Chemical reactions and equations
Example: Conservation of mass
Consider the following reaction for the electrolysis of water:
2 H2O
electricity
2 H2
+
O2
If 22.4 g of hydrogen and 177.6 g of oxygen are formed, how
many grams of water reacted?
Conservation of mass
mass of reactants = mass of products
mass of H2O = mass of H2 + mass of O2
mass of H2O = 22.4 g + 177.6 g
mass of H2O = 200.0 g
Review: Balanced chemical equations
In a balanced chemical equation, the total number of atoms of each
element must be the same on both sides of the equation
Calculations from chemical equations
What information about a chemical reaction can you get from a
balanced chemical equation?
-- you can think of this as applying accounting principles to chemistry
Equation:
C3H8(g) + 5 O2(g)
propane
oxygen
3 CO2(g) + 4 H2O(g)
carbon
dioxide
C3H8(g) + 5 O2(g)
3 CO2(g) + 4 H2O(g)
water
If you are given the amount of any reactant or product involved
in the reaction:
• you can calculate the amounts of all the other reactants and
3 carbon atoms, 8 hydrogen atoms,
10 oxygen atoms
3 carbon atoms, 8 hydrogen atoms,
10 oxygen atoms
A balanced chemical equation is an expression of the Law of Conservation of Mass
Matter can not be created nor destroyed -- it can only shift from one form to another
In a chemical reaction, no atoms are created or destroyed
-- they are just recombined to form new substances
products that are consumed or produced in the reaction
Baking recipe analogy
1 bag
flour
1 carton
milk
+
+ 6 eggs
Baking recipe analogy
24 pancakes
24 pancakes
1 bag flour
X pancakes
=
2 bags flour
1 carton
milk
+
(2 bags flour)
(36 pancakes)
6 eggs
24 pancakes
Back to chemical reactions…
Example: Copper and oxygen, when heated, combine to form copper (II) oxide
X eggs
=
36 pancakes
Remember: In a balanced chemical equation, coefficients
indicate the relative numbers of formula units and
relative numbers of moles of each reactant / product
Cu
O
Cu
+
O
Cu
O
+ (1) O2
2 molecules CuO
2 atoms Cu
O
Cu
O
2 CuO
If you react 6 atoms of Cu, how many molecules of CuO will you get?
(6 atoms Cu)
Cu
O
Cu
2 Cu
(36 pancakes)
O
Cu
Cu
O
24 pancakes
9 eggs = X
48 pancakes = X
+
+ 6 eggs
If you want to make 36 pancakes, how many eggs will you need to use?
If you use 2 bags of flour, how many pancakes will you get?
(2 bags flour)
1 bag
flour
=
X molecules CuO
6 molecules CuO = X
6 atoms Cu
(6 atoms Cu)
2 Cu
2 Cu atoms
+ (1) O2
1 O2 molecule
2 CuO
2 CuO molecules
Remember: In a balanced chemical equation, coefficients
indicate the relative numbers of formula units and
relative numbers of moles of each reactant / product
Calculations from chemical equations
If you know the amount of any reactant or product involved in the reaction:
• you can calculate the amounts of all the other reactants and products
Cu
+
O
Cu
O
Cu
O
O
that are consumed or produced in the reaction
C3H8(g) + 5 O2(g)
Cu
3 CO2(g) + 4 H2O(g)
BUT REMEMBER!
+ (1) O2
2 Cu
2 moles Cu
1 mole O2
2 CuO
2 moles CuO
2*(6.022 x 1023 atoms/mol) 1*(6.022 x 1023 molecules/mol) 2*(6.022 x 1023 molecules/mol)
= 1.204 x 1024 atoms Cu
= 6.022 x 1023 molecules O2
= 1.204 x 1024 molecules CuO
The coefficients in a chemical equation provide information ONLY about
the proportions of MOLES of reactants and products
• given the number of moles of a reactant/product involved in a
reaction, you CAN directly calculate the number of moles of other
reactants and products consumed or produced in the reaction
• given the mass of a reactant/product involved in a reaction, you can
NOT directly calculate the mass of other reactants and products
consumed or produced in the reaction
Mole - mole calculations
Given: • A balanced chemical equation
• A known quantity of one of the reactants/product (in moles)
Calculate: The quantity of one of the other reactants/products (in moles)
Example:
How many moles of ammonia are produced from 8.00 mol of hydrogen
reacting with nitrogen?
Equation:
3 H2 + N2
2 NH3
Mole ratio between unknown
substance (ammonia) and
known substance (hydrogen):
Use ratio between coefficients
of substances A and B from
balanced equation
Moles of
substance A
Mole - mole calculations
Moles of
substance B
( 8.00 moles H2 )
n
8.00 moles H2
2 moles NH3
3 moles H2
=
2 moles NH3
3 moles H2
n = 5.33 moles NH3
( 8.00 moles H2 )
Mole - mole calculations
Mole - mole calculations
Given the balanced equation:
K2Cr2O7 + 6 KI + 7 H2SO4
Given the balanced equation:
Cr2(SO4)3 + 4 K2SO4 + 3 I2 + 7 H2O
K2Cr2O7 + 6 KI + 7 H2SO4
Cr2(SO4)3 + 4 K2SO4 + 3 I2 + 7 H2O
Calculate:
Calculate:
a) The number of moles of potassium dichromate (K2Cr2O7) required
to react with 2.0 mol of potassium iodide (KI)
b) The number of moles of sulfuric acid (H2SO4 ) required to produce
2.0 moles of iodine (I2 )
Mole ratio between the unknown
substance (potassium dichromate) and
the known substance (potassium iodide):
( 2.0 mol KI )
n
2.0 mol Kl
=
Mole ratio between the unknown
substance (sulfuric acid) and the
known substance (iodine):
1 mol K2Cr2O7
6 mol Kl
1 mol K2Cr2O7
( 2.0 mol KI )
6 mol Kl
( 2.0 mol l2 )
n
2.0 mol l2
n = 0.33 mol K2Cr2O7
Given: • A balanced chemical equation
• A known mass of one of the reactants/product (in grams)
Calculate: The mass of one of the other reactants/products (in grams)
Grams of
substance B
Use ratio between coefficients
of substances A and B from
balanced equation
Moles of
substance A
7 mol H2SO4
3 mol l2
( 2.0 mol l2 )
Mass - mass calculations
How many grams of nitric acid are required to produce 8.75 g of
dinitrogen monoxide (N2O)?
The balanced equation is:
4 Zn (s) + 10 HNO3 (aq)
Use molar mass
of substance A
3 mol l2
n = 4.7 mol H2SO4
Mass - mass calculations
Grams of
substance A
=
7 mol H2SO4
Use molar mass
of substance B
4 Zn(NO3)2 (aq) + N2O (g) + 5 H2O (l)
Step 1: Convert the amount of known substance (N2O) from
grams to moles
Molar mass N2O: ( 2 x 14.01 g/mol ) + 16.00 g/mol = 44.02 g/mol
8.75 g N2O ( 1 mol N2O / 44.02 g N2O )
Moles of
substance B
=
0.199 mol N2O
Mass - mass calculations
Mass - mass calculations
How many grams of nitric acid are required to produce 8.75 g of
dinitrogen monoxide (N2O)?
The balanced equation is:
4 Zn (s) + 10 HNO3 (aq)
The balanced equation is:
4 Zn(NO3)2 (aq) + N2O (g) + 5 H2O (l)
Step 2: Determine the number of moles of the unknown substance (HNO3) required to
produce the number of moles of the known substance (0.199 mol N2O)
Mole ratio between the unknown
substance (nitric acid) and the known
substance (dinitrogen monoxide):
( 0.199 mol N2O )
How many grams of nitric acid are required to produce 8.75 g of
dinitrogen monoxide (N2O)?
n
0.199 mol N2O
=
10 mol HNO3
1 mol N2O
4 Zn(NO3)2 (aq) + N2O (g) + 5 H2O (l)
Step 3: Convert the amount of unknown substance (1.99 moles HNO3)
from moles to grams
Molar mass HNO3: 1.008 g/mol + 14.01 g/mol + ( 3 x 16.00 g/mol )
1 mol N2O
10 mol HNO3
4 Zn (s) + 10 HNO3 (aq)
= 63.02 g/mol
( 0.199 mol N2O )
1.99 mol HNO3 ( 63.02 g HNO3 / 1 mol HNO3 )
=
125 g HNO3
n = 1.99 mol HNO3
Mass - mass calculation: Another example
Mass - mass calculation: Another example
How many grams of carbon dioxide are produced by the complete
combustion of 100. g of pentane (C5H12)?
How many grams of carbon dioxide are produced by the complete
combustion of 100. g of pentane (C5H12)?
The balanced equation is:
The balanced equation is:
C5H12 (g)
+
8 O2(g)
5 CO2 (g)
+
6 H2O(g)
Step 1: Convert the amount of known substance (C5H12) from
grams to moles
Molar mass C5H12: ( 5 x 12.01 g/mol ) + ( 12 x 1.008 g/mol )
= 72.15 g/mol
100. g C5H12 ( 1 mol C5H12 / 72.15 g C5H12 ) = 1.39 mol C5H12
C5H12 (g)
+
8 O2(g)
5 CO2 (g)
+
6 H2O(g)
Step 2: Determine the number of moles of the unknown substance (CO2) required to
produce the number of moles of the known substance (1.39 mol C5H12)
Mole ratio between the unknown
substance (carbon dioxide) and the
known substance (pentane):
( 1.39 mol C5H12 )
n
1.39 mol C5H12
5 mol CO2
1 mol C5H12
=
5 mol CO2
1 mol C5H12
n = 6.95 mol CO2
( 1.39 mol C5H12 )
Mass - mass calculation: Another example
How many grams of carbon dioxide are produced by the complete
combustion of 100. g of pentane (C5H12)?
Chapter 6 Problems:
The balanced equation is:
C5H12 (g)
+
8 O2(g)
Homework assignment
5 CO2 (g)
+
6 H2O(g)
Step 3: Convert the amount of unknown substance ( 6.95 moles CO2 )
from moles to grams
Molar mass CO2: 12.01 g/mol + ( 2 x 16.00 g/mol ) = 44.01 g/mol
n = 6.95 mol CO2 ( 44.01 g CO2 / 1 mol CO2 )
= 306 g CO2
6.63, 6.64, 6.66, 6.67, 6.68 6.69