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:
CH
4
+ O
2
 H
2
O + CO
2 a) Balance this chemical equation.
CH
4
+ 2O
2
 2H
2
O + CO
2 b) How many moles of oxygen gas are needed to react with 5 moles of methane (CH
4
)?
5 molsCH
4


2 molO
2
1 molCH
4



10 molsO
2 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 O
2
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.
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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
6)
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 left-over nuts
______6_____
______0_____ ii) 100 bolts and 223 nuts bolt/nut combinations left-over bolts left-over nuts
_____100____
______0_____
______23____ iii) 73 bolts and 146 nuts bolt/nut combinations left-over bolts
______73____
______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  BN
2
2

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
C
S
C S
C S
C
C
C
S
S
C
S C S
S
C
C C
C
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_____ CS
2
molecules
_____4_____ S atoms _____4_____ C atoms
B) Which reactant (C or S atoms) is in excess?
C) Which reactant (C or S atoms) limits the reaction?
_____C_______
_____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). carbon
Just as you did with the nuts and bolts combination, we want a general statement that tells us how combines with sulfur.
D) Write a chemical equation to represent how carbon atoms react with sulfur atoms.
C + 2S  CS
2
8) In the chemical equation below:
2Na + S  Na
2
S
What is the difference between using 2 as a coefficient (e.g. 2Na) and using it as a subscript (e.g. Na
2
S)?
2Na represents 2 separate Na atoms, while Na
2
represents 2 Na atoms combined in a compound.
3

9) The diagram below represents the chemical reaction between nitrogen gas and oxygen gas.
= hydrogen atom atom nitrogen atom
A) What is the limiting reactant in this chemical change? ____N
2
___________
B) What is the excess reactant in this chemical change? ____H
2
___________
C) Write a chemical equation to represent this reaction.
N
2
+ 3H
2
 2NH
3
10) The diagram below represents the chemical reaction between krypton gas and fluorine gas.
= Kr
= F
A) What is the limiting reactant in this chemical change? _____F
2
______
B) What is the excess reactant in this chemical change? _____Kr______
C) Write a chemical equation to represent this reaction.
Kr + 2F
2
 KrF
4
4

11) The diagram below represents the chemical reaction between fluorine and iodine gas.
A) What is the limiting reactant in this chemical change? ______F
2
______
B) What is the excess reactant in this chemical change? ______I
2
______
C) Write a chemical equation to represent this reaction.
I
2
+ 5F
2
 2IF
5
12) The diagram below shows the chemical reaction between sulfur tetrafluoride and fluorine gas.
= S
= F
A) What is the limiting reactant in this chemical change? ______SF
4
_____
B) What is the excess reactant in this chemical change? _______F
2
_____
C) Write a chemical equation to represent this reaction.
SF
4
+ F
2
 SF
6
5

13) The diagram below shows the chemical reaction between ethyne gas and oxygen gas.
= C
= O
= H
A) What is the limiting reactant in this chemical change? _____C
2
H
2
_____
B) What is the excess reactant in this chemical change? ______O
2
______
C) Write a chemical equation to represent this reaction.
2C
2
H
2
+ 5O
2
 4CO
2
+ 2H
2
O
14) Carbon monoxide gas reacts with oxygen gas to produce carbon dioxide gas. The balanced chemical equation is:
2CO + O
2
 2CO
2
A) In the diagram below draw what you would expect to see in the right circle after this chemical reaction has occurred.
= C
= O
B) What is the limiting reactant in this chemical change? _______CO
2
____
C) What is the excess reactant in this chemical change? _______O
2
_____
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15) Sulfur dioxide gas reacts with oxygen gas to produce sulfur trioxide gas. The balanced chemical equation is:
2SO
2
+ O
2
 2SO
3
A) In the diagram below draw what you would expect to see in the right square after this chemical reaction has occurred.
B) What is the limiting reactant in this chemical change? ______SO
2
_____
C) What is the excess reactant in this chemical change? _______O
2
_____
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16) Hydrogen gas and chlorine gas react to produce hydrogen monochloride gas. The balanced chemical equation is:
H
2
+ Cl
2
 2HCl
A) In the diagram below draw what you would expect to see in the right circle after this chemical reaction has occurred.
Cl
Cl
H
H
Cl
Cl
H
H
H
H
H
H
Cl
H H
H
H
Cl
H
H
Cl
H
Cl
H
B) What is the limiting reactant in this chemical change? ______Cl
2
_____
C) What is the excess reactant in this chemical change? ______H
2
______
17) Fluorine gas and water vapor react to produce oxygen difluoride gas and hydrogen monofluoride gas. The balanced chemical equation is :
2F
2
+ H
2
O  OF
2
+ 2HF
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A) In the diagram below draw what you would expect to see in the right circle after this chemical reaction has occurred.
F
H
O
H
F
H
H
F
H
H
O
F
H F
F
F
F
F
H
F
O
F F
H
O H
H
O
H
H
H
F
O H
F
O H
F
O
F
B) What is the limiting reactant in this chemical change? ______F
2
______
C) What is the excess reactant in this chemical change? ______H
2
O__
18) Under high pressure and temperature xenon gas will react with oxygen gas to produce xenon trioxide. The balanced chemical equation is:
2Xe + 3O
2
 2XeO
3
A) In the diagram below draw what you would expect to see in the right circle after this chemical reaction has occurred.
= Xe
= O
B) What is the limiting reactant in this chemical change? _____O
2
_______
C) What is the excess reactant in this chemical change? _____Xe_______
19) Chlorine gas will react with fluorine gas to produce chlorine pentafluoride gas. The balanced chemical equation is: Cl
2
+ 5F
2
 2ClF
5
9

A) Draw what you would expect to see in the circle on the right at the completion of the reaction described above. b)
= Cl
= F
B) What is the limiting reactant in this chemical change? ______Cl
2
_____
C) What is the excess reactant in this chemical change? _______F
2
_____
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