Acid-Base Equilibria

14A Strong and Weak Acids

The Brønsted–Lowry Theory

The Brønsted–Lowry theory defines acids as proton (H+) donors and bases as proton acceptors. This concept expands the definition of acids and bases beyond those that contain hydroxide or hydrogen ions, allowing for a broader range of acid-base reactions.

• Acid: A substance that donates a proton to another substance.

• Base: A substance that accepts a proton.

• Example: In the reaction between hydrochloric acid and water: HCl acts as the acid, and water acts as the base.

Hydrogen Ion Concentration and the pH Scale

The pH scale is a logarithmic measure of hydrogen ion concentration in a solution. It is used to quantify the acidity or basicity of a solution.

• pH Formula:

• Acidic solutions: pH < 7

• Neutral solutions: pH = 7

• Basic solutions: pH > 7

• Example: A solution with M has a pH of 3.

Ionic Product of Water, Kw

The ionic product of water () is the equilibrium constant for the self-ionization of water. It is important for understanding the relationship between hydrogen and hydroxide ion concentrations in aqueous solutions.

• Equation:

• At 25°C,

• Example: In pure water, M

Analysing Data from pH Measurements

pH measurements are used to determine the concentration of hydrogen ions in a solution, which can be used to calculate other properties such as acidity, basicity, and equilibrium constants.

• Key Point: Accurate pH measurements allow for the determination of acid and base strengths, calculation of equilibrium constants, and analysis of buffer systems.

• Example: Using pH data to calculate the concentration of an unknown acid in solution.

14B Acid-Base Titrations

Acid-Base Titrations, pH Curves and Indicators

Acid-base titrations are analytical techniques used to determine the concentration of an acid or base by reacting it with a solution of known concentration. pH curves and indicators are used to monitor the progress of the titration and identify the equivalence point.

• Titration Curve: A plot of pH versus volume of titrant added.

• Indicator: A substance that changes color at a specific pH, signaling the endpoint of the titration.

• Example: Titrating acetic acid with sodium hydroxide and using phenolphthalein as an indicator.

Buffer Solutions

Buffer solutions are mixtures of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resist changes in pH when small amounts of acid or base are added.

• Buffer Equation:

• Example: A mixture of acetic acid and sodium acetate acts as a buffer.

Buffer Solutions and pH Curves

Buffer solutions are often analyzed using pH curves to observe their resistance to pH changes. These curves demonstrate the effectiveness of a buffer in maintaining a stable pH.

• Key Point: Buffers maintain pH within a narrow range, even when acids or bases are added.

• Example: Addition of HCl to a buffer solution causes only a slight decrease in pH compared to pure water.

Thinking Bigger: A Problem Grows

This section likely presents a complex, real-world problem involving acid-base equilibria, titrations, or buffer systems, requiring the application of multiple concepts discussed above.

• Key Point: Integrating knowledge of acid-base theory, pH, titrations, and buffers to solve advanced problems.

Exam Practice

Practice questions and problems are provided to reinforce understanding and prepare for exams on acid-base equilibria, titrations, and buffer solutions.

• Key Point: Applying theoretical knowledge to practical and exam-style questions.