

• Formation of a gas when a substance is added to a solution

• Emission of light

• Emission or absorption of heat

Distinguishing Chemical and Physical Changes

Not all observable changes are chemical reactions. For example, boiling water produces bubbles and steam, but no new substance is formed; this is a physical change.

Chemical Equations

Representing Chemical Reactions

Chemical equations use formulas to represent reactants and products. States of matter are indicated in parentheses: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).

• Reactants: Substances on the left side of the equation

• Products: Substances on the right side

• State symbols: Indicate physical state

Balancing Chemical Equations

Balanced equations reflect the conservation of atoms. Coefficients are used to ensure the number of each atom is equal on both sides.

• Write correct formulas for reactants and products

• Balance elements that appear in only one compound first

• Balance free elements last

• Adjust coefficients, not subscripts

• Check atom counts on both sides

Example: Combustion of methane:

$\mathrm{CH_4(g) + 2\,O_2(g) \rightarrow CO_2(g) + 2\,H_2O(g)}$

Aqueous Solutions and Solubility

Terminology and Properties

An aqueous solution is a homogeneous mixture of a substance dissolved in water. Soluble compounds dissolve; insoluble compounds do not. Ionic compounds often dissociate into ions in water.

• Strong electrolytes: Substances that completely dissociate into ions in solution

• Non-conductors: Pure water does not conduct electricity

• Conductors: Solutions with ions conduct electricity

Solubility Rules

Solubility rules help predict whether an ionic compound will dissolve in water. These rules are essential for understanding precipitation reactions.

Compound Type	Solubility
Group 1 ions, NH4+	Soluble
Nitrates (NO3-), acetates (CH3COO-)	Soluble
Chlorides, bromides, iodides	Soluble (except Ag+, Pb2+, Hg2+)
Sulfates (SO4 2-)	Soluble (except Ba2+, Pb2+, Ca2+)
Carbonates, phosphates	Insoluble (except Group 1, NH4+)
Hydroxides	Insoluble (except Group 1, Ba2+)
Sulfides	Insoluble (except Group 1, Group 2, NH4+)

Precipitation Reactions

Formation of a Precipitate

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid (precipitate). Only insoluble compounds form precipitates.

• Mixing potassium iodide and lead(II) nitrate forms a yellow precipitate of lead(II) iodide.

• Solubility rules determine if a precipitate will form.

When No Reaction Occurs

If all products are soluble, no precipitate forms and no reaction occurs. For example, mixing potassium iodide and sodium chloride results in no reaction.

Molecular, Complete, and Net Ionic Equations

Types of Chemical Equations

Chemical reactions can be represented in three ways:

• Molecular equation: Shows complete neutral formulas for all compounds.

• Complete ionic equation: Shows all ions as they exist in solution.

• Net ionic equation: Shows only the ions and molecules directly involved in the reaction, omitting spectator ions.

Example:

Molecular: $\mathrm{2\,KI(aq) + Pb(NO_3)_2(aq) \rightarrow 2\,KNO_3(aq) + PbI_2(s)}$

Complete ionic: $\mathrm{2\,K^+(aq) + 2\,I^-(aq) + Pb^{2+}(aq) + 2\,NO_3^-(aq) \rightarrow 2\,K^+(aq) + 2\,NO_3^-(aq) + PbI_2(s)}$

Net ionic: $\mathrm{Pb^{2+}(aq) + 2\,I^-(aq) \rightarrow PbI_2(s)}$

Acid–Base Reactions

Neutralization and Salt Formation

Acid–base reactions (neutralization reactions) occur when an acid reacts with a base to form water and an ionic compound (salt).

• Acid: Forms H+ ions in solution, tastes sour, dissolves metals

• Base: Forms OH- ions in solution, tastes bitter, feels slippery

• Net ionic equation: $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$

Example: Reaction between nitric acid and calcium hydroxide:

$\mathrm{2\,HNO_3(aq) + Ca(OH)_2(aq) \rightarrow 2\,H_2O(l) + Ca(NO_3)_2(aq)}$

Gas-Evolution Reactions

Formation of Gaseous Products

Gas-evolution reactions produce a gas either directly or through decomposition of an intermediate product.

• Direct formation: $\mathrm{Na_2S(aq) + 2\,HCl(aq) \rightarrow 2\,NaCl(aq) + H_2S(g)}$

• Intermediate decomposition: $\mathrm{Na_2CO_3(aq) + 2\,HCl(aq) \rightarrow 2\,NaCl(aq) + H_2O(l) + CO_2(g)}$

Oxidation–Reduction (Redox) Reactions

Electron Transfer Processes

Redox reactions involve the transfer of electrons between substances. They are responsible for processes such as rusting, bleaching, and battery operation.

• Oxidation: Loss of electrons

• Reduction: Gain of electrons

• Mnemonic: OIL RIG (Oxidation Is Loss; Reduction Is Gain)

Combustion Reactions

Combustion as Redox Reactions

Combustion reactions are a type of redox reaction where a substance reacts with oxygen to form oxygen-containing compounds, often releasing heat.

• Combustion of hydrocarbons produces carbon dioxide and water.

• Example: $\mathrm{CH_3OH(l) + O_2(g) \rightarrow CO_2(g) + H_2O(g)}$

Classifying Chemical Reactions

Types of Reactions Based on Atom Movement

Chemical reactions can be classified by the behavior of atoms:

• Synthesis (Combination) Reaction: Two simple substances combine to form a more complex substance.

• Decomposition Reaction: A complex substance breaks down into simpler substances.

• Single-Displacement Reaction: One element displaces another in a compound.

• Double-Displacement Reaction: Two elements or groups exchange places between compounds.

Review and Learning Objectives

• Identify evidence of a chemical reaction

• Write and balance chemical equations

• Determine solubility of compounds

• Predict and write equations for precipitation, acid–base, gas-evolution, redox, and combustion reactions

• Classify chemical reactions