AP Chemistry College Board

Acid-Base Reactions and Buffers

Acid-Base Reactions

• An acid-base reaction is also known as a neutralisation reaction. It involves the direct transfer of a proton (H+) from an acid to a base. The products usually include water and a salt.
• Acids are proton donors. They typically taste sour, react with metals to produce hydrogen gas, and turn blue litmus paper red. Common examples include hydrochloric acid (HCl) and sulfuric acid (H2SO4).
• Bases are proton acceptors. They taste bitter, feel slippery to the touch and turn red litmus paper blue. Common examples include sodium hydroxide (NaOH) and ammonia (NH3).
• The reaction of a strong acid with a strong base results in a completely neutral solution with a pH of 7. For example: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l).
• However, the reaction of a weak acid with a strong base, or a strong acid with a weak base, does not necessarily give a neutral solution. It depends on the relative strengths of the acid and base.

pH Scale and the Role of Water

• The pH scale is used to measure how acidic or basic a solution is. It ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, and values above 7 indicate basicity.
• Water plays a critical role in pH. It can act as both an acid and a base. When pure water self-ionises, it produces equal amounts of H+ (hydrogen ion) and OH- (hydroxide ion). This state of balance means that pure water has a neutral pH of 7.

Buffer Solutions

• A buffer solution is one that resists changing its pH, even when small amounts of acid or base are added. This is accomplished by the presence of significant amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• In a buffer, the weak acid reacts with any added base and the conjugate base reacts with any added acid, helping to keep the pH more or less constant.
• An example of an acid buffer system consists of ethanoic acid (CH3COOH) and its conjugate base, the ethanoate ion (CH3COO-).
• Buffer solutions are important in many biological systems. For example, the human blood stream is buffered to maintain a pH of approximately 7.4.

Remember: Accurately determining pH levels and understanding how to create and manipulate buffer solutions is essential in many scientific and industrial applications.