1. Sec.2 – chemical formulas & equations
Ch. 2 Chemical reactions
Sec.2 – chemical formulas & equations

2. Chemical formulas

3. Chemical formulas Shortened way to represent a substance
Use chemical symbols & numbers
Show the type & number of atoms in a substance

4. Subscripts Number written to lower right of element
Tell how many atoms of each element are in the substance

5. How many atoms of each element?
1 carbon, 1 oxygen
1 carbon, 2 oxygen
1 nitrogen, 4 hydrogen
6 carbon, 12 hydrogen, 6 oxygen CO
CO2
NH4
C6H12O6

6. Writing formulas for covalent compounds

7. Writing formulas for covalent compounds
Made up of 2 nonmetals
Common prefixes used in covalent compound names
Mono – 1
Di – 2
Tri – 3
Tetra - 4
Penta – 5

8. Carbon dioxide
Write chemical symbol of 1st element. Add a subscript if there is a prefix.
Write chemical symbol of 2nd element. Add a subscript if there is a prefix.
CO2

9. Now you try…write a formula for each covalent compound
Carbon tetrachloride
Dihydrogen monoxide
Dinitrogen tetroxide
CCl4
H2O
N2O4

10. Writing formulas for ionic compounds

11. Remember…
Ionic compounds are made up of a metal & a nonmetal chemically bonded together.
The METALLIC element is always named 1st in the compound name.
Sodium chloride – sodium is the metal &
chlorine is the nonmetal

12. Steps for writing ionic compound formulas
Write the chemical symbol for the metallic ion first.
Write the chemical symbol for the nonmetallic ion.
Make sure the ions’ charges equal zero.
Metallic ions always have a positive charge.
Nonmetallic ions always have a negative charge.
An ion’s charge is the same as the number of electrons its group will gain or lose to become stable.
Add subscripts to the chemical symbols to make the compound’s charge equal zero.

13. Sodium chloride Sodium’s chemical symbol is Na. Na
Chlorine’s chemical symbol is Cl.
Sodium is in group 1. It’s charge is 1+.
Chlorine is in group It’s charge is 1-.
(+1) + (-1) = 0. No subscripts are needed. Na Cl
Chemical formula: NaCl

14. If the ions’ charges do not equal zero…
Aluminum oxide
Find the LCM of the ions’ charges.
Add subscripts where needed to equal the LCM.
Al – group 13 – charge 3+
O – group 16 – charge 2-
LCM of 3 & 2 is 6
Subscript for Al is 2 (2 x 3=6)
Subscript for O is 3 (3 x 2=6)
4. Formula: Al2O3

15. Now You try…..
Potassium chloride
Magnesium chloride
Sodium fluoride
Aluminum sulfide
KCl
MgCl2
NaF
Al2S3

16. Chemical equations

17. Chemical equations
Shortcut way to describe a chemical reaction
Use symbols to show the relationship between the reactants & products
Can be understood by scientists worldwide

18. reactants
Substances or molecules that participate in a chemical reaction
The starting substances in a chemical reaction
Are ALWAYS written on the LEFT side of the arrow in a chemical equation

19. Identify the reactants in these equations
Na + Cl NaCl
2 H2O H2 + O2
Zn + 2 HCl ZnCl2 + H2
Na + Cl2
2 H2O
Zn + 2 HCl

20. products
The substance that forms in a chemical reaction
The ending substance(s) in a chemical reaction
Are ALWAYS written on the RIGHT side of the arrow in a chemical equation

21. Identify the products in these equations
Na + Cl NaCl
2 H2O H2 + O2
Zn + 2 HCl ZnCl2 + H2
2 NaCl
2 H2 + O2
ZnCl2 + H2

22. Accuracy is important in chemical equations
Incorrect symbols or formulas do not correctly describe a chemical reaction.
Small mistakes will make a HUGE difference!

23. Balancing chemical equations

24. Law of conservation of mass
Mass is neither created nor destroyed in an ordinary chemical or physical change.
Chemical equations must show the same numbers & kinds of atoms on each side of the arrow.

25. Subscripts vs. coefficients
Written to the lower right of a chemical symbol
Written smaller than chemical symbol
Only apply to single atoms
Written to the left of the chemical symbol
Written the same size as the chemical symbol
Apply to every atom in molecule, including subscripts

26. Subscript or coefficient?
H2O2
2 CO2
Subscript
Coefficient
Both!

27. Calculating the number of atoms
A coefficient applies to every atom & every subscript in a molecule.
If there is a subscript, multiply the coefficient by the subscript to find the number of atoms.
Coefficients in an equation are “stopped” by the plus sign or the arrow.

28. 2 H2 + O H2O
Reactant side has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies only to the hydrogen.
Product side also has 4 hydrogen atoms & 2 oxygen atoms. The coefficient 2 applies to everything here. There is no plus sign or arrow to stop the coefficient.

29. Calculate the total number of atoms
2 HCO3
3 C6H12O6
NaF
4 CaO

30. Steps for balancing chemical equations
Set up a T-chart for the reactants & products.
reactant product

31. Ca + O2 CaO Write the chemical symbol & number of atoms for each side.
reactant product
Ca – 1 Ca – 1
O – O – 1
Compare to see if the equation is balanced.
This equation is not balanced.

32. Ca + O2 CaO Find the LCM for each element that is unbalanced.
reactant product
Ca – 1 Ca – 1
O – O – LCM = 2

33. Ca + O2 2 CaO Add coefficients to balance the elements.
NEVER CHANGE A SUBSCRIPT!!
Recount the number of atoms for each
element. Repeat steps as needed.
reactant product
LCM = 2 Ca – 1 Ca – O – O – 1 2

34. 2 Ca + O CaO
reactant product LCM = 2 Ca – 1 2 Ca – 1 2 O – 2 O – 1 2 LCM = 2 The equation is now balanced.