Chemistry 05.05 Limiting Reactant

If 35.0 grams of coal (carbon) burns in 58.5 grams of oxygen gas, how many grams of carbon dioxide can be produced? Describe the steps. C + O2 → CO2

35.0 g C and 58.5 g O2 already balanced equation Molar mass of O2 = (15.999 g × 2) = 31.998 g/mol Molar mass of C = 12.011 g/mol 35.0 g C.......1 mol C...........1 mol CO2 ____________ × _____________ × _________________ 1.......................12.011 g C.......1 mol C 58.5 g O2.......1 mol O2...........1 mol CO2 ______________ × ________________ × ________________ 1.........................31.998 g O2.......1 mol O2 Molar mass of CO2 = 44.009 g/mol convert each to grams 128 g CO2 80.5 g CO2 80.5 g CO2—theoretical yield

Describe limiting reactant problems.

In a limiting reactant problem, there are given amounts of more than one reactant, and one will run out before the others. So, that reactant stops, or limits, the reaction, and it determines how much product can be made.

Show how the limiting reactant is determined for 3.80 g of magnesium reacting with 12.5 g of oxygen.

2 Mg + O2 -> 2MgO Mg = 24.31 g/mol O2 = 32.0 g/mol 2 mol MgO........2 mol MgO _________________...,...__________________ 2 mol Mg.............1 mol O2 MgO = 40.31 g/mol .........................1 mol Mg..........2 mol MgO........40.31 g MgO 3.8 g Mg x ________________ x __________________ x ____________________ ........................24.31 g Mg..........2 mol Mg...........1 mol MgO = 6.30 g MgO .........................1 mol O2.........2 mol MgO........40.31 g MgO 12.5 g O2 x ______________ x __________________ x ____________________ ..........................32.0 g O2.......1 mol O2..............1 mol MgO = 31.5 g MgO

If 3.0 moles of HF react with 4.5 moles of SiO2, how many moles of SiF4 can be formed? Describe the steps. SiO2(s) + HF(g) → SiF4(g) + H2O(l)

1. Determine if this is a limiting reactant problem by identifying the given amounts of the reactants and what product is being solved for. 3.0 moles of HF and 4.5 moles of SiO2 2. Write a balanced chemical equation and use it to create a mole ratio for the given reactants and product you are solving for. SiO2(s) + 4HF(g) → SiF4(g) + 2H2O(l) for SiO2(s): 1 mol SiF4 .........................______________ .........................1 mol SiO2 for HF: 1 mol SiF4 ................_______________ .................4 mol HF 3. Set up and solve a stoichiometry calculation for each reactant using the mole ratios. 3.0 mol HF.........1 mol SiF4 _________________ × _______________ = 0.75 mol SiF4 1...............................4 mol HF 4.5 mol SiO2.......1 mol SiF4 ___________________ × ________________ = 4.5 mol SiF4 1..................................1 mol SiO2 4. The lower amount is the theoretical yield. The reactant that produced the lower amount is the limiting reactant. The reactant (or reactants) that produced the higher amount is the excess reactant. 0.75 mol SiF4—theoretical yield

According to the video, how do you identify the limiting and excess reactants of a reaction?

Set up one stoichiometry problem for each given reactant. Solve each for the same product, to make it easy to compare. The reactant that can make less is the limiting reactant, and the reactant that could produce more is the excess reactant.

How is the mass of the theoretical yield determined?

The mass produced by the limiting reactant is the mass of the theoretical yield.

What about the other product?

To find the mole amount of the second product, use the starting amount of the limiting reactant to calculate the amount for the second product.

Which reactant is the limiting reactant and which is the excess reactant when there are 15.0 g of hydrogen gas and 10.0 g of nitrogen gas? 3H2(g) + N2(g) → 2NH3(g)

nitrogen gas = limiting reactant (12.2 g NH3) hydrogen gas = excess reactant (84.5 g NH3)

The amount of _____________ is always limited by the amounts of available _______________.

product, reactants

To solve for limiting and excess reactants, you will repeat the same stoichiometry steps using the same ___________________ in the chemical reaction for all _____________________.

stoichiometry steps, reactants

What is theoretical yield?

the maximum amount of product you would expect from the reaction due to a reactant running out before the others

What is the limiting reactant?

the reactant that controls the amount of product that can be produced by a chemical reaction because it is used up completely

What is the excess reactant?

the reactant that is not used up completely in a chemical reaction