pH and pK

pH and pK

pH Scale

  • The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution.
  • pH is defined as the negative logarithm (base 10) of the activity of hydrogen ions (H+ ions) in a solution.
  • A solution with a pH less than 7 is acidic; a solution with a pH greater than 7 is basic; a solution with a pH of 7 is neutral.
  • Each unit on the pH scale represents a tenfold difference in acidity or basicity; for instance, a solution with a pH of 2 is ten times more acidic than a solution with a pH of 3.

Acids, Bases and the pH Scale

  • Acids are substances that release hydrogen ions (H+) when dissolved in water. The greater the concentration of H+ ions, the lower the pH and the stronger the acid.
  • Bases, on the other hand, are substances that release hydroxide ions (OH-) when dissolved in water. The greater the concentration of OH- ions, the higher the pH and the stronger the base.
  • Neutralisation is the reaction between an acid and a base to form a salt and water. The resulting solution has a pH of 7.

pKa and Acid Strength

  • pKa is the negative logarithm of the acid dissociation constant (Ka), and it provides a measure of the strength of an acid in solution.
  • The smaller the pKa value, the stronger the acid and the greater its ability to donate H+ ions.
  • A high pKa value means the acid is weak, donates fewer H+ ions and has a higher pH.
  • The pKa value is specific to each acid and is used to compare the strengths of different acids.

Brief Overview of the pKa-pH Relationship

  • The pH of an aqueous solution can be calculated using the pKa and the ratio of concentrations of the conjugate base to the weak acid.
  • The relationship between pKa and pH is given by the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid in the solution.

Remember: Understanding the concepts of pH and pKa is vital to grasp the behaviour of acids and bases in solution and an essential part of mastering the topic of acid-base equilibrium.