Thermal effects

1. Thermal effects The chemical reactions take place through the breaking of chemical bonds and the forming of new ones. Therefore, chemical reactions are accompanied by important thermal effects (heat release or absorption). Exothermic reactions = reactions that take place with heat release. Endothermic reactions = reactions that take place with heat absorption.

2. Chemical reactions are represented using chemical equations. Reactants = substances initially involved in a chemical reaction. They are written in the left term of the equation. Reaction products = substances formed in a chemical reaction. They are written in the right term of the equation Because in a chemical reaction, the nature of atoms of the substances is not changed, the chemical equations are equalized so that the number of atoms of a certain element from the left term is equal to the one from the right term.

3. Let’s consider the chemical reaction between hydrogen and chlorine, when hydrochloric acid is formed: H2 + Cl2 = 2HCl For the hydrochloric acid we chose the coefficient 2 so that the number of chlorine atoms, as well as the number of hydrogen atoms is not modified. The primary signification of this chemical reaction is that a hydrogen molecule interacts with a chlorine molecule in order to form two molecules of hydrochloric acid. During this transformation, the covalent bonds: H – H and Cl – Cl are broken, and a new bond is formed: H – Cl.

4. NA  H2 + NA Cl2 = 2 NA HCl The chemical equations have the same properties as mathematical equations. Thus, the equation can be multiplied with Avogadro’s number, and we obtain: The second signification of the chemical equation is: that 1 mole of hydrogen reacts with 1 mole of chlorine to obtain 2 moles of hydrochloric acid.

5. In some situations, in order not to create confusion, chemical formulas of the reactants and the reaction products are followed by the symbol of the aggregation state written between brackets: 2Na (s) + 2H2O (l) = 2NaOH (aq) + H2 (g) The next symbols are used: s – solid, l – liquid, g – gas, aq – aqueous solution.

6. Classification of chemical reactions It is very difficult to choose unique and well defined criteria for the chemical reactions classification. One criterion can be the way the reactants interact in order to form the reaction products. Based on these criteria, we can distinguish: • combination reactions (synthesis), • decomposition reactions, • single displacement reactions, • double displacement reactions.

7. a) Combination reactions (synthesis) are reactions in which two substances interact to form a single compound. There are many examples for this: N2 + 3H2 = 2NH3 Fe + S = FeS Ca + Cl2 = CaCl2 SO3 + H2O = H2SO4

8. b) Decomposition reactions are transformations in which from one substance, two or more substances are formed: CaCO3 = CaO + CO2 4NH4NO3 = 3N2 + N2O4 + 8H2O Fe2(SO4)3 = Fe2O3 + 3SO3

9. c) Single displacement or substitution reactions are transformations in which one element or one group of elements from a combination is replaced with another element or group of elements: Fe + CuSO4 = Cu + FeSO4 Mg + 2H2O = Mg(OH)2 + H2 Zn + 2HCl = ZnCl2 + H2 Cl2 + 2KI = 2KCl + I2

10. d) Double displacement or coupling substitution is transformation in which two elements or groups of elements are exchanged between two chemical combinations: Pb(NO3)2 + 2KI = PbI2 + 2KNO3 AgNO3 + KCl = AgCl + KNO3 H2SO4 + BaCl2 = BaSO4 + 2HCl CaCl2 + K2CO3 = CaCO3 + 2KCl A special case of double substitution reactions is the reaction between acids and bases: H2SO4 + 2NaOH = Na2SO4 + 2H2O

11. Based on the nature of the reactants or products there are: - combustion reactions - hydrolysis reaction - precipitation and complexation reactions a) Combustion reactions: oxygen reacts with a carbon compound containing hydrogen and/or other element like O, S, N. Example: the combustion of hydrocarbons (toluene, methane, acetylene), alcohols (methanol) or sulfur compounds (thiophene) C6H5-CH3 + 9O2 = 7CO2 + 4H2O CH4 + 2O2 = CO2 +2H2O

12. 2C2H2 + 5O2 = 4CO2 + 2H2O 2CH3OH + 3O2 = 2CO2 + 2H2O C4H4S + 6O2 = 4CO2 + 2H2O + SO2 The burning of carbon can also be considered a combustion reaction: C + O2 = CO2 b) Hydrolysis reaction: the reactant is water; this reactions are frequent in inorganic chemistry as well as in organic chemistry: Al2(SO4)3 + 6H2O = 2Al(OH)3 + 3H2SO4 R-CN + H2O = R-CONH2

13. c) The precipitation and complexationreactions: the classification criteria is the nature of the reaction products: Pb(NO3)2 + K2SO4 = PbSO4 + 2KNO3 CoCl3 + 6NH3 = [Co(NH3)6]Cl3

14. 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 + 2H2O = 2NaOH + H2 Atomic masses: Na – 23, H – 1, O – 16.

15. In a vessel filled with sufficiently enoughwater 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 H2O 0.23g Na………………………………x g H2O ________________________________________

16. 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 _____________________________________

17. We can calculate directly the volume of H2 that results from the reaction in normal conditions of temperature and pressure: 2·23g Na………………………………22.4L H2 (cn) 0.23g Na………………………………x L H2 (cn) ___________________________________________