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CHMB31H3 (8)
Chapter 4

CHMB31 Chapter 4

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Alen Hadzovic

Chapter 4: Acids and Bases Bronsted Acidity • Bronsted Acid: a proton donor. • Bronsted Base: a proton acceptor. • A proton has no separate existence in chemistry and it is always associated with other species. • A simple representation of a hydrogen ion in water is as the hydronium ion, H O . + 3 • Amphiprotic: a substance that can act as both a Bronsted acid and a Bronsted base like water. • When a species donates a proton, it becomes the conjugate base; when a species gains a proton, it becomes the conjugate acid. • Conjugate acids and bases are in equilibrium in solution. • The strength of a Bronsted acid is measured by its acidity constant and the strength of a Bronsted base is measured by its basicity constant; the stronger the base, the weaker is its conjugate acid. o Acidity Constant (K a: K a [H ][A ]/[HA] + - o Basicity Constant (K b: K b [HB ][OH ]/[B] + - -14 o Autoprotolysis Constant (K )w K =w[H ][OH ] = 1.00 x 10 o Kw= K Ka b o pKa= -logK a o pKa+ pK =bpK w • An acid or base is classified as either weak or strong depending on the size of its acidity constant. o Strong Acid: if the proton transfer equilibrium lies strongly in favor of donation of a proton to the solvent (pK a 0, K >a1). o Weak Acid: proton transfer equilibrium lies in favor of nonionized acid (pK > 0 a and K < 1). a o Strong Base: reacts with water to become almost fully protonated. o Weak Base: only partially protonated in water. o The conjugate base of any strong acid is a weak base because it is thermodynamically unfavorable for such a base to accept a proton. • A polyprotic acid loses protons in succession and successive deprotonations are progressively less favorable; a distribution diagram summarizes how the fraction of each species present depends on the pH of the solution. o Ka1= [H ][A ]/[H 2] o Ka2= [H ][A ]/[HA ]- o Because additional electrostatic work must be done to remove the positively charged proton, the deprotonation is less favorable (K < K ). a2 a1 • Proton affinity is the negative of the gas phase proton-gain enthalpy. o Proton-Gain Enthalpy (ΔH ᶿ)pgthe standard enthalpy of the reaction. o Proton Affinity (A ): the negative of the proton-gain enthalpy. p o If the proton gain enthalpy is only slightly negative, then the proton affinity is low, indicating a weaker basic character. • Proton affinities of p-block conjugate bases decrease to the right along a period and down a group. o HF is a stronger acid than H O a2d HI is the strongest acid of the hydrogen halides. - - - o The order of proton affinities of their conjugate bases is I < OH < F . o The gas-phase acidity of HA increases across a period as the electron affinity of A increases. • Solution proton affinities for binary acids are lower than gas phase proton affinities. o Effective Proton Affinity (A’ p: the negative of the accompanying proton-gain enthalpy o Reaction is exothermic if the effective proton affinity of A (aq) is lower than that of H O2and provided entropy changes are negligible and enthalpy changes are a guide to spontaneity. - o Reaction is exothermic if the effective proton affinity of A (aq) is higher than - that of OH . • Small highly charged ions are stabilized in polar solvents. 2 o Born Equation: ΔG solv = -N A ξ/8πε (0-1/ε )r where z is charge number of the ion, r is the effective radius, ε0is the vacuum permittivity and ε is rhe relative 2 permittivity and ξ = z /r. o The larger the relative permittivity, the more negative the value of ΔG ᶿ. solv • A solvent with a large autolysis constant can be used to discriminate between a wide range of acid and base strengths. o Levelling Effect: brings all stronger acids down to the acidity of H (water has this effect). o The leveling effect can be expressed in terms of the pKa of the acid and the base leveling effect can be expressed in terms of the pKb of the base. • The solvent system definition of acids and bases extends the Bronsted-Lowry definition to include species that do not participate in proton transfer. + + o An acid increases the concentration of H O or 3 ions and a base increases - the concentration of OH ions. o Solvent-System Definition: any solute that increases the concentration of the cation generated by autoionization of the solvent is defined as an acid and any that increase the concentration of the corresponding anion is defined as a base. Characteristics of Bronsted Acids • Aqua acids, hydroxoacids and oxoacids are typical of specific regions of the periodic table. o Acidic Proton: a donatable proton from an –OH group attached to a central atom. o Aqua Acid: the acidic proton is on a water molecule coordinated to a central metal ion like [Fe(OH ) ] 3+ (aq)  central atoms in low oxidation states of the s 2 6 and d-block metals and of metals on the left of the p-block. o
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