1. Balancing Chemical Equations Notes

2. Parts of an equation A. Reactants: the original, starting substances B. Products: the new substances produced C. Coefficients: whole numbers in front of a formula; indicates the number of molecules needed in the balanced equation D. Subscripts: small numbers in formulas that tell the number of atoms of each element in the formula

3. E. Correctly written formulas: chemical formulas must be written to represent 1 molecule/formula unit of a substance F. Yields sign: the arrow; points to the products; means gives, equals, or produces G. States of matter: shows what state of matter each substance is in (solid, liquid, gas, or aqueous)

4. Label the parts of this equation 2H 2 (g) + O 2 (g)  2H 2 O (g)

5. Word Equations Describe a chemical reaction with a sentence Use correct names of reactants and products Example –Hydrogen gas reacts with oxygen gas to produce water vapor

6. Formula (skeletal) Equations Describe a reaction using the correct chemical formulas of the reactants and products State of substance may be given Example H 2 + O 2  H 2 O Or H 2 (g)+ O 2 (g)  H 2 O(g)

7. Balanced Equations Formula Equations that show the law of conservation of mass is obeyed Coefficients are used to make the total number of atoms equal on each side Example 2H 2 + O 2  2H 2 O *Note that a coefficient applies to the whole formula; multiply the coefficient by the subscript to get the total number of atoms of each element in the formula

8. Tips for balancing equations Always start with correctly written formulas Keep a tally of how many atoms of each element are present on each side Example: CaCl 2 + AgBr  CaBr 2 + AgCl 1-Ca-1 2-Cl-1 1-Ag-1 1-Br-2.

9. Place coefficients in front of formulas as needed to balance and change your tally numbers as you do so. Continue until you have equal #’s of each type of atom on each side Coefficients should be in the simplest ratio CaCl 2 + AgBr  CaBr 2 + 2AgCl 1-Ca-1 2-Cl-1 2 1-Ag-1 2 1-Br-2

10. CaCl 2 + 2AgBr  CaBr 2 + 2AgCl 1-Ca-1 2-Cl-1 2 2 1-Ag-1 2 2 1-Br-2

11. Other Tips Sometimes balancing is just trial and error, but –Sometimes you need to find the least common multiple to balance a certain element AlBr 3 + Cl 2  AlCl 3 + Br 2

12. Notice when you tally your atoms, you have 3 Br’s on the left side and 2 Br’s on the right side. You have to use the LCM to balance (2x3=6 on both sides) 2AlBr 3 + 3Cl 2  2AlCl 3 + 3Br 2 2 1-Al-1 2 6 3-Br-2 6 6 2-Cl-3 6

13. –In ionic equations it is often helpful to keep polyatomic ions together as a unit NaNO 3 + Ca 3 (PO 4 ) 2  Na 3 PO 4 + Ca(NO 3 ) 2 1-Na-3 1-NO 3 -2 3-Ca-1 2-PO 4 -1 Try to finish balancing this one!

14. –In combustion reactions, balance C first, then H, then O –If you end up with an odd number of oxygens, double all the coefficients, then balance the oxygen C 3 H 8 + O 2  CO 2 + H 2 O Try to finish balancing this one!