1 Stoichiometry It is the part of chemistry that has as aim the establishment of the quantitative relations between the reactants and reaction products. The name stoichiometry derives from Greek stoicheon that means element and metron that means measurement. So, stoichiometry is the science of the elements measuring. As it was seen before, the atomic mass unit (uam) was introduced, that represents the 12th part of the mass of the C: 1 uam = · kg

2 Based on the atomic mass unit the relative atomic masses of all elements have been determined. Knowing the atomic masses one can calculate the (relative) molecular masses, as the sum of the relative masses of all the atoms in the molecule. For example, the molecular mass of water is M H2O = 2·1+16=18 M H2SO4 = 2·1+32+4·16=98 M NaCl = =58.5 M CuSO4 = ·16=159.5.

3 The mole was initially defined as the mass of substance, expressed in grams, equal to the molecular mass of the substance. Thus, 1 mole of H 2 SO 4 is the quantity of substance that contains 98 g H 2 SO 4. The definition of the mole, as a fundamental unit in the International System of Units, is the following:

4 The mole is the quantity of substance of a system that contains 6.022·10 23 (the Avogadro’s number N A ) elementary particles. Avogadro’s number refers to different elementary particles that can be: molecule, atoms, ions or electrons.

5 Stoichiometric calculation Stoichiometric calculation is based on the law of conservation of mass: In a chemical reaction, the mass of the reactants is equal to the mass of the reaction products. Let us consider the reaction between metallic sodium and water that occurs according to the chemical equation: 2Na + 2H 2 O = 2NaOH + H 2 Atomic masses: Na – 23, H – 1, O – 16.

6 In a vessel filled with sufficiently enough water we introduce 0.23 g sodium. Calculate the quantity (mass) of water that has reacted, as well as the quantities (masses) of sodium hydroxide and hydrogen that have resulted. The quantity of water that has reacted with sodium: 2·23g Na………………………………2·18g H 2 O 0.23g Na………………………………x g H 2 O ________________________________________

7 Similarly, we calculate the mass of the resulted NaOH: 2·23g Na………………………………2·40g NaOH 0.23g Na………………………………x g NaOH _________________________________________ The resulted hydrogen mass: 2·23g Na………………………………2g H2 0.23g Na………………………………x g H2 _____________________________________

8 We can calculate directly the volume of H 2 that results from the reaction in normal conditions of temperature and pressure: 2·23g Na………………………………22.4L H 2 (cn) 0.23g Na………………………………x L H 2 (cn) ___________________________________________

9 CHAPTER 7 CHEMICAL EQUILIBRIUM

10 Reactions that may proceed in both directions are called reversible reactions. H 2 + I 2 2HI Example: The reversible equation is represented using arrows in both ways instead of the equality sign. Reversible reactions

11 The law of mass action We consider the reversible reaction: CH 3 -COOH + C 2 H 5 -OH CH 3 COOC 2 H 5 + H 2 O The ratio between the product of the reaction products concentrations and the product of the reactants concentrations, all taken to the power of their stoichiometric coefficients, is constant. K c is the equilibrium constant.

12 For a general reversible reaction: aA + bB mM + nN the expression of the law of mass action is: Le Chatelier’s principle If a dynamic equilibrium is disturbed by changing the conditions (concentrations, temperature and pressure) the position of equilibrium moves to counteract the change.

13 3. Increasing the pressure will shift the equilibrium so that molecules with smaller volume are being formed. Consequences of Le Chatelier’s principle: 1. Increasing the concentration of one of the components will shift the equilibrium in the direction in which this component reacts; 2. Increasing the temperature of the system will shift the equilibrium in the direction of endothermic reaction, so that the heat will be absorbed;

14 Electrolytic dissociation of water The water molecules dissociates according to the reaction: H 2 O + H 2 O H 3 O + + HO - The equilibrium is shifted far to the left. Experimentally, it was determined that at 25°C, only one molecule of water, out of 556,000,000 is dissociated, which means the dissociation degree of water is α = 18· The equilibrium constant for the dissociation reaction of water is:

15 The ionic product of water depends on the temperature. At 25°C, the value of K W is mol L -1. In pure water the concentration of the H 3 O + ions is equal to that of the HO -, which means that at 25°C: mol L -1 Kw is called the ionic product of water