CHEMISTRY FOR ENGINEERS - LEC
2nd Semester Ay 2020 - 2021
CHEMICAL REACTION &
STOICHIOMETRY
Learning Outcomes:





Define chemical reaction, chemical equation,
coefficients, reactants and product.
Identify the types of chemical reaction
Write correctly the chemical formulas of reactants
and products in a chemical reaction
Balance a chemical equation
Analyze and solve problems on the amount of
reactants or products used/form in a chemical
reaction
Chemical Reaction

A process that transform one or more
substances into another set of substances

Chemical changes

Change in composition
Example:
Ethyl alcohol + Oxygen
Lead (IV) oxide
Gasoline + Oxygen
Carbon dioxide +
Water
Lead (II) Oxide +
Oxygen
Carbon dioxide +
water
Law of Conservation of Mass
In a chemical reaction:
Mass before
reaction
Reactants
Mass of reactants
consumed
Mass after
reaction
Products
Mass of products
produced
Reactants – substances that are consumed in a
reaction
Products – substances that are produced or formed in
a reaction
Sample Problem:
If 2 g of iron is heated with chlorine gas, 5.81 g of
ferric chloride forms. How much chlorine was used
up in the reaction?
Iron + Chlorine
Mass of Fe + Mass of Cl =
Ferric Chloride
Mass of FeCl 3
Mass of Cl = Mass of FeCl 3 – Mass of Fe
= 5.81 g – 2g
= 3.81 g
Chemical Equation


The symbolic representation of a chemical reaction where the
reactant entities are given on the left hand side and the
product entities on the right hand side
A chemical equation uses symbols and formula instead of the
names of the elements and compounds participating in the
reaction
Ex. Sodium + Chlorine
2Na
+ Cl2
Sodium Chloride
2NaCl2
Notations Used in Chemical Equation
1. (g) – for gas
2. (l) – for liquid
3. (s) - for solid
4. (aq) – for aqueous
5. Δ - denotes heat
Coefficient



Is the number placed before the symbol or formula
of the participating substances
Represents the number of moles of reactants
needed or product formed in the reaction.
If no coefficient appears before a given formula,
the number 1 is understood.
N2
+
3 H2
Coefficients
2NH3
Writing and Balancing Chemical Equation
1. Identify the reactants and products. Look for some
keywords.
2. Write the correct formula of the reactants and
products
3. Balance the equation by trial and error using
coefficients. Balance the atom that appears only once
either in the reactants or in the products. The number
of atoms in the reactants should be equal to the
number of atoms in the products.
4. Indicate the states of matter of the reactants and
products
Consider this reaction:
Water by electrolysis process produces hydrogen gas
and oxygen gas:
H2 O
2H2O(l)
2 mole(18g/mole)
36g
36g
H2
+ O2
2H2(g) + O2(g)
2moles(2g/mole) +
1 mole(32g/mole)
4 g + 32 g
36g
Sample
? CH4
+
? O2
→
? CO2
+
?H2O
CH4 + O2 → CO2 + H2O
CH4 + O2 → CO2 + 2 H2O
CH4 + 2 O2 → CO2 + 2 H2O
CH4 + 2 O2 → CO2 + 2 H2O
(balance equation)
Balance these equation:
1.)
Fe
+
O2
2.)
KClO3
3.)
Ca(OH)2 + H2SO4
Fe2O3
KCl + O2
H2O +
CaSO4
Balance these equation
1.)
4 Fe
+
3O2
2.)
2KClO3
3.)
Ca(OH)2 + H2SO4
2Fe2O3
2KCl + 3O2
2H2O +
CaSO4
Types of Chemical Reactions
1. Combination
- occurs when two or more substances either
elements or compounds react to form one
product
A + B
Ex.
C(s) + O2(g) →
AB
CO2(g)
2. Decomposition
- occurs when one compound decompose into two or more
new substances.
AB
Ex.
2H2O(l) →
CaCO3(s) →
A
+
B
2H2(g) + O2(g)
CaO(s) + CO2(g)
3. Single Displacement or Single Substitution
- a more active element takes the place of another
element in a compound and sets the less active one
free.
•
•
A + BX → AX + B
AX + Y → AY + X
A free metal can generally displace a less active metal
in a compound.
A free nonmetal can generally displace a less active
nonmetal in a compound.
Ex.
Fe(s) + CuSO4(aq)
FeSO4(aq) + Cu(s)
Cu(s) + FeSO4(s)
N. R. (no reaction)
2Na(s) + 2H2O(l) →
Zn(s) + 2HCl(aq) →
2NaOH(aq) + H2(g)
ZnCl2(aq) + H2(g)
ACTIVITY SERIES OF NON-METALS
Most Active
Least Active
Example:
F
Cl
Br
I
NaF + Cl2 →
N. R.
ACTIVITY SERIES OF METALS
Most Active Li
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Cd
Ni
Sn
Pb
H
Cu
Hg
Ag
Au
Least active
4. Double Substitution or Double Displacement
- the reaction of two compounds in which two
other compounds are produced, as a result of
a trade of the anions by the cations.
AB + CD →
AD + CB
Ex.
NH4Cl(aq) + AgClO3(aq)
NH4ClO3(aq) + AgCl(s)
Ca(OH)2(s) + 2HNO3(aq)
Ca(NO3)2(aq) + 2H2O(l)
5.
Combustion
- a process also known as “burning”.
- rapid reaction of a wide variety of substances
with oxygen gas.
Ex. S(s) + O2(g) → SO2(g)
C + O2(sufficient Amount) → CO2
C + O2(insufficient Amount) → CO
Exercise:
Balance the following chemical reaction and identify its type.
Mg(OH)2 + 2HCl → MgCl2 + 2H2O
P2O5 + 3H2O → 2 H3PO4
3(NH4)2S(aq) + 2FeCl3(aq) → Fe2S3 + 6NH4Cl
Fe + 2 AgNO3 → Fe(NO3)2 + 2 Ag
2KClO3(s) → 2 KCl + 3O2
Exercise:
Propane, C3H8, is used as a fuel for gas barbecue grills, where it
burns in a controlled fashion. But if a mixture of propane is ignited
in a closed space, like a gas pipeline an explosion can easily result.
In either of these cases, the propane combines with oxygen O2 to
form carbon dioxide and water. Write a balanced chemical
equation describing this reaction.
Solution:
Identify the reactants and products. Look for keywords (italics). Write the
correct formula and the chemical equation. Reactants on the left side of
the equation while products on the right side of the equation.
ANSWER:
C3H8 + 5O2 → 3CO2 + 4H2O
Mass Relations from Equations/Stoichiometry
Given the following chemical equation:
+ N2O4 → 3N2 + 4H2O
2N2H4
This is read as:
2 mols N2H4 + 1 mol N2O4 → 3 mols N2 + 4 mols H2O
The quantities 2 moles N2H4, 1 mol N2O4, 3 mols N2 and 4 mols H2O
are chemically equivaleny to each other in this reaction. Hence they can
be use in conversion factors such as:
2 𝑚𝑜𝑙𝑠 𝑁2 𝐻4
3 𝑚𝑜𝑙 𝑁2
;
3 𝑚𝑜𝑙 𝑁2
1 𝑚𝑜𝑙 𝑁2 𝑂4
; or a variety of other combinations
These are called stoichiometric ratios. Use the balanced equations for
the ratios.
Problem:
Ammonia is used to make fertilizers for lawns and
gardens by reacting nitrogen gas with hydrogen gas.
a.) Write a complete balanced chemical equation.
b.) How many moles of ammonia are formed when
1.34 mol of nitrogen react?
c.) How many grams of hydrogen are required to
produced 2750 g of ammonia?
Solution:
a.
b.
N2 + 3H2 → 2NH3
nN2 = 1.34 mole
nNH3 = 1.34 mols of N2 x
2 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑁𝐻3
1 𝑚𝑜𝑙𝑠 𝑜𝑓 𝑁2
nNH3 = 2.68 mols
c. Mass of NH3 = 2750 g , m of H2 = ?
Stoichiometric
ratios
Solution:
c.
Mass of NH3 = 2750 g , m of H2 = ?
Find no. of moles of NH3 = mass/molar mass
= 2750g/17 g/mols
= 161.76 moles
nH2 = 161.76 moles NH3 x
= 242.65 moles
3 𝑚𝑜𝑙𝑠 𝐻2
2 𝑚𝑜𝑙𝑠 𝑁𝐻3
mH2 = nH2 x molar mass = 242.65 moles x 2 g/moles
= 485.3 g
Problem Set:
A.
Balance the following chemical equations:
1. CaC2 + H2O → Ca(OH)2 + C2H2
2. (NH4)2Cr2O7 → Cr2O3 + N2 + H2O
3. CH4 + NH3 + O2 → HCN + H2O
4. SO2 + H2O → H2SO4
5. HNO3 + P4O10 → HPO3 + N2O5
6. Al + HCl → AlCl3 + H2
7. H2SO4 + NaOH → Na2SO4 + H2O
Problem Set:
B.
1.
Answer the following problems
Ethanol, C2H5OH , is responsible for the effects of
intoxication felt after drinking alcoholic beverages.
When ethanol burns in oxygen, carbon dioxide and
water are produced.
a. White a balanced chemical equation for the
reaction
b. How many liters of ethanol (density = 0.789 g/cm3
will be produced 1.25 L of water (density = 1.0
g/cm3)
Problem Set:
B.
2.
Answer the following problems
The combustion of liquid chloroethylene, C2H3Cl,
yields carbon dioxide, steam and hydrogen
chloride gas.
a. Write a balanced chemical equation for the
reaction.
b. How many moles of oxygen are required to
react with 35.00 g of chloroethylene?
References
Hill, John W. (2013). Chemistry for Changing Times. New York:
Prentice Hall
Masterton, William, et al. General Chemistry. Philippines:
Cengage Learning Asia Pte Ltd
Stoker, H. Stephen (2016). General Chemistry. Andover:
Cengage Learning Asia Pte Ltd.
Zumdahl, Steven S. (2015). Introductory Chemistry:
Foundation. USA: Cengage Learning Asia Ptd Ltd., USA
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