2 Abdullah Zreqat Laith Abu Shekha Mamoun Ahram
In this sheet we will talk about carbon, water, acid and bases. Carbon: Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules. Properties of carbon: 1) It can form four bonds, which can be the same or different in a serious form which means that carbon atom can bind with four different atoms such as (F, Cl, H) in the same time. 2) The bonds are very stable so it requires a lot of energy to break it. (Remember: strength of bonds: triple > double > Single) 3) Carbon can form rings or can form chains, they serve as backbones. 4) Carbon bonds have angles giving molecules three dimensional structure. In a carbon backbone, some carbon atoms rotate around a single covalent bond producing molecules of different shapes. 5) Electronegativity of carbon, for example: the covalent bond between carbon and hydrogen is nonpolar but the covalent bond between carbon and nitrogen or oxygen will form polar covalent bond. 1 P a g e
6) Pure carbon is not water soluble, but when carbon forms covalent bonds with other elements like O or N, the molecule that makes carbon compounds to be soluble. Functional groups are important so return to slides 21-22 and have a look at them because we need to know the structure of functional groups. Water: Properties of water: 1) Water is a polar molecule, as we mentioned before that not only the bond is polar, the whole molecule is polar. 2) Water is highly cohesive, because it has the ability to form hydrogen bond, which is a strong bond not as strong as covalent bonds but collectively the are strong. Because of formation of hydrogen bonds it can form cohesive networks. 3) Water is an excellent solvent because It is small, for example: if you simply add water to a compound like NaCl (salt) which is form by ionic bond, so just by adding water it can compete with sodium and chloride forming electrostatic interactions with the ions. 4) Water itself can form hydrogen bonds, and it can also form dipole-dipole interactions due to the presence of the partial positive and negative charges, and it can also form Dipole-charge interaction. Dipole means that it has covalent bonds one with a partial positive charge and the other with partial negative charge. 2 P a g e
5) Water is a reactive molecule because it is a nucleophile, it s a nucleophilic molecule so it loves protons because it has a lot of electrons. A nucleophile is an electron-rich molecule that is attracted to positively-charged or electron-deficient species (electrophiles) 6) Water molecules can be ionized by itself to become a positively charged hydronium ion (or proton), and a hydroxide ion: Acids and Bases: Acid: a substance that produces proton (H+) when dissolved in water. Note: H+ Reacts with water producing hydronium ion (H3O+) Base: a substance that produces hydroxyl ion (OH-) when dissolved in water. Monoprotic acid: HCl, HNO3, CH3COOH Diprotic acid: H2SO4 Triprotic acid: H3PO3 Water can act as an acid and as a base, so at one point is can accept a proton or it can donate a proton, so depending on this definition water is an amphoteric molecule. Ampho: dual 3 P a g e
For example: when water reacts with ammonia it can donate a proton so it acts as an acid, and when it reacts with hydrochloric acid it will accept a proton so it acts as a base. Acids and bases differ in strength, so there is a strong and a weak acids and in the other hand there is a strong and weak bases, it depends on the ability to dissociate. Strong acids dissociate 100% Strong bases have strong affinity for protons Note: the strongest acid is hydrochloric acid because it s mostly found as an ion. For multi-protic acids (H2SO3, H3PO4), each proton is donated at different strengths. When a weak acid reacts with a weak base it will produce conjugates. Note: The stronger the acid, the weaker the conjugate base, so when the acid is strong it has a high ability to dissociation Remember: the acid is a substance which has the ability to dissociate and giving protons, but conjugate base has the ability to obtain that protons. 4 P a g e
Note: Strong acids and bases are one-way reactions Weak acids and bases do not ionize completely Equilibrium constant: It s a constant equals the concentration of the products (Ions) divided by the concentration of the reactants. The value of the Ka indicates direction of reaction, so: When Ka is greater than 1 the product side is favoured. When Ka is less than 1 the reactants are favoured. Note: Equilibrium constant for acids is Ka and for bases Kb We can calculate Ka and pka values by using the following equation: The higher the Ka value the stronger the acid, and the lower the pka value the stronger the acid. 5 P a g e
Base dissociation constant: We can express concentration in different ways and in this sheet we will talk about three of them: molarity, equivalence and normality. Molarity of solutions: Molarity equals moles divided by volume (in Litres). Moles of a solution are the amount in grams in relation to its molecular weight Note: a molar solution is one in which 1 litre of solution contains the number of grams equal to its molecular weight. o Since (mol = grams / MW), you can calculate the grams of a chemical you need to dissolve in a known volume of water to obtain a certain concentration (M) using the following formula: Exercise: How many grams do you need to make 5M NaCl solution in 100 ml (MW 58.4)? Equivalents: grams = 58.4 x 5 M x 0.1 litre = 29.29 g When it comes to acids, bases and ions, it is useful to think of them as equivalents. 1 equivalent of a strong acid contains 1 mol of H+ ions, and 1 g-eq of an acid is the mass in grams that contains 1 mol of H+ ions. Similarly, 1 equivalent of a strong base contains 1 mol of OH- ions, and 1 g-eq of a base is the mass in grams that contains 1 mol of OH- ions. 6 P a g e
Remember: One equivalent of any acid neutralizes one equivalent of any base. Examples: 1. 1 mol HCl = 1 mol [H+] = 1 equivalent 2. 1 mol H2SO4 = 2 mol [H+] = 2 equivalents 3. One equivalent of Na+ = 23.1 g 4. One equivalent of Cl- = 35.5 g 5. One equivalent of Mg+2 = (24.3)/2 = 12.15 g Exercise: Calculate milligrams of Ca+2 in blood if total concentration of Ca+2 is 5 meq/l. 1 Eq of Ca+2 = 40.1 g/2 =20.1 g Grams of Ca+2 in blood = (5 meq/l) x (1 Eq/1000 meq) x (20.1 g/ 1 Eq) = 0.1 mg/l =100 mg/l Normality: Normality equals the number of protons that the base can accept (or the number of protons can acid donates) Multiply by molarity. Note: Normality (N) considers both the molarity of the solution and the equivalent content of the acid or base. For an acid solution, n is the number of protons provided by a formula unit of acid. Similarly, for a base solution is the number of hydroxide (OH-) a base can donate. Examples: 3 M H2SO4 solution is the same as a 6 N H2SO4 solution. 1 M Ca(OH)2 solution is the same as a 2N Ca(OH)2 solution. Note: The normality of a solution is NEVER less than the molarity. Exercise: What is the normality of H2SO4 solution made by dissolving 6.5 g into 200 ml? M = moles / volume = 6.5 g / (98 x 0.2 L) = 0.33 N = M x n = 0.33 x 2 = 0.66 N 7 P a g e
Normality and equivalents: Based on the equation above, since n Eq of an acid is neutralized by the same n Eq of a base, then (N x litres) of an acid is neutralized by (N x litres) of a base. Note: return to slides (48-50) and answer the exercises. Past papers : 1-Which of the following has ion-dipole interaction: a- Hcl...Hcl b-bf3...ccl4 C-NH3...CH4 d- Na+ (H2O) 2- For hydrogen bonding.. it s between an electronegative atom and a hydrogen connected to : a)iodine b)electronegative atom c)carbon d)sulfur Answers: 1-D. 2-B Success consists of going from failure to failure without loss of enthusiasm. 8 P a g e