Transcript Chapter 3 Sections 3.7-3.8 Powerpoint

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•Chemical change – reorganization of the atoms in one or
more substances.
•Represented by a chemical equation with the reactants on
the left side of an arrow and the products on the right side.
CH4 + O2 → CO2 + H2O
Reactants
Products
•Notice that the atoms have been reorganized.
•Bonds have been broken, and new ones have been formed.
•In a chemical reaction, atoms are neither created nor
destroyed.
•All atoms present in the reactants must be accounted for
among the products.
•In other words, there must be the same number of each
type of atom on the product side and on the reactant side of
the arrow.
•Known as balancing a chemical equation.
CH4 + O2 → CO2 + H2O
This equation is not balanced.
+
+
CH4
+
O2
CO2
+
H2O
•Notice that the number of oxygen atoms (in O2) on the left of
the arrow is two, but on the right there are three O atoms (in
CO2 and H2O).
•Notice there are four hydrogen atoms (in CH4) on the left and
only two (in H2O) on the right.
•Remember atoms are not created nor destroyed in a
reaction so the numbers of each type must occur in the
reactants and products.
+
+
•By balancing each type of atom in the above representation
we now have the same number of each type of atom in the
reactants and the products.
•Written in shorthand by a chemical equation:
CH4 +
2O2
CO2 +
2H2O
1C 4H
In summary:
4O
1C 2O
Reactants
Products
1C
1C
4H
4H
4O
4O
4H
2O
•Chemical equations give two important pieces of
information: the nature of the reactants and products and
the relative numbers of each.
•An equation usually gives the physical states of the
reactants and products.
Physical State
Symbol
Solid
(s)
Liquid
(l)
Gas
(g)
Dissolved in water (aqueous)
(aq)
•For example, when hydrochloric acid in aqueous solution is
added to solid sodium hydrogen carbonate, the products
carbon dioxide gas, liquid water, and sodium chloride (which
dissolves in the water) are formed:
HCl (aq) + NaHCO3 (s)
CO2 (g) + H2O (l) + NaCl (aq)
•The relative numbers of reactants and
products in a reaction are indicated by
the coefficients in the balanced
equation.
•Notice in the above reaction no
coefficients are given because they are
all understood to be one.
Balancing Chemical Equations
•When balancing a chemical equation you may add
coefficients in front of the compounds to balance
the equation but you may NOT change the
subscripts.
•Changing the subscripts changes the compound.
Steps to Balancing Chemical Equations
•There are four basic steps to balancing a chemical
equation.
•Write the correct formulas for the reactants and
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!
Aluminum sulfate and calcium hydroxide react to produce aluminum
hydroxide and calcium sulfate.
Al2(SO4)3 +
Ca(OH)2
→
Al(OH)3 +
CaSO4
•Find the number of atoms of each element on the
left side (reactant).
•Compare those against the number of atoms of the
same element on the right side (product).
•You may find a t-chart helpful.
Reactant
2 Al
3S
14 O
1 Ca
2H
Al2(SO4)3 +
Ca(OH)2
→
Al(OH)3 +
CaSO4
Product
1 Al
1S
7O
1 Ca
3H
•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 to
balance the equation.
Reactant
2 Al
3S
14 O
18
1 Ca
3
2H
6
Al2(SO4)3 + 3 Ca(OH)2
→ 2 Al(OH)3 + 3 CaSO4
Product
2 Al
1
31SS
10
18
7 OO
31Ca
Ca
63HH
•Check your answer to see if:
•The number of atoms on both sides of the equation
are now balanced.
•The coefficients are in the lowest possible whole
number ratios (reduced).
Reactant
Product
2 Al
3S
18 O
3 Ca
6H
Al2(SO4)3 + 3 Ca(OH)2
→ 2 Al(OH)3 + 3 CaSO4
2 Al
3S
18 O
3 Ca
6H
Helpful Hints
•Take one element at a time, working left to right
except for H and O. Save H for next to last and O
until last.
•IF everything balances except for O, and there is
not way to balance O with a whole number, double
all the coefficients and try again. (Because O is
diatomic as an element.)
•Polyatomic ions that exist on both sides of the
equation should be balanced as independent units.