ACIDS AND BASES

COMPARISON Acid – a substance whose water solution Turns litmus paper red Turns litmus paper red Has a sour taste Has a sour taste Neutralizes bases Neutralizes bases Reacts with active metals to produce hydrogen gas Reacts with active metals to produce hydrogen gas Base – a substance whose water solution Turns litmus paper blue Turns litmus paper blue Has a bitter, biting taste Has a bitter, biting taste Has a slippery, soapy feel Has a slippery, soapy feel Neutralizes acids Neutralizes acids

Theories of Acids and Bases - 1 Arrhenius acid A substance which gives hydrogen ions [H + ] on dissociation in water A substance which gives hydrogen ions [H + ] on dissociation in water HCl === H + + Cl - Arrhenius Base A substance which gives hydroxide ions [OH - ] on dissociation in water A substance which gives hydroxide ions [OH - ] on dissociation in water NaOH === Na + + OH -

Theories on Acids and Bases - 2 Bronsted-Lowry Acid Proton donor Proton donor H + A - + H 2 O == A - + H 3 O + acid(protondonor) Bronsted-Lowry Base Proton acceptor Proton acceptor H + Cl - + NH 3 == Cl - + NH 4 + base base proton acceptor proton acceptor

Conjugate Acid-Base Pairs H + A - + H 2 O ==== A - + H 3 O H + A - + H 2 O ==== A - + H 3 O + acid 1 base 2 base 1 acid 2 When HA (acid) donates a proton [H + ] to H 2 O, it becomes a potential proton acceptor [A - ] and therefore a base. When H 2 O accepts a proton, it acts as a bases and become a potential acid [H 3 O + ]. An acid and a base that are related by a transfer of protons are called a conjugate acid-base pair. The sets HA (acid 1)/A - (base 1) and H 2 O (base 2/H 3 O + (acid 2) are said to be acid-base pairs.

Exercise Identify the acid-base pairs in the following reactions HCN + H 2 PO 4 - ==== CN - + H 3 PO 4 HCN + H 2 PO 4 - ==== CN - + H 3 PO 4 HS - + OH - ==== S -2 + H 2 O HS - + OH - ==== S -2 + H 2 O H 2 O + NH 2 - ==== OH - + NH 3 H 2 O + NH 2 - ==== OH - + NH 3 H 2 O + HCl ==== H 3 O + + Cl - H 2 O + HCl ==== H 3 O + + Cl -

Amphoteric Compounds Substances that can act either as an acid or a base are called amphoteric compounds Substances that can act either as an acid or a base are called amphoteric compounds 1. H 2 O + NH 2 - ==== OH - + NH 3 acid 1 acid 1 1. H 2 O + HCl ==== H 3 O + + Cl - base 2 base 2

Strength of Acids and Bases Strong acid ionizes completely in water solutions producing H 3 O + (HCl ---- H + + Cl - ) Strong base ionizes completely in water solutions producing OH - (NaOH ---- Na + + OH - ) Weak acids and weak bases ionize slightly in water solutions HAc ==== H + + Ac - HAc ==== H + + Ac -

Exercise 2 Write an equation for the ionization of the following substances in water 1. HNO 3 2. KOH 3. H 2 CO 3 4. CO H 2 O

pH and pOH pH = -log [H + ] 01 2 increasing 3 acidity Neutral increasing 12 basicity 1314 pOH = -log [OH - ] 01 2 increasing 3 basicity Neutral increasing 12 acidity 1314

Exercise 3 Complete the table SolutionpHpOH[H+][OH-] Nature of solution A12.3 B6.8 C 2 x D 1 x 10 -7

Buffers Substances that resist sudden changes in pH Substances that resist sudden changes in pH Blood pH is 7.4 Blood pH is 7.4 Blood pH above 7.4 = alkalosis Blood pH above 7.4 = alkalosis Blood pH below 7.4 = acidosis Blood pH below 7.4 = acidosis Body must get rid of ~15 moles potential acid/day (mostly CO 2 ) Body must get rid of ~15 moles potential acid/day (mostly CO 2 ) In neutralization H+ and OH- react to form water In neutralization H+ and OH- react to form water pH changes change charges on molecules, especially proteins pH changes change charges on molecules, especially proteins

Buffers The Mammalian Body Regulates pH in Several Ways Buffers (such as bicarbonate/CO 2 ) minimize pH change Buffers (such as bicarbonate/CO 2 ) minimize pH change CO 2 is eliminated by the lungs CO 2 is eliminated by the lungs Other acids and bases are eliminated by the kidneys Other acids and bases are eliminated by the kidneys Blood pH Must be Kept Close to 7.4 in Mammals Hydrogen ion is extremely reactive and effects many molecules which regulate physiological processes Hydrogen ion is extremely reactive and effects many molecules which regulate physiological processes Blood pH is set at a slightly alkaline level of 7.4 (ph 7.0 is neutral) Blood pH is set at a slightly alkaline level of 7.4 (ph 7.0 is neutral) A change of pH of 0.2 units in either direction is considered serious A change of pH of 0.2 units in either direction is considered serious Blood pHs below 6.9 or above7.9 are usually fatal if they last for more than a short time Blood pHs below 6.9 or above7.9 are usually fatal if they last for more than a short time

Buffers are Mixtures of Chemicals Which Stabilize pH Buffers are mixtures of two chemicals that resist pH changes:  if the pH is too low one chemical will bind some of the hydrogen ions and raise the pH  if the pH is too high the other chemical will donate some hydrogen ions to lower the pH

The Chief Mammalian Blood Buffer is a Mixture of Bicarbonate and Carbon Dioxide  All body fluids, inside or outside cells have buffers which defend the body against pH changes  The most important buffer in extracellular fluids, including blood, is a mixture of carbon dioxide (CO 2 ) and bicarbonate anion (HCO 3 - )  CO 2 acts as an acid (it forms carbonic acid when it dissolves in water), donating hydrogen ions when they are needed  HCO 3 - is a base, soaking up hydrogen ions when there are too many of them  There are also other buffers in blood, such as proteins and phosphate, but they are less important

Blood pH is determined by a balance between bicarbonate and CO 2

Too Much CO 2 or Too Little HCO3 Will Cause Acidosis  The balance will swing toward a low pH, producing acidosis, if CO 2 is raised or HCO 3 lowered  CO 2 can be raised by hypoventilation (pneumonia, emphysema)  Metabolic conditions such as ketoacidosis caused by excess fat metabolism (diabetes mellitus) will lower bicarbonate

Too Much HCO3 or Too Little CO2 Will Cause Alkalosis  The balance will swing the other way, producing alkalosis, if CO 2 is lowered or HCO 3 - raised  CO 2 can be lowered by hyperventilation  Vomiting removes stomach acid and raises bicarbonate  Alkalosis is less common than acidosis

Blood pH is Chiefly Regulated by the Lungs and Kidneys in Mammals  Normal metabolism produces large amounts of CO 2 continuously (about 14 moles/day)  If this CO 2 were not removed we would rapidly develop fatal acidosis  Almost all of the CO 2 is removed, as a gas, from the lungs  If blood pH is low respiration is stimulated so that more CO 2 is removed, raising the pH to the normal level  Bicarbonate is adjusted in the kidney  The kidneys can generate new HCO 3 - when it is low  The kidneys also dispose of acids produced in metabolism which are not volatile

Many Serious Medical Problems Involve Abnormalities of Salt, Water or pH Examples:  Hyperkalemia: caused by kidney disease & medical malpractice ; high K+ in blood- can stop the heart in contraction (systole)  Dehydration: walking in desert- can lose 1-2 liters/hour through sweat; blood becomes too viscous to circulate - > loss of temperature regulation -> hyperthermia -> death  Acidosis: many causes including diabetes mellitus and respiratory problems; can cause coma, death