Most of my first year students have a hard time understanding what a limiting reagent is. I found this website through the Computational Chemistry for Chemistry Educators workshop I attended during summer 2011. This is one of the many interactive simulations from University of Colorado-Boulder.
A student should be able
- to explain what a limiting reagent is
- identify what is the limiting reactant in a chemical reaction
Here is my typical script:
Me: I open the simulation in the sandwich shop tab, then say:
Our topic is Limiting Reagents, but before we talk about this, I want to make a cheese sandwich. If I am making one cheese sandwich, how many pieces of bread do I need?
Class: 2 (I then place 2 next to the bread)
Me: How many cheese strips do I need?
Class: 1 (This number will vary, but for the sake of simplicity I normally guide them to tell me 1 here. I place 1 next to the cheese)
Me: (They now see that I produce 1 cheese sandwich.) So, I need 2 bread and 1 cheese to make 1 cheese sandwich.
Me: If I have 6 pcs of bread and 2 pcs of cheese, how many cheese sandwiches do I have?
Class: 2 (Usually in chorus)
Me: Are you sure? See I am not strong with my math skills. I will need help with a simulation. (I will put 6 next to the bread and 2 next to the cheese)
You are right. I have 2 cheese sandwiches.
What else do I have?
Class: 2 bread
Class: Because you do not have enough cheese. (At this point, some of the students are seeing where I am going)
Me: So, the cheese limits the sandwich making. In chemistry language, it is the limiting reagent.
Me: Now if I have 6 pcs of bread instead, and keep 10 pcs of cheese. How many sandwiches do I have?
Me: What else do I have?
Class: 7 pcs of cheese
Me: So which one is the limiting reagent? Cheese or bread?
Me: I now switch to the real reaction tab, and use the make water button.
If I synthesize water, I need 2 moles of H2 and 1 mole of O2 to make 1 mole of water.
If I have 6 moles of H2 and 2 moles of O2, how many moles of water do I make?
Me: What is the limiting reagent?
Me: What if I have 6 moles of H2 and 10 moles of O2, how many moles of water do I make?
Me: What is my limiting reagent?
After this, I will tell them that the link to the website is posted and they can go back to it as much as they can. I then proceed to do a sample problem from the end chapter of the book on the board.