Limiting Reactant Demo

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Initial Procedures

Separate into groups of 2 with 1 group of 3 if needed.
Each group needs to obtain a bag labelled A and one labelled B.
Each group get a balance and make sure it is zeroed.
Mass each bag and put masses in following blanks.

A = Black Eye Peas (White)
B = Red Kidney Beans (Red)

Mass of A: __________________ g Mass of B: ______________

Background Information and more procedures

We are going to use two Substance (A and B) as our reactants. These reactants will form products (actually different groupings of A & B) by the following chemical reaction:

4 A + 3 B --> 1 A₂B₁ + 2 A₁B₁

Question: With your knowledge of the mass of each bag (do not do any counting) and how these material are going to combine (from above chemical reaction),

Which Substance A or B with run out first (write guess in here): _____________________

Now open each bag and count how many items are in each bag and record that number here:

Number of A in bag: ___________________ A Number of B in bag: _____________________B

Question: Again without calculating and from your knowledge of how many items you have of each (and chemical reaction),

Which Substance A or B will run out first (write guess here): ________________

Do Actual Experiment

Now do the reaction by putting/grouping together the reactants into the correct grouping as products. After doing this fill in the following:

Hint: Each person take a different product to make and keep track of the 1:2 product grouping ratio as you make your products.

a. Experimental Leftover of A : _____________

b. Experimental Leftover of B: ______________

c. Experimental Number of A₂B₁: ____________

d. Experimental Number of A₁B₁: ____________

Place all Substance A and B back in their separate "Reactant" bag, seal the bags.
End of "Do Actual Experiment"

Now determine the limiting reactant, reactant in excess (how much is leftover).
Limiting Reactant determines how much products you make and how much excess reactant there is.

Now, we can not do the above experiment for actual chemical since there are significantly more particles (hence the use of the mole concept). You will need to guess which of the reactant (limiting reactant).
Now fill in the following information:
a. Which substance you assumed all of it reacted: ______________
Now, how much of the other reactant must react, calculation here:

b. What were the number of the other reactant needed to react all the substance in a. _____________________.

Now, you will write the following: Assumed: ____________ goes to completion.

Since I have ___________ (reactant not completely used up in reaction) initially and I need ______________ numbers of _______ (reactant not completely used up) to completely react all of _________(substance that you did assume completed reacted), therefore, _________________ is the limiting reactant (one that runs out first).

Now, using the limiting reactant information (one that runs out first), determine the number of each product you will get. Hint: will be using the limiting information as your starting point for stoichiometry (one calculation for each of the different product formed). Use space below for calculations.

Now, determine how much of reactant that did not get completely used up in reaction (called "in excess") by the following equation: Initial - reacted = leftover. You will need to do stoichiometry to figure out how much of that reactant used up in reaction ("reacted"). Again you are starting stoichiometry with limiting reactant information. From calculations, fill in the following:

Calculated Leftover of A: ___________________

Calculated Leftover of B: ____________________

Calculated Number of A₂B₁: ____________

Calculated Number of A₁B₁: ____________

Compared calculated verse experimental answers and answer the following Questions.

Questions

i . Was the substance you guessed went to completion using mass information correct?

ii. Was just counting how many items of each reactant and comparing the two totals a good way of determining which reactant ran out first? Why?

iii. What do we call the reactant that runs out first? What do you call the other reactant?

iv. What is your "starting point" for stoichiometry in order to calculate how much of each product you obtain and how much of the "in excess" reactant has reacted?

v. Do you do a "Limiting Reactant Problem Calculation" for all stoichiometry problems? If not when do and why?