1. 1 CHAPTER 6 CHEMICAL REACTIONS

2. 2 Chemical reaction represents the phenomenon through which one or more substances are transformed in other substances, without affecting the nature of the constituent atoms of the transformed substances. In the environment several reactions can be observed, although most of them have a slow rate. Some examples in this way are rusting of the steel pieces, alcoholic fermentation, green turning of leaves due to the forming of chlorophyll, the ignition of fuels. Chemical reactions can be emphasized through the next manifestations:

3. 3 a) Evolution of gas bubbles If we introduce a piece of zinc in a hydrochloric acid solution, we may observe the hydrogen evolution reaction. Zn + 2HCl  ZnCl 2 + H 2

4. 4 A more violent reaction occurs between sodium and water. The reaction product is also hydrogen. 2Na + 2H 2 O  2NaOH + H 2

5. 5 b) Forming of precipitates By pouring sodium dichromate solution in a lead nitrate solution we observe the appearance of a yellow-coloured precipitate consisting of slightly soluble lead dichromate. Pb(NO 3 ) 2 + Na 2 Cr 2 O 7  2NaNO 3 + PbCr 2 O 7

6. 6 c) Changing of colour Substances absorb light of different wave lengths, so they appear differently coloured. Changing the nature of a substance through a chemical reaction can sometimes lead to color modifications. So, if in a colourless solution of ammonium thiocyanate we pour an iron (III) and ammonia sulphate solution we observe the colouring of the solution in deep red because of the forming of the iron (III) tiocyanate. Sometimes the modifying of colour can be the sign of a physical process, not necessary chemical. 3NH 4 SCN + FeNH 4 (SO 4 ) 2  Fe(SCN) 3 + 2(NH 4 ) 2 SO 4

7. 7 d) Appearance of flame This is another sign that a chemical reaction takes place. An example is the ignition reactions of hydrocarbons. The flame that appears at the Bunsen bulb is the sign of the oxidation reaction of methane with oxygen from air. CH 4 + 2 O 2  CO 2 + 2 H 2 O

8. 8 e) Modification of physical properties of solutions This is another proof of a chemical reaction. Such kind of property is conductivity. If in a vessel with hydrochloric acid solution we add a sodium hydroxide solution, with the help of a conductivity meter one can measure the decreasing of the solution’s conductivity until the complete neutralization of the acid.