Limiting Reactant Activity

UNIT-Atomically Correct

In Class Practice – Limiting Reactants

The Nuts & Bolts of Chemical Changes

Pre-Lab

Methane gas reacts with oxygen gas to produce carbon dioxide and water vapor. The chemical equation is:

CH4 + O2 à H2O + CO2

a) Balance this chemical equation.

CH4 + 2O2 à 2H2O + CO2

b) How many moles of oxygen gas are needed to react with 5 moles of methane (CH4)?

c) What would happen if you used twice as many moles of oxygen gas as you calculated in b) and still had 5 moles of methane to react with?

No reaction with extra 10 mols of O2

In this activity we will use nuts and bolts to represent atoms in involved in a chemical change.

1) Obtain one of the numbered sets of nuts and bolts from your instructor. Record the number of your set.

Set # ______

2) Count the number of bolts and nuts. Record these numbers.

# Bolts ______# Nuts ______

3) Now put as many nuts as you can completely fit on each bolt in your set. The nuts must completely fit on the bolt, none of the nut should be hanging off the bolt.

Which did you not have enough of to combine, nuts or bolts?

In chemical reactions atoms combine in specific numbers. In the nuts and bolts example above the bolts would only hold two nuts for each bolt. Atoms behave this way also; they have a specific combining capacity (valence) when they make a compound with other atoms.

In chemical changes, the reactant that is left over and doesn’t have anything to combine with is called the excess reactant, and the reactant that is completely combined is called the limiting reactant.

4) A) What was the excess reactant (nuts or bolts) in your set?

B) What was the limiting reactant (nuts or bolts) in your set?

C) Define the terms limiting reactant and excess reactant in your own words.

5) Take an inventory of your nut and bolt combinations:

A) How many bolt/nut combinations were you able to make with your set? ______bolt/nut combinations

B) How many total bolts did it take to make all your nut/bolt combinations? ______bolts

C) How many total nuts did it take to make all your nut/bolt combinations? ______nuts

A) For each of the following sets of nuts and bolts state: the number of bolt/nut combinations and how many nuts or bolts are left-over. Use the combining pattern you observed with your set.

i) 12 bolts and 12 nuts

bolt/nut combinations ______6_____

left-over bolts ______6_____

left-over nuts ______0_____

ii) 100 bolts and 223 nuts

bolt/nut combinations _____100____

left-over bolts ______0_____

left-over nuts ______23____

iii) 73 bolts and 146 nuts

bolt/nut combinations ______73____

left-over bolts ______0_____

left-over nuts ______0_____

B) For each of the combinations above in 6A, state whether bolts or nuts represents the limiting reactant.

i) limiting reactant ___nuts____

ii) limiting reactant ___bolts___

iii) limiting reactant ___both or none

C) Using the symbol B to represent bolt and N to represent nut, write a “chemical equation” to represent the simplest way bolts combined with nuts. In other words, all your classmates were given different numbers of the SAME type of nut and bolt; write a chemical equation that would represent what happens in EVERY set when nuts and bolts are combined.

B + 2N à BN2

7) The diagram below, the left circle represents gaseous carbon and sulfur before they react; the diagram on the right represents the compound that results when these atoms combine.

BEFORE REACTION AFTER REACTION

A) Take an inventory. Count the particles in each diagram and list them in the table below.

Before Reaction / After Reaction
_____6_____ C atoms / _____2_____ CS2 molecules
_____4_____ S atoms / _____4_____ C atoms

B) Which reactant (C or S atoms) is in excess? _____C______

C) Which reactant (C or S atoms) limits the reaction? _____S______

When writing an equation to represent this chemical change, remember, the equation shows how carbon would combine with sulfur no matter how many total atoms you start with, or what is leftover (in excess). Just as you did with the nuts and bolts combination, we want a general statement that tells us how carbon combines with sulfur.

D) Write a chemical equation to represent how carbon atoms react with sulfur atoms.

C + 2S à CS2

8) In the chemical equation below:

2Na + S à Na2S

What is the difference between using 2 as a coefficient (e.g. 2Na) and using it as a subscript (e.g. Na2S)?

2Na represents 2 separate Na atoms, while Na2 represents 2 Na atoms combined in a compound.

9) The diagram below represents the chemical reaction between nitrogen gas and oxygen gas.

A) What is the limiting reactant in this chemical change? ____N2______

B) What is the excess reactant in this chemical change? ____H2______