AP Chemistry 4.6 Introduction to Titrations Study Notes - New Syllabus Effective fall 2024
AP Chemistry 4.6 Introduction to Titrations Study Notes- New syllabus
AP Chemistry 4.6 Introduction to Titrations Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.
Key Concepts:
- Equivalence Point in Titration — Relationship Between Titrant and Analyte
Equivalence Point in Titration — Relationship Between Titrant and Analyte
A titration is a quantitative laboratory technique used to determine the amount or concentration of an analyte (unknown substance) by reacting it with a titrant (solution of known concentration). The equivalence point occurs when the amount of titrant added exactly reacts with the total amount of analyte present, according to the stoichiometric relationship of the balanced chemical equation.
1. Components of a Titration:
- Analyte: The unknown solution whose concentration or amount is to be determined.
- Titrant: The standard solution of known concentration that reacts quantitatively with the analyte.
- Equivalence Point: The point at which the moles of titrant added are exactly stoichiometrically equivalent to the moles of analyte present.
- Endpoint: The experimental signal (e.g., color change of an indicator or pH jump) used to detect the equivalence point.
2. Concept of Equivalence Point:
- At the equivalence point, the reaction between analyte and titrant has gone to completion — no analyte remains unreacted.
- The mole ratio between analyte and titrant is determined by the coefficients in the balanced equation.
- The relationship between the titrant and analyte is given by:
\(\mathrm{\dfrac{M_1V_1}{n_1} =\dfrac{ M_2V_2}{n_2}}\)
- \(\mathrm{M_1, V_1}\) — molarity and volume of the titrant
- \(\mathrm{M_2, V_2}\) — molarity and volume of the analyte
- \(\mathrm{n_1, n_2}\) — stoichiometric coefficients from the balanced equation
3. Types of Titrations:
- Acid–Base Titrations: The titrant and analyte neutralize each other. Example: \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\)
- Redox Titrations: Involve electron transfer between oxidizing and reducing agents.
- Precipitation Titrations: Reaction leads to the formation of an insoluble solid (precipitate).
- Complexometric Titrations: Formation of a complex ion between analyte and titrant (e.g., EDTA titration of metal ions).
4. Relationship Between Equivalence Point and Endpoint:
| Term | Definition | Description |
|---|---|---|
| Equivalence Point | Exact stoichiometric completion of reaction between titrant and analyte | Theoretical point calculated from balanced equation |
| Endpoint | Observable signal indicating equivalence point has been reached | Detected experimentally (e.g., color change of indicator) |
Note: The endpoint should coincide closely with the equivalence point for accurate titration results.
5. Identifying the Equivalence Point:
- At the equivalence point:
- The moles of titrant added equal the moles of analyte reacted, according to the balanced equation.
- All analyte is consumed; further titrant addition leaves excess titrant unreacted.
- Graphically, in an acid–base titration, the equivalence point corresponds to a sharp inflection in the pH curve.
- In a redox titration, it corresponds to a sudden change in potential (voltage).
6. Example Reactions and Equivalence Conditions:
| Reaction Type | Example Reaction | Equivalence Point Condition |
|---|---|---|
| Acid–Base | \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\) | Moles of \(\mathrm{H^+}\) = moles of \(\mathrm{OH^-}\) |
| Redox | \(\mathrm{Fe^{2+} + Ce^{4+} \rightarrow Fe^{3+} + Ce^{3+}}\) | Electrons lost = electrons gained |
| Precipitation | \(\mathrm{Ag^+ + Cl^- \rightarrow AgCl(s)}\) | All ions have combined to form the precipitate |
The equivalence point represents the exact stoichiometric balance between titrant and analyte. It is determined by the reaction’s balanced equation and is independent of the measurement method. The endpoint is the observable event that experimentally indicates the equivalence point.
Example:
In a titration, 25.00 mL of \(\mathrm{HCl}\) is titrated with \(0.100\ \mathrm{M}\ \mathrm{NaOH}\). The equivalence point is reached when 30.00 mL of \(\mathrm{NaOH}\) is added. Calculate the molarity of the \(\mathrm{HCl}\) solution.
▶️ Answer / Explanation
Step 1: Write the balanced equation:
\(\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}\)
At equivalence point: \(\mathrm{n_{HCl} = n_{NaOH}}\)
Step 2: Calculate moles of \(\mathrm{NaOH}\).
\(\mathrm{n = M \times V = 0.100\ mol/L \times 0.0300\ L = 0.00300\ mol}\)
Step 3: Moles of \(\mathrm{HCl} = 0.00300\ mol\)
Step 4: Calculate molarity of \(\mathrm{HCl}\).
\(\mathrm{M = \dfrac{n}{V} = \dfrac{0.00300\ mol}{0.0250\ L} = 0.120\ M}\)
Final Answer: The molarity of the \(\mathrm{HCl}\) solution is \(\mathrm{0.120\ M}\).