6.3 Acid-base reactions and titrations

Acid-base reactions and titrations are key to understanding how acids and bases interact. These processes help us measure and control pH levels in various solutions, from everyday products to complex chemical processes.

Titrations allow us to determine the concentration of unknown solutions by reacting them with known solutions. This technique is crucial in many fields, including environmental science, food production, and medicine, where precise pH control is essential.

Balancing Chemical Equations for Neutralization

Acid-Base Neutralization Reactions

• Acids react with bases to produce a salt and water in a neutralization reaction
• The general form of the neutralization reaction equation is: acid + base → salt + water
• Examples of neutralization reactions include:
  • Hydrochloric acid and sodium hydroxide: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
  • Acetic acid and ammonia: CH3COOH(aq) + NH3(aq) ⇌ CH3COONH4(aq)

Balancing Equations for Strong and Weak Acids/Bases

• Strong acids and strong bases completely dissociate in aqueous solutions and react in a 1:1 mole ratio
  • The balanced equation for the reaction between hydrochloric acid and sodium hydroxide is: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
• Weak acids and weak bases partially dissociate in aqueous solutions
  • The balanced equation for the reaction between acetic acid and ammonia is: CH3COOH(aq) + NH3(aq) ⇌ CH3COONH4(aq)
• Polyprotic acids, such as H2SO4 and H3PO4, can undergo multiple neutralization reactions, depending on the mole ratio of the acid and base
  • For example, the neutralization of sulfuric acid by sodium hydroxide can occur in two steps:
    • H2SO4(aq) + NaOH(aq) → NaHSO4(aq) + H2O(l)
    • NaHSO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)

Titration: Concept and Applications

Titration Technique and Procedure

• Titration determines the concentration of an unknown solution by reacting it with a solution of known concentration (the titrant) until the reaction is complete (the equivalence point)
• In an acid-base titration, an acid of unknown concentration is titrated with a base of known concentration, or vice versa
• Titrations are performed using a buret to accurately dispense the titrant into the analyte solution
  • The analyte solution is typically placed in an Erlenmeyer flask with an indicator that changes color at the end point of the titration

Applications of Titration in Various Fields

• Titrations have applications in various fields:
  • Environmental monitoring (determining water hardness)
  • Food science (measuring acidity in wine)
  • Pharmaceutical analysis (assessing drug purity)
• Titrations are used in quantitative analysis to determine the concentration of unknown solutions, such as:
  • Determining the concentration of vitamin C in fruit juices
  • Measuring the acidity of soil samples
  • Quantifying the amount of calcium carbonate in antacid tablets

Concentration Calculations from Titration

Determining Concentration Using Titration Data

• The concentration of the unknown solution (analyte) can be determined using:
  • The balanced chemical equation
  • The volume and concentration of the titrant
  • The volume of the analyte solution
• The mole ratio between the acid and base in the balanced equation is used to set up a proportion between the moles of titrant and analyte at the equivalence point

Calculation Steps

1. Calculate the moles of titrant using the molarity (M) and volume (V) of the titrant: moles of titrant = M(titrant) × V(titrant)
2. Determine the moles of analyte using the mole ratio from the balanced equation
3. Calculate the concentration of the analyte by dividing the moles of analyte by the volume of the analyte solution

• For example, if 25.0 mL of 0.100 M NaOH is required to neutralize 20.0 mL of an HCl solution:
  • Moles of NaOH = 0.100 M × 0.0250 L = 0.00250 mol
  • From the balanced equation, HCl and NaOH react in a 1:1 mole ratio, so moles of HCl = 0.00250 mol
  • Concentration of HCl = $\frac{0.00250\text{ mol}}{0.0200\text{ L}} = 0.125\text{ M}$

Titration Curves: Equivalence vs End Point

Interpreting Titration Curves

• A titration curve is a plot of the pH of the analyte solution against the volume of titrant added
• The shape of the curve depends on the strength of the acid and base being titrated
• The equivalence point is the point in the titration where the moles of acid and base are equal, and the reaction is complete
  • On a titration curve, the equivalence point is the point of inflection, where the slope of the curve is steepest
• The end point is the point in the titration where the indicator changes color, signaling the completion of the reaction
  • The end point should be as close to the equivalence point as possible for accurate results

Equivalence Point pH for Different Titrations

• For a strong acid-strong base titration, the equivalence point occurs at pH 7
• For weak acid-strong base titrations, the equivalence point is at a pH > 7
  • For example, the titration of acetic acid with NaOH has an equivalence point at pH ≈ 8.7
• For strong acid-weak base titrations, the equivalence point is at a pH < 7
  • For example, the titration of HCl with ammonia has an equivalence point at pH ≈ 5.3

Indicator Selection for Titration

Indicator Properties and Selection Criteria

• Indicators are weak acids or bases that change color over a specific pH range
• The choice of indicator depends on the expected pH at the equivalence point of the titration
• The ideal indicator should have a pKa value close to the pH at the equivalence point and should undergo a sharp color change over a narrow pH range

Suitable Indicators for Different Titrations

• For strong acid-strong base titrations, indicators with a pH transition range around 7 are suitable:
  • Bromothymol blue (pH 6.0-7.6)
  • Phenol red (pH 6.8-8.4)
• For weak acid-strong base titrations, indicators with a pH transition range above 7 are appropriate:
  • Phenolphthalein (pH 8.2-10.0)
  • Thymol blue (pH 8.0-9.6)
• For strong acid-weak base titrations, indicators with a pH transition range below 7 are suitable:
  • Methyl orange (pH 3.1-4.4)
  • Bromocresol green (pH 3.8-5.4)