BIOMEDICAL SCIENCE-2 Aaser Abdelazim Assistant professor of Medical Biochemistry aaserabdelazim@yahoo.com
Theme 3 Water, acids, bases and buffers 1. Structure and chemistry of water 2. Acids and bases 3. Dissociation of water and ph scale 2
Lecture 1(water, acids, alkalis and bases) 1. Structure of water 2. Water bonding 3. Properties of water 4. Acids alkalis and bases At the end of this lecture the student will be able to 1. Describe the structure of water. 2. Draw water bonding. 3. List the characteristics that make water vital to our bodies. 4. Differentiate among acids alkalis and base 3
Water is very important due to: 1. Water constitutes 45%-73% of total human body weight. 2. It is distributed in intracellular (55%) and extracellular (45%) compartments. 3. It provides a continuous solvent phase between body compartments so it has a role in (metabolsim-absorption-digestion-transportation and excretion). 4. Regulates body temerature. 4
STRUCTURE OF WATER 1 molecule of water is made up of 2 hydrogen atoms bonded with 1 oxygen atom H H O 5
STRUCTURE OF WATER O NOTE: The bond that forms one water molecule is a covalent bond while water molecules join each other by hydrogen bond. 6
Water Bonding One molecule of water 7
Electronegativity of water molecules Electronegativity describes the degree to which an atom attracts electrons in a chemical bond. The difference in the electronegativity of two atoms determines their bond type. Electronegativity difference Chemical bond >1.7 An ionic bond 0.4-1.7 Polar covalent bond < 0.4 Non-polar covalent bond In water molecules, Oxygen atom has electronegativity =3.5 while hydrogen is 2 so the difference is 1.5 this indicate that water is polar covalent molecule. In water, an electron from each of the hydrogen atoms is drawn towards the oxygen atom. In this case, the oxygen atom has a partial negative charge, whereas the hydrogen atoms have partial positive charges. 8
WATER CHEMISTRY A. water is the most abundant chemical in the body. B. Water has many characteristics that make it vital to our bodies.. Size Heat capacity Water properties Polarity Buffer Crystals Cohesio n and Adhesio n 9
WATER PROPERTIES (1) Size 1. Water is a very small molecule (10-1 nm). 2. It moves fast and can squeeze into tiny gaps between other molecules. 3. Its size enable it to be the fastest absorbed molecule in the body. NOTE: nm=10-9 meter 10
(2) Polarity Hydrogen has a slightly positive charge while oxygen has a + DURACELL Polar molecules that carry a charge _ slightly negative charge. This makes it easy for water to attarct other charged molecules, dissolving them. DURACELL Non-Polar molecules that do not carry a charge Water is Called as[universal Solvent]. + + Hydrogen (1 proton) bind to oxygen by covalent bond carry +ve charge Oxygen (8 protons/8 neutrons) bind to hydrogen by covalent bond- carry ve charge NOTES The positive hydrogen ends of 1 molecule are attracted to the negative end of the oxygen of another molecule. Cohesion, adhesion, specific heat, universal solvent, capillary action, surface tension ALL HAPPEN BECAUSE WATER IS A POLAR SUBSTANCE! - Bohr model of water 11
(3) Crystal structure Due to polarity, water forms a crystal structure with metals that is less dense than liquid water. Examples calcium chloride (CaCl 2 2H 2 O)- Zinc chloride (ZnCl 2 (H 2 O) 4 )- cupper sulphate ([Cu(H 2 O) 4 ]SO 4 H 2 O) (4) Heat capacity water absorbs and releases heat energy slowly, and can hold a great deal of heat energy. This helps organisms maintain their body temperature in the safe range. (5) Cohesion and adhesion Polarity allows water to stick to itself (cohesion) and to any charged material (adhesion). Water can glue materials together. (6) Buffer Water can act as either an acid or a base, maintaining a stable ph in our bodies (will be discussed later). 12
ACIDS, ALKALIS AND BASES ACID Any substance gives H + ions in a solution CH 3 COOH CH 3 COO - + H + Acid Conjugate base Hydrogen ion ACID Strong Intermediate Weak HCl H 3 PO 4 H 2 CO 3 H 2 SO 4 Acidity is the concentration of H ions in a solution Types: 1. Free (true/ actual): amount of H present free in solution it is high in strong acids 2. Combined: amount of H present combined with conjugate base 3. Total (titratable) = total amount of free H + combined H 13
ALKALI Is a substance that gives (OH - ) in a solution NaOH Na Na + + ALKALI OH - Strong Weak BASE NaOH NH 4 OH Is the substance that can accept (H + ) ion in a solution Examples for bases Ammonia Nucleic acid bases 14
All alkalis are bases but not all bases are alkalis why? That is because all alkalis can produce (OH - ) and by role (OH - ) can accept (H + ) to form H2O while bases like NH 3 (ammonia) only can accept (H + ) to form NH 4 (ammonium). Example (1): ( Sodium hydroxide is an alkali and base NaOH Na + Na + + Example (2): ( Ammonia is only a base OH - H + H 2 O NH 3 H + NH 4 It can accepts H + but not give OH - 15
QUIZ on water chemistry 1. The bond which join two molecules of water together is ---------------- a) Hydrogen bond b) Electrostatic bond c) Covalent bond d) Hydrophobic bond 2. The cohesion of water is the attachment of water molecules to ----------------------------- a) Non-polar surfaces b) Polar surfaces c) Each others. d) Hard surfaces 3. The polarity of water is due to ---------------- a) Presence of polar bonds. b) Partial negative charge of oxygen c) Partial positive charge of hydrogen. d) Positive charge of hydrogen and negative charge of oxygen. 4. Adhesion of water molecules is known as ---------------- a) Sticking to charged surfaces. b) Joining to each others. c) Holding non-polar surfaces. d) Dissolving polar molecules. 5. One molecule of water is made up of ----------------- a) Two molecules of hydrogen b) Two molecules of oxygen c) One molecule of hydrogen and two molecules of oxygen d) One molecule of oxygen and two molecules of hydrogen. 16
6. The bond H-O-H angle in water equals----------- a) 104º b) 104.5º c) 105º d) 105.4º 7. Which one is true regarding electronegativity in water molecules? a) Oxygen is more electronegative than hydrogen b) Hydrogen is more electronegative than oxygen c) Hydrogen and oxygen have the same electronegativity d) Both have no electronegativity 8. Theoretically, one molecule of water can form how many hydrogen bonds? a) Two b) Three c) Four d) Five 9. The hydrogen bond in water is ------------------- a) 10% covalent and 90% electrostatic b) 25% covalent and 75% electrostatic c) 20% covalent and 80% electrostatic d) 15% covalent and 85% electrostatic 10. The life span of hydrogen bond between two molecules of water in solution is ------------ a) 1-20 seconds b) 1-20 microseconds c) 1-20 nanoseconds d) 1-20 picoseconds One picosecond= 10-12 second 17
QUIZ on acid, alkalis and bases 1. Acid is known as any substance gives --------------------in solution. a) Hydrogen b) Hydroxyl c) Water d) Oxygen 2. Which one is a strong acid? a) Hydrochloric acid b) Phosphoric acid c) Acetic acid d) Carbonic acid 3. Which of the following structures represents the conjugate acid of HPO 2-4? a) H 2 PO - 4 b) H 3 PO 4 c) H 4 PO + 4 d) PO - 43 4. Which of the following relationships is true for an acidic solution at 25ºC? a) [H+] > [OH-] b) ph > 7.00 c) K w > 1 10-14 d) The solution is negatively-charged 5. Which one of the following relationships is true in water at 25ºC? a) [H + ] = [H 2 O] b) [OH - ] = [H 2 O - ] c) K w > 1 10-14 05-02-2018 d) [H + ] = [OH - ] AASER ABDELAZIM BMS-2-221 18
QUIZ on acid, alkalis and bases 6. Which of the following is NOT an example of a weak acid? a) Lactic acid b) Carbonic acid c) Sulfuric acid d) Pyruvic acid 7. In Universal indicators, red color shows---------- a) Strong acids b) Strong alkalis c) Weak acids d) Weak bases 8. When an acid (H + ) is added to alkali (OH - ), product is---------- a) Hydroxides b) Water c) Salts d) Hydrogen gas 9. More corrosive acid is------------------------- a) Carbonic acid b) Nitric acid c) Gluconic acid d) Sulforic 10. Which one is a phosphoric acid? a) H 2 PO 3 b) H 2 PO 4 c) H 3 PO 4 05-02-2018 d) H 3 PO 3 AASER ABDELAZIM BMS-2-221 19