Law of Conservation of Mass
Mass can neither be created nor destroyed
• This means that both sides of the arrow in
a chemical equation (reactants and
products) must have the same number of
atoms of each element (an atom cannot
disappear or appear from nothing).
Balancing Chemical Equations
• Use coefficients (whole numbers in front of
substances) in the equation to make it obey
the law of conservation of mass.
– Coefficients can only be placed in front of
substances between the elements in a compound
– Coefficients must be whole numbers in the
finished equation
– Coefficients multiply all of the atoms in a
compound/elements
Balancing Examples
•
•
•
H20 →
H2 +
Fe + O2 →
CH4 +
O2 →
O2
Fe2O3
CO2 +
H2O
• Tips:
•
•
•
•
Save single elements for last
Fix odds vs. evens by making both even
Count atoms carefully and keep changing coefficients until fixed
Don’t ever change the subscripts in a compound formula (ex. C + O2 → CO
Types of Chemical Reactions
• 5 General types of chemical reactions
– Single Replacement
– Double Replacement
– Synthesis
– Decomposition
– Combustion
Single Replacement RxNs
• A single element reacts with a compound and
replaces one of the element in the compound
– A + BX → AX + B
– Metals can replace metals or hydrogen
– Nonmetals can replace other nonmetals (except
hydrogen) ex: Mg + H2SO4 → H2 + MgSO4
Double Replacement RxNs
• Two compounds react and trade partners in
the reaction
– AX + BY → BX + AY
– Metals trade with metals (2 metals never make a
compound)
– When acids and bases trade partners they make
water and an ionic compound
• Ex: NaCl + AgNO3 → AgCl + NaNO3
– HCl + NaOH → NaCl + HOH
H2O
Synthesis RxNs
• Two elements combine to make a compound
or two simple compounds combine to make a
more complicated compound.
– A + X → AX
– Only one product is made
– Ex: Zn + S → ZnS
• NH3 + H2O → NH4OH
Decomposition RxNs
• A compound breaks apart into two or more
simpler substances (elements or simple
compounds)
– BY → B + Y
– There is only 1 reactant and more than 1 product
is made
• Ex: H2CO3 → CO2 + H2O
– H2O2 → H2O + O2
Combustion RxNs
• Burning a fuel in the presence of oxygen.
• Burning a carbon based (organic) fuel always
creates CO2 and H2O. Combustions are always
exothermic.
– Ex: ________ + O2 → CO2 + H2O
• Fuels: CH4 (methane)
C6H12O6 (glucose)
C8H10 (octane)
C3H8 (propane)