1. Balancing Chemical Equations SCIENCE 10 MS. MCGRATH

2. Counting Atoms How many of each type of atom are present in the compounds below? ◦H 2 SO 4 ◦CaCl 2 ◦4NaF ◦2KNO 3 ◦3K 2 SO 4 ◦2Mg 3 (PO 4 ) 2 Type of Atom# of Atoms

3. Law of Conservation of Mass Law of conservation of mass: mass of products = mass of reactants Video: http://www.youtube.com/watch?v=dExpJAECSL8http://www.youtube.com/watch?v=dExpJAECSL8 ◦the # of atoms of each type in the reactants is the same as the # of atoms of each type in the products ◦Matter can’t be created or destroyed: atoms can’t be gained or lost during a chemical reaction A + B → C + D Reactants Products “produces/ gives” “reacts with”

4. Balancing Chemical Equations Skeleton equation: CH 4(g) + O 2(g)  CO 2(g) + H 2 O ( l ) The skeleton equation needs coefficients to balance both sides. Balanced equation: CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O ( l ) State of matter (aq) = aqueous/dissolved in water (s) = solid ( l ) = liquid (g) = gas Coefficients - indicate the ratio of compounds in the reaction - here, there is twice as much NO and NO 2 than there is O 2 Subscripts - show the # of atoms *YOU CANNOT CHANGE THESE TO BALANCE AN EQUATION

5. Types of Chemical Equations Word Equation (words only, no formulas): Methane + oxygen  carbon dioxide + water Skeleton Equation (formulas, but not balanced): CH 4(g) + O 2(g)  CO 2(g) + H 2 O (l) Balanced Equation:

6. Remember: You can only change the coefficients (the number in front) to balance You can not have coefficients that are fractions Double check to make sure coefficients can’t be reduced

7. Examples 1.Hg + O 2  HgO 2.Aluminum reacts with oxygen to form aluminum oxide. 3.CaO + H 2 O  Ca(OH) 2

8. Strategies for Balancing Equations Balance chemical equations by following these steps: ◦Trial and error will work, but can be very inefficient ◦Balance compounds first, elements last ◦Balance one compound at a time ◦Only add coefficients; NEVER change subscripts! ◦If H and O appear, attempt to balance them LAST ◦Polyatomic ions (such as SO 4 2– ) can be balanced as a whole group. They don’t break apart! ◦Always double-check after you think you are finished!

9. More examples Balance the following: 1.__ N 2 (g) + ___ H 2 (g) → ___ NH 3 (g) __ N 2 (g) + _3_ H 2 (g) → _2_ NH 3 (g) 2.__ Fe (s) + ___ H 2 SO 4 (aq) → ___ H 2 (g) + Fe 2 (SO 4 ) 3 (aq) 2_ Fe (s) + _3_ H 2 SO 4 (aq) → _3_ H 2 (g) + Fe 2 (SO 4 ) 3 (aq)

10. Balancing Word Equations 1.Write the skeleton equation. Remember to ensure ionic compounds have a neutral charge (+ and – charges must add to zero) 2.Then balance 3.Check coefficients for common factors. If yes, reduce them.

11. Try It! 1.sodium + chlorine  sodium chloride 2.iron (II) oxide  iron + oxygen 3.methane + oxygen  carbon dioxide + water 4.copper (I) oxide  copper + oxygen

12. Practice Fe + Br 2  FeBr 3 CaCl 2 + Na  NaCl + Ca Sn(NO 2 ) 4 + K 3 PO 4  KNO 2 + Sn 3 (PO 4 ) 4 C 2 H 6 + O 2  CO 2 + H 2 O

13. To Do: Text: ◦p.133 #13, 18 ◦p.137 #8, 9, 11-18 ◦p.134 #11-13