Balancing Chemical Equations

Review: Particle Model All matter is made up of small, indivisible particles called atoms The arrangement of the atoms differs for each state of matter We represent these atoms as “spheres” (circles)

Review: Law of Conservation of Mass Matter cannot be created or destroyed during a chemical reaction The mass of the reactants will always be equal to the mass of the products

Chemical Equations A chemical equation is made up of reactants and products: Reactant + Reactant  Product Reactant + Reactant  Product + Product

The number of atoms of each element must be the same on both “sides” of the equation because matter cannot be created or destroyed during the chemical reaction

Balancing Chemical Equations Defn: Placing a coefficient before each reactant and product so that the number of atoms of each element is equal (reactants and products)

Overview:

“Rules” for Balancing Equations The number of atoms must be the same for both sides Use PENCIL ALWAYS start by writing down the equation, draw a line to divide reactant vs products First step is to count the number of each type of atom Leave Hydrogen and Oxygen for last Remember to re-calculate the total after each “change” You CANNOT touch (change) the subscripts (little numbers) When you add a coefficient, it multiplies everything after it

Balancing-Step 1: Counting Atoms The first step is to count the number of each type of atom in each molecule of the equation Examples: NaCl 3NaCl Na2SO4 2NaSO4

3CuSO4 C5H11OH 3C5H11OH Ca3(PO4)2 3Ca3(PO4)2 Cu(NO3)2 2Cu(NO3)2 3NH4Cl

Hint: Draw the Molecules We use a different symbol to represent each element 4Al + 3O2  2Al2O3 Fe2O3 + 3CO  2Fe + 3CO2 HNO3 + 4H2  NH3 + 3H2O

Balancing Equations- Step 2: Change your Totals with Every Step Remember: A coefficient multiplies the entire molecule A subscript (little number) outside of brackets multiplies everything inside the brackets (ex: (X2Y3)2 = 4X and 6Y)

Old Exam Questions: 1) One litre of nitrogen (N2) reacts with three litres of hydrogen (H2) to produce two litres of ammonia, according to the following equation : N2(g) + 3 H2(g)  2 NH3(g) Which of the following models best describes the chemical change that occurs?

2) Consider the chemical reactions represented by the equations below 2) Consider the chemical reactions represented by the equations below. Which one represents a neutralization equation in which the law of conservation of matter is respected? A) 2 NO + O2  NO2 B) H3PO4 + 3 KOH  2K3PO4 + 3 H2O C) 2 Na + 2 H2O  2 NaOH + H2 D) 3 HBr + Fe(OH)3  FeBr3 + 6 H2O

3. The reaction involving magnesium (Mg) and hydrochloric acid (HCl) produces magnesium chloride (MgCl2) and hydrogen gas (H2). The balanced equation for this reaction is as follows: Mg + 2 HCl  MgCl2 + H2 In the laboratory, 6.00 g of magnesium (Mg) reacts with a certain amount of hydrochloric acid (HCl) to form 23.75 g of magnesium chloride (MgCl2) and 0.50 g of hydrogen gas (H2). What mass of hydrochloric acid (HCl) is needed for this reaction to occur? A) 9.13 g B) 18.25 g C) 24.25 g D) 48.50 g

4. One of the causes of acid rain is the sulfur released when fossil fuels such as coal and oil are burned. The following two reactions take place when these fuels are burned: 1) Sulfur dioxide + oxygen  sulfur trioxide 2) Sulfur trioxide + water  sulfuric acid With the above reactions in mind, a student combined 128 g of sulfur dioxide with 32 g of oxygen to produce sulfur trioxide. He then combined all the resulting sulfur trioxide with 36g of water to produce sulfuric acid. What mass of sulfuric acid did he produce?