1. Balancing Chemical Equations

2. Remember! Atoms can’t be created nor destroyed (Law of Conservation)
Balanced Equations
Remember! Atoms can’t be created nor destroyed (Law of Conservation)
The atoms that we start with… we MUST end up with.
A balanced equation has the same number of atoms of each element on both sides of the equation.

3. ® C + O2 ® CO2 This equation is already balanced
What if it isn’t already?

4. ® C + O2 ® CO We need one more oxygen in the products.
We can’t change the formula, because it describes what actually happens

5. ® 2 C + O2 ® 2 CO Another Carbon must be used to make another COSo C C O C O C O + O
Another Carbon must be used to make another CO
But where did the other C come from?
Must have started with two C
2 C + O2 ® 2 CO

6. Rules for balancing equations
Write the correct formulas for all the reactants and products
Count the number of atoms of each type appearing on both sides
Balance the elements one at a time by adding coefficients (the numbers in front of formulas or elements)
Check to make sure it is balanced.
You can see that this is balanced !
2 C + O2 ® 2 CO

7. NEVER Change a subscript to balance an equation
Never, Never, Never…
NEVER Change a subscript to balance an equation
If you change the formula, you are describing a different reaction.
H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula
2 NaCl is okay, Na2Cl is not.

8. Example 2
H2 + O H2O 2 R P H O 2 1 4 4 2
Need twice as much H in the reactant
The equation is balanced, has the same
number of each kind of atom on both sides
Make a table to keep track of where you
are at
Recount
Changes the O
Need twice as much O in the product
Also changes the H

9. Example 2 H2 + O2 2 H2O R P 4 2 H 2 4 2 O 1 2 ® This is the answer
Not this 2 O 1 2

10. Try these examples Al + N2 ® Al2N3 4Al + 3N2 ® 2 Al2N3
And the answer is….
4Al + 3N2 ® 2 Al2N3

11. 2AgNO3 +Cu ® Cu(NO3)2+ 2Ag AgNO3 + Cu ® Cu(NO3)2 + Ag
And the answer is….
2AgNO3 +Cu ® Cu(NO3)2+ 2Ag

12. P + O2 ® P4O10
And the answer is….
4P O2 ® P4O10

13. CH4 + O2 ® CO H2O
And the answer is….
CH O2 ® CO H2O

14. If an atom appears more than once on a side, balance it last.
Techniques
If an atom appears more than once on a side, balance it last.
If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there..
How would you balance this one?
C4H10 + O2  CO2 + H2O

15. Then What? Then What? C4H10 + O2  CO2 + H2O Double it. 4

16. & Predicting the Products
Types of Reactions
&
Predicting the Products
Click on types of reactions to get a you tube video introducing types of rxns.

17. Tons of reactions, too many to remember!
Types of Reactions
Tons of reactions, too many to remember!
Divided into 5 categories
Will be able to predict the products.
For some we will be able to predict whether they will happen at all. You must recognize them by the reactants

18. #1 Synthesis Reactions 2Ca +O2 ® 2CaO Synthesis – means put together
A + X  AX
2 elements, or compounds combine to make 1 compound.
2Ca +O2 ® 2CaO

19. Remember! Synthesis Reaction: A + X  AX
We can predict the products if there are two elements.
Mg + N2 ® ?
Remember! Synthesis Reaction: A + X  AX
3Mg + N2 ® Mg3N2

20. Write and balance
Ca + Cl2 ®
And the answer is….
Ca + Cl2 ® CaCl2

21. Another practice example
Write and balance
Al + O2 ®
Remember 1st step is to write the formula
2nd step balance
(also called a combination reaction)
4Al O2 ® 2 Al2O3

22. Decomposition Reaction
AX → A + X
Decompose - means fall apart
Just like a Decomposition Reaction! They were together in the beginning and the ended up breaking apart.
Yields egg shells + baby turtle
Egg Hatching

23. Decompose - means fall apart
Decomposition Reactions
AX → A + X
Decompose - means fall apart
2 NaCl Na + Cl2
CaCO CaO + CO2

24. Decomposition Reactions
More about Decompostion Reactions…
Can predict the products if it is a binary compound
Binary compound means that it is made up of two different elements
Simply falls apart into its elements!
Decomposition Reactions
What do you think?
2H2O
2H2 + O2

25. Great Job ! HgO 2 Hg + O2 2 No! Guess the product Decompostion Rxns
Balanced or not?
Great Job !

26. One element replaces another A + BX → AX + B Y + BX → BY + X
Single Replacement
One element replaces another A + BX → AX + B Y + BX → BY + X
Reactants must be an element and a compound.
Products will be a different element and a different compound.
Na + KCl ® K + NaCl
F2 + LiCl ® LiF + Cl2

27. Na + KCl ® K + NaCl Look at some examples
A + BX → AX + B Y + BX → BY + X
Na + KCl ® K + NaCl Cl K Na

28. More examples
F2 + LiCl ® LiF + Cl2 2 2 Cl Li Li F F Cl Li Li

29. Na + HOH ® Single Replacement NaOH + H2 Example: Na + H2O®
Metals replace metals (and hydrogen)
Example: Na + H2O®
Think of water as HOH
Na + HOH ®
NaOH + H2
Metals replace one of the H, combine with hydroxide.

30. 2Na + 2H2O → 2NaOH + H2 Na + HOH → NaOH + H2
This equation isn’t balanced…
How would you balance it?
Na + HOH → NaOH + H2
2Na + 2H2O → 2NaOH + H2

31. Single Replacement We can tell whether a reaction will happen
Some elements are more active than others
More active replaces less active

32. Single Replacement Higher on the list replaces lower.
If the element by itself is higher; it happens.
If the element by itself is lower; it doesn’t happen.

33. Single Replacement Note the *
H can be replaced in acids by everything higher
Only the first 4 (Li - Na) react with water.

34. Try finishing this equation2 2
Al+ HCl ® 6
AlCl3 + H2 3
No!
Is this equation
balanced?
Fe + CuSO4 ®
FeSO4 + Cu
Is this equation
balanced?
YES!!!

35. #3 Single Replacement
What does it mean that Ag is on the bottom of the list?

36. #3 Single Replacement Nonmetals can replace other nonmetals
Limited to F2 , Cl2 , Br2 , I2
The order of activity is that on the periodic table. ?????????
Higher replaces lower.
F2 + HCl ®
Br2 + KCl ®

37. Double Replacement Reaction
Two things replace each other.
AX + BY → AY + BX
Reactants must be two ionic compounds or acids.
Usually in aqueous solution
NaOH + FeCl3
Just like a double displacement reaction!
Let’s try an example!

38. The positive ions change place.
NaOH + FeCl3 ®
Fe3+OH- + Na+Cl-
The final equation will be…
NaOH + FeCl3 ® Fe(OH)3 + NaCl
Is this balanced or not?
NO!
3NaOH + FeCl3 ® Fe(OH)3 + 3NaCl

39. 3NaOH + FeCl3 ® Fe(OH)3 + 3NaCl

40. #4 Double Replacement Will only happen if one of the products
doesn’t dissolve in water and forms a solid (precipitate)
or is a gas that bubbles out.
or is a covalent compound usually water (water forming) .

41. Complete and balance Assume all of the reactions take place.
CaCl NaOH ®
Ca(OH)2+ NaCl 2 2
Balanced?
No!

42. Complete and balance CuCl2 + K2S → CuS + KCl 2 NO!
Is this equation balanced?

43. Practice these on your own...
Complete and Balance
KOH + Fe(NO3)3 ®
H3PO4 + Ca(OH)2 ®
And the answers
Are…
3KOH + Fe(NO3)3 ® 3KNO3 + Fe(OH)3
2H3PO Ca(OH)2 → 6H2O + Ca3(PO4)2

44. How to recognize which type
Look at the reactants
E for element
C for compound
E + E Synthesis
C Decomposition
E + C Single replacement
C + C Double replacement

45. Last Type
Combustion
A compound composed of only C and H is reacted with oxygen
If the combustion is complete, the products will be CO2 and H2O.
If the combustion is incomplete, the products will be CO and H2O.
O2 will always be the second reactant

46. Examples Complete combustion of C4H10
Assume all combustion is complete.
What are the products of combustion?

47. No! C4H10 + O2 → CO2 + H2O Combustion example continued…
Hint:
Balance C 1st
Balance H 2nd
Balance O last
No!
Is this balanced?
C4H /2O2 → 4CO H2O

48. C4H10 + 6 1/2O2 → 4CO2 +5H2O 2C4H10 + 13O2 → 8CO2 + 10H2O
Multiply everything by 2 to get rid of fractions.
2C4H O2 → 8CO H2O
Let’s Review our 5 chemical reactions.
Or you can rap about it like this guy

49. Oxidation-Reduction Reactions
Redox Reactions
Oxidation-Reduction Reactions
Oxidation-reduction or redox reactions involve the transfer of electrons from one atom to another.
2 Mg + O2  2 [Mg2+][O2-]
In this reaction, each magnesium atom loses two electrons to form an
Mg2+ ion.
Mg  Mg e This is OXIDATION (atoms lose)
And, each O2 molecule gains four electrons to form a pair of O2- ions.
O2 + 4 e-  2 O This is REDUCTION (atoms gain)

50. Lithium is oxidized: Li  Li1+ + 1e- (loss of electrons)
Determine which element is oxidized and which is reduced when lithium reacts with nitrogen to form lithium nitride.
6 Li(s) + N2(g)→2 Li3N(s)
Lithium is oxidized: Li  Li e-
(loss of electrons)
Nitrogen is reduced: N + 3 e-  N3-
(gain of electrons)
Created by Belinda Sensiba