1. ENTRY QUIZ 1 1.What is chemical reaction? 2.Give an example? 3.What is at the left? 4.What is at the right? 5.What the arrow means?

2. 14 th ELEMENT 2 SILICON Si Atomic number 14 Atomic number 28.09 Metalloid, widely used in computers Atomic structure 14 p, 14 e, 14n

3. Chemical Reactions and Equations Chemical Reactions and Equations 3

4. Phase Notation A chemical equation can also show the physical state of the materials in the reaction. This is done with phase notations: (s) solid(l) liquid(g) gas (aq) aqueous or dissolved in water (ppt) or ( ) means “a precipitate forms” ( ) means “a gas bubbles off” means “is heated” Phase notations are written to the lower right of each formula. Clues to phase notations are often given in equations written in words. 4

5. An example When pellets of sodium hydroxide are dropped into an aqueous solution of iron(III) chloride, iron(III) hydroxide precipitates and sodium chloride stays dissolved. 3NaOH (s) + FeCl 3(aq) Fe(OH) 3 (ppt) + 3NaCl (aq) 5

6. Reactants  Products Methane + Oxygen  Carbon Dioxide + Water The arrow (yield sign) and indicates that a reaction took place. Word Equations: show the names of the reactants and the products

7. Skeleton Chemical Equations The formulas of the reactants and products are written without indicating their relative amounts. Example: CH 4 + O 2  CO 2 +H 2 O

8. Balanced Chemical Equations The chemical formulas and the relative amounts of the reactants and products are written. Example: CH 4 + 2O 2  CO 2 + 2H 2 O The coefficients indicate the relative amounts of each substance. Equal number of atoms of each element must appear on both sides of the balanced equation.

9. Why does a chemical reaction have to be balanced? In any chemical or physical change, mass is neither created or destroyed Mass is CONSTANT Law of Conservation of Mass (Antoine Lavoisier)

10. Combustion of Methane CH 4 + 2O 2  CO 2 + 2H 2 O Atoms are rearranged!

11. Example: Decomposition of hydrogen peroxide in the presence of a catalyst. Word equation: hydrogen peroxide  water + oxygen

12. Example: Decomposition of hydrogen peroxide in the presence of a catalyst. Skeleton equation: H 2 O 2  H 2 O + O 2

13. Example: Decomposition of hydrogen peroxide in the presence of a catalyst. Balanced Equation: 2H 2 O 2  2H 2 O + O 2

14. Example: Decomposition of hydrogen peroxide in the presence of a catalyst. Balanced equation showing the catalyst (MnO 2 ) the state of the reactants and products: MnO 2 2H 2 O 2 ( l )  2H 2 O ( l ) + O 2 ( g )

15. Activity 4. Calculate balance at both sides 15 Balance each decomposition equation first! 1.H2O ---  H2 + O2 2.HCl --  H2 + Cl2 3.MgO --  Mg + O2 4.NH3 --  N2 + H2 5.Fe2O3 ---  Fe + O2 6.Al2O3 --  Al + O2 7.CO2 ---  C + O2 8.CO ----  C + O2 9.NaCl ----  Na + Cl2 10.KCl ----  K + Cl2

16. EXIT QUIZ 16 1.What is chemical reaction? 2.What are signs of chemical reaction? 3.How do we describe chemical reaction? 4.Do we have to balance chemical equations? 5.What if we don’t balance equations?