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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R2.1 - How much? The amount of chemical change

2.1.1 Chemical Equations and State Symbols 2.1.2 Stoichiometry and Mole Reaction 2.1.3 + 2.1.4 Limiting Reactants and Yield 2.1.5 Atom Economy

Writing Chemical Equations with State Symbols

Specification Reference R2.1.1

Quick Notes:

  • Chemical equations represent the molar ratio of reactants to products in a reaction.
  • Equations must always be balanced – same number of atoms of each element on both sides.
  • State symbols show the physical state of reactants and products:
    • (s) – solid
    • (l) – liquid
    • (g) – gas
    • (aq) – aqueous (dissolved in water)

Full Notes:

What a Chemical Equation Shows

A chemical equation is a symbolic way of representing a chemical reaction. It shows:

Steps for Writing a Chemical Equation

  1. Identify the reactants and products.
  2. Write correct formulas for all substances.
  3. Balance the equation by adjusting coefficients (not subscripts).
  4. Add state symbols.

State Symbols in Chemical Equations

Symbol Meaning
(s) solid
(l) liquid
(g) gas
(aq) aqueous – dissolved in water

Example 1: Neutralization Reaction

Example Hydrochloric acid reacts with sodium hydroxide

Word equation:
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

Balanced symbol equation:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Example 2: Metal + Acid Reaction

Example Zinc reacts with hydrochloric acid

Balanced symbol equation:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

Example 3: Precipitation Reaction

Example Silver nitrate reacts with sodium chloride

Balanced symbol equation:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Tips for Writing Equations

How to Write Ionic Equations

Ionic equations show only the species that are involved in the reaction, they ignore spectator ions.

Example Reaction between NaOH and HCl

IB Chemistry diagram showing full ionic equation with spectator ions removed, leaving H⁺(aq) + OH⁻(aq) → H₂O(l).

Full equation: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Ionic equation: OH⁻(aq) + H⁺(aq) → H2O(l)

Summary

Reactivity 3.2 – Linked Course Question

When is it useful to use half-equations?

Half-equations are useful in redox reactions because they separate the oxidation and reduction processes, making it easier to:

  • Track the movement of electrons
  • Identify oxidising and reducing agents
  • Balance complex redox reactions, especially in acidic or basic solutions
  • Construct or interpret electrochemical cells

Example: Reaction between zinc and copper(II) sulfate

Full redox reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

Half-equations:

  • Oxidation (zinc loses electrons): Zn(s) → Zn2+(aq) + 2e⁻
  • Reduction (copper gains electrons): Cu2+(aq) + 2e⁻ → Cu(s)

By using half-equations, it's easier to see that zinc is oxidised and copper(II) ions are reduced, with 2 electrons transferred. This method helps ensure charges and atoms are correctly balanced.