1. Unit 2 – Lecture 8

2. Chemical Equations A shorthand way to describe a chemical reaction using chemical symbols and formulae Three Parts to a Chemical Equation Reactants substances present before the reaction Yield Arrow indicates the direction of a reaction some rxns are reversible [double-sided arrow] Products substances present after the reaction

3. Chem. Equations – cont’d Chemical symbols give a “before-and-after” picture of a chemical reaction Reactants Products MgO + CCO + Mg magnesium oxide to form carbon monoxide reacts with carbon and magnesium or… magnesium oxide reacting with carbon yields carbon monoxide and magnesium

4. Chemical Equations – cont’d Coefficient – (large) # in front of an element applies to all elements in the group to the right of it ex: 4C 4 carbon atoms ex: 5KI 5 potassium atoms & 5 iodine atoms ex: 4SO 4 4 Sulfur atoms & 16 Oxygen atoms (4x4)

5. Chemical Equations – cont’d Subscript – (small) # to the lower right of an element applies only to the element it is attached to if outside of parentheses [(XY) 3 ] applies to everything inside of parentheses ex: H 2 2 hydrogen atoms ex: CO 2 1 carbon atom, 2 oxygen atoms ex: (SO 4 ) 3 3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)

6. Chemical Equations - cont’d Try this: Al 2 (SO 4 ) 3 Aluminum? 2 atoms Sulfur? 3 atoms Oxygen? 12 atoms (4x3)

7. Chemical Equations reactants and products reactants and products 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) coefficients coefficients The letters (s), (g), and (l) are the physical states of compounds.

8. Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) (dissolved in water) Catalyst H 2 SO 4 or Pt Escaping gas (  ) Change of temperature/ heat energy (  or + 3kJ or – 3kJ) *there is no subtraction…a negative sign means released/exothermic Symbols Used in Equations

9. Balancing Chem Equations Law of Conservation of Matter matter is neither created nor destroyed in a chemical reaction, it is conserved biology: energy is lost, matter is RECYCLED (changed from one thing to another) ie, if you begin a reaction with 5 atoms of Hydrogen, you must end up with 5 atoms of Hydrogen

10. Balancing – cont’d Balanced = same # of each element on each side Al + S Al 2 S 3 2Al + 3S Al 2 S 3 Unbalanced Balanced

11. Balanced or Unbalanced? C 3 H 8 + 5O 2 4H 2 O + 3CO 2 Carbon? = 3 vs 3 Hydrogen? = 8 vs 8 Oxygen = 10 vs 10 (4 + 6) Is it balanced? Yes.

12. Balanced or Unbalanced? Fe 3 O 4 + H 2 Fe + H 2 O Iron = 3 vs 1 Oxygen = 4 vs 1 Hydrogen = 2 vs 2 Is it balanced? No. So, now what?

13. Balancing a Chemical Reaction Unequal? Balance Coefficients. [can’t change subscripts] you can treat Polyatomic Ions as one if they appear on both sides of the equation like (SO 4 ) in parentheses change your greatest inequality first leave solitary elements for last

14. Practice Fe 3 O 4 + H 2 Fe + H 2 O Fe O H 342342 112112

15. Practice Complete the two problems on the front of your sheet. ____H 2 + ____O 2  ____H 2 O ____Cl 2 + ____NaBr  ____Br 2 + ____NaCl 21 2 1 2 1 2