Understanding Chemical Reactions
 What is a chemical reaction?
 A chemical reaction is a chemical change which forms
new substances.
 Examples:
- The chemical reaction H2(g) + ½ O2(g) → H2O(l)
describes the formation of water from its elements.
- Butane burns in air to produce carbon dioxide and
water: 2C4H10(g) + 13O2 (g) → 2CO2 (g) + 10H2O (g)
 Reactant: Starting substance in a reaction.
Written on the left before the arrow.
 Product: End substance in a reaction.
Written on the right after the arrow.
 Example: Al + Br2 → AlBr3
(arrow reads as “yields” or “produces”)
 A word equation represents a chemical reaction.
 All the reactants are written on the left of the yield
arrow, separated by a plus sign (if needed).
 All the products are written on the right of the yield
arrow, separated by a plus sign (if needed).
 Reactant 1 + Reactant 2 → Product 1 + Product 2
 Example: Rust
 iron + oxygen → iron (III) oxide
 What are the reactants?
 What are the products?
propane + oxygen → carbon dioxide + water
 a) List all the reactants in the reaction.
 b) List all the products in the reaction.
 c) What is the purpose of the arrow?
 #3 Write word equations for the following reactions:
 a) CaCl2 and Na2SO4 react to form CaSO4 and NaCl
 b) BaCO3 reacts when heated to produce BaO and CO2
 c) AgNO3 reacts with KCl to produce AgCl and KNO3
4. Write word equations to represent the
following chemical reactions:
a) Carbon dioxide and water are produced in a human cell
during respiration. The reactants are sugar and oxygen.
b) B) Stalactites form in caves when calcium bicarbonate
reacts to form calcium carbonate, water and carbon
dioxide gas.
Can you figure out what is missing in the following chemical
reactions?
1. Aluminum resists corrosion (rust) because it reacts with a
gas in the air to form a protective coating of aluminum oxide.
Aluminum + ____________ → aluminum oxide
2. Zinc Metal, used as a coating in galvanized iron, also reacts
with air to form a coating that resists further corrosion.
zinc+ oxygen → ___________________
3. When aluminum foil is placed in a solution of copper (II)
chloride, copper metal and another solution are formed:
aluminum + copper (II) chloride → copper + ___________
Count the number of atoms of each type in the reactants
and the products. In this case we can leave NO3- as one
group because it is a polyatomic ion and stays together.
Mg + 2HNO3 → H2 + Mg(NO3)2
Reactants
Mg: ______
H: _______
NO3- : _______
Products
Mg: _____
H: ______
NO3- : _______
2C4H10(g) + 13O2 (g) → 2CO2 (g) + 10H2O (g)
Reactants
C: ________
H: _______
O : _______
Products
C: _______
H: _______
O : _______
H2(g) + ½ O2(g) → H2O(l)
Reactants
H: _______
O : _______
Products
H: _______
O : _______
A skeleton equation is a representation of a chemical
reaction in which the formulas of the reactants and
products are used instead of the names of the
compounds.
Reactants are still separated from each other by a “+” and
reactants and products are separated by an arrow
Example: CH4 +O2 → CO2 +H2O
(Instead of : butane + oxygen → carbon dioxide + water)
****Here is where the HOFBrINCl’s begin to come in to play!
Change the word equations to skeleton equations:
1) Iron reacts with hydrogen sulfate to form iron (III)
sulfate and hydrogen gas
2) Potassium hydroxide reacts with hydrogen
phosphate to produce potassium phosphate and
hydrogen gas
3) Ammonia and oxygen gas react to form nitrogen
monoxide and water.
4) Boron trifluoride and lithium sulfite produce
diboron trisulfite and lithium floride
But wait...
There's a problem with our skeleton equations....
Can you spot it?
Example:
CH4 + O2 → CO2 + H2O
The law of conservation of mass states that: in a
chemical reaction, the total mass of the reactants is
always equal to the total mass of the products.
Experiments show that atoms in a chemical reaction
are not changed themselves, and the number of atoms
has to stay the same from before the reaction to after.
How many atoms of each element are there in each
molecule of:
a) H2SO4
b) Al2(SO4)3
c) CH3COOH
1. When a log burns in your fire place, why are is
the mass of the ashes less than the mass of the log?
2. a) Count the number of atoms of each type of
reactant in the reaction below and record them in the
table below. Count the number of atoms of each
type of product and record them below. b) What do your
results say about conservation of mass?
# of atoms
Reactants
Carbon
Oxygen
Hydrogen
CH4 + 2O2 → CO2 + 2H2O
Products
3. A solid has a mass of 35g. When it is mixed
with a solution, a chemical reaction happens. If
the total mass of the products is 85g, what was
the mass of the solution?
4. Solution A gas a mass of 60g. Solution B has
a mass of 40g. When they are mixed, a chemical reaction
occurs in which a gas is produced. If the mass of the final
mixture is 85g, what mass of gas was produced?
What's wrong with this reaction?
Al +Br2 → AlBr3
It does not follow the Law of Conservation of Mass.
How do we fix that?
By adding coefficients to increase the number of molecules.
Remember: 3 Na2SO4
Coefficient Subscripts
***Remember this rule above all else:
YOU CANNOT CHANGE SUBSCRIPTS, only coefficients
Example:
Iron reacts with oxygen to form magnetic iron oxide (Fe3O4)
 Step 1: Write the word equation.
Iron + Oxygen → magnetic iron oxide
 Step 2: Write the skeleton equation, replacing words for
formulas.
Fe + O2 → Fe3O4
 Step 3: Count the numbers of atoms of each
type in reactants and products.
Fe + O2 → Fe3O4
Reactants:
Fe: 1
O: 2
Products:
Fe: 3
O:4
Step 4: Multiply each of the formulas by
appropriate coefficients to balance the numbers
of atoms.
To balance the three irons on the right, multiply the ones
on the left by 3 multiply the 2 oxygens on the left by two
to balance with the 4 on the right.
3 Fe + 2 O2 → Fe3O4
Magnesium metal reacts with nitric acid (HNO3) to form
hydrogen gas and magnesium nitrate.
Step 1: Write the word equation for the reaction
Magnesium + nitric acid → hydrogen +magnesium nitrate
Step 2: Write the skeleton equation by replacing the
formulas for the name.
Mg+ HNO3 → H2 + Mg(NO3)2
Step 3: Count the number of atoms of each
type in reactants and products.
****(in this case we can count the NO3 as one atom because
it stays together)****
Mg+ HNO3 → H2 + Mg(NO3)2
Reactants:
Mg:1
H: 1
NO3: 1
Products:
Mg: 1
H: 2
NO3: 2
Place a 2 in front of the HNO3
Mg+ 2HNO3 → H2 + Mg(NO3)2
1. a) Why is the following equation not balanced?
N2 + H2 → NH3
b) What is wrong with the way the above equation was
balanced here?
N 2 + H3 → N 2 H 3
 2. Balance:
 a) Na + Cl2 → NaCl
 b) K + O2 → K2O
 c) H2 + O2 → H2O
 d) H2 + Cl2 → HCl
 e) N2 + H2 → NH3
 f) CO + O2 → CO2
 g) Al + Br2 → AlBr3
 h) N2H4 + O2 → H2O + N2
 i) CH4 + O2 → CO2 + H2O
 3.For each of the following, write the correct skeleton
equation, and then balance it to form a chemical
equation.
→ copper + water
b) lead (II) nitrate + potassium iodide → lead (II)
 a) copper (II) oxide + hydrogen

iodide + potassium nitrate
 c) calcium + water → calcium hydroxide + hydrogen
 d) lead (II) sulfide +oxygen → lead + sulfur dioxide
 e) hydrogen sulfide → hydrogen +sulfur