Difficulty: Intermediate | Prerequisites: Oxidation-reduction fundamentals (Doc 1), solubility rules, ionic compound nomenclature.
This is the companion to the fundamentals notes. It covers how to write net ionic equations for redox reactions, the main categories of redox reaction types (combination, decomposition, combustion, double-displacement, disproportionation), and how redox titrations work. You should be comfortable assigning oxidation numbers and balancing half-reactions before tackling this material.
Net ionic equations strip away spectator ions to show only the species that change. Redox reactions come in several flavours: combination, decomposition, combustion, disproportionation, and more. Redox titrations track electron transfer (charge) rather than proton transfer to find the equivalence point.
Complete ionic equation
An equation in which every soluble ionic compound is written as its individual ions. Solid salts remain as formula units. This is the starting point before you identify spectator ions.
Spectator ion
An ion that appears in identical form on both sides of the equation and does not participate in the reaction. Polyatomic anions are frequently spectator ions.
Net ionic equation
The equation that remains after spectator ions are removed. It shows only the species that undergo a chemical change.
Combination reaction
A reaction where two or more substances combine to form a single product. General form: A + B → AB.
Decomposition reaction
A reaction where one compound breaks apart into two or more simpler substances. General form: X → Y + Z.
Combustion reaction
A reaction in which a fuel reacts with an oxidant (usually O₂) to produce CO₂ and H₂O.
Double-displacement (metathesis) reaction
A reaction in which the cations and anions of two compounds switch partners. These are usually not redox reactions because the ions retain their original oxidation states.
Disproportionation (dismutation)
A reaction in which a single element is simultaneously oxidised and reduced, producing at least two products containing that element in different oxidation states.
Redox titration
A titration that uses an oxidation-reduction reaction to determine the concentration of an analyte. It tracks charge transfer rather than proton transfer.
Potentiometric titration
A redox titration performed without a colour indicator. Instead, a voltmeter measures the electrical potential of the solution, and the equivalence point is identified by a sharp voltage change.
A net ionic equation strips a reaction down to the species that actually change, removing everything that just floats along unchanged.
How to write one:
Write the balanced molecular equation.
Split every soluble ionic compound (aqueous) into its individual ions. Keep solid salts, pure liquids, and gases as formula units.
Identify spectator ions: any ion that appears identically on both sides.
Cancel spectator ions. What remains is the net ionic equation.
Key points to remember:
Solid salts stay together as formula units. Only aqueous compounds are split into ions.
Polyatomic anions (SO₄²⁻, NO₃⁻, PO₄³⁻) are common spectator ions. Watch for them.
If both reactants and both products are aqueous, all ions cancel and there is no net ionic reaction.
Combination reactions
Two or more substances form a single product.
General form: A + B → AB
Net ionic form: A + B → A⁺ + B⁻ (when the product is ionic)
Example: 2Na + Cl₂ → 2NaCl
Decomposition reactions
One compound breaks into two or more simpler substances.
General form: X → Y + Z
Example: 2H₂O → 2H₂ + O₂ (electrolysis of water)
Combustion reactions
A fuel combines with an oxidant (typically O₂) to produce CO₂ and H₂O.
Example: CH₄ + 2O₂ → CO₂ + 2H₂O
The fuel is oxidised; oxygen is reduced.
Double-displacement (metathesis) reactions
Cations and anions switch partners.
These are generally not redox reactions because no oxidation states change.
If both reactants and both products are aqueous, spectator ions cancel everything and there is no net ionic equation.
Example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Net ionic: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
Disproportionation (dismutation) reactions
A single element is both oxidised and reduced in the same reaction, ending up in at least two different oxidation states.
Example: 2H₂O₂ → 2H₂O + O₂. Oxygen starts at -1 in hydrogen peroxide, goes to -2 in water (reduced) and 0 in O₂ (oxidised).
Common in biological systems. Enzymes such as catalase drive the decomposition of H₂O₂ this way.
The superoxide radical O₂⁻ has an unusual oxidation state of -1/2 per oxygen atom.
Redox titrations
Similar in setup to acid-base titrations, but the equivalence point is determined by charge transfer rather than proton transfer.
The titrant is a strong oxidising or reducing agent of known concentration.
Indicators that change colour at a specific voltage mark the endpoint. Potassium permanganate (KMnO₄) is a classic example: it is intensely purple when oxidised and nearly colourless when reduced, so it acts as its own indicator.
Potentiometric titration
A redox titration that uses no colour indicator at all. A voltmeter measures the electrical potential of the solution continuously, and the equivalence point shows up as a sharp jump in voltage.
Redox titrations are used in water-treatment plants to measure dissolved oxygen and residual chlorine. Disproportionation of hydrogen peroxide is how your body neutralises a toxic by-product of metabolism: the enzyme catalase splits H₂O₂ into water and oxygen almost instantly. Combustion reactions power every internal combustion engine and gas turbine.
Students often assume all double-displacement reactions are redox. They are not. In metathesis the ions simply swap partners and keep their original oxidation states.
Confusing "no net ionic equation" with "no reaction". If all species are aqueous on both sides, the spectator ions cancel everything. That does not mean no reaction occurred at the molecular level; it means there is no observable net change in ionic form.
Thinking disproportionation requires two different elements. It involves one element changing into two different oxidation states.
Forgetting to keep solids together. Only aqueous compounds are broken into ions. Precipitates, gases, and pure liquids stay as formula units.
⚠️ Writing net ionic equations is heavily tested. Know the procedure: split aqueous compounds, identify spectator ions, cancel.
⚠️ Be able to classify a reaction by type (combination, decomposition, combustion, metathesis, disproportionation) and state whether it is a redox reaction.
⚠️ Double-displacement reactions are a common exam trap: students label them as redox when no oxidation states change.
⚠️ Know the difference between a redox titration and an acid-base titration. The key distinction: redox titrations track charge transfer, acid-base titrations track proton transfer.
⚠️ Be ready to identify disproportionation: a single element appearing in two different oxidation states in the products.
True or false: A spectator ion is one that changes oxidation state during the reaction.
False. Spectator ions are unchanged and appear on both sides.
Fill in the blank: In a combustion reaction, the fuel is ___ and oxygen is ___.
Oxidised; reduced.
True or false: Double-displacement reactions are usually redox reactions.
False. The ions typically retain their oxidation states.
Fill in the blank: A disproportionation reaction involves one element being simultaneously ___ and ___.
Oxidised; reduced.
True or false: A potentiometric titration uses a colour indicator to find the equivalence point.
False. It uses a voltmeter to detect a sharp change in electrical potential.
Q: Write the net ionic equation for the reaction between AgNO₃(aq) and NaCl(aq).
A: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). The Na⁺ and NO₃⁻ are spectator ions.
Q: Is the decomposition of H₂O₂ into H₂O and O₂ a redox reaction? Why?
A: Yes. It is a disproportionation reaction. Oxygen in H₂O₂ (oxidation state -1) is both reduced to -2 in water and oxidised to 0 in O₂.
Q: Why is a double-displacement reaction between two aqueous reactants forming two aqueous products said to have no net ionic equation?
A: Because all ions appear on both sides and cancel out as spectator ions. No species undergoes a net change.
Q: What distinguishes a redox titration from an acid-base titration?
A: A redox titration reaches its equivalence point through charge (electron) transfer, while an acid-base titration reaches its equivalence point through proton transfer.
Q: Give an example of a disproportionation reaction and identify the element that is both oxidised and reduced.
A: 2H₂O₂ → 2H₂O + O₂. Oxygen is the element: it goes from -1 in H₂O₂ to -2 in H₂O (reduced) and 0 in O₂ (oxidised).
Net ionic equations tie directly back to solubility rules (you need those to know which compounds are aqueous and which precipitate). Redox titrations link to electrochemistry and standard reduction potentials, which quantify how readily a species gains electrons. Combustion reactions connect to thermochemistry and enthalpy calculations. Disproportionation appears again in biochemistry when studying enzyme kinetics and reactive oxygen species.
Net ionic equation, complete ionic equation, spectator ion, combination reaction, synthesis reaction, decomposition reaction, combustion, metathesis, double displacement, disproportionation, dismutation, redox titration, potentiometric titration, equivalence point, indicator, KMnO₄, catalase, hydrogen peroxide decomposition, Chemistry Ch. 11, Michigan State University