intro to balancing equations q: how could you represent this reaction (rusting bicycle), besides...
TRANSCRIPT
A: Scientists use a standard method to represent a chemical reaction, called a chemical equation.
Fe + O2 Fe2O3 (Rust)
Q: What is the general equation for the reaction in which iron rusts?
Iron + Oxygen → Iron Oxide
Or, using symbols:
Fe + O2 → Fe2O3 (rust)
Because of the principle
of the conservation of matter, an equation must be balanced.
Lavoisier, 1788
Chemical Equations
When is an equation called balanced?
when an equation has the same number of atoms of the same kind on both sides.
Learning Check:A chemical reaction is balanced
whena. The reactants are the same as the products
b. The same atoms and molecules appear on each side of the equation
c. The same number of each type of atom appears on both sides of the arrow.
d. None of the above
Chemical Equations
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
The letters (s), (g), and (l) are the physical states of compounds. (aq) – aqueous solution
coefficients subscripts
The arrow means: to yield = to produce = to form
Learning Check – Cold Call
What does “2“ mean in front of LiOH?
2Li (s) + 2H2O (l) 2LiOH (aq) + H2 (g)
What does “2“ to the right of “H2” mean?
What does “(aq)“ to the right of LiOH mean?
Number of molecules
Number of atoms of H
Aqueous solution
What does ““ mean?Yield, form, produce
Q: What is the chemical equation for this reaction?
A: The chemical equation is:
C + O 2 → CO 2
1 min – answer in your IAN
Q: How have the atoms of the reactants been rearranged in the products of the reaction?
C + O 2 → CO 2
Classroom discussion
A: Carbon atom chemically bonds with two oxygen atoms.
HOW ? What bonds are broken and what new bonds have formed?
A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms.
C + O=O →
Classroom discussion
C + O 2 → CO 2
O=C=O
Bonds Break New Bonds Form
Learning Check 1
30 sec - Turn to your elbow partner…..
Q: What needs to happen for a chemical reaction (change) to take place?
A: Bonds between atoms of the reactants
break and NEW bonds form, producing
new substance.
Learning Check 210 sec Cold Call
Q: Is this chemical reaction correct? Explain why.
A: No. There are no “pink atoms” on the
reactant side – NEW matter (pink) on the
product side. Matter canNOT be created.
+ +
Learning Check 2
Why is this reaction correct?
B/C atoms must REARRANGE for a chemical reaction to take place
+ +
NEW PRODUCTS
Count the number of atoms of each element below:
An ATOM is the smallest particle of an element.
O2
H2O
HCl
Mg(NO3)2
2 atoms of oxygen (O)2 atoms of hydrogen & 1 atom of oxygen (O)1 atom of hydrogen (H) & 1 atom of chlorine (Cl)1 atom of magnesium (Mg), 2 atoms of nitrogen (N) & 6 atoms of oxygen (O)2x3 =6
2x1 =2
Count the number of molecules
below:
A MOLECULE is a pure substance
composed of two or more atoms.
O2
2H2O
17HCl
One molecule of oxygen gas
Two molecules of water
Seventeen molecules of an acid
1
Chemical Equations
4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
4 Al atoms
+ 3 O2 molecules
--->2 molecules
of Al2O3
This equation means:
Q: Is this reaction Balanced? When called on, be ready to
justify your answer
C + O 2 → CO 2
Cold Call
Which of the following equations is balanced?
a. 3Fe + 3O2 3Fe2O3
b. 2Fe + O2 Fe2O3
c. 4Fe + 2O2 3Fe2O3
d. 4Fe + 3O2 2Fe2O3
When called on, be ready to
justify your answer
Does this equation obey the Law of Conservation of Mass?
(Is it balanced?)
2H2O2 2H2O + O2
What is the coefficient ratio?
2:2:1
YES
Does this equation obey the Law of Conservation of Mass?
(Is it balanced?)
2Al + O2 Al2O3
Why?2 atoms of Al of left side2 atoms of Al on right side
NO
Al is balanced
2 atoms of O on left side3 atoms of O on right side
O is NOT balanced
REMEMBER:To balance chemical equation, you must add COEFFICIENTS (big numbers) in front of the chemical formulas in the equation. You cannot add or change subscripts!!! Why?
Because it will alter the identity of the substance otherwise.Ex. O2 – oxygen, if a subscript changed to 3, O3 = ozone
Balancing using shapes
____H2 + ____O2 ____H2O
Is this reaction balanced?
Now fix the H on the left.
Coefficient ratio: 2:1:2 BALANCED !
Double Hydrogen too.
2 21
Balancing using shapesYou try!
__Al2O3 ___Al + ___O2
Now fix the Al on the right.
Coefficient ratio: 2:4:3 BALANCED !
2 34
+
Balancing Equations using T-chart
___ H2(g) + ___ O2(g) ---> ___ H2O(l)
What Happened to the Other Oxygen Atom?????
This equation is NOT balanced!
1. Count the number of atoms of each element found in the reaction.
R P
2 H 2 H
2 O 1 O
Balancing Equations using T-chart
___ H2(g) + ___ O2(g) ---> ___ H2O(l)2 2
3. Don’t forget to
recount.
2. Add coefficients
to balance the atoms
1
2
4 4
Now fix the H on the left.
R P
2 H 2 H
2 O 1 O
Don’t forget there is a 1 in front of O2.
Now it is balanced!
Easy Balancing
Al + O2 Al2O33
RULE: Balance H and O last
R P
1 Al 2 Al
2 O 3 O
2
2
3 x 2 = 6
2:3 – find a common multiple
2
2 x 2 = 44
1
2 x 3 = 6
Coefficient ratio: 4:3:2
The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction,
C + O2 CO2,
Is this a balanced equation?
YES
R PCount the number of atoms of each element found in the reaction.
1 C
1 C2 O 2 O