2 Acid and Bases - 2010 Department of Chemistry The University of Western Ontario 2.1 2 Acids and Bases(text 2.1 2.5 Acid-base reactions are one of

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
© 2010, Department of Chemistry, The University of Western Ontario 2.1 2. Acids and Bases (text 2.1 – 2.5) Acid-base reactions are one of the most important classes of reactions in organic chemistry and biology, e.g. : o Enzyme-catalyzed reactions o Control of blood pH by CO 2 (H 2 CO 3 , HCO 3 , CO 3 2– ) Of special relevance is proton exchange, which fits into the Bronsted-Lowry definition of acids and bases: o An acid is a proton donor HA H + + A (acid) (proton) (conjugate base) o A base is a proton acceptor B: + H + BH + (base) (proton) (conjugate acid) Every acid-base reaction must have both an acid and a base (to supply and accept the proton, respectively). Such reactions often occur in water, so we more properly write: o For an acid HA: HA + H 2 O A + H 3 O + o For a base B: B: + H 2 O BH + + OH
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2.2 In first-year chem, we measured [H + ] in the form of pH for strong and weak acids: pH = –log 10 [H + ] Recall that strong acids and bases are those that completely dissociate (ionize) in water. These are usually inorganic compounds, such as HCl and NaOH. On the contrary, organic compounds act as weak acids or bases: they do not dissociate or protonate completely. For the general weak acid (HA), we can write: 1. The equilibrium: HA + H 2 O H 3 O + (aq) + A (aq) 2. The equilibrium constant: O] [HA][H ] ][A O [H K 2 3 eq 3. Define the acid dissociation constant as K a = K eq [H 2 O] [HA] ] ][A O [H K 3 a o K a increases as the acid becomes stronger, but for organic weak acids, K a usually << 1. o Why? Charges are bad… they are high in energy. 4. Like pH, we can define: pKa = –log 10 K a Therefore, as acids get stronger, their K a _____________ and their pK a _____________. K a and pK a values of selected acids
Background image of page 2
2.3 Acid Name K a pK a HF hydrofluoric acid 6.9 × 10 –4 3.16 CH 3 COO H acetic acid 1.8 × 10 –5 4.76 H 2 O water 2.0 × 10 –16 15.7 NH 3 ammonia 1.0 × 10 –38 38.0 CH 3 CH 3 ethane 1.0 × 10 –51 51.0 Organic chemists concentrate on structural features of molecules that influence acid or base strength. Consider again the equilibrium: HA + H 2 O H 3 O + (aq) + A (aq) Reactants Products As seen in first-year chemistry, K eq is related to the Gibbs free energy of the reaction by G = RT(ln K eq ). So, when G is more negative, the reaction is more favourable, K eq is larger, and the products are thermodynamically more stable.
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2.4 The strength of an acid is primarily dependent on the stability of the products. If we assume that the reaction is happening in water, then one of the products is H 3 O + , but this is always formed regardless of what the acid HA is.
Background image of page 4
Image of page 5
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/01/2011 for the course CHEMISTRY Chemistry taught by Professor Dr.felixlee during the Fall '11 term at UWO.

Page1 / 15

2 Acid and Bases - 2010 Department of Chemistry The University of Western Ontario 2.1 2 Acids and Bases(text 2.1 2.5 Acid-base reactions are one of

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online