Reactions of acids common mistakes

Common mistakes

• Common Mistake in Writing Ionic Equations

  Students often forget to include the correct charges on the ions when writing the ionic equation for neutralisation.

  Fix itAlways ensure to write the ionic equation with the correct charges, such as H+ for hydrogen ions and OH- for hydroxide ions.

• Misinterpreting pH change as a direct ion count

  Students often think that because neutralisation removes H⁺ ions, the pH simply jumps to a fixed value, ignoring the logarithmic scale and concentration of OH⁻ added.

  Fix itExplain that pH is calculated from the negative log of the H⁺ concentration; when an alkali supplies OH⁻, the H⁺ concentration falls exponentially, so the pH rises by a number of units that depends on the amount of OH⁻ added and the initial H⁺ concentration, not by a fixed amount.

• Misunderstanding pH Changes

  Students often think that the pH decreases immediately when an alkali is added to an acid, without considering the neutralisation process.

  Fix itEmphasize that pH decreases gradually as the alkali neutralises the acid, and explain the role of hydrogen ions reacting with hydroxide ions.

• Misidentifying pH Levels

  Students often confuse the pH values and incorrectly classify a solution with a pH of 7 as acidic instead of neutral.

  Fix itRemind students that a pH of 7 is neutral, while pH values below 7 indicate acidity and values above 7 indicate alkalinity.

• Misunderstanding Strong Acids

  Students often think that strong acids are just more concentrated than weak acids, rather than being completely ionised in solution.

  Fix itEmphasize that a strong acid is defined by its complete ionisation in aqueous solution, regardless of its concentration.

• Weak Acid Misconception

  Students often think that weak acids do not ionise at all in solution.

  Fix itEmphasize that weak acids do ionise, but only partially, meaning some molecules remain un-ionised in solution.

• Misunderstanding Ionisation

  Students often think that all acids of the same concentration have the same pH, not realizing that strong acids are completely ionised while weak acids are only partially ionised.

  Fix itTo fix this, remember that strong acids fully dissociate in solution, leading to a higher concentration of hydrogen ions and a lower pH compared to weak acids at the same concentration.

• pH–concentration relationship

  Students think a lower pH simply means a lower concentration of hydrogen ions, or they confuse pH with the amount of acid present.

  Fix itExplain that pH is the negative logarithm of the hydrogen ion concentration: pH = –log[H⁺]. A lower pH value indicates a higher [H⁺] because the logarithm function is decreasing. For example, a solution with pH 3 has ten times the [H⁺] of a solution with pH 4, and a pH of 1 has 100 times the [H⁺] of a pH 3 solution. Emphasise that pH is a measure of acidity, not the quantity of acid added.

• Misunderstanding pH Changes

  Students often think that a decrease in pH by one unit results in a linear increase in hydrogen ion concentration, rather than an exponential increase.

  Fix itEmphasize that a decrease in pH by one unit means the hydrogen ion concentration actually increases by a factor of ten, highlighting the logarithmic nature of the pH scale.

• Confusing Acid Strength with Concentration

  Students often think that a more concentrated acid is necessarily a stronger acid, leading to incorrect conclusions about their properties.

  Fix itRemember that acid strength refers to the degree of ionisation in solution, while concentration refers to the amount of acid in a given volume of solution. A strong acid is fully ionised regardless of its concentration.