Chemical Reactions and Equations

Introduction to Chemical Reactions

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more different substances (products). The atoms in reactants are rearranged to form new bonds, producing entirely new chemical species.

Key Insight: The total number of atoms of each element is conserved during a chemical reaction. This is the Law of Conservation of Mass.

Types of Chemical Reactions

1. Combination Reactions

Two or more reactants combine to form a single product.

General form: $A + B \rightarrow AB$

Example: Burning of magnesium in air.

$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$

The magnesium burns with a brilliant white flame and produces magnesium oxide.

2. Decomposition Reactions

A single compound breaks down into two or more simpler substances.

General form: $AB \rightarrow A + B$

Example: Thermal decomposition of calcium carbonate (limestone).

$\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)$

This reaction is used industrially to produce quicklime.

3. Displacement Reactions

A more reactive element replaces a less reactive element from its compound.

Example: Iron displacing copper from copper sulfate solution.

$\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)$

The blue colour of the solution fades as copper(II) ions are reduced.

4. Double Displacement Reactions

Two compounds exchange ions to form two new compounds.

$\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s)\downarrow + \text{NaNO}_3(aq)$

The white precipitate of silver chloride forms immediately.

Balancing Chemical Equations

Balancing ensures the Law of Conservation of Mass is satisfied. Use Python to verify a balanced equation:

def count_atoms(formula: str) -> dict[str, int]:
    """
    Counts atoms in a simple molecular formula.
    E.g., 'H2O' → {'H': 2, 'O': 1}
    """
    import re
    atoms: dict[str, int] = {}
    pattern = r'([A-Z][a-z]?)(\d*)'
    for element, count in re.findall(pattern, formula):
        atoms[element] = atoms.get(element, 0) + (int(count) if count else 1)
    return atoms

# Verify: 2H₂ + O₂ → 2H₂O
reactants = {'H': 4, 'O': 2}   # 2×H₂ + 1×O₂
products  = {'H': 4, 'O': 2}   # 2×H₂O

print("Balanced:", reactants == products)  # True

Oxidation and Reduction (Redox)

Oxidation and reduction always occur simultaneously — this is called a redox reaction.

| Concept | Definition | |---------|-----------| | Oxidation | Loss of electrons / Gain of oxygen / Loss of hydrogen | | Reduction | Gain of electrons / Loss of oxygen / Gain of hydrogen | | Oxidising agent | The species that is reduced (gains electrons) | | Reducing agent | The species that is oxidised (loses electrons) |

Example — Rusting of iron:

$4\text{Fe} + 3\text{O}_2 + 6\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3$

Iron is oxidised (loses electrons); oxygen is the oxidising agent.

Reaction Rate Factors

The speed of a chemical reaction depends on:

1. Temperature — Higher temperature increases kinetic energy and collision frequency
2. Concentration — More particles per unit volume increases collision probability
3. Surface area — Powdered zinc reacts faster with acid than zinc lumps
4. Catalyst — Lowers activation energy without being consumed
5. Pressure (for gases) — Increases concentration of gaseous reactants

Activation Energy

Every reaction requires a minimum energy called activation energy ($E_a$). The Arrhenius equation describes this relationship:

$k = A \cdot e^{-E_a / RT}$

Where:

• $k$ = rate constant
• $A$ = frequency factor
• $E_a$ = activation energy (J/mol)
• $R$ = gas constant (8.314 J/mol·K)
• $T$ = temperature (K)

Quick Reference Code: Molar Mass Calculator

ATOMIC_MASS = {
    'H': 1.008, 'He': 4.003, 'C': 12.011, 'N': 14.007,
    'O': 15.999, 'Na': 22.990, 'Mg': 24.305, 'S': 32.06,
    'Cl': 35.45, 'Ca': 40.078, 'Fe': 55.845, 'Cu': 63.546
}

def molar_mass(formula: str) -> float:
    import re
    total = 0.0
    for element, count in re.findall(r'([A-Z][a-z]?)(\d*)', formula):
        total += ATOMIC_MASS.get(element, 0) * (int(count) if count else 1)
    return round(total, 3)

print(molar_mass('H2O'))    # 18.015
print(molar_mass('CaCO3'))  # 100.086
print(molar_mass('H2SO4'))  # 98.072