Balancing Equations

1
Chapter 9

• Balancing Equations
• Part 1

2
Terms

• A chemical reaction is a process that always
  results in the change of chemical substances.
• A chemical equation is a means of writing out a
  chemical reaction.
• A reactant or reagent is a substance consumed
  during a chemical reaction.
• A product is a substance that forms during
  chemical reaction.

3
Potassium metal is added to water.
4
K H2O
5
Water can also be written HOH
6
K HOH
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What are the products?
8
What gas might be produced?
9
What has happened to the liquid?
10
The litmus test

• Blue litmus paper turns red under acidic
  conditions.
• Red litmus paper turns blue under basic
  conditions.
• If both litmus tests result in no change then the
  solution is neutral.

11
The resulting liquid is a base.
Most common bases contain hydroxide (OH).
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What are the products?
13
K H2O ? KOH H
What is wrong here?
?

14
K H2O ? KOH H
HONClFIBr mean H should be H2
?

15
K H2O ? KOH H
2
HONClFIBr mean H should be H2
?

16
K H2O ? KOH H2
Now what is wrong?
?

17
Chemical Equations

• Because of the principle of the conservation of
  matter,
• an equation must be balanced.
• It must have the same number and type of atoms
  
• on both sides of the equation.

18
Balancing Equations

• When balancing a chemical reaction you may add
  coefficients in front of the compounds to balance
  the reaction, but you may not change the
  subscripts.
• Changing the subscripts changes the compound.

19
Subscripts vs. Coefficients

• The subscripts tell you how many atoms of a
  particular element are in a compound. The
  coefficient tells you about the quantity, or
  number, of molecules of the compound.

20
K HOH ? KOH H2
We must balance the equation.
?

21
Steps to Balancing Equations

• There are four basic steps to balancing a
  chemical equation.
• Write the correct formula for the reactants and
  the products. DO NOT TRY TO BALANCE IT YET! You
  must write the correct formulas first. And most
  importantly, once you write them correctly DO NOT
  CHANGE THE FORMULAS!
• Find the number of atoms for each element on the
  left side. Compare those against the number of
  the atoms of the same element on the right side.
• Determine where to place coefficients in front of
  formulas so that the left side has the same
  number of atoms as the right side for EACH
  element in order to balance the equation.
• Check your answer to see if
• The numbers of atoms on both sides of the
  equation are now balanced.
• The coefficients are in the lowest possible whole
  number ratios. (reduced)

22
_K _HOH ? _KOH _H2
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2K 2HOH ? 2KOH H2
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2K 2H2O ? 2KOH H2
Consider the form of these substances.
?

25
2K 2H2O ? 2KOH H2
gas
solid
?

liquid
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2K(s) 2H2O(l) ? 2KOH H2(g)
gas
solid
?

What about the KOH?
liquid
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potassium hydroxide (KOH) is a solid at room
temperature.

• However KOH is soluble in water. Substances
  dissolved in water are said to be aqueous and are
  indicated as such by an (aq) in the equation.

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2K(s) 2H2O(l) ? 2KOH H2(g)
(aq)
gas
solid
?

aqueous solution
liquid
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Other Symbols Used in Equations

• Solid (cr) or (s)
• Precipitate (?)
• Heated
• Escaping gas (?)
• Catalyst H2SO4
• A word may be written above an arrow to indicate
  something is necessary for the reaction to occur.

?
electricity