Understanding Acids, Bases, and Chemical Reactions: A Comprehensive Guide
Chemical Reactions and Electrochemistry
Redox Reactions
Zn + CuSO4 > Cu + ZnSO4 / Cu2+ + Zn > Cu + Zn2+ (Blue to colorless)
Electrolysis of Water
Passing a direct current through water containing an electrolyte (e.g., H2SO4) results in the formation of oxygen at the anode (positive electrode) and hydrogen at the cathode (negative electrode).
Anode: 2H2O > O2 + 4H+ + 4e- (oxidation)
Cathode: 2H+ + 2e- > H2 (reduction)
Overall: 2H2O > O2 + 2H2
Fuel Cells
Fuel cells directly convert chemical energy into electrical energy by continuously feeding hydrogen and oxygen into the cell. They can be connected in series or parallel to increase voltage or power output, respectively. Fuel cells find applications in various sectors, including transportation, domestic power supplies, and industrial settings.
Advantages:
- Clean technology with water as the only waste product
- High efficiency due to direct energy conversion
- Quiet and compact
Disadvantages:
- Hydrogen storage challenges due to flammability and explosiveness
- Net energy loss in hydrogen production
- Requirement of expensive catalysts due to low operating temperatures
Oxidizing and Reducing Agents
Oxidizing agents:
- MnO4- + 8H+ + 5e- > Mn2+ + 4H2O (purple to pale pink)
- Cr2O72- + 14H+ + 6e- > 2Cr3+ + 7H2O (orange to green)
Reducing agent:
- 2S2O32- > S4O62- + 2e-
Standard Electrode Potential
Standard electrode potential is the potential of a half-cell measured against the standard hydrogen electrode under standard conditions (298K, 1 atm, 1 mol/dm3 solutions).
Acids, Bases, and pH
Acid-Base Pairs and Dissociation
An acid-base pair consists of two species that transform into each other by gaining or losing a proton. The acid dissociation constant (Ka) measures the strength of an acid.
Strong acids: Ionize almost completely in water, have a large Ka value.
Weak acids: Ionize partially in water, have a small Ka value.
pH and Concentration
pH measures the concentration of hydrogen ions (H+) in a solution: pH = -log10[H+]. A lower pH indicates a higher H+ concentration and greater acidity.
pH of Strong and Weak Acids
The pH of a strong acid can be calculated assuming complete dissociation. For weak acids, the degree of dissociation and the equilibrium constant (Ka) must be considered.
Self-Ionization of Water and Kw
Water can act as both an acid and a base, undergoing self-ionization: H2O > H+ + OH-. The ionic product of water (Kw) is the product of the concentrations of H+ and OH- ions, which is 1×10^-14 mol^2/dm^6 at 298K.
pH of Strong Bases
The pH of a strong base depends only on its concentration, as it is fully ionized in water.
Salt Hydrolysis
Solutions of some salts are not neutral due to the hydrolysis of their ions. Salts of weak acids and strong bases result in alkaline solutions, while salts of strong acids and weak bases result in acidic solutions.
Titration and Buffers
Equivalence Point and End Point
The equivalence point in a titration is when the amounts of acid and base are stoichiometrically equal. The end point is when the indicator changes color, signaling the completion of the reaction.
Titration Curves
Titration curves show the pH change during a titration. The shape of the curve depends on the strength of the acid and base involved.
Buffer Solutions
Buffer solutions resist changes in pH upon addition of small amounts of acid or base. They consist of a weak acid and its conjugate base or a weak base and its conjugate acid.
Buffer action:
- Addition of acid: H+ ions are neutralized by the conjugate base.
- Addition of base: OH- ions are neutralized by the weak acid.