COUNTING ATOMS & BALANCING EQUATIONS LESSON 6

DO NOT COPY THE FOLLOWING! In grade 9 you learned how to count the number of different types of atoms in a compound. LETS REVIEW! In order to balance equations you must have a very strong understanding of counting atoms. Please make sure to see me if after the review you are still unsure about counting atoms.

Eg. Water H 2 O Eg. 4 molecules of water 4H 2 O

Rule #1: Multiply coefficient by each subscript. Remember: If there is no subscript written, it is 1. Eg. 5H 2 SO 4 Coefficients apply to the ENTIRE molecule.

Brackets – If there is brackets around any part of the compound, atoms need to be counted differently. Eg. Pb(NO 3 ) 2 Rule #2: The subscript outside of the bracket only applies to the atoms inside the bracket. Multiply. Pb(NO 3 ) 2

Eg. H 2 (SO 4 ) 3 Combining the Rules Eg. 5 H 2 (SO 4 ) 3 Remember: The coefficient applies to the whole compound.

Eg. 3 Pb(NO 2 ) 3

Activity 1 Practice the following. Indicate the number of each type of atom. 1.3 Ca(NO 3 ) H 2 3.2HCl 4.2NaOH 5.FeSO K(ClO 3 ) 2

Chemical Equations Chemistry is about the reactions of compounds to form new compounds. Compounds are made up of elements. When a chemical reaction takes place these elements do not disappear but they rearrange themselves to make new compounds. Therefore, matter is not made up out of nothing and it also doesn’t disappear. “Matter is not created or destroyed!” This is called the Law of Conservation of Mass. This law states that the mass of the reactants in a chemical equation must equal the mass of the products.

Example: A chemical equation: H 2 + O 2  H 2 O (________)(___________) A word equation: REACTANTS  PRODUCTS (things that ______) (things that are ____________) (mass of reactants) = (mass of products)

Because of the Law of Conservation of Mass, all chemical reactions must be equal. The number of atoms of each different element on the left hand side must be the same as the number of atoms of each different element on the right hand side.

Balancing Chemical Equations: Steps 1. You do not change the subscripts (small numbers) in the formulas. 2. You do change the coefficients (large numbers in front of the formula). 3. Keep groups that you see on the left hand side and the right hand side of the reaction together. 4. Balance oxygen and hydrogen last.

Example 1: Balance H 2 + O 2  H 2 O Example 2: Write the chemical equation and balance: sulfur dioxide reacts with oxygen gas to produce sulfur trioxide

Example 3: Write the chemical equation and balance: carbon tetrahydride reacts with chlorine gas to produce carbon tetrachloride and hydrogen chloride

Example 4: Balance and write the word equation: NO + O 2  NO 2

Example 5: Balance: KOH + H 2 SO 4  K 2 SO 4 + 2H 2 O

Example 6: Balance: Al 2 (SO 4 ) 3 + Ca(OH) 2  Al(OH) 3 + CaSO 4

Example 7: Balance: Fe 2 (SO 4 ) 3 + KOH ---> K 2 SO 4 + Fe(OH) 3

Assignment Chemical Equation Worksheet – Due next class