Limiting Reactant Give Me S More Chemistry KEY

Name: ______Date: ______KCI 2016 Sem1

Limiting reactant – Give me S’more chemistry KEY

A S’more can be made from the following ingredients and amounts:

Reactants / à / Product
2 graham crackers
/ 1 chocolate rectangle / 1 marshmallow /
G = Graham cracker / C= chocolate / M= marshmallow / The formula of one complete S’more is:
____ G2CM __

A-  Write the balanced chemical equation that makes one mole of S’more:

B-  Get a S’more ingredient bag from Ms. Pham. Write down the # of each items in the bag

S’more ingredients / # of each item in bag
G / 5
C / 4
M / 2

3. List the item(s) in excess and how much in excess:

Item(s) in excess / Quantity remaining
G / 1
C / 2

4. Given 14 Graham crackers, 5 marshmallow and 12 chocolate rectangles.

a. What is the maximum yield of S’more? (i.e. how many S’more can you make?) / 5 S’more
b. Which reactant(s) are in excess (left over)? / G and C
c. What is the excess amount of each? / The # of marshmallows dictates the maximum # of the S’more product;
based on stoich., 5 marshmallows means 5 S’more get made (1:1 ratio)
5 S’more require 10 Graham crackers and 5 Choc.
Thus, 4 G and 15 C are left over
d. Which are the limiting reactant(s) (i.e. those that were completely used up)? / Using the mole ratio from the balanced reaction:
2G : 1M: 1C
àThe one with the lowest ratio is the limiting reagent (i.e. Marshmallow)

Misconception addressed:

5. Given 16 Graham crackers, 10 marshmallow and 20 chocolate rectangles.

a.  The ingredients in the lowest quantity is: __marshmallow______

b.  Which is the limiting reactant(s)?

àGraham cracker is the limiting reagent

c.  Is the reactant with the lowest quantity the limiting reactant?

No, they are not the same. Not in this case!

è  Conclusion: The limiting reactant is NOT ALWAYS the one in the lowest quantity (i.e. smallest mass)

Why is learning about limiting reactant important?

Real-Life Scenario example

1. In an assembly line, 4 tires & 2 wiper blades are needed to equip a car. Suppose in the warehouse 140 tires & 74 wiper blades are available. How many cars can be equipped?

The # cars that can be made is limited by the car parts that are used up first (i.e the limiting parts)

Establish ratios:

140 tires/ 4 tires = 35 cars 74 wiper blades /2 wiper blades = 37 cars

2. Are the tires or the wiper blades the limiting reagent? Explain.

It’s the # tires is the limiting reagent because it will be the part that will be completely consumed first and it dictates the max. # of cars that can be made

In the context of chemical reactions

1. Collaborate to complete the following table

a)

Reaction / ___2__ H2 / + / ______O2 / / __2____H2O
Mole Ratio from balanced rxn / 2 / 1 / 1
Given / 0.500 mol / 0.500 mol / Not yet available
If H2 is limiting, how much water is made? / 0.500 mol available / N/A / 0.500 mol (1:1 ratio)
If O2 is limiting, how much water is made? / N/A / 0.500 mol available / 1.00 mol (for every mol O2 reacted, 2 mole water get made)
Therefore what is the limiting reagent? / H2 made less product than oxygen à H2 is the limiting reactant or reagent
How much of each reactant is used up in this reaction? / All 0.500 mol / / N/A
How much of the excess reagent remains unreacted? / 0 mole remaining as all is used up / 0.500 mol initially – 0.250 mol reacted = 0.250 moles remaining / 0.500 moles formed
Based on # mole H2 & stoich.

b)

Reaction / _____ N2 / + / ___3___ H2 / / ___2___NH3
Mole Ratio from balanced rxn / 1 / 3 / 2
Given / 3.00 g / 2.00g / Not yet available
Convert given mass into moles / n=m/M = 3.00/ 28 = 0.107 mol / n=m/M = 2.00/2.02
=0.99mol / Not yet available
If N2 is limiting, how much water is made? / 0.107 mol available / N/A / 0.214 mol (1:2 ratio)
If H2 is limiting, how much water is made? / N/A / 0.99 mol available /
Therefore what is the limiting reagent? / N2 made less product than hydrogen à N2 is the limiting reactant or reagent
Once again, you’ve seen that the reactant present in lower quantity is not necessarily the limiting reagent
How much of each reactant is used up in this reaction? / All 0.107 mol / 0.321 mol (1:3 ratio) / N/A
How much of the excess reagent remains unreacted? / 0 mole remaining as all is used up / 0.99 mol initially – 0.321 mol reacted = 0.67 moles remaining / 0.214 moles formed
Based on # mole N2 & stoich.

Exit card:

à

Go to https://goo.gl/8gu8M0 to play the game.

What’s your level?

Name: ______Date: ______

Limiting reactant (Cont.)

Model how to solve a limiting reagents example problem

Given: ___Na3PO4 (aq) + ___Ba(NO3)2 (aq) → __Ba3 (PO4)2(s) + __ NaNO3 (aq)
If 4.13 g of sodium phosphate and 8.05 g of barium nitrate are combined in solution,
a)  Which is the limiting reactant? / b)  How much precipitate is produced (in grams)? / c)  How much reactant is leftover?
How to recognize if it’s a limiting reactant problem?
a)  a) Calculate the moles of each substance
Choose a product and calculate the moles produced by each
Identify the limiting reactant and the reactant in excess
b)  b) Calculate the mass of designated product based on moles of limiting reagent.
c)  c) Calculate moles of excess reagent is consumed based on the limiting reagent
Subtract to obtain the moles of reagent leftover
Calculate the mass of reagent leftover.

HW practice: # 49, 50 p 311 & 9 p 313

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