Year 12 Chemistry: Acids & Bases Explained
Acids and bases are fundamental chemical substances characterized by their proton donation or acceptance, or electron pair behavior. Measured by pH, they indicate acidity or alkalinity. These compounds play crucial roles in various chemical reactions and biological systems, including maintaining physiological balance through buffer systems.
Key Takeaways
Acids increase H+, bases increase OH- in solution.
pH scale measures acidity: <7 acidic, >7 basic.
Three main theories define acid-base behavior.
Buffers resist pH changes, crucial for stability.
Acid/base strength relates to dissociation constants.
What are the fundamental concepts of acids and bases?
Acids increase hydrogen ions (H+) in aqueous solutions, while bases increase hydroxide ions (OH-). The pH scale measures their strength: pH less than 7 is acidic, 7 is neutral, and greater than 7 is basic. This fundamental understanding is crucial for chemical reactions.
- Acid: Increases H+.
- Base: Increases OH-.
- Strength: Measured by pH.
What are the main acid-base theories and their limitations?
Acid-base behavior is defined by theories like Arrhenius (H+/OH- in water), Brønsted-Lowry (proton donor/acceptor), and Lewis (electron-pair acceptor/donor). Each theory expands the definition, with Lewis being the most comprehensive, covering a wider range of reactions.
- Arrhenius: H+ or OH- in water.
- Brønsted-Lowry: Proton transfer.
- Lewis: Electron-pair transfer.
How do conjugate acid-base pairs form and what defines their strength?
Conjugate acid-base pairs form via proton transfer. An acid donating a proton becomes its conjugate base. A base accepting a proton becomes its conjugate acid. Stronger acids yield weaker conjugate bases, demonstrating an inverse relationship in strength.
- Acid donates H+, becomes conjugate base.
- Base accepts H+, becomes conjugate acid.
- Stronger acids have weaker conjugate bases.
What are the different classifications of acids based on proton donation?
Acids are classified by the number of protons they can donate. Monoprotic acids donate one, diprotic two, and triprotic three. Polyprotic acids donate more than one proton, undergoing multi-step ionization processes.
- Monoprotic: Donates one H+.
- Diprotic: Donates two H+.
- Triprotic: Donates three H+.
- Polyprotic: Donates more than one H+.
What are amphiprotic substances and how do they behave?
Amphiprotic substances can act as both an acid and a base, depending on the environment. They can either donate or accept a proton. Water (H2O) and bicarbonate (HCO3-) are common examples, showcasing this dual nature in reactions.
- Acts as acid or base.
- Examples: water, HCO3-.
How does successive ionization or protonation affect acid/base behavior?
For polyprotic acids, successive proton removal becomes increasingly difficult, causing dissociation constant (Ka) values to decrease with each step. Similarly, adding protons to a base becomes harder. This impacts overall strength and buffering capacity.
- Ka/Kb values decrease.
- Each removal/addition is harder.
How is acid and base strength quantified in chemistry?
Acid and base strength are quantified by dissociation constants. Ka measures acid strength, Kb measures base strength. Larger Ka or Kb values indicate stronger acids or bases, meaning they dissociate more completely in solution.
- Ka: Acid dissociation constant.
- Kb: Base dissociation constant.
- Large Ka/Kb: Strong acid/base.
What is the self-ionization of water and its significance?
Water undergoes self-ionization, forming hydronium (H3O+) and hydroxide (OH-) ions. The ion product constant (Kw) is 1.0 x 10^-14 at 25°C. This constant is fundamental for the pH scale, where pH + pOH always equals 14.
- 2H2O ⇌ H3O+ + OH-.
- Kw = [H+][OH-] = 1.0 x 10^-14.
- pH + pOH = 14.
What are chemical buffers and how do they maintain pH stability?
Buffers are solutions that resist significant pH changes when small amounts of acid or base are added. They consist of a weak acid and its conjugate base. Buffer capacity depends on component concentrations and the weak acid's pKa.
- Resist pH changes.
- Weak acid + conjugate base.
- Capacity depends on concentration/pKa.
What is acidosis and how does the body manage it?
Acidosis is a physiological condition where blood pH drops below 7.35. The body's natural buffer systems actively minimize these critical pH changes by absorbing excess H+ or OH- ions, maintaining vital physiological balance.
- Blood pH < 7.35.
- Body buffers minimize pH change.
How does the phosphate buffer system function in biological systems?
The phosphate buffer system (H2PO4-/HPO42-) is crucial for maintaining pH balance within biological systems, especially inside cells. Its effectiveness near physiological pH makes it essential for regulating intracellular fluid pH and ensuring proper cellular function.
- H2PO4-/HPO42- system.
- Effective near physiological pH.
How are chemical indicators used in acid-base titrations?
Chemical indicators change color within a specific pH range. In titrations, they visually signal the endpoint where acid and base neutralize. Proper indicator choice is crucial, depending on the titration type, to ensure accurate results.
- Change color at specific pH.
- Signal titration endpoint.
- Choice depends on titration type.
Frequently Asked Questions
What is the primary difference between an acid and a base?
Acids typically increase hydrogen ion concentration, while bases increase hydroxide ion concentration in aqueous solutions. Modern theories expand this to proton donation/acceptance or electron pair behavior.
How is the strength of an acid or base determined?
Acid or base strength is determined by its dissociation constant (Ka for acids, Kb for bases). A larger Ka or Kb value indicates a stronger acid or base, meaning it dissociates more completely.
What is a conjugate acid-base pair?
A conjugate acid-base pair consists of two species that differ by the presence or absence of a proton (H+). When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.
Why are buffer solutions important in chemistry and biology?
Buffer solutions are crucial because they resist significant changes in pH when small amounts of acid or base are added. This stability is vital for maintaining constant conditions in chemical reactions and biological systems, like blood.
What is the significance of water's self-ionization?
Water's self-ionization establishes the fundamental relationship between hydrogen and hydroxide ion concentrations (Kw). This constant is essential for defining the pH scale and understanding neutrality, acidity, and basicity in aqueous solutions.
