Limiting Reactants

Limiting Reactants

Suppose that you are in a car factory. In order to assemble a car, 4 tires and 2 headlights are needed (among other things). In this example, imagine that the tires and headlights are reactants while the car is the product formed from the reaction of 4 tires and 2 headlights. If you have 20 tires and 14 headlights, how many complete cars can you make? + 2 4

With 20 tires, you can produce 5 cars because there are 4 tires to a car With 14 headlights, 7 cars can be built because each car needs 2 headlights. But which one limits the number of cars that can be put together? Although you can make more cars from the amount of headlights available, you can only create 5 full cars because of the number of tires available, making the tires the limiting reactant in this "reaction". In this case, the headlights are in excess.

The limiting reactantis the reactant that limits the amounts of the other reactants that can combine and the amount of product that can form in a chemical reaction. The substance that is not used up completely in a reaction is sometimes called the excess reactant.

Another real world example of limiting reactants:2 pieces of bread + 3 pieces of ham + 1 slice of cheese sandwich If I give you 10 pieces bread, 10 slices of lunch meat and 10 slices of cheese, which ingredient would limit the number of sandwiches you are able to make? 10 pieces of bread = 5 sandwiches 2 pieces per sandwich 10 slices of ham = approx 3 sandwiches 3 slices per sandwich 10 slices of cheese = 10 sandwiches 1 slice per sandwich Ham is the limiting reactant, bread and cheese are in excess.

So how do you apply this concept to a chemical reaction? First you need to be able to recognize a limiting reactant problem- they give you two chemical amounts. For example: What mass of carbon dioxide forms in the reaction of 25 grams of glucose with 40 grams of oxygen?

Steps for finding Limiting reactant and Excess reactant • Just like with the car and sandwich examples, we first have to determine which one will make the least product- the limiting reactant. Choose one product and use stoichiometry to convert both given amounts to amount of the product you chose. -The reactant that produces the least is the limiting reactant. What mass of carbon dioxide forms in the reaction of 25 grams of glucose with 40 grams of oxygen? C6H12O6 + 6 O2 → 6 CO2 + 6 H2O 25 grams C6H12O6 1 mole C6H12O6 6 mole CO2 = .83 mol CO2 180.18g C6H12O6 1 mole C6H12O6 40 grams O2 1 mole O2 6 mole CO2 = 3 mol CO2 16.00 g O2 6 mole O2 C6H12O6 is the limiting reactant because it makes the least CO2

2. Now that we have determined the limiting reactant, we can finish the problem and calculate the mass of carbon dioxide formed. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O 25 grams C6H12O6 1 mole C6H12O6 6 mole CO2 44.01 grams CO2 = 37 grams 180.18 g C6H12O6 1 mole C6H12O61 mole CO2 CO2 Many limiting reactant problems will also ask you to find the amount of excess reactant. 3. Many limiting reactant problems will also ask you to find the amount of excess reactant. To calculate the excess reactant remaining, use limiting reactant to calculate the amount of other reactant consumed. Subtract the amount consumed from the amount available, or the given amount. 25 grams C6H12O6 1 mole C6H12O6 6 mole O2 32.00 grams O2 = 27grams 180.18 g C6H12O6 1 mole C6H12O61 mole O2O2 The information from the original problem gave us 40 grams of O2 40 grams - 27 grams = 13 grams of excess O2

Steps Re-cap • Determine the limiting reactant • Calculate amounts of each product • Calculate excess. Tips: • Once you determine the limiting reactant, you should ALWAYS start with it! • Be sure to pick one product when determining limiting reactant! • You may have to do up to six or seven stoichiometry calculations in order to solve one limiting reactant problem

Limiting Reactants