Balancing Chemical Equations Notes

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

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)

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

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

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)

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

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.

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 Ag Br-2

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

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

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 Al Br Cl-3 6

–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 Ca-1 2-PO 4 -1 Try to finish balancing this one!

–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!