 1. Balancing equations
2. Limiting reactants

A balanced equation is a model of a chemical reaction that shows the formulae of the reactants and the products. In order for an equation to be balanced, the number of atoms must be equal on both sides of the equation. The limiting reactant is the first reactant to be used up in a chemical reaction. When the limiting reactant is used up, the reaction stops and no more of the product can be formed. Therefore, the mass of the product is dependent on the mass of the limiting reactant. The left-over reactant is referred to as the reactant in excess. # 1

2.5 g of zinc are reacted with 9.8 g of hydrogen chloride. Which is the limiting reactant?

Zn + 2HCl → ZnCl₂ + H₂. Calculate the number of moles for each reactant. Zn: n = m/Mᵣ = 2.5/65 = 0.04. HCl: n = m/Mᵣ = 9.8/36 = 0.3. Consider the ratio of the reactants: 1 mol Zn : 2 mol HCl. 0.15 mol Zn : 0.3 mol HCl. As there are only 0.04 mol of Zn, Zn is the limiting reactant. # 2

5.7 g of sodium are reacted with 6.1 g of sulfur. Which is the limiting reactant and which reactant is in excess?

2Na + S → Na₂S. Moles of Na = 5.7/23 = 0.25. Moles of S = 6.1/32 = 0.19. Molar ratio = 2 : 1 = 0.38 : 0.19. There are only 0.25 moles of Na, so Na is the limiting reactant and S is in excess. # 3

Ethane reacts with oxygen to produce carbon dioxide and water. Write the balanced equation for this reaction.

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O # 4

Calculate the limiting reactant in the reaction of 5.6 g of iron(II) oxide with 6.7 g of hydrogen.

FeO + H₂ → Fe + H₂O. Moles of FeO = 5.6/72 = 0.08. Moles of H₂ = 6.7/2 = 3.35. Molar ratio = 1 : 1 = 3.35 : 3.35. There are only 0.08 moles of FeO, so FeO is the limiting reactant. # 5

14.5 g of ammonia are reacted with 7.2 g of oxygen to produce nitric oxide and water. Which reactant is in excess?

4NH₃ + 5O₂ → 4NO + 6H₂O. Moles of NH₃ = 14.5/17 = 0.85. Moles of O₂ = 7.2/32 = 0.225. Molar ratio = 4 : 5 = 0.85 : 1.06. There are only 0.225 moles of O₂, so NH₃ is in excess. End of page