In the late eighteenth century, Antoine
Lavoisier, a French chemist,
recognized the importance of accurate
measurements. He extensively studied
and explained the nature of
combustion. He found out that
combustion involved reaction with
Law of Conservation of Mass
Mass is neither
chemical or physical
Total mass of reactants
Total mass of products
Reactants are the starting material in
a chemical reaction.
Products are the substance formed as
a result of a chemical reaction.
In a chemical equation, reactants are found
on the left side and the products are on the
right side. A chemical reaction can therefore
be summarized as:
Reactants → Products
sodium metal + chlorine gas ---- table salt
The Law of conservation of mass
states that matter cannot be
created or destroyed in any chemical
The atoms in the reactants are
rearranged to form new compounds,
but none of the atoms disappear, and
no new atoms are formed.
O Al O Al O
O Al O Al O
Balancing an equation:
2 H2 +
Second: Determine which atoms are sides of and
First: Inventory the atoms on both not equalthe
add coefficients to balance these atoms.
4 H atoms
2 H atoms
4 2 H atoms
atoms are ≠
atoms are =
2 O atoms
2 O atoms Oxygen
2 1 O atom
atoms are =
When a coefficient is added you must multiply 2.
After redoing the balancedmultiplied by
Oxygen,is now a inventory, add coefficients
This inas a product, mustthis number.
allA coefficient compound in With each additional
atoms the is placed by
to balanced other atoms. front of the
Redo the inventorycontains O.
compounds that inventory must be updated.
So: Chemical equations are balanced, when the
numbers and kinds of atoms on each side of the
reaction arrow are equal.
Because atoms are conserved, the mass of
the reactants in the chemical reaction is
equal to the mass of the products.
H2 + F2
2g H+ 38g
= 2( 20g)
If 27.0 g of mercuric oxide (the red
solid compound) is heated and
completely decomposes to give the
elements oxygen and mercury. 2.0 g of
oxygen are produced. How many grams
of mercury are produced?
mercuric oxide mercury + oxygen
Hg + O2
2.0 g O2
? g Hg
27 g HgO = 2 g O2 + ? Hg = 25 g Hg
Mg + Cl2 2 MgCl
B. 2 Ca + O2 2 CaO
C. Zn + S 2 ZnS
D. C + O2 2 CO
Because atoms must be equal on each side
of the equation:
The correct answer is B
2 Mg + 2 HCl → MgCl2 + H2
When the above equation is balanced,
the coefficient for magnesium is —
answer is C
Iron + Oxygen Rust
100 g + ?g 143g
What mass of
oxygen is needed?43 g Oxygen Choice D