Ch. 11 Chemical Reactions
Ch. 11.1 Describing Chemical
Reactions
Vocabulary
Reactants: original substances before
the reaction (Rx)
Products: new substances formed in
the Rx
‘arrow’ → symbolizes chemical Rx
Definition of a Reaction
 One or more reactants change into
one or more products.
 Reactants → products
1. Chemical Word Rx
Hydrogen peroxide decomposes to form water and
oxygen gas.
 Hydrogen peroxide → water + oxygen
2. Chemical Equations
They show
 Chemical formulas
 Number of atoms or molecules of
reactants and products
 Solid (s) , liquid (l) , gas (g), aqueous
solution (aq)
 Direction of the reaction → or ↔
 What facilitates the Rx heat
→
Examples
 Fe + O2 → Fe2O3
Unbalanced skeleton equation
Add physical states of the substances
info:
 Fe(s) + O2(g) → Fe2O3 (s)
Balanced with coefficients:
 4Fe(s) + 3O2 (g) → 2Fe2O3 (s)
Balancing Chemical Equations
 Making a bicycle
 Frame (F), Wheel (W), handlebar (H),
pedals (P)
 Bicycle: FW2HP2
Rx:__F + _W + __H + __P → _FW2HP2
Balanced:
Rx: F + 2W + H + 2P → FW2HP2
C (s) + O2 (g) → CO2 (g)
Name the reactants and products
 Reactants C, O2
 Product: CO2
Is the equation balanced? Meaning are the
same # of elements on both sides of the
arrow?
1 x C, 2 x O → 1 x C, 2 x O
 yes
Rules for Balancing Equations
1.
2.
3.
4.
5.
6.
Determine the correct formulas for all reactants and
products.
Write the skeleton equation, with the reactants on the
left side, and the products on the right side of the
yield arrow.
Count the # of atoms on both sides, counting
polyatomic ions as single units if they appear
unchanged on both sides of the arrow.
Balance single elements first by using coefficients.
When no coefficient is used it is assumed to be 1.
Never balance by changing the subscripts of a
formula.
Check that each atom or polyatomic ion is equal on
both sides of the reaction.
All coefficients have to be in the lowest possible ratio.
Practice
 Hydrogen gas and oxygen gas react to
form water
1/2. __H2 (g) +__O2 (g) →__H2O (l)
3. 2H, 2O
2H, 1O
4. __H2 (g) +__O2 (g) →2H2O (l)
3. 2H, 2O
4H, 2O
4. 2H2 (g) +__O2 (g) →2H2O (l)
3. 4H, 2O
4H, 2O balanced!!!
Practice
 Hydrogen peroxide solution (H2O2) will
decompose into water and oxygen gas
1/2:__H2O2 (aq) →__H2O (l) +__O2 (g)
3.
2H, 2O
2H, 3O
1/2: 2H2O2 (aq) →__H2O (l) +__O2 (g)
 4H, 4O
2H, 3O
1/2: 2H2O2 (aq) → 2H2O (l) +__O2 (g)
3. 4H, 4O
4H, 4O balanced!!
Practice
 Balance the following skeleton Rx
 ___ Hf + ___N2 → ___Hf3N4