1. 34. What is the chemical formula for ammonium oxide?
A. Al2O3
B. Am2O3
C. NH4O
D. (NH4)2O
Answer: D

2. 38. What is the formula for the compound dinitrogen pentoxide?
A. NO4
B. NO5
C. N2O4
D. N2O5
Answer D

3. 4.3 Balancing Chemical Equations
Objective: Understand the law of conservation of mass and how it applies to balancing equations

4. Chemical reactions result in chemical changes.
Chemical changes occur when new substances are created.
The original substance(s), called reactants, change into new substance(s) called products.
Can you think of an example where you have seen this?
What is an example of a physical change?

5. Chemical reactions can be written in different ways.
A word equation:
The simplest form of chemical equation
Nitrogen monoxide + oxygen  nitrogen dioxide
Reactant: on left side arrow
Product on the right
Arrow means “produce”

6. Indicate the ratio of compounds in the reaction.
A symbolic equation:
2NO(g) + O2(g)  2NO2(g)
State of matter
Letters indicate the state of each compound.
(aq) = aqueous/dissolved in water
(s) = solid
( l ) = liquid
(g) = gas
Coefficients
Indicate the ratio of compounds in the reaction.
Here, there is twice as much NO and NO2 than as is O2.

7. February 21 cont’d Review- cons. Of mass
Lesson, balancing chemical equations
Quiz next class on balancing
CH 4 test Tuesday Feb. 27th

8. Back to Law of conservation of mass and balancing

9. ELEMENT RIDDLES 
1. If you give up your seat on a bus to an elderly person, you would be considered a good ________ 
Samarium
2. Sometimes teachers can be so darn ______
Boron

10. Conservation of Mass in Chemical Change
Chemical change means new compounds are created.
But no new matter is created or destroyed; atoms are just rearranged
All matter in the reactants = all matter in the products.
No new matter is created or destroyed; atoms just rearranged.
Number of each atom in reactants = number of each atom in products

11. Law of conservation of mass:
In chemical reactions, atoms are neither created nor destroyed.
Mass of reactants = mass of products

13. Writing and Balancing Chemical Equations
The simplest form of chemical equation is a word equation:
A skeleton equation shows formulas of atoms, but not quantities of atoms.
K + O2  K2O
A balanced chemical equation shows all atoms and their quantities
Balancing ensures the number of each atom is the same on both sides of the reaction arrow.
Always use the smallest whole-number ratio.
4K + O2  2K2O
Writing and Balancing Chemical Equations

14. Try the previous example…
Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations.
Word equation:
methane + oxygen  water + carbon dioxide
Skeleton equation:
CH4 + O2  H2O + CO2
To balance, find values that
equate atoms on both sides
Try the previous example…
Balanced equation:
CH4 + 2O2  2H2O + CO2
The same number of atoms must be on each side.

15. Example balance the following:
Fe + Cl2  FeCl3
ZnSO4 + BaCl2  ZnCl2 + BaSO4
C3H8 + O2  CO2 + H2O

16. Hints for Writing Word Equations
The chemical symbol is used for most elements not in a compound.
Be careful of diatomic and polyatomic elements such as O2, P4 and S8 .
The “special seven” are all diatomic elements
H2, N2, O2, F2, Cl2, Br2, I2
Several common covalent molecules containing hydrogen have common names that you should know.
For example, methane = CH4, glucose = C6H12O6, ethane = C2H6, ammonia = NH3

17. Strategies for Balancing Equations
Balance chemical equations by following these steps:
Trial and error will work but can be very inefficient.
Balance compounds first and elements last.
Balance one compound at a time.
Only add coefficients; NEVER change subscripts.
If H and O appear in more than one place, attempt to balance them LAST.
Polyatomic ions (such as SO42–) can often be balanced as a whole group.
Always double-check after you think you are finished.

18. Try Balancing the following:
Fe + Br2  FeBr3
Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
C2H6 + O2  CO2 + H2O

19. Quiz next day (balancing)
“Balancing practice”
Quiz next day (balancing)
Review Next class
Test Feb.27

20. Practice: Page 211 1(a-f) 2(a-f)

21. Practice Exam Question Q40
ade10/english/release/exam/1112sc_p_a.pdf