7‐1 CHEM 0011
Experiment 7 – Energy Accompanying Reactions
Objectives
1. To observe the techniques involved to remove water from a hydrated salt,
copper (II) sulphate pentahydrate, CuSO45H2O, quantitatively.
2. To calculate the percentage of water in copper (II) sulphate pentahydrate,
CuSO45H2O, theoretically and experimentally.
3. To classify reactions as endothermic or exothermic.
Apparatus:
Part A - Instructor Demonstration:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Porcelain crucible with lid
Tongs
Triangle
Ring clamp
Bunsen burner and lighter
Desicooler
Spatula
Analytical balance
Solid copper (II) sulphate pentahydrate, CuSO45H2O
Part B
Apparatus:
1.
2.
3.
4.
4 test tubes
Test tube rack
Stir rod
Water bottle
Solids:
1.
2.
3.
4.
Solid anhydrous copper (II) sulphate, CuSO4
Solid ammonium chloride, NH4Cl
Solid potassium hydroxide, KOH
Solid potassium nitrate, KNO3
Updated November 2014 7‐2 Introduction
Salts are compounds composed of a metal ion plus a non-metal (or polyatomic)
ion, e.g., sodium chloride (NaCl), and sodium phosphate (Na3PO4).
Hydrated salts (or Hydrates) are salts, which have a definite amount of water
chemically combined. Some common hydrates are:
CuSO45H2O Copper (II) sulphate pentahydrate
MgSO47H2O Magnesium sulphate heptahydrate
CoCl26H2O Cobalt (II) chloride hexahydrate
SnCl22H2O Tin (II) chloride dihydrate
The dot indicates an attractive force between the polar water molecules and the
positively charged metal ion. On heating, the attractive forces are overcome and
the water molecules are released leaving behind the anhydrous salt.
CuSO 4 5H2O (s)
hydrated
copper (II) sulphate

CuSO 4(s) + 5H2O (g)
anhydrous1
copper (II) sulphate
The water released on heating is called the water of hydration. Since this is a
heat absorbing process, the reaction is endothermic. (In an "exothermic"
reaction heat is liberated.)
In Part A, your instructor set up the apparatus to carry out the proper techniques
to remove the water molecules from CuSO45H2O. Gentle heating will be carried
out by holding the Bunsen burner in a sweeping motion. Care will be taken to
avoid letting the flame of the Bunsen burner rest on one spot of the crucible.
CuSO45H2O decomposes at temperatures greater than 560oC. When
CuSO45H2O decomposes a black solid, CuS, will formed. When this happens, it
will introduce experimental error in the calculation of percent water in
CuSO45H2O.
A weighed sample of CuSO45H2O will be heated in a porcelain crucible at
temperatures slightly above 100oC until all of its water content will be driven off as
steam. After the heating, the mass of the anhydrous CuSO4, which remains in the
crucible, will be determined. According to the chemical equation, we will observe
a colour change and a decrease in mass after heating since the molar mass of
1
anhydrous means “without water”
Updated November 2014 7‐3 anhydrous CuSO4 is less than the molar mass of CuSO45H2O. The loss in mass
will be the amount of water that is driven off as steam. Therefore, using the
following equation and experimental data, the percentage of water in the
CuSO45H2O sample can be determined.

0
0
1
d
e O
s 2
a H
e 5
l
e
4
r
r O
e
S
t
a
u
w C
f o
f
o
s s
s
s
a a
m m
4
O
2
H
5
O
S
u
C
n
i
r
e
t
a
w
%



The percentage of water in CuSO45H2O can be determined theoretically using
the following equation.


0
0
1

O
r H2
e
t
5
a
w 4
f
o
s
s
a
m
r
a
l
o
m
5

O
S
u
C
f
o
s
s
a
m
r
a
l
o
m
4
O
2
H
5
O
S
u
C
n
i
r
e
t
a
w
%

In Part B, you will study the dissolving process of several compounds and
classifying them as endothermic or exothermic. The dissolving process of
anhydrous CuSO4 is the formation of the hydrated form, CuSO45H2O.
CuSO4(s) + 5H2O(l)
CuSO4 5H2O(aq)
Notice that the phase of the anhydrous CuSO4 is a solid, (s). The phase of the
dissolved CuSO4 in water is aqueous, (aq).
The chemical equation of the dissolving process of solids that are not hydrates is
simply a change in phase. For example,
NH4Cl (s)  NH4+ (aq) + Cl- (aq)
When the dissolving process is endothermic, the temperature of the solution will
decrease as it is accompanied by a net absorption of energy. Heat can be
included on the left-hand side of the chemical equation. When the
dissolving process is exothermic, the temperature of the solution
will increase as it is accompanied by a net release of energy. Heat
can be included on the right-hand side of the chemical equation.
An application of an endothermic reaction is used to manufacture
athletes’ chemical ice packs, which usually contains water and a
packet of ammonium chloride. The cold pack is activated by
breaking the barrier which separates the ammonium chloride, and
water. Once the barrier is broken, ammonium chloride begins to
dissolve in water, and almost immediately, the pack becomes cold
as the chemical reaction absorbs heat from its environment.
Updated November 2014 7‐4 Practice Question – to be attempted before lab
1) The combustion of propane (CH3CH2CH3) produces carbon dioxide, water
vapour and heat. Write the balanced chemical equation for this reaction and
include the heat on the correct side of the reaction.
2) Write the word equation for the following two reactions and determine if the
reactions are endothermic or exothermic:
a) H2(g) + Cl2(g)  2HCl(g) + 185 kJ
b) 285 kJ + 3O2(g)  2O3(g)
Updated November 2014 7‐5 Procedure:
Part A - Removing Water from a Hydrated Salt
Instructor Demonstration
1. Obtain a clean crucible and lid. Inspect the crucible for cracks.
2. Use an analytical balance and determine the mass of the empty crucible.
Record the mass of the empty crucible to four decimal places on the data
sheet. Record the mass of the empty crucible to four decimal places on the
data sheet.
3. Use an analytical balance and weigh approximately 3.9 to 4.1 grams of
CuSO45H2O into the crucible. Record the mass of the crucible and the
hydrate to four decimal places on the data sheet.
4. Place the crucible on the clay triangle and heat gently by holding the
Bunsen burner in a sweeping motion for about 12 minutes.
5. Allow to cool to room temperature inside the desicooler and weigh the
crucible and the content. Record the mass of the crucible and the content
(after 1st heating) to four decimal places on the data sheet.
6. After weighing, place the crucible back on the clay triangle and reheat
content for about 5 minutes. Allow to cool and reweigh. Record the mass
of the crucible and the content (after re-heating) to four decimal places on
the data sheet. If you have done a good job in heating, the mass of the
crucible and residue after the 1st heating and the 2nd heating should not
differ by more than 0.01 gram.
Procedure:
Part B - Energy Changes Associated with Substances Dissolving in Water
1. Place 4 large test tubes in a test tube rack. Label the test tube as #1, #2,
#3, and #4.
2. Fill each test tube with 5 mL of distilled water.
3. Use a thermometer and record the temperature of the water in one of the
test tubes. You may assume that the water is the same temperature of the
other test tubes.
4. Use a spatula to place a heaping scoop of NH4Cl in the first test tube. Mix
with a glass rod to dissolve it and record the maximum or minimum
temperature reached. You need to add enough salt to observe a
Updated November 2014 7‐6 temperature change.
5. Clean the thermometer by rinsing with water and then wiping it dry.
Repeat step 4 for KOH, KNO3, and anhydrous CuSO4 using the other 3
test tubes.
Updated November 2014 7‐7 Datasheet:
Part A - Removing Water from a Hydrated Salt
1. Mass of empty crucible
2. Mass of crucible and
CuSO4·5H2O
3. Mass of crucible and content
(after 1st heating)
4. Mass of crucible and content
(after re-heating)
5. Mass of CuSO4·5H2O used
6. Mass of anhydrous CuSO4
which remains after heating
7. Mass of water released
Updated November 2014 7‐8 Datasheet:
Part B - Changes Associated with Substances Dissolving in Water
Observations
Test tube #1
NH4Cl
Temperature
of the water
in the test tube
Appearance
of the solid
Appearance of
the solution
Maximum or
minimum
temperature the
solution reached
as the solid
dissolved
Classify the
dissolution
process as
endothermic or
exothermic
Updated November 2014 Test tube #2
KOH
Test tube #3
KNO3
Test tube #4
CuSO4
7‐9 Calculations:
Calculation of % water in CuSO45H2O (experimental):
Calculation of % water in CuSO45H2O (theoretical):
Updated November 2014 7‐10 Percent Error Calculation
Using the results from the two previous calculations, the % error for the experimentally determined
percent water in CuSO45H2O can be carried out using the following equation:
% Error = | theoretical % water in CuSO45H2O - experimental % water in CuSO45H2O | * 100
theoretical % water in CuSO45H2O
Show work here and report your answer with proper number of significant figures.
Updated November 2014 7‐11 Questions:
1. Write the balanced chemical equation which occurred in:
(a) Part A. For an endothermic reaction, include heat on the left-hand side
of the equation. For an exothermic reaction, include heat on the righthand side.
(b) Part B. For endothermic reactions, include heat on the left-hand side of
the equation. For exothermic reactions, include heat on the right-hand
side. e.g. An endothermic reaction is written as:
heat + MX(s)  M+(aq) + X-(aq)
(i) NH4Cl
(ii) KOH
(iii) KNO3
(iv) Anhydrous CuSO4
Updated November 2014 7‐12 2. Calculate the percent water in sodium dichromate dihydrate,
Na2Cr2O72H2O.
Updated November 2014