Reactivity 3.1.8 — pH Curves for Neutralization Reactions

A pH curve is a graphical representation of the change in pH of a solution during an acid–base titration. It helps visualize how pH evolves as one reactant is gradually added to another and plays a key role in understanding acid–base behavior, particularly in neutralization reactions.

Neutralization Reactions:

These are reactions between acids and bases that result in the formation of water and a salt. For example:

\( \text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l) \)

During this process, the acidic and basic properties cancel out, moving the pH toward neutrality depending on the strength of the acid and base involved.

Shape of the Curve:

The exact shape of a pH curve depends on the strength of the acid and base involved in the neutralization. Key elements that define the shape include:

• Initial pH: Determined by the strength and concentration of the solution in the flask (acid or base).
• pH change before equivalence: As titrant is added, the pH increases slowly, especially for weak acids forming buffer systems with their conjugate base.
• Equivalence Point: The point at which stoichiometrically equal amounts of acid and base have reacted. A sharp, nearly vertical rise in pH is observed here for strong–strong titrations.
• Final pH: After the equivalence point, the added titrant is in excess, determining the final pH of the solution.

Why pH Curves Matter:

• Allow determination of equivalence point volume.
• Help in selecting a suitable indicator.
• Reveal buffer behavior and acid–base strength.

Note:

A pH curve provides not just qualitative, but quantitative insight into acid–base interactions, essential for experimental and theoretical understanding in IB chemistry.

Sketching and Interpreting the General Shape of pH Curves

A pH curve visually displays how pH changes during a titration as a strong or weak acid is gradually neutralized by a base (or vice versa). Proper interpretation of these curves allows chemists to identify key features such as the strength of acids and bases, buffer regions, and equivalence points.

Important IB Focus: Only monoprotic neutralization reactions are assessed. These involve the transfer of one proton per acid or base molecule and produce a single, clear equivalence point on the graph.

Key Features to Include When Sketching:

1. Initial pH:
   • Determined by the acid or base initially present in the flask.
   • Strong acid: very low pH (~1).
   • Weak acid: higher pH (~3–6) due to partial ionization.
2. Buffer Region (Only in Weak Acid or Base Titrations):
   • Occurs before the equivalence point when both the weak acid and its conjugate base are present.
   • pH changes gradually and resists sudden changes — this is the buffer zone.
3. Equivalence Point:
   • Volume at which stoichiometric amounts of acid and base have reacted.
   • Appears as the center of the steep vertical rise in pH.
   • The pH at equivalence varies:
     • Strong acid + strong base: pH = 7.
     • Weak acid + strong base: pH > 7 (due to formation of basic salt).
     • Strong acid + weak base: pH < 7 (due to formation of acidic salt).
4. Vertical (Steep) Rise:
   • Occurs near the equivalence point.
   • Indicates a rapid pH shift with small volume addition.
   • Most pronounced in strong acid–strong base titrations.
   • Used to determine the most appropriate indicator (must change color within this range).
5. Final pH:
   • After the equivalence point, the added titrant is in excess.
   • For excess strong base, pH levels off between 12–14.
   • For excess weak base, final pH remains lower (~9–11).

Visual Characteristics by Titration Type (Strength of Acids and Bases):

How to Interpret a pH Curve Accurately:

• Identify the acid or base strength from the shape and initial pH.
• Use the equivalence point to determine when neutralization is complete.
• Recognize buffer regions for weak acid/base titrations.
• Match the indicator color change range with the steep part of the curve.

Sketching Tips:

• Label axes: volume of titrant (x-axis) and pH (y-axis).
• Include clear changes: initial slope, buffer, equivalence jump, and final plateau.
• Mark the equivalence point clearly with an annotation.

Conclusion: pH curves reveal vital information about acid–base reactions, and their interpretation is key for identifying reaction endpoints, suitable indicators, and acid–base strength. Mastery of sketching and analyzing these graphs is essential in IBDP Chemistry, especially during titration-based questions and experimental design.