This MCQ module is based on: Acids and Bases Further Reactions and pH Scale
Acids and Bases Further Reactions and pH Scale
Study Notes and Summary
Title Suggestion: Chemical Dynamics: Neutralization, Oxides, and the pH Spectrum
Chapter Notes (Moderate to Advanced Level):
- Reaction of Metal Carbonates and Metal Hydrogen Carbonates with Acids:
- Acids react with metal carbonates and metal hydrogen carbonates to produce a salt, carbon dioxide gas (CO2​), and water.
- General equation: \(\text{Metal Carbonate/Hydrogen Carbonate} + \text{Acid} \rightarrow \text{Salt} + \text{Carbon Dioxide} + \text{Water}\)
- Examples:
- \(\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)\)
- \(\text{NaHCO}_3(s) + \text{HCl}(aq) \text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)​\)
- Test for Carbon Dioxide: \(\text{CO}_2\)​ turns lime water \(\text{Ca(OH)}_2\) milky due to the formation of insoluble calcium carbonate (CaCO3​).
- \(\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)\)
- Excess CO2​ causes the milkiness to disappear due to the formation of soluble calcium hydrogen carbonate \(\text{Ca(HCO}_3)_2\).
- \(\text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \text{Ca(HCO}_3)_2(aq)​\)
- Reaction of Acids and Bases with Each Other (Neutralisation Reaction):
- Acids and bases react to form salt and water. This reaction is called a neutralisation reaction.
- General equation: \text{Acid} + \(\text{Base} \text{Salt} + \text{Water}\)
- Example:\(\text{HCl}(aq) + \text{NaOH}(aq) \text{NaCl}(aq) + \text{H}_2\text{O}(l)\)
- Neutralisation reactions are exothermic (release heat).
- Reaction of Metallic Oxides with Acids:
- Metallic oxides are generally basic in nature. They react with acids to form salt and water, similar to a neutralisation reaction.
- Example: \(\text{CuO}(s) + 2\text{HCl}(aq) \text{CuCl}_2(aq) + \text{H}_2\text{O}(l)\)
- Reaction of Non-metallic Oxides with Bases:
- Non-metallic oxides are generally acidic in nature. They react with bases to form salt and water.
- Example: \(\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \text{CaCO}_3(s) + \text{H}_2\text{O}(l)​\)
- This reaction is similar to the reaction between a base and an acid, indicating the acidic nature of non-metallic oxides.
- What Happens to an Acid or a Base in a Water Solution?
- Acids produce H+ ions (which exist as H3​O+ hydronium ions, combining with water molecules) in water.
- Bases produce OH− ions in water.
- The process of dissolving an acid or a base in water is highly exothermic. Care must be taken while diluting acids (add acid slowly to water, not water to acid).
- Strength of Acid or Base Solutions (pH Scale):
- pH Scale: A scale to measure the concentration of H+ ions in a solution. ‘p’ stands for ‘potenz’ (German for power).
- Ranges from 0 (very acidic) to 14 (very alkaline/basic). A value of 7 is neutral.
- Higher \(\text{H}^+\) ion concentration means lower \(\text{pH}\) (more acidic).
- Lower \(\text{H}^+\) ion concentration (or higher \(\text{OH}^-\) concentration) means higher pH (more basic).
- Universal Indicator: A mixture of several indicators that shows different colors at different pH values, allowing for a more precise pH determination.
- Importance of pH in Everyday Life:
- Plants and Animals: Sensitive to pH changes. The human body works within a narrow pH range of 7.0 to 7.8.
- Acid Rain: Rainwater with pH less than 5.6 is acid rain, harmful to aquatic life and historical monuments.
- pH in Soil: Plants require a specific pH range for healthy growth. Farmers use lime (basic) or gypsum (acidic) to adjust soil pH.
- pH in Digestive System: Stomach produces HCl for digestion (pH ~1.2). Antacids are used to relieve acidity.
- Tooth Decay: Caused by acid produced by bacteria in the mouth (pH below 5.5).
- Self-Defence by Animals and Plants: Bees inject acidic venom (treated with baking soda). Wasp stings inject alkaline venom (treated with vinegar). Nettle leaves release methanoic acid (treated with dock plant).
Practice MCQs
Assessment Worksheets
This assessment will be based on: Acids and Bases Further Reactions and pH Scale
Experiment-Based Theories for Olympiad Preparation
- Hypothetical Experiment: Titrimetric Determination of Acid/Base Strength and Unknown Concentration via Neutralization Calorimetry
- Objective: To determine the precise concentration of an unknown acid/base solution and to quantify the heat released during its neutralization, allowing for a comparison of acid/base strengths.
- Materials: Standardized solution of a strong acid (e.g., 0.1 M HCl), standardized solution of a strong base (e.g., 0.1 M NaOH), an unknown concentration of an acid or base, a weak acid (e.g., acetic acid), a weak base (e.g., ammonium hydroxide), phenolphthalein indicator, burette, pipette, conical flask, calorimeter (e.g., two nested polystyrene cups with a lid and stirrer), precise thermometer (0.1°C readability).
- Procedure:
- Standard Acid-Base Titration (Volumetric Analysis):
- Carefully prepare a titration setup. Pipette a known volume of the unknown acid into a conical flask. Add a few drops of phenolphthalein indicator.
- Fill the burette with the standardized strong base. Slowly add the base from the burette to the acid in the flask while swirling, until the indicator changes color (endpoint).
- Record the volume of base used. Repeat for accuracy.
- Calculate the unknown acid concentration using \(M_1V_1 = M_2V_2\)​.
- Neutralization Calorimetry (Thermodynamic Analysis):
- In a calorimeter, measure a known volume of the strong acid and record its initial temperature.
- Add a known volume of the strong base (at the same initial temperature) that is just enough to neutralize the acid (determined from titration results).
- Stir the mixture and record the highest temperature reached.
- Repeat the calorimetry for neutralization of a strong acid with a weak base, or a weak acid with a strong base, or weak acid with weak base.
- Calculate the heat released (q) using \(q = mc\Delta T\), where ‘m’ is the total mass of solution, ‘c’ is the specific heat capacity (assume for water), and \(ΔT\) is the temperature change.
- Calculate the molar enthalpy of neutralization \(\Delta H_{\text{neut}} = \frac{q}{\text{moles of water formed}}\)
- Standard Acid-Base Titration (Volumetric Analysis):
- Expected Observations:
- Titration: A sharp color change at the equivalence point.
- Calorimetry: A measurable temperature increase upon neutralization. The temperature increase will be significantly higher for strong acid-strong base neutralization compared to reactions involving weak acids/bases (due to less energy required to break bonds for ionization).
- Theoretical Outcomes: This multi-part experiment provides a comprehensive understanding of acid-base chemistry. Titration reinforces stoichiometry and concentration calculations. Calorimetry demonstrates the exothermic nature of neutralization and provides a quantitative measure of the heat released, which is directly related to the strength of the acid and base (stronger acids/bases ionize completely, releasing more heat upon forming water). Differences in \Delta \(H_{\text{neut}}\)​ for strong vs. weak acid/base reactions highlight the energy consumed in the partial ionization of weak electrolytes. This experiment can lead to a deeper understanding of buffer solutions and their applications.
Real-Life Connections: Pharmaceutical industry for quality control of drugs, chemical manufacturing, environmental monitoring (e.g., acid mine drainage treatment), understanding the effectiveness of antacids, and the energy considerations in industrial chemical processes.
