Reactions of acids common mistakes

Common mistakes

• Apparatus Selection Mistake

  Students often confuse the apparatus needed for crystallisation with that needed for filtration, leading to incorrect setups.

  Fix itReview the specific functions of each piece of apparatus: use a beaker for mixing, a filter paper and funnel for filtration, and an evaporating dish for crystallisation.

• Misidentifying the solid reactant as the salt

  Students write the insoluble solid (e.g. ZnS) as the product of the soluble salt preparation, rather than recognising it as the reactant that is added in excess

  Fix itExplain that in a soluble salt preparation the insoluble solid is the reactant that reacts with the acid to form the soluble salt and a gas; the product is the soluble salt, not the solid that was added.

• Incorrectly balancing the equation for soluble salt preparation

  Students often write the balanced symbol equation for a soluble salt preparation as 2NaCl + H2SO4 → Na2SO4 + 2HCl, treating the acid as a reactant and the salt as a product, but they forget that the acid is consumed and the salt is produced

  Fix itThe correct balanced symbol equation for preparing a soluble salt from an acid and an insoluble metal salt is: 2NaCl + H2SO4 → Na2SO4 + 2HCl. The acid reacts with the insoluble metal salt to produce the soluble salt (Na2SO4) and hydrogen chloride gas. Ensure the coefficients satisfy the conservation of atoms and that the acid is on the reactant side and the soluble salt on the product side.

• Common Mistake in Salt Preparation

  Students often forget to add the insoluble solid in excess when preparing a soluble salt, leading to incomplete reaction and impurities in the final product.

  Fix itAlways add the insoluble solid in excess to ensure that all the acid reacts and to filter out any unreacted solid, resulting in a pure dry sample of the soluble salt.

• Ignoring Safety Equipment

  Students often forget to mention the use of safety goggles and gloves when handling acids and heating solutions.

  Fix itAlways include the necessity of wearing safety goggles and gloves to protect against splashes and burns when working with acids and heating solutions.

• Misclassifying pH Levels

  Students often confuse the classification of solutions, mistakenly labeling a neutral solution as acidic or alkaline.

  Fix itTo fix this, remember that a pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. Use a pH meter or universal indicator to accurately measure and classify the solution.

• Misunderstanding pH Measurement Methods

  Students often confuse the use of universal indicators and pH meters, thinking they measure pH in the same way.

  Fix itClarify that universal indicators provide a color change to indicate pH levels, while pH meters give a numerical value for pH directly.

• Misidentifying the species released by acids

  Students often think that acids release oxygen atoms or hydroxide ions when they dissolve in water, rather than hydrogen ions (H⁺).

  Fix itExplain that in aqueous solution acids dissociate to give H⁺ (or H₃O⁺) and the conjugate base; the hydrogen ion is the species that determines acidity.

• Confusing Alkalis with Acids

  Students often confuse the ions produced by alkalis and acids, thinking that alkalis produce hydrogen ions instead of hydroxide ions.

  Fix itRemember that alkalis produce hydroxide ions (OH-) in aqueous solution, while acids produce hydrogen ions (H+).

• Confusing ionic and molecular equations

  Students write the neutralisation reaction as H⁺ + OH⁻ → H₂O instead of the full ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) and omit the state symbols, or they write a molecular equation such as HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) and then claim it is the ionic equation.

  Fix itRemind students that the ionic equation shows only the ions that actually participate in the reaction. The correct ionic equation for neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l). The molecular equation is fine for describing the overall reaction, but the ionic form is required when describing the mechanism of neutralisation.