Limiting & Excess Reactants

What we did
Today in class, we measured the mass of the silver in the beaker and recorded the amount. We also went over a new concept, limiting and excess reactants. It's pretty straightforward, and as long as you can do gram to gram stoichiometry, the concept is similar.
Limiting &Excess Reactants

 Basically, a limiting reactant is the reactant that runs out the quickest, meaning it is the reactant that reduces the total amount of products that can be produced. An excess reactant, on the other hand, is the reactant you have left over, or the one that produces the most amount of product. When comparing limiting and excess reactants, the limiting reactant is naturally the smaller of the two yields.

Conceptual Problems

First, to get the concept of the limiting reactant, it's important to cover a few analogies.

1. You have 18 cars bodies and 43 tires. How many cars could you build?

A. You can only build 10 cars. This is because for each car, you need 4 tires. Although there are enough car bodies to build 18 cars, there is a lack of tires, therefore, the amount of cars built is dramatically reduced.In this particular problem, the tires would be the limiting reactant, and by process of elimination, the car bodies the excess reactant. 

2. In a  typical lunch, there are two napkins given along with one sandwich and three fruits. There are 100 napkins, 60 sandwiches, and 21 fruits. What's the most number of complete lunches you can make?

A. 7 lunches. The fruits are the limiting reactant.

Sample Problems

A 50.6 g sample of Mg(OH)₂ is reacted with 45.0 g of HCl according to the reaction: What is the theoretical yeild of MgCl_2?

Mg(OH)₂ + 2 HCl --> MgCl₂ + 2 H₂O The first step to find out the limiting reactant is to convert each grams of reactant to grams of any product, it doesn't matter whether you choose MgCl₂ or H₂O to convert to. However, please read the next question, as it asks for MgCl2 (Don't do extra work!) First, Convert grams of Mg(OH)2 to grams of MgCl2...  50.6 g Mg(OH)2 x 1 mol  Mg(OH)2/58.3 g x 1 mol MgCl2 /1 mol Mg(OH)2 x 95.g MgCl2/1 mol MgCl2 = 82.6 g MgCl2

Then, convert gams of HCl to to grams of MgCl2 as well

45 g HCl x 1 mol HCl/36.5 g x 1 mol MgCl2/2 mol HCl x 95.2g MgCl2 /1 mol MgCl2 = 58.7 g MgCl2

Okay, so now that you've found out how many grams of MgCl2 you will get by reacting with the two reactants, now you can compare to find out the theoretical yield.

The theoretical yield of MgCl2 is 58. 7 g. As stated before, it is the smaller of the two numbers you get. Naturally, the limiting reactant is the HCl and the excess reactant was the Mg(OH)2.

Another question might be "Find the amount of excess reactant". To find the excess reactant, all you do is calculate how much of the non-limiting reactant (MgCl2) reacts with the limiting reactant (HCl).

So first, you take the amount of limiting reactant given, and convert that to grams of the excess reactant to compare.
45 g HCl x 1 mol HCl/36.5 g x 1 mol Mg(OH)2/ 2 mol HCl x 58.3 g Mg(OH)2/1 mol Mg(OH)2 = 35.9 g Mg(OH)2
That is the amount of Mg(OH)2 reacted, so to find out the excess amount of the reactant, all you do is subtract 35.9 grams from the initial amount of Mg(OH)2, which was 50.6 g. So...the excess amount is 14.7g
Tips 
-always make sure to convert the given amount of a reactant to grams of any product at least. If you can do at least this gram to gram stoichiometry, the rest will come naturally. (Once you have yields of both reactants, just pick smaller of the two and that is limiting reactant.)
-Just remember the idea that limiting is the smaller number, and excess is the larger.
-Always read ahead to see which product's yield it asks for, so you don't do extra work
- don't assume that just because one reactant has fewer moles it is the limiting reactant (it is not true in many cases)
Reminders
-Formula of a nitrate lab due Monday
-Several limiting & excess reactants worksheets due Monday as well
- Don't forget about the one webassign
In case you still don't understand...
http://www.youtube.com/watch?v=Vaiz0zLesHk
http://www.youtube.com/watch?v=-biLP2acKqg
The first video is actual sample problems using chemical formulas.
The second video is more about the concept, using hamburgers, etc.
Both videos helped solidify my understanding of this concept.

Another stoichiometry problem in case mine didn't make sense. This is again just converting grams to grams, and the smaller amount is again the limting reactant.
The next scribe will be Jaehee