.MODULE TITLE - CHEMICAL REACTIONS
This
module is especially made for Grade 11 STEM students to enjoy learning General
Chemistry 1 hassle-free and effectively at home. This module provides is based
on the competency/ies from the Most Essential Learning Competencies (MELC)
developed by the Department of Education as a part of the Learning Continuity
Plan (LCP).
OVERVIEW:
Topics to
be discussed in this module:
· Writing and balancing chemical equations
· Types of Reactions
· Stoichiometry: mole-to-mole
relationships;
· Stoichiometry:
grams-to-mole-to-mole-to-grams relationships;
· Limiting Reactant and Reaction Yields
MOST ESSENTIAL LEARNING COMPETENCIES:
At the end of this
module, you should be able to answer the following questions:
1. How do you balance a
chemical equation?
2. What are the
indications of a chemical change?
3. What are the types of
chemical reactions?
4. How are limiting and
excess reagents determined in a chemical reaction?
Chemical Reaction
When a substance undergoes changes in its composition, a chemical
change or chemical reaction occurs. In a chemical reaction, one or more
reactants change into one or more products, and this is represented by chemical
equations. Chemical equations are symbolic representations of chemical
processes that use standardized notation to communicate information in an
efficient manner. These equations follow the general form:
Reactants → Products
where the reactants are written on the left and products
on the right. The reactants and the products are separated by an arrow,
which indicates some change occurs as a result of the process. The arrow is
read as yields, gives, or reacts to produce.
Evidences for Chemical Reactions
1. Change
in color;
2.
Formation of solid products (precipitates);
3.
Release of gas;
4. Change in
temperature; and
5. Production of flame.
Examples:
1. Smooth
and shiny steel becomes reddish-brown and flaky when it rusts;
2. A
colored dress discolored when bleached;
3. Blue
flame is produced when natural gas burns;
4. CO2
gas fizzes when vinegar is poured on eggshells;
5. Milk
curdles when you add lime juice;
6. White
precipitates are formed when aqueous solutions of NaCl and AgNO3 are
mixed
Writing Chemical Equations
Chemical
reactions are represented by chemical equations.
Chemical
reactions involve the re-grouping of atoms in molecules or compounds. For example, when methane gas (CH4)
reacts with oxygen gas, atoms in CH4 and O2 molecules break up and then recombine
to form H2O and CO2. This reaction is conveniently represented by a
chemical equation. For example,
Word
equation: Methane gas + oxygen gas → carbon dioxide gas + water vapor
Chemical
equation: CH4(g) + 2 O2(g) → CO2(g)
+ H2O(g)
Exercise-1: Write the
unbalanced chemical equation for each of the following reactions:
1. Magnesium reacts with oxygen gas to form solid
magnesium oxide.
2. Calcium metal reacts with water to form calcium
hydroxide precipitate and hydrogen gas.
3. Solid ammonium
carbonate decomposes when heated to produce solid ammonia gas, carbon dioxide
gas, and water vapor.
4. Nitrogen reacts
with hydrogen to form ammonia gas.
Balancing Chemical Equations
According to the law of
conservation of mass, atoms are neither created nor destroyed in chemical reactions. Therefore, a chemical equation should have
equal numbers of each type of atoms on both reactant and product sides. We say
that the equation is balanced.
Example:
Not
balanced: C3H8(g) + O2(g) → CO2(g) + H2O(g)
(3C, 8H, 2O)
(1C, 2H, 3O)
Balanced
eqn.: C3H8(g) + 5 O2(g) → 3CO2(g) + 4H2O(g)
(3C, 8H, 10 O) (3C, 8H, 10 O)
How to
write and balance equations?
1. All reactants and products of a reaction
must be known before attempting to write an equation.
2. Identify the reactants and products and
write their chemical symbols or formulas CORRECTLY.
3. Indicate the state of substances: (g)
for gas, (l) for liquid, (s) for solid, and (aq) for aqueous solution.
4. Balance the equation by introducing
smallest integer (whole number) coefficients in front of each reactant and
product as needed, (coefficient "1" is not shown). The chemical
formula of any substances MUST NOT be changed. For example, the formula for
sodium chloride is NaCl; it CANNOT be written as Na2Cl2
or any other forms.
Finally, inspect the
equation – check that each type of atoms (or group of atoms) is equal on both
sides of the equation.
Balancing
Process: the following steps may be used to make balancing easier:
1. Balance first elements that occur in only one
compound on each side of the equation;
2. Balance last elements that occur in more than
one compound.
Rules
in Balancing Equations:
· NEVER CHANGE the chemical formula of any of the substances
(reactants or products);
· NEVER introduce extra formulas that are not parts of the
reaction.
Examples:
1. Hydrogen and
oxygen gases react to form liquid water.
Write the chemical equation.
Unbalanced equation: H2(g) + O2(g) → H2O(l);
Balanced equation: 2 H2(g) + O2(g) → 2 H2O(l)
Inspection: ( 4H, 2O) (4H, 2O)
2. Propane gas (C3H8) burns
in air (reacts with oxygen) to form carbon dioxide gas and water vapor. Write a balanced chemical equation for the
reaction
Unbalanced equation: C3H8(g) + O2(g) → CO2(g) + H2O(g)
Balanced equation: C3H8(g) + 5 O2(g) → 3CO2(g) + 4H2O(g)
Inspection: (3C, 8H, 10O) (3 C, 8H, 10O)
Balanced
chemical equations provide the following information:
· The identity of reactants and products;
and
· The molar relationships between one substance to another
represented in the
equation.
Exercise-2:
A. Write a balanced equation for each of
the following reactions:
1. Solid ammonium nitrite decomposes to
form nitrogen gas and water vapor.
2. Baking powder (sodium hydrogen
carbonate) decomposes to produce solid sodium carbonate, carbon dioxide gas,
and water vapor.
3. Solid calcium carbonate reacts with
aqueous hydrochloric acid solution to yield aqueous calcium chloride solution
and carbon dioxide gas.
4. Solid ammonium dichromate, (NH4)2Cr2O7(s),
readily decomposes to produce solid chromium(III) oxide, nitrogen gas, and
gaseous water.
B. Balance the following equation using
smallest whole number coefficients:
1. ___C4H10O(l) + ___O2(g) → ___CO2(g) + ___H2O(g)
2. ___Al2O3(s) +
___HCl(aq) → ___AlCl3(aq) + ___H2O(l)
3. ___PCl5(s) + ___H2O(l) → ___H3PO4(aq) +
___HCl(aq)
4. ___H3PO4(aq) +
___NaOH(aq) → ___H2O(l) +
___Na3PO4(aq)
5. ___NH3(g) + ___O2(g) → ___NO(g) + ___H2O(g)
6. ___Ba(OH)2(aq) + ___H3PO4(aq) → ___H2O(l) +
___Ba3(PO4)2(s)
7. ___Mg3N2(s) + ___H2O(l) → ___Mg(OH)2(s) +
___NH3(g)
Classification of Reactions
Oxidation-Reduction
or Redox Reactions - these are reactions that involve the transfer of
electrons.
· Oxidation è lost of electrons;
· Reduction è gain of electrons;
· Oxidizing agent è the reactant that gains electrons;
· Reducing agent è the reactant that
loses electrons;
Types
of Redox Reactions:
1. Combination
Reactions:
(a) Reaction between
sodium metal and chlorine gas: 2 Na(s) + Cl2(g) → 2 NaCl(s)
(1) Oxidation half-reaction - lost of
electrons: 2 Na → 2 Na+ + 2e-;
(2) Reduction half-reaction - gain of
electrons: Cl2 + 2e- → 2Cl-;
————–———-———————————————————————————
Oxidation-reduction
reaction: 2 Na(s) + Cl2(g) → 2 Na+ +
2 Cl- → 2NaCl(s)
———————————————————————————————————
Net: 2 Na(s) + Cl2(g) → 2 NaCl(s)
———————————————————————-
In the
above reaction, sodium loses an electron and is oxidized to Na+,
while chlorine gains an electron and is reduced to Cl-. In this reaction, sodium is the reducing
agent and chlorine the oxidizing agent.
(b) Reaction of magnesium with oxygen: 2Mg(s) + O2(g) → 2MgO(s);
(1) Oxidation half-reaction - lost of
electrons: 2Mg → Mg2+ + 4e-;
(2) Reduction half-reaction - gain of
electrons: O2 + 4e- → 2 O2-;
————————————————–——————————————————
Oxidation-reduction
reaction: 2Mg(s) + O2(g) → 2Mg2+ +
2 O2- → 2MgO(s)
————————————————————–——————————————
Net:
2Mg(s) + O2(g) → 2MgO(s)
—————————–-——————————
In the
above reaction, Mg is the reducing agent and it is oxidized to Mg2+,
while oxygen is the oxidizing agent and it is reduced to O2-.
(c) Reaction of aluminum metal with chlorine gas: 2Al(s) + 3Cl2(g) → 2AlCl3(s)
(1) Oxidation half-reaction - lost of
electrons: 2Al → 2Al3+
+ 6e-;
(2) Reduction half-reaction - gain of
electrons: 3Cl2 + 6e- → 6Cl-;
————————————————————————–——————————
Oxidation-reduction
reaction: 2Al(s) + 3Cl2(g) → 2Al3+
+ 6Cl- → 2AlCl3(s)
————————————————————————————–——————
Net: 2Al(s) + 3Cl2(g) → 2AlCl3(s)
———————————————————–———--
(d) Reaction between
aluminum and sulfur: 2 Al(s) + 3 S(s) → Al2S3(s)
(1) Oxidation half-reaction - lost of
electrons: 2 Al → 2 Al3+ +
6 e-;
(2) Reduction half-reaction - gain of
electrons: 3 S + 6e- → 3 S2-;
Exercise-3:
For each
reaction below, show how electrons are lost and gained by the elements involved?
1. 2 K(s) + Br2(l) → 2KBr(s); 3. Ca(s) + F2(g) → CaF2(s);
2.
2 Cu(s) + S(s)
→ Cu2S(s); 4. 6 Na(s) + N2(g) → 2Na3N(s)
__________________________________________________________________________________
Other
types of Redox Reactions:
2. Decomposition
Reactions
A
reaction in which a compound breaks up into two or more elements or
simpler compounds.
Examples: (1) 2
KClO3(s) → 2 KCl(s) + 3 O2(g)
(2) (NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4 H2O(g)
(3) 2
HgO(s) → 2 Hg(l) + O2(g);
3. Single Replacement Reactions
This is a
redox reaction in which an element in a compound is replaced by another element.
Examples: (1)
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g); (Zn replaces H)
(2)
Mg(s) + CuSO4(aq) → MgSO4(aq)
+ Cu(s); (Mg replaces Cu)
(3)
Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)
; (Cl replaces I)
4. Combustion Reactions
These are
reactions with molecular oxygen (oxygen gas), which are always accompanied by
the release of a large amount of heat and flame.
Examples: (1)
CH4(g) + 2 O2(g) → CO2(g)
+ 2H2O(g)
(2)
C3H8(g)
+ 5 O2(g) → 3CO2(g) + 4H2O(g)
(3)
2C4H10(g)
+ 13 O2(g) → 8CO2(g)
+ 10H2O(g)
(4)
2C8H18(l)
+ 25 O2(g) → 16CO2(g)
+ 18H2O(g)
(5)
C(s) + O2(g) → CO2(g)
__________________________________________________________________________________
Examples
of combination and decomposition reactions which are NOT redox reactions:
1. CaO(s) + SO2(g) → CaSO3(s); 4. LiOH(aq) + CO2(g) → LiHCO3(aq)
2. CaCO3(s) → CaO(s) + CO2(g); 5. Mg(OH)2(s) → MgO(s)
+ H2O(g)
3. 2NaHCO3(s) → Na2CO3(s) + H2O(g)
+ CO2(g)
__________________________________________________________________________________
Exercise-4 : Classify each of the following reactions,
first as redox or nonredox. Then further classify as combination,
decomposition, single replacement, or combustion:
1. Fe2O3(s) + 2
Al(s) → Al2O3(s) + 2 Fe(s)
_______________________________________________
2. 3 Mg(s) + N2(g) → Mg3N2(s)
_______________________________________________
3. 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)
_______________________________________________
5. 4 HNO3(aq) → 2 H2O(g) + 4 NO2(g) + O2(g)
_______________________________________________
6. C2H5OH(l)
+ 3 O2(g) → 2 CO2(g)
+ 3 H2O(l)
_______________________________________________
7. SO3(g) + H2O(l) → H2SO4(aq)
_______________________________________________
8. (NH4)2CO3(s) → CO2(g) +
2NH3(g) + H2O(l)
_______________________________________________
__________________________________________________________________________________
Other Types of Reactions
Double-Displacement Reactions
These are reactions in aqueous solution that involve the
exchange of ions between the reacting species of the type:
AB + CD → AD + CB
The two
types of double-displacement reactions are precipitation and neutralization or
acid-base reactions.
A. Precipitation
reactions are reactions that result in the formation of precipitates (solid
products) because at least one of the products of the reaction is not soluble
in water.
For
example,
1. Pb(NO3)2(aq) + K2C
rO4(aq) → PbCrO4(s) + 2
KNO3(aq)
2. AgNO3(aq) + KBr(aq) → AgBr(s) +
KNO3(aq)
3. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) +
2NaCl(aq)
The
products, PbCrO4, AgBr, and BaSO4 are only slightly
soluble or insoluble in water.
B. Acid-base
(or Neutralization) reactions are reactions in aqueous solution that form
salts and water as the products. Examples:
1. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
2. H2SO4(aq) + 2
KOH(aq) → K2SO4(aq) + 2 H2O(l)
3. HC2H3O2(aq) + NaOH(aq) → NaC2H3O2(aq) + H2O(l)
Some
double-displacement reactions may result in both the formation of precipitates
and water. They can be classified as
both precipitation and acid-base reactions.
For
examples:
1. Ba(OH)2(aq) + H2SO4(aq) → BaSO4(s) +
2 H2O(l)
2. 3Ca(OH)2(aq) + 2H3PO4(aq) → Ca3(PO4)2(s) + 6 H2O(l)
Information obtained from Chemical Equations
A
chemical equation tells us what type of chemical changes take place during a
chemical reaction. While a balanced chemical equation also tells the molar
relationships between one reactant to another, and between reactants and
products. Consider the following balanced chemical equation:
CO(g) + 2H2(g) → CH3OH(l)
Mole-to-Mole Relationships
The coefficients in balanced equation give mole
relationships between reactants and products.
Since mole quantity is directly related to mass, the equation also
provide information regarding mass relationships between substances involved in
the reaction. For example, the above equation implies that:
One CO molecule reacts with two H2 molecules to
form one CH3OH molecule.
Or, one mole of CO reacts with two moles
of H2 to form one mole of CH3OH.
The
stoichiometric ratios in the above reaction are: 1 mol CH3OH and 1
mol CH3OH
1
mol CO 2 mol H2
These
mole ratios can serve as conversion factors in stoichiometric calculations.
The
combustion reaction of propane gas, C3H8(g), represented
by the following equation,
C3H8(g) + 5 O2(g)
→ 3 CO2(g) + 4 H2O(g)
implies
that one mole of C3H8 reacts with 5 moles of O2
to yield 3 moles of CO2 and 4 moles of H2O.
Exercise-5:
1. For
the combustion of propane gas according to the following reaction:
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g);
(a) How many moles of oxygen gas will react with one mole
of propane?
(b) How many moles of CO2 and H2O,
respectively, are formed from one mole of propane reacted?
(c) How many mole moles of O2 gas are required
to react exactly with 0.25 mol of C3H8?
(d) How many moles of C3H8 are
combusted if 3.75 moles of O2 were consumed during the reaction?.
(e) How many moles of CO2 and H2O,
respectively, will be produced from the
complete combustion of 0.25 mole
of propane gas?
2. Ammonia is produced by the following
reaction: N2(g) + 3 H2(g) → 2 NH3(g)
(a) How many moles of
ammonia are produced if 12.0 moles of H2 are completely reacted?
(b) How many moles nitrogen gas are needed to react
completely with 12.0 moles of hydrogen gas according to the above reaction?
(c) How many moles
each of N2 and H2, respectively, are needed to produce
12.0 moles of ammonia according to the above equation?
__________________________________________________________________________________
Calculations of Masses
Mass-to-Mole-to-Mole-to-Mass Relationships
In any given reaction, if the
mass of one of the reactants is given, its quantity in moles can be obtained.
Then the moles of the other reactant or the product can be determined from the balanced equation.
This quantity in moles of the other reactant or product(s) can be converted
back to their respective masses.
Consider
the formation of CH3OH from CO and H2 according to the
equation:
CO(g) + 2 H2(g) → CH3OH(l)
Suppose
that the reaction uses 454 g of CO gas, how many grams of hydrogen gas are also
reacted, and how many grams of methanol, CH3OH, are formed?
Solution: First we have to convert 454 g of CO to moles
of CO gas:
? Moles of CO = 454 g x 1
mol CO = 16.2
mol CO;
28.01 g CO
Using the
balanced equation, we find how many moles and grams of H2 that will
react with 454 g of CO:
? Moles of H2 = 16.2
mol CO x 2 mol H2 = 32.4 mol H2;
1 mol CO
? grams of H2 = 32.4
mol H2 x 2.016 g H2 = 65.3 g H2
1 mol H2
Next, we
calculate the moles and grams of methanol that would be formed if the reaction
occurs to completion. That is, all of CO are reacted.
? Moles of CH3OH =
16.2 mol CO x 1 mol CH3OH = 16.2
mol CH3OH
1 mol CO
?
Grams of CH3OH = 16.2 mol CH3OH x 32.04
g CH3OH = 519 g CH3OH
1 mol CH3OH
Exercise-6:
1. In the following reaction to produce
ammonia: N2(g) + 3H2(g) → 2NH3(g)
(a) How many grams of H2
gas are needed to react completely with 454 g of nitrogen gas? (b) How many grams of ammonia will be formed
if 454 g of N2(g) are completely reacted? (c) How many grams of N2 and H2,
respectively, are needed to produce 907 g of ammonia?
2. What mass of oxygen gas is required to
react completely with 680. g of octane, C8H18, in the
following reaction:
2 C8H18(l) + 25 O2(g) → 16 CO2(g)
+ 18 H2O(g)
3. Baking soda, NaHCO3, and milk
of magnesia, which is an aqueous suspension of magnesium hydroxide, Mg(OH)2(s),
are used as antacids. Both neutralize
excess hydrochloric acid secreted by the stomach. (a) Write a balanced equation for the
reaction: (i) between NaHCO3(s) and HCl(aq); (ii) between
Mg(OH)2 and HCl(aq).
(b) How many moles of
HCl(aq) will be reacted by: (i) 10.0 g of NaHCO3;
(ii) 10.0 g of Mg(OH)2 ? Which is the more effective antacid,
NaHCO3 or Mg(OH)2?
__________________________________________________________________________________
Reactions Involving Limiting Reactant
When reactants are mixed in stoichiometric quantities, all
reactants will be completely consumed
at the same time. At the end of the
reaction, only products but none of the reactants exist. For example, in the reaction,
CH4(g) + H2O(g) → CO(g) + 3 H2(g)
if
exactly 16 g of CH4 and 18 g of steam, (or in other quantity of
similar proportion), are allowed to react completely, there will not be any
reactants remains at the end of the
reaction. The products are 28 g of CO and 6 g of H2 (or other
quantities of similar
proportions).
On the
other hand, if 16 g of CH4 are mixed with 25 g of steam and allowed
to react completely, all of CH4
and only 18 g of steam will be completely reacted, but 7 g of steam will remain
in excess. We say that CH4 is
a limiting reagent because it gets used up
before steam. The products of
this reaction are 28 g CO and 6 g H2. The yield of products formed
is determined by the amount of limiting reactant that is reacted.
Steps for solving stoichiometric problems involving
limiting reactants
1. Write a balanced equation for the
reaction
2. Convert given masses, in grams, of
reactants into moles.
3. Compare the mole ratio of available
reactants to the stoichiometric mole ratio according to the balanced equation.
Then determine which reactant is the limiting reagent.
4. Alternatively, you may calculate the
theoretical yield of the product based on the available amount of each
reactant. The one that gives the lower product yield is the limiting reactant.
Exercise-7:
1. In the
following reaction to produce ammonia: N2(g) + 3 H2(g) → 2 NH3(g)
(a) if 91.3 g of
nitrogen gas and 18.0 g of hydrogen gas are mixed, which is the limiting reactant?
(b) How many grams of
ammonia are formed when the limiting reactant is completely reacted.
2. In a thermite reaction,
31.9 g of iron(III) oxide, Fe2O3, is reacted with 12.0 g
of aluminum. (a) Which is the limiting
reagent? (b) How many grams of iron are
formed in this reaction?
2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s)
__________________________________________________________________________________
Theoretical Yield, Actual Yield, and Percent Yield
Theoretical yield = the expected amount
products formed based on the amount of limiting reactant.
· Actual yield = actual amount of product
obtained in an experiment;
· Percent Yield =
(Actual Yield/Theoretical Yield)
x 100%
Exercise-8:
1. 685 g of CO(g)
is reacted with excess of H2(g) according to the following equation:
2 H2(g) + CO(g) → CH3OH(l)
(a) Assuming that the reaction goes
to completion, calculate the theoretical yield of methanol. (b) If
357 g of CH3OH is actually obtained, what is the percent yield of
methanol in this reaction?
2. Silicon (Si)
is prepared from silicon tetrachloride, SiCl4, by reaction with
magnesium metal according to the following equation:
2 Mg(s) + SiCl4(l) → 2 MgCl2(s) + Si(s)
(a)
How many grams of silicon are expected when 454 g of SiCl4
are reacted with an excess of magnesium?
(b) If only 60.5 g of silicon are
actually obtained, what is the percent yield of silicon from this reaction?
__________________________________________________________________________________
REFERENCES:
Ilao, L., Lontoc, B.,
& Gayon, E. E. (2016). General
Chemistry 1. Quezon City, Ph: Rex Bookstore, Inc.
Kim, A. (2020, 5 31).
Clipartkey. Retrieved from
clipartley.com/view/Ttibwnjk
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