Download CHEM MINI-COURSE SERIES M1.2___

EXPERIMENT NO. 5
Name _______________________
Class time_____
Chemical Reactions
In this Learning Activity Packet (LAP), you will begin to study chemical reactions, a topic which could be
considered the heart of chemistry. You will learn (1) why there is a need to balance chemical equations, (2)
how to balance simple chemical equations, and (3) how to classify different types of chemical reactions. You
will also conduct several chemical reactions, and learn to identify new substances produced in chemical
changes. You will then write and balance the chemical equation corresponding to each reaction.
After completing this exercise, you should be able to do the following:
(1) Discuss chemical reactions in terms of the reactants, the products, and the energy involved.
(2) Explain how the law of conservation of mass and energy relates to chemical reactions.
(3) Describe four types of chemical reactions: combination, decomposition, single replacement and
double replacement; classify given chemical equations as to type.
(4) Balance simple chemical equations.
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I. Components of a Chemical Reaction
Chemical reactions are processes in which elements and/or compounds react to make other elements and/or
compounds. A chemical equation describes the essence of a chemical reaction. Most chemical equations
take this general format:
Reactant(s)
original substance(s)
Product(s)
arrow meaning "reacting to produce"
new substance(s)
Every chemical reaction follows the law of conservation of mass and energy, which describes the fact that
mass and energy are neither created nor destroyed through a chemical change. This means a correct
chemical equation must show the fact that no atom can be destroyed or created; i.e., the same number of
each type of atoms must appear on both sides of an equation. The atoms merely rearrange or regroup into
different elements or compounds; they will not change into other atoms or be lost through a chemical
reaction. For example, hydrogen gas and oxygen gas react to produce water. The following equation
describes this chemical change. The (g) or (l) next to each formula indicates whether the substance is a gas,
(g), or a liquid, (l). If the substance were aqueous, or dissolved in water, the notation would be (aq) and for a
solid the notation would be (s).
H2(g) + O2(g) 
H2O(l) (unbalanced)
This equation as written is unbalanced. On the left side of the arrow (reactant side), there are 2 atoms of
oxygen. On the right side of the arrow (product side), there is only one atom of oxygen. A quick-and-easy
attempt to balance the two sides may be to add a subscript to the H2O formula, so it appears as H2O2. Doing
this, however, will change the reaction into one that produces hydrogen peroxide, H 2O2, instead of water,
H2O. This is a serious mistake! Never try to balance an equation by changing subscripts in any formula!
Rather, balance this equation in two steps. First, place a "2" in front of H2O. (A number placed in front of a
chemical formula in a chemical equation is called a coefficient. ) This step equalizes the number of oxygen
atoms on both sides.
H2(g) + O2(g) 
2 H2O(l) (unbalanced)
Second, place a coefficient, "2", in front of the H2 at the left side of the arrow:
2 H2(g) + O2(g) 
2 H2O(l) (balanced)
Now both sides of the equation have four H atoms, and two O atoms. The equation is balanced!
A chemical equation may also show the energy involved in the reaction. Energy exists in many forms; there
are kinetic energy (related to motion), chemical energy (related to an element or compound), heat, light, etc.
Total energy remains a constant for every chemical reaction. Energy is neither created nor destroyed in any
chemical change; it may, however, change from one form to another.

An exothermic reaction releases the chemical energy originally present in the reactant(s), to the
surrounding environment.

An endothermic reaction absorbs energy from the surrounding environment, and converts it into
chemical energy of the product(s).
For example, when hydrogen and oxygen react to produce water, heat is released. This is an exothermic
reaction. A greater amount of chemical energy is present in 2H2 and O2 (the reactants) than in 2H2O, the
product. Therefore, the reaction releases energy to the surroundings in the form of heat. The diagrams and
equation in Figure 1 illustrate the energy conversion process in this reaction.
Figure 1. Reaction Diagram For The Formation of Water from Hydrogen and Oxygen.
2 H2


O2
2 H2 O

energy (heat) !!
greater chemical
energy
Energy difference turns
into heat and is released!
2H2 + O2
Energy
less chemical
energy
2H2O
reactants

product
II. Chemical Equations
A. Compounds and Their Formulas
Before proceeding further with the study of chemical reactions, you should become more familiar with
compounds and their formulas. The following discussion and hands-on exercise foucs on how to describe a
compound: both in terms of the correct chemical formula and its appearance.
Compounds are composed of two or more elements. For example, ammonia is a compound made by three H
atoms and one N atom, with the formula NH3. Each compound has a definite composition; that is, the
proportions of elements in a compound are specific to each compound; they can never change. This is why
each compound has a definite formula which specifies how many atoms of each element are present in one
unit of the compound.
For example, H2O and H2O2 are different compounds because the proportions between hydrogen and
oxygen atoms differ, as indicated in the formulas. H2O is water and H2O2 is hydrogen peroxide.
Q1. Examine the rack of compounds in test tubes labeled “H” through “O” and complete Table 1.
Table 1.
Some Compounds and Their Formulas
Name
Formula
Physical Appearance
H. Sodium Chloride
NaCl
I. Sodium Bromide
NaBr
J. Cobalt(II)Chloride
CoCl2
K. Iron(III) Oxide
Fe2O3
L. Copper(II)Sulfate
pentahydrate
CuSO4·5H2O
M. Barium Chloride
dihydrate
BaCl2·2H2O
N. Cobalt(II)Nitrate
Co(NO3)2
O. Calcium Hydroxide
Ca(OH)2
Elemental Composition
1 sodium and 1 bromine
Deep blue crystals (May become
pale blue upon losing H2O.)
1 barium, 2 chlorine, 2 oxygen and
4 hydrogen
1 calcium, 2 oxygen, 2 hydrogen
* Compounds present in test tubes L and M are hydrates. A hydrate is a compound with H2O built into its
crystal structure; this is indicated by a dot (·) between the compound and the number of H2O present in the
formula. The same compound becomes anhydrous when it loses all the H2O in its crystal structure.
B. Balancing Chemical Equations
The following is a list of equations describing some common chemical reactions. These equations need to be
balanced according to the law of conservation of mass. Here are some hints to help you balance them.

Add whole numbers, or coefficients, in front of formulas to balance the number of each type of atoms
on both sides of the equation. A coefficient pertains to the entire formula. For example, 2 NH 3 denotes
two nitrogen atoms and six hydrogen atoms. If no number appears in front of a formula, the coefficient
"1" is implied.

Never, never change subscripts in any formula!

Balance one, and only one, element at a time.

Start with an element that appears only in one formula on each side of the arrow. (Avoid starting with
O, oxygen, for example. It appears frequently in many formulas, and is difficult to balance first.)

After balancing an element, recheck if previously balanced elements are still balanced. If not, change
one or more previously assigned coefficients to re-establish balance.
Q2. Balance the following chemical equations.
(a) _____ Mg + _____ O2  _____ MgO
(b) _____ Zn +
_____ HCl  _____ ZnCl2 + _____ H2
(c) _____ Fe + _____ O2  _____ Fe3O4
(d) _____ N2 + _____ H2 
_____ NH3
(e) _____ KClO3  _____ KCl + _____ O2
(f) _____ Fe + _____ H2O  _____ Fe3O4 + _____ H2
(g) _____ Cu(NO3)2 + _____ Na2S  _____ CuS + _____ NaNO3
(h) _____ Al(OH)3 + _____ HNO3  _____ Al(NO3 )3 +
(i) _____ BaCl2 + _____ H2SO4  _____ BaSO4 +
(j) _____ H2CO3  _____ H2O +
_____ H2O
_____ HCl
_____ CO2
C. Four Types of Chemical Reactions
A simple way to understand chemical reactions is to recognize that many reactions belong to one of four
common types. These types are named according to the way reactants and products behave in a reaction.
They are summarized in Table 2.
Q3. Under the “Examples” column in Table 2, Place ALL letters, a to j, from the reactions in Q2 under
the ”Examples” column that should fit the description and format of each reaction type.
Table 2. Four Types of Chemical Reactions
Type of Reaction
Description
Format
Combination
Two reactants combine to give
the product.
A+B C
Decomposition
One reactant breaks down into
two or more products.
A B + C
Single
Replacement
One reactant replaces a part of
the second reactant.
A + BX  B + AX
Double
Replacement
Two reactants “switch”
partners with each other.
AX + BY  AY + BX
Examples (from Q2)
III. Chemical Reactions In Action
The one common feature of all chemical reactions is that they all produce new substances. These new
substances have distinctly different properties when compared to the reactants, and they have new chemical
formulas. In the laboratory, we may identify the new substances formed in a chemical reaction by noting
changes in properties as the reaction proceeds. Examples of such changes include: color changes, the
appearance of a solid, or of a gas, or the detection of a different odor, and/or changes in temperature. Of
course, some chemical reactions proceed without any apparent changes. In these cases, chemists need the
help of various instruments and probes to detect and identify new substances.
Q4. Based on the discussion in the previous paragraph, list at least five common ways by which new
substances can be identified in chemical reactions.
(a) _________________________________________________________________________
(b) _________________________________________________________________________
(c) _________________________________________________________________________
(d) _________________________________________________________________________
(e) _________________________________________________________________________
In the following hands-on exercises, you will conduct four chemical reactions. Practice being a keen
observer to identify the new substance(s) formed. Refer to the list you made in Q4 as you conduct each
reaction. You may start with any of the following sections and continue to do another one, until you
complete all four reactions.
REACTION A
Put on Safety Goggles!!
Deliver 3 mL of hydrochloric acid solution, HCl(aq) into a clean test tube. The subscript (aq) denotes aqueous
solution, indicating that HCl is dissolved in water. Obtain a piece of magnesium with a pair of tweezers.
Drop the metal into the acid solution. Observe the contents of the test tube.
Q5. Complete the following concerning reaction A.
(a) At least one new substance (with properties distinctively different from the reactants) can be identified in
every chemical reaction. Describe a new substance produced in this reaction in terms of color, physical
state (liquid, solid, or gas), and any other notable characteristics.
__________________________________________________________________________________
(b) Reactants are being consumed during all chemical reactions. Can you tell if this is taking place in this
particular reaction? ______ Explain.
____________________________________________________________________________________
(c) In Reaction A, the reactants are _______________________________________; the products are
aqueous magnesium chloride and hydrogen gas.
(d) Write a balanced equation for this reaction. _______________________________________________
(e) Classify this reaction according to type. __________________________________________________
Empty the content of this test tube into a designated waste jar. Clean the test tube with a test tube brush and
tap water.
REACTION B
Put on Safety Goggles !!
Deliver 1 mL lead nitrate solution (Pb(NO3)2 ), into a clean test tube. Deliver 1 mL potassium iodide solution
(KI) to the same test tube. Observe the contents of the test tube.
Q6. Complete the following.
(a) Chemists use the word precipitation to describe what you have observed. One of the new substances
formed in this reaction is a precipitate, which refers to a solid appearing from a clear solution. Fine
crystals of a solid, PbI2, cause the originally clear solution to turn “cloudy.” Describe other
characteristics of this precipitate.
_________________________________________________________________________________
(b) The reactants of Reaction B are __________________________________________; the products are
aqueous potassium nitrate and solid lead(II)iodide.
(c) Write the balanced equation for this reaction. ____________________________________________
(d) Classify this reaction according to type. ________________________________________________
(e) Set this test tube in a test tube stand for la few minutes. Then describe and record any changes observed.
Empty the entire content of this test tube into a designated waste jar. Clean the test tube with a test tube
brush and tap water.
REACTION C
Put on Safety Goggle !!
Note: Obtain proper instruction on heating with a Bunsen burner before starting this exercise.
Obtain a clean, dry test tube. With a metal spatula, transfer a
small amount (about the size of a pea) of ammonium
carbonate, (NH4)2CO3, into this test tube.
Bring this test tube, a test tube clamp and a Bunsen burner to
the fume hood. Start the burner; adjust air and gas inlets to
obtain a double-layered, blue flame. Position the lower part of
the test tube in the outer layer of the blue flame at an angle to
the flame. (See Figure 2.) The mouth of the test tube must be
pointed into the fume hood and away from any persons.
Figure 2.
Heating with
a Bunsen
Burner
Test tube
positioned at
an angle in
the outer layer
Double-layered
blue flame
After a few seconds, carefully detect the odor of a gas emitted from the test tube as follows: position the test
tube about 3 inches below your nose. Gently waft vapor from the mouth of the test tube toward your nose. If
no odor is detected, continue heating and try again. Stop heating when odor is detected.
Examine the upper portion of the interior wall of the test tube for evidences of condensation. Notice changes
taking place with the ammonium carbonate in the test tube. Place leftover ammonium carbonate, if any, in a
designated waste jar. Clean the test tube with a test tube brush and tap water.
Q7. Complete the following:
(a) In Reaction C, the reactant is __________________________________; the products are ammonia,
NH3(g) , gaseous carbon dioxide and liquid water.
(b) Write the balanced equation for this reaction. _____________________________________________
(c) Classify this reaction according to type. _________________________________________________
(d) Is the reaction endothermic or exothermic? _____________ How do you know? __________________
(e) The formula of ammonia is given as one of the products (NH3). Did you detect ammonia? _______
Explain.
____________________________________________________________________________________
(f) Can you tell from your observations that water is one of the products also? _____________ Explain.
____________________________________________________________________________________
REACTION D
Put on Safety Goggles!!
Note: Obtain proper instruction on heating with a Bunsen burner before starting this exercise.
(Be prepared to observe an exciting explosive reaction.) Obtain a small strip of magnesium ribbon, approx.
3 cm in length. In the fume hood, light your bunsen burner. Use your crucible tongs to grip the magnesium
ribbon and hold in the flame of the bunsen burner. The ribbon will ignite and burn with a bright, white light.
Remove from the flame immediately after the ribbon catches on fire and burns. Close the fume hood shield
all the way and carefully watch the process as it continues on its own. Sweep the inside of the fume hood,
collect the waste with a dust pan and dispose of it in a designated container.
Q8. (a) The initial burning with a bright, white flame is a reaction between magnesium and oxygen (O 2) in
the air. Balance this equation.
_____ Mg(s) + _____ O2(g)  ____ MgO(g)
(b) Classify the reaction in (a) according to type. ____________________________________________
(c) Examine the new substance (MgO) as well as the original substances (Mg and O2). Complete Table 3.
Table 3.
Contrasting Properties Between Reactants and Product
Contrasting Property
Original Substance Mg
Original Substance O2
color
colorless
consistency
gas
chemical class
metallic element
New Substance (MgO)
ionic compound
FURTHER STUDIES
Q9. Balance the following chemical equations and classify each as to type listed on Table 2.
(a) This is an explosive reaction occurring when ammonium nitrate (NH4NO3, a fertilizer) is heated.
___ NH4NO3  ___ N2 + ____ O2 + ___ H2O
Type: _________________
(b) This is an exothermic reaction in which sulfuric acid (H2SO4) is neutralized by lye (NaOH).
___ H2SO4 +
___ NaOH  ___ Na2SO4 + ___ H2O
Type: _________________
(c) This exothermic reaction occurs when nitric acid (HNO3) neutralizes magnesium hydroxide, Mg(OH)2.
___ HNO3 + ___ Mg(OH)2
 ___ Mg(NO3)2 + ___ H2O
Type: _________________
(d) Scrap iron (Fe) reacts with stibnite (Sb2S3, a black mineral) to produce pure antimony (Sb).
___ Fe
+ ___ Sb2S3
 ___ FeS + ____ Sb
Type: _________________
(e) Charcoal (C) burns with insufficient oxygen (O2) to produce carbon monoxide (CO), a toxic gas.
___ C + ___ O2  ___ CO
Type: _________________
(f) A precipitation reaction in which a black precipitate (CuS) is formed.
___ Cu(NO3)2 + ___ Na2S
 ___ CuS + ___ NaNO3
Type: ________________
(g) An exothermic reaction in which sulfuric acid (H2SO4) neutralizes aluminum hydroxide (Al(OH)3).
___ Al(OH)3 + ___ H2SO4
 ___ Al2(SO4)3 + ___ H2O
Type: ________________
(h) An endothermic reaction in which limestone (CaCO3) is converted to quicklime (CaO) in a 800C kiln.
___ CaCO3  ___ CaO + ___ CO2
Type: ________________
(i) Copper (Cu, a cheap metal) reacts with silver nitrate (AgNO3, a silver-containing mineral) to yield
metallic silver (Ag, a precious metal).
____ AgNO3 + ____ Cu  ___ Ag + ____ Cu(NO3)2
Type: ________________
(j) A violent, exothermic reaction in which phosphoric acid (H3PO4) is produced.
____ P4O10 + ____ H2O  ___ H3PO4
Type: ________________
Q10. Balance the following chemical equations, and classify each as to type listed on table 2.
(a) ____ K + ____ H2O  ____ KOH + ____ H2
Type: _______________
(b) ____ Ag2CO3 + ____ NaBr  ____ AgBr + ____ Na2CO3
(c) ____ Al + ____ Br2  ____ Al2Br6
(d) ____ NH4NO3
Type: _______________
Type: _______________
 ____ N2O + ____ H2O
Type: _______________
Q11. Explain why all chemical equations need to be balanced.
Q12. (a) Should the product(s) of an endothermic reaction have more or less chemical energy than the
reactant(s) of that reaction? (b) Explain.
Q13. What do all chemical reactions have in common? (List at least three aspects.)
Q14. When hexane, C6H14 (a hydrocarbon and a gasoline component) burns to produce water and carbon
dioxide (CO2).
(a) This reaction would be classified as: ________________________
(b) Identify the reactant(s) and the product(s) of this reaction.
REACTANTS: ________________________
(c) Write a balanced equation for the reaction.
PRODUCTS: ____________________
Calculations based on reaction in (c).
(e) Calculate the theoretical yield of CO2 from 6.58 grams of hexane. _________ g CO2
(f) If the actual yield is 16.5 g, what is the percent yield, % yield? __________ %
(g) If you started with 28.6 g of oxygen (reactant in excess), how much remains? _________ g left