Weak Acid Base Chemistry Dr Mullins 1 Water Ionizes Water partially ionizes to H and OH Water is an amphiprotic substance Amphiprotic substance can act either as an acid or a base The ionizable species of weak acids bases are in equilibrium Base Acid Weak Acids and Bases Weak acids only partially dissociate HA H A H A Keq Ka HA Ka is the ionization constant for a weak acid HA in water log Ka pKa The smaller the pKa the more acidic stronger the weak acid The larger the Ka the more acidic stronger the weak acid The proton has a stronger tendency to dissociate In other words the dissociation is pushed to the right or to H 3 Strong Weak Acid Weak Weak Acid Weak Weak Base Strong Weak Base 4 Relationship between pH and pKa Weak acids will ionize dissociate in water HA H A The resulting change in pH is determined by two factors the pKa of the conjugate acid base pair the concentration of the conjugate acid base pair deprotonated pH pKa log protonated This equation is known as the Henderson Hasselbalch equation 5 Henderson Hasselbalch Relationships deprotonated pH pKa log protonated A pH pKa log HA A HA From this equation we see that When pH pKa A HA when pH pKa 10 7 10 10 3 A HA H on or substance protonated 10 pH pKa A HA If the pH pKa The ratio of A HA form 3 1 1000 2 1 100 1 1 10 when pH pKa 10 7 4 10 3 A HA 0 1 1 1 10 1 H off or substance deprotonated 2 100 1 3 1000 1 6 H H H H H H pH H H H H H H Solution is acidic High H so protonated form predominates the soln does not need the protons H H pKa pH H Solution is less acidic Low H so deprotonated form predominates the soln is low on protons so it wants the protons more Titration Curves Titration curves give an experimental way to determine pKa A titration is an experiment in which measured amounts of a strong base are added to solution of acid The strong base is added in small amounts until the acid is neutralized A titration curve is a plot of the pH vs the amount of strong base added The titration curve is analyzed to give the X Inflection point the point in an acid base titration at which enough base has been added to neutralize 50 of the acid Acid Base pH pKa 8 Comparison of several titration curves 9 Mono Di and Triprotic acids Compounds are classified according to the number of protons that can dissociate One dissociable proton monoprotic acid 2 possible ionic forms HA A Two dissociable protons Diprotic acid 3 possible ionic forms H2A HA A2 Three dissociable protons triprotic 4 possible ionic forms H3A H2A HA2 A3 Structure ionic form varies as a function of pH H3A is not the same as HA210 H3A H2A HA2 A3 11 Biological Systems are Buffered Buffers are solutions that resist changes in pH as acid and base are added Most buffers consist of a weak acid and its conjugate base The titration curve is flat near the pKa of the buffer i e structure is resistant to change in pH Best area to buffer 1 pH unit around pKa Buffer capacity is related to the concentrations of the weak acid and its conjugate base the greater the concentration of the weak acid and its conjugate base the greater the buffer capacity 12 Biological Buffers H2PO4 HPO42 is the principal buffer in cells H2CO3 HCO3 is an important but not the only buffer in blood CO2 g CO2 aq H2 O l CO2 aq H2 CO3 aq H2 CO3 aq H aq HCO3 aq CO2 g H2 O l H aq HCO3 aq 13 Changes in Blood Chemistry CO2 H2O H HCO3 Breathing induced hyperventilation can result in increased blood pH Remove CO2 so H combines with HCO3 to produce more CO2 H or pH hypoventilation can result in decreased blood pH Increase CO2 so it combines with H2O to produce more H pH or Metabolically induced decreases in blood pH will result in increased breathing rate pH or H so H combines with HCO3 to produce more CO2 Increases in blood pH very rare will results in decreased breathing rate pH or H so to compensate CO2 combines with H2O 14