A balanced chemical equation is quantitative portion of a recipe that converts elements and compounds into different substances. It describes the ingredients (reactants) and lists the ratios in which they combine to make the final product(s). These ratios are called the . . . . from Greek meaning "element measuring" of the reaction.
Activity: explore stoichiometry with something you can see and understand . . . . follow the directions below to assemble a bolt, washer(s) and nut(s).
- Click the radio buttons below the washers and nuts to display only one of each).
- Click the Play button to "start the reaction".
- The starting number of bolts, washers and nuts are displayed in the Initial Row.
- Use the stoichiometry of the reaction (1 bolt + 1 washer + 1 nut → 1 BWN) to determine the maximum number of bolts, washers and nuts that "can react". Place these numbers in the Change Row. Place a minus sign in front of the reactants in this row.
- Sum the Initial Row and the Change Row to calculate the number in the Final Row.
- Click the Check button (lower right) to check your answers. Change any incorrect answers.
- Click the Continue button (lower right).
- Complete the conversion from kilograms to gross for the bolts, washers and nuts.
- Click the Check button (lower right) to check your answers. Change any incorrect answers.
- Click the Continue button (lower right).
- Complete the table using "gross units" just as you did using actual numbers.
- Repeat this activity two more times using different combinations (2 or 3) of washers and nuts. Click the Reload button to restart the animation.
In working with the animation above you should have realized that the easiest way to determine the "limiting reactant" is to . . . .
- determine how many products (BWxNy) can be made from the starting number of each reactant:
- starting number of bolts (B) produces ___ BWxNy
- starting number of washers (W) produces ___ BWxNy
- starting number of nuts (N) produces ___ BWxNy
- identify the limiting reactant as the one that produces the least number of products (BWxNy)
- place the "initial" value of the limiting reactant in the Change row and make it negative.
- calculate the other values in the Change row using the ratio of the reaction's coefficients as conversion factors . . . . the ratio of the numbers entered in the Change row must equal the ratio of the reaction's coefficients.
- sum the Initial and Change rows to find the Final row values
- convert all reactant quantities into moles (not gross) using the Periodic Table (not the Table Of The Fasteners)
- pick a product and determine how many moles of it can be produced from the moles of each reactant.
- identify the limiting reactant - it produces the least number of moles of the chosen product.
- subtract ALL of the moles of the limiting reactant in the Change row.
- use the coefficients in the balanced equation to determine the moles of the other reactants that will react with the limiting reactant's moles. Enter these values in the Change row as a negative number.
- use the coefficients in the balanced equation to determine the moles of products formed. Enter these values in the Change row as a positive number.
- enter the moles of reactants and products in the Final row by adding the Initial and Change rows.
- calculate the following (described more completely in the next section)
- theoretical yield - the mass of product formed from the limiting reactant. Use the product's molar mass (Periodic Table) to convert moles of product to grams of product.
- mass of an "excess" reactant - convert the moles of the excess reactant in the Final row to grams using the reactant's molar mass.
- percent yield - when performing a chemical reaction in the laboratory, the last step involves measuring the mass of the purified product. The final mass is not very significant unless you know the mass you were supposed to get (the theoretical yield). A simple percentage equation gives the percent yield of the reaction . . . .
% Yield =
Experimental Yield
Theoretical Yield
× 100
Activity: calculate the answer to the question in the Limiting Reagent tutorial. There are five steps needed to reach the answer:
- balanced equation
- initial moles
- change in moles
- final moles
- final grams
If you can work the problem without "guidance", then enter your answer on the initial screen. If your initial answer is incorrect or you need help, enter any answer (to 3 SigFigs) and click the Check button. The Continue button will take you to the tutorial page where you will be guided through the process and your progress checked at each of the five steps.
To work a different limiting reagent problem, click the Reload button.
Activity: complete the Lab 4.3: Thermal Decomposition of NaHCO3 assignment. The weight of the solid residue that remains after heating the baking soda is used to determine which of the four proposed reactions has occurred.
Activity: complete the iTRQ 4.3a: Limiting Reagents assignment. You must correctly answer 2 problems in less than an hour to get full credit, or answer 1 problem in less than 10 minutes without guidance.
Activity: the day after completing the iTRQ 4.3a: Limiting Reagents assignment, complete the HW 4.3b: Limiting Reagents assignment. You must correctly answer 2 problems in less than an hour to get full credit, or answer 1 problem in less than 10 minutes without guidance.
Activity: the day after completing the iTRQ 4.3b: Limiting Reagents assignment, complete the HW 4.3c: Limiting Reagents assignment. You must correctly answer 3 problems in less than an hour to get full credit, or answer 1 problem in less than 10 minutes without guidance.
Activity: complete the HW 4.3: Reaction Stoichiometry assignment.
Activity: watch the video on Stoichiometry to the right).
