6 Chemical Reactions Worth Understanding
Not every chemical reaction is governed by the same principle. Sometimes electron transfer is crucial, sometimes proton transfer, while in other cases the key issue is equilibrium or the formation of a sparingly soluble product.
REDOX
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
EXAM: WHAT YOU NEED TO KNOW
oxidation • reduction • oxidizing agent • reducing agent • oxidation state • electrons
Zn → Zn²⁺ + 2e⁻ – oxidation
Cu²⁺ + 2e⁻ → Cu – reduction
⊖ Zinc is the reducing agent - it donates electrons.
⊕ Cu²⁺ is the oxidizing agent - it accepts electrons.
💡 In an exam question: determine the oxidation states, identify the oxidizing and reducing agents, and then write the electron-transfer half-equations.
BRØNSTED ACID-BASE REACTION
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
EXAM: WHAT YOU NEED TO KNOW
H⁺ donor • H⁺ acceptor • Brønsted acid • Brønsted base • conjugate acid-base pair
CH₃COO⁻ accepts H⁺ → Brønsted base.
H₂O donates H⁺ → Brønsted acid.
Conjugate acid-base pairs: CH₃COOH / CH₃COO⁻ oraz H₂O / OH⁻.
The OH⁻ ions formed are responsible for the alkaline nature of the acetate solution.
CHROMATE ⇌ DICHROMATE EQUILIBRIUM
2 CrO₄²⁻ + 2 H⁺ ⇌ Cr₂O₇²⁻ + H₂O
EXAM: WHAT YOU NEED TO KNOW
equilibrium • shift in equilibrium position • Le Chatelier's principle • effect of pH • colour change
🟡 CrO₄²⁻ - yellow
🟠 Cr₂O₇²⁻ - orange
↑ [H⁺] → equilibrium shifts towards Cr₂O₇²⁻
↓ [H⁺] → equilibrium shifts towards CrO₄²⁻
💡 In an exam answer, it is not enough to state that “the colour changes”. Link the observation to the change in ion concentration and to the shift in the equilibrium position.
PRECIPITATION REACTION
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)↓
EXAM: WHAT YOU NEED TO KNOW
precipitate • sparingly soluble salt • net ionic equation • solubility product, Kₛₚ
⚪ A white precipitate of AgCl forms.
AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)
Whether precipitation occurs depends on the relationship between the ionic product and Kₛₚ.
💡 In an exam answer, distinguish an observation (“a white precipitate formed”) from a conclusion (“sparingly soluble AgCl was formed”).
ESTER HYDROLYSIS
CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH
EXAM: THAT YOU NEED TO KNOW
ester • hydrolysis • carboxylic acid • alcohol • reversible reaction
ESTER + H₂O ⇌ CARBOXYLIC ACID + ALCOHOL
In this example, ethyl ethanoate is hydrolysed to ethanoic acid and ethanol.
💡 In an exam question, identify the -COO- functional group and correctly determine the carboxylic acid and alcohol from which the ester is derived.
ESTERIFICATION
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
EXAM: THAT YOU NEED TO KNOW
carboxylic acid • alcohol • ester • reversible reaction • equilibrium • catalys
CARBOXYLIC ACID + ALCOHOL ⇌ ESTER + H₂O
Increasing the amount of one reactant or removing a product can shift the equilibrium towards ester formation.
A catalyst increases the rate at which equilibrium is reached but does not change the equilibrium position.
💡 In an exam question, remember that hydrolysis and esterification can be treated as opposite directions of the same reversible reaction.
