Stoichiometry Calculations based on Chemical Reactions

Stoichiometry  Stoichiometry is the study of the quantitative relationships between chemical formula and chemical equations  It uses the fixed ratio of reactants and products from a balanced reaction equation to determine the quantitative relationships

Essential Skills  Stoichiometry Requires 3 skills:  1) Writing Balanced Equations  2) Determining Mole Ratios  3) Utilizing Molar Masses

The Stoichiometry Pathway:

Mole Ratios from Coefficients  Write the balanced reaction equation for the complete combustion of propane (C 3 H 8 )  C 3 H O 2  3 CO H 2 O  The coefficients from the balanced equation are used to establish the relationship between two chemical species in the reaction.  (The coefficients establish the slope of the line when graphically analyzing two species in the reaction)  Mole ratios can be considered conversion factors that represent equal quantities of different species within that specific reaction.

Mole Ratios from Coefficients  C 3 H O 2  3 CO H 2 O  How many moles of carbon dioxide can be produced from 1.5 moles of propane?  How many moles of oxygen are required to produce 0.25 moles of water?  How many moles of propane are needed to react with 1.5 x moles of oxygen?

From Grams to Grams  C 3 H O 2  3 CO H 2 O  How many grams of water can be produced when 25 grams of propane is combusted?  How many grams of water are produced when 75 grams of carbon dioxide are produced according to this reaction?  How many grams of carbon dioxide can be produced when 2.5 x grams of propane is combusted?

Stoichiometry  If we can understand how to do these basic problems, the remaining types of stoichiometry problems are simple applications of same process.  Determining Limiting Reactants (or Limiting Reagents)  Calculating Theoretical Yields  This type of problem is essentially the same as a Limiting Reactant problem

Limiting Reactant Problems  The goal of this type of problem is to determine which reactant is present in the lowest stoichiometric amount.  In simple terms: Which reactant will be completely consumed first.  When a reactant is completely consumed, the reaction stops.  Steps to solving LR problems:  1) Read the entire question to determine what it is asking*.  2) Convert the given quantities of reactants to the same product*  3) Determine which reactant makes the smallest amount of that product. The one that does, is the limiting reactant

Limiting Reactant Problem  Consider the following reaction for this question:  4 NH 3 (g) + 5 O 2 (g)  4 NO(g) + 6 H 2 O(g)  What is the limiting reactant when 2.0 g of NH 3 reacts with 2.5 g of O 2 ?  Determine the moles of NO that can be produced under these conditions.

Theoretical Yield Problems  These are essentially limiting reactant problems.  They ask how many moles or grams of a specific product that can be produced based on a limiting reactant  You follow the same format of a LR problem, the smallest quantity of product from the LR is the Theoretical Yield  Be able to calculate % yield if necessary.  (Actual Yield / Theoretical Yield) x 100

Theoretical Yield Problem  Consider the combustion of propane we discussed earlier, C 3 H O 2  3 CO H 2 O. Determine the theoretical yield of carbon dioxide in grams when 10.0 grams of propane are reacted with 35.0 grams of oxygen. Calculate the % yield for this reaction if 26.4 grams of carbon dioxide are collect in a laboratory exercise.

Calculating Quantities of Remaining Reactants  If a reaction has a limiting reactant, it also has excess reactant(s)  Many types of questions will ask you to determine the amount of the excess reactant(s) remaining.  This type of problem is treated the same way as a LR problem, but an additional subtraction step is needed.

Example Problem Sodium hydroxide reacts with carbon dioxide as follows: 2 NaOH(s) + CO 2 (g)  Na 2 CO 3 (s) + H 2 O(l)  1) Which is the limiting reactant when 1.85 moles NaOH and 1.00 mole of CO 2 are allowed to react?  2) How many grams of solid product will be produced?  3) How many moles of excess reactant will remain when the reaction is complete?