Transcript Chemical Reactions-2007

Chemical Reactions
Ms. Haut
Pre-AP Biology
Chemical Reactions
• Reactants—starting material
• Products—ending material
• Chemical Equation
Reactants → Products
Chemical Reactions
• Occur when bonds
between the
outermost parts of
atoms are formed or
broken
• Involve changes in
matter, the making of
new materials with
new properties, and
energy changes
Chemical Equations
• Show the kind of reactants and
products and their relative amounts in
the reaction
4 Al + 3 O2 → 2 Al2O3
Chemical Equations
• Because of the
principle of the
conservation of
matter,
• an equation
must
be balanced.
• It must have the same
number of atoms of
the same kind on both
sides
Balancing Chemical
Equations
• You may add coefficients in front of the
compounds to balance the reaction, but
you may not change the subscripts.
3 CO2
• Changing subscripts changes the
compound.
Subscripts vs. Coefficients
• 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.
4 Al + 3 O2 → 2 Al2O3
This equation means
4 Al atoms + 3 O2 molecules →produces→
2 molecules of Al2O3
Four basic steps to balancing
a chemical equation.
• Write the correct formula for the reactants and the
products in an unbalanced equation.
• Inventory the number of each kind of atom on
both sides of the unbalanced equation.
• Determine where to place coefficients in front of
formulas to balance the equation.
• Take another inventory to determine if:
– The numbers of atoms on both sides of the
equation are now balanced.
– The coefficients are in the lowest possible
whole number ratios.
Let’s Practice…
Write and balance the chemical equation for:
methane reacting with oxygen gas to produce
carbon dioxide and water
1) What are the formulas of the reactants and products?
REACTANT
PRODUCT
Methane: CH4
Carbon dioxide: CO2
Oxygen gas: O2 Water: H2O
2) Write the general formula of the reaction
CH4 + O2  CO2 + H2O
CH4(l) + O2(g)  CO2 (g) + H2O(l)
Count the number of same elements on both sides
ELEMENT
REACTANT
PRODUCT
C
H
1
4
1
2
O
2
3
C is balanced by H and O are not…H is off by 2 and O is off by 1
CH4 + O2  CO2 + H2O
C is balanced but H and O are not…H is off by 2 and O is off by 1
For H: What number do you multiply the 2 H on the product by
to get 4H? 2
CH4 + O2  CO2 + 2 H2O
So now there are 4 H’s on both sides
Now there are 2 O on the reactant side and 4 O on the product side
For O: what number do you multiply the 2O on the reactant
side to get 4O?
2
CH4 + 2 O2  CO2 + 2 H2O
CH4 + 2 O2  CO2 + 2 H2O
ELEMENT
REACTANT
PRODUCT
C
H
1
4
1
4
O
4
4
Reaction is balanced
Now Balance this Reaction
•
N2 + 3 H2 → 2 NH3
N=2
H = 26
N=12
H=36
Now Balance this Reaction
• 2 Na + Cl2 → 2 NaCl
Na = 1 2
Cl = 2
Na = 1 2
Cl = 1 2
Now Balance this Reaction
• 42 Al + 3 O2 → 2 Al2O3
Al = 12 4
O = 26
Al = 2 4
O=36