Transcript Chapter 9 Balancing Equations

Chapter 9
Balancing Equations
Terms
• A chemical reaction is the process by which
one or more substances are changed into new
substances.
• A chemical equation uses symbols to
represent a chemical reaction.
• A reactant or reagent is a substance used to
begin a chemical reaction.
• A product is a substance that forms during
chemical reaction.
Potassium metal is added to water.
K + H2O
Water can also be written HOH
K + HOH
What are the products?
What gas might be produced?
What has happened to the liquid?
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.
The resulting liquid is a base.
Most common bases
contain hydroxide (OHˉ).
What are the products?
K + H2O → KOH + H
What is wrong here?
+
→
K + H2O → KOH + H
H,O,N,Cl,F,I,Br mean H should be H2
+
→
K + H2O → KOH + H 2
H,O,N,Cl,F,I,Br mean H should be H2
+
→
K + H2O → KOH + H2
Now what is wrong?
+
→
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.
Lavoisier, 1788
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.
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.
K + HOH → KOH + H2
We must balance the equation.
+
→
Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
1. 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!
2. 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.
3. 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.
4. 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)
_K + _HOH → _KOH + _H2
2K + 2HOH → 2KOH + H2
2K + 2H2O → 2KOH + H2
Consider the form of these substances.
+
→
2K + 2H2O → 2KOH + H2
gas
solid
+
→
liquid
2K(s) + 2H2O(l) → 2KOH + H2(g)
gas
solid
+
→
What about the KOH?
liquid
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.
2K(s) + 2H2O(l) → 2KOH (aq)+ H2(g)
gas
solid
+
→
liquid
aqueous solution
Other Symbols Used in Equations
• Solid (cr) or (s)
• Precipitate (↓)
• “Heated” ∆
• Escaping gas ()
H2SO4
• Catalyst
• A word may be written above an arrow
to indicate something is necessary for
the reaction to occur. electricity
Balancing Equation Strategy
• Balance elements that occur in only one
compound on each side first.
• Balance free elements last.
• Balance unchanged polyatomic ions as groups.
• Fractional coefficients are acceptable but must
be cleared at the end using multiplication.
Balancing Example
Chemical Equation:
C6H14O4 + 15 O2 → 6 CO2 + 7 H2O
2
1. Balance C.
2. Balance H.
3. Balance O.
4. Multiply by two
2 C6H14O4 + 15 O2 → 12 CO2 + 14 H2O
and check all elements.
Ca3(PO4)2 + H2SO4 → CaSO4 + H3PO4
• In this equation, the polyatomic ions do not
break up.
• Instead of counting individual atoms,
polyatomic ion groups may be counted.
Ca3(PO4)2 + H2SO4 → CaSO4 + H3PO4
Cu + H2SO4 → CuSO4 + H2O + SO2
• The sulfate group breaks up. Each
atom must be counted individually.
Cu + H2SO4 → CuSO4 + H2O + SO2
Balancing Equations
• This method of
balancing equations
is the inspection
method.
• The method is trial
and error.
• You need to Practice.
Homework
• Worksheet: “Balancing Equations Ch. 9”