Brønsted-Lowry Acids and Bases Conjugate Acid, Lecture notes of Chemistry

Download Brønsted-Lowry Acids and Bases Conjugate Acid and more Lecture notes Chemistry in PDF only on Docsity! Acid-Base Chemistry & Organic Compounds Chapter 2 Brønsted – Lowry Acids & Bases Brønsted-Lowry Acid: Proton (H+) Donor Brønsted-Lowry Base: Proton (H+) Acceptor   General reaction: HA + B: A- + BH+ Brønsted-Lowry Acids and Bases Base Acid + Acid Base + Conjugate Pair Conjugate Pair Reaction 4 H2PO4 - OH- + Reaction 5 HPO4 2- SO3 2- + Reaction 1 HF H2O + F- H3O+ + Reaction 3 NH4 + CO3 2- + Reaction 2 HCOOH CN- + HCOO- HCN + NH3 HCO3 - + HPO4 2- H2O + PO4 3- HSO3 - + Conjugate Acid – Base Pairs Brønsted-Lowry Acids and Bases in Water   A) Hydrochloric acid – Strong Acid – Complete dissociation in water   B) Ammonia – Weak Base – Partial deprotonation of water Examples of Brønsted-Lowry Acid-Base Reactions General Reactivity These reactions illustrate a general pattern of reactivity: •  Electron-rich species react with electron-deficient species. •  Bases are electron rich. •  The H of an acid has a partial positive charge making it electron deficient. •  Therefore, acids and bases tend to react with one another. Consider the mixing of a solution of hydrochloric acid and sodium hydroxide.   As discussed on a previous slide, hydrogen chloride fully dissociates in aqueous solution.   Sodium hydroxide also fully dissociates:   When the two solutions are mixed, the actual reaction is between hydronium and hydroxide ions. Strong Acid – Strong Base Neutralization Reaction Strengths of conjugate acid-base pairs Acidity of Some Common Compounds Outcome of Acid-Base Reactions •  The position of the equilibrium depends on the relative strengths of the acids and bases. •  Equilibrium always favors formation of the weaker acid and base. •  Because the pKa of the starting acid (25) is lower than that of the conjugate acid (38), equilibrium favors the products. Steps in Solving Acid-Base Reaction Equilibria 1)  Identify the acid and base in the starting materials. Assume –NH2 is the base because it bears a net negative charge. That makes HC≡CH the acid. 2)  Draw the products of proton transfer and identify the conjugate acid and base in the products. Acetylene gives up its proton to –NH2. 3)  Compare the pKa values of the acid and the conjugate acid. Equilibrium favors formation of the weaker acid with the higher pKa. The pKa of NH3 is higher; therefore products are favored.   Acid-base reaction always favor the formation of the weaker acid / weaker base pair.   The weaker acid / weaker base are always on the same side of the equation.   Example: Acetic acid reacts with sodium hydroxide to greatly favor products: water & sodium acetate Predicting the Outcome of Acid-Base Reactions Predicting K for an acid–base reaction ]pK [pK acid)(reactant a,acid)(product a10K −= Acid-Base Chemistry & Solubility   When an organic acid is ionized by a base, it will become more soluble in water, and less soluble in organic solvents. 24 Factors that Determine Acid Strength •  Anything that stabilizes a conjugate base A:¯ makes the starting acid H-A more acidic. •  Four factors affect the acidity of H-A. These are: • Element effects •  Inductive effects • Resonance effects • Hybridization effects Comparing the Acidity of Any Two Acids •  Always draw the conjugate bases. •  Determine which conjugate base is more stable. •  The more stable the conjugate base, the more acidic the acid. Element Effects—Trends in the Periodic Table Why does H2O have a much lower pKa than CH4? Since oxygen is much more electronegative than carbon, it more readily accepts a negative charge. Element Effects—Trends in the Periodic Table Across a row of the periodic table, the acidity of H-A increases as the electronegativity of A increases. Element Effects Down a Column in the Periodic Table •  Down a column of the periodic table, size, and not electronegativity, determines acidity. The acidity of H-A increases as the size of A increases. Positive or negative charge is stabilized when it is spread over a larger volume.