Download Balancing Equations

Balancing Equations
Warm-Up 9/22
1. List 4 signs of chemical change you observed in the reaction you
carried out last class period.
2. State the law of conservation of mass.
3. Chemical equations must always be written as “balanced”. What
does balanced mean in these context?
Power Standard 1.2
Students understand chemical and physical changes of
matter.
Today’s Learning Targets
• I can list the main types of chemical reactions.
• I can identify reactants and products in a chemical equation.
• I can balance chemical equations when all reactants and products are
given.
• I can classify reactions by major type and can predict products of
simple reactions.
• I can apply the state symbols (s), (l), (g) and (aq) in equations.
D. Describing Chemical Change
1. Word Equations—Show the names of reactants and products.
Example: Sodium + Chlorine  Sodium chloride
2. Chemical equations—Show the formulas of reactants and products.
Example: Na + Cl2  NaCl (Not Balanced Yet!)
3. Skeleton equations—Equations that are not yet balanced to
represent what actually occurs in the real world (see example
above)
4. State symbols—
• (s)--solid, (g)--gas, (l)--liquid, (aq)—in water solution
• are listed after each formula to give the physical states of the
substances.
• Example:
Na(s) + Cl2(g)  NaCl (s)
5. Balancing chemical equations
A. Everyday example
• 2 slices of bread + 1 slice of cheese + 1 slice of turkey  1 sandwich
(multiple parts combine to 1 new whole “product”)
B.
What does it mean for an equation to be “balanced”?
• Equal numbers of atoms of each type on both sides of the arrow.
c. Why must equations be balanced?
• Equations always occur in a balanced way since atoms are not
created or destroyed in chemical reactions
d. How are chemical equations balanced?
Steps for writing balanced equations.
1. Write a skeleton equation including correct formulas for all
reactants and products.
At this point in our course you will be given the skeleton
equation.
Example: Mg + O2  MgO
2. Count the number of atoms of each element on each side of the
arrow.
Left: 1 Mg, 2 O
Right: 1 Mg, 1 O
3. Place coefficients in front of chemical formulas to make the number
of atoms of each element equal on both sides of the arrow.
2Mg + O2  2MgO (This means there are now 2 units of MgO)
Left: 2 Mg, 2 O
Right: 2 Mg, 2 O
4. Make sure coefficients are in the lowest whole number ratio.
They are!
5. Add the appropriate state symbols.
Do this if the state symbols are known.
2Mg (s) + O2 (g) 2MgO (s)
Examples
1. C + O2  CO
2C + O2  2 CO
2. Pb(NO3)2 + KI  KNO3 + PbI2
• Pb(NO3)2 + 2 KI  2 KNO3 + PbI2
II. Types of Reactions
A. Classifying reactions
• Reactions are classified into several general types
B. Combination/Synthesis Reactions
• Key Characteristic: 2 or more reactants  1 product
• General Form: A + B  AB
• Example: 2Na + S  Na2S
C. Decomposition Reactions
• Key Characteristic: 1 reactant  2 or more products
• General Form: AB  A + B
• Example: 2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g)
4. Combustion Reactions
• Key Characteristic: 1 reactant is oxygen gas (O2)
• General Form: CH compound + O2  CO2 + H2O
• Example: CH4 +2O2  CO2 + 2H2O
Warm-Up 9/26
Balance the following equations. Identify the reaction type.
1.
2.
3.
4.
NaOH  Na2O + H2O
KClO3  KCl + O2
K + Cl2  KCl
C3H8 + O2  CO2 + H2O
Warm-Up Solution
Balance the following equations. Identify the type of reaction.
1. 2K + Cl2  2KCl
Combination
2. 2KClO3  2KCl + 3O2
Decomposition