Introduction to Chemical Reactions and Equations

1. Introduction to Chemical Reactions and Equations

2. Chemical Reactions are Everywhere Cooking Respiration

3. Chemical Reactions are Everywhere Hair Dye Auto Fuel

4. What are the clues that a chemical reaction has occurred?

5. A gas bubbles off.

6. A new solid material is formed (a precipitate.)

7. Color change.

8. Heat and/or light are emitted.

9. The key to chemical reactions! The Law of Conservation of Mass In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to form new molecules. Since atoms aren’t created or destroyed, no mass is created or destroyed either. Mass is “conserved.”

10. Terminology • In a chemical reaction, the materials to the left of the arrow are the reactants. Reactants are the substances that exist before a chemical change (or reaction) takes place. • Reactants are consumed in the reaction. • The arrow is like an equal sign in math. It can be read “react to produce” or “produces” or “forms”. • The materials to the right of the arrow are the products. The new substance(s) that are formed during the chemical changes. • H2O + SO3 H2SO4

11. 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.

12. Ways to represent a chemical reaction • Word Equations: • decomposition of hydrogen peroxide • Iron + oxygen yields rust (iron (III) oxide) • Burning gas in lab – methane + oxygen

13. Skeleton Equations • Base equation – identifies the substances involved in the chemical reaction, but does not obey law of conservation of mass • Get the chemical formulas of reactants and products correct first. • REMEMBER, diatomic molecules • Once the substances are identified for the reaction, then add…….

14. Coefficients • In a chemical reaction, if more than 1 molecule of a substance reacts or is produced, the number of molecules is shown with a coefficient. • 6CO2 + 6H2O C6H12O6 + 6O2 • Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms.

15. Count the atoms, before & after 2H2 + O2 2H2O

16. How many? • How many of each atom are shown by 4Al2(SO4)3 • How many of each atom are shown by 3(NH4)3PO4

17. 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) + FeCl3(aq) Fe(OH)3 (ppt) + 3NaCl(aq)

18. Example 2 0 Aluminum reacts with oxygen gas to form aluminum oxide 4 Al(s) + 3 O2(g)2 Al2O3(s)

19. Rules for Balancing Equations • 1) Write reactants and products • 2) Must have correct formulas • Remember the 7 diatomic molecules • Watch out especially for ionic compounds with correct charges. • 3) Count number of atoms • 4) Balance by changing the coefficients • Save Oxygen and Hydrogen usually for last • 5) Check answer • You’ll know if you made a mistake • 6) Make sure that you have the lowest ratio of coefficients

20. Do the other practice problems

21. Main Ideas • Chemical Reactions are represented by Chemical Equations. • Chemical Equations are balanced to show the same number of atoms of each element on each side. • The Law of Conservation of Mass says that atoms won’t be created or destroyed in a chemical reaction. That is why you have to balance chemical equations!