Calorimetry Lab: Enthalpy of Solution (∆Hsoln) of Ionic Compounds
Dhivya Umasuthan
Tuesday, November 24, 2020
SCH4U0 – Mr.Wang
November 2020
SCH4U0 – Calorimetry Lab: Enthalpy of Solution (∆Hsoln) of Ionic Compounds
Introduction
The process of dissolving a solid in water always has an energy change associated with it. Examples of
dissolving of ionic compounds exist for both endothermic and exothermic enthalpies of solution.
The dissolving process itself is really a two-step process.
1) Lattice dissociation or breaking down the solid crystal. This process is endothermic.
2) Hydration or the surrounding of an ion with a layer of water molecules. This process is exothermic.
For an ionic compound, ionic bonds within the crystal lattice are broken while ion-dipole forces with water are
formed. Ion-dipoles are formed when the slightly positive hydrogen atoms (of the highly polar water
molecules) are attracted to the negative ions. The slightly negative oxygen atoms of the polar water molecules
are attracted to the positive ions. This is the hydration of the ions.
The overall heat of solution depends on the relative amounts of energy involved in these two individual steps –
the lattice dissociation energy and the hydration energy. In this experiment, you will determine the molar
enthalpies of solution for the dissolving of some ionic compounds in water using a simple Styrofoam cup as a
calorimeter.
Prelab assignment
1. For each compound used in this experiment
a. Calculate the number of moles present in a 1.00 g sample of each of the ionic compounds to be used
in this experiment (see Reagents)
b. Write the chemical equation for each compound’s dissociation in water.
Calcium Chloride, CaCl2
Dissociation Equation:
CaCl2  Ca2+ + Cl-
M= 110.98g/mol
n = m/M
n = 1.00g / 110.98g/mol
n = 0.009 mol
Sodium Chloride, NaCl
Dissociation Equation:
NaCl  Na+ + Cl-
M= 58.44g/mol
n = m/M
n = 1.00g / 58.44g/mol
n = 0.017 mol
Ammonium chloride, NH4Cl
Dissociation Equation:
NH4Cl  NH4+ + Cl-
M= 53.491g/mol
n = m/M
n = 1.00g / 53.491g/mol
n = 0.019 mol
Sodium hydroxide, NaOH
Dissociation Equation:
NaOH  Na+ + OH-
M= 39.997g/mol
n = m/M
n = 1.00g / 39.997g/mol
n = 0.025mol
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2. What is calorimetry and draw a basic diagram of a calorimeter.
Calorimetry is a process of determining the amount of heat absorbed or released during a chemical
reaction. The reaction can be differentiated between exothermic (releases heat) and endothermic (absorbs
heat) through measuring the change in heat.
3. Describe two factors that affect the strength of an ionic bond in a compound.
The strength of an ionic bond in a compound would be directly dependant on charge and distance. The
quantity of charges (electronegativity) affects the strength of an ionic bond because a cation with a charge of
2+ would create a stronger ionic bond than a cation with a 1+ charge. The strength also depends on the
distance (atomic radius) between charged particles because larger ions display a farther distance between its
nucleus and electrons which shows that this ionic bond weaker than smaller ions. These factors can be proven
through the trends depicted on a periodic table.
4. a. Define crystal lattice energy and hydration energy
Crystal lattice energy is the measure of the bond strength within an ionic compound. Hydration
energy, otherwise known as hydration enthalpy is the amount of energy released when 1 mole of a gaseous
ion is diluted in sufficient water.
b. Which of the properties in part (a) reduces the solubility of an ionic compound?
When Crystal lattice increases the solubility of an ionic compound would decrease. Crystal lattice
energy represents the disassociation of ions and water molecule. Therefore, it would decrease solubility as a
result.
c. Which property, when increased, increases the solubility?
When hydration energy increases, the solubility increases as well. The hydration energy is the
association of ions and water molecules. Specifically, when the hydration energy is equal or larger than the
lattice energy the substance would be soluble.
November 2020
SCH4U0 – Calorimetry Lab: Enthalpy of Solution (∆Hsoln) of Ionic Compounds
Objectives
 To find the molar enthalpy of solution for group of ionic compounds.
 To relate the enthalpy of solution of an ionic compound to the compound’s crystal lattice energy and
hydration
Apparatus
Balance
Styrofoam cup calorimeter
Thermometer
Goggles
Reagents
Calcium Chlorine, CaCl2
Sodium Chloride, NaCl
Ammonium chloride, NH4Cl
Sodium hydroxide, NaOH
50 or 100 mL graduated cylinder
Safety
1. Sodium hydroxide is extremely corrosive. Eye protection must be worn at all times.
2. Ammonium chloride is a very fine powder that is potentially dangerous if it comes in contact with your
eyes.
Method – CREATE A LAB DOODLE (or FLOW-CHART) to summarize the procedure in your lab report
1. Add 25 mL of distilled water to a Styrofoam cup calorimeter and record the initial temperature of the
water.
2. Using a balance, measure approximately 1.00 g of one of the ionic compounds and add it to the
calorimeter. Record the final (maximum) temperature.
3. Rinse the calorimeter and repeat the method for a different ionic compound.
4. Record all data in a table. Include a description of the physical properties of each compound.
Observations:
Create an observation table (or tables) to record quantitative data you capture in this experiment. For some
hints on what to include in your data table, look at analysis question 1 below:
Ionic Compounds
NaCl
NaOH
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Quantitative Properties
Initial Temperature (Ti)
Final Temperature (Tf)
Change in Temperature (ΔT)
Molar Mass (M)
Mass (m)
Volume (v)
Initial Temperature (Ti)
Final Temperature (Tf)
Change in Temperature (ΔT)
Molar Mass (M)
Mass (m)
Volume (v)
= 22oC
= 20oC
= -2oC
= 39.997 g/mol
= 1.00 g
= 25 mL (0.025L)
= 21oC
= 27oC
= 6oC
= 58.44 g/mol
= 1.00 g
= 25mL (0.025L)
Analysis
1. From your data, calculate the following for each part of the experiment. (make sure it is organized)
a. The temperature changes of the water
NaCl
NaOH
Tf - Ti = ΔT
Tf - Ti = ΔT
20oC - 22oC = ΔT
27oC - 21oC = ΔT
-2oC = ΔT
6oC = ΔT
Therefore, the temperature change of the water of the
ionic compound NaCl is -2oC.
Therefore, the temperature change of the water of the
ionic compound NaOH, is 6oC.
b. The mass of the water
25mL = 25g
Therefore, the mass of the water would be 25g.
c. The quantity of heat absorbed (or given off) by the water during the dissolving, given that specific heat
of water is 4.184 J/g oC
NaCl
NaOH
Given:
Given:
- c = 4.184 J/g oC
- c = 4.184 J/g oC
- m = 1.00g
- m = 1.00g
Q = mc ΔT
Q = (25g)(4.184 J/g oC)(-2oC)
Q = -209.2J
Q = mc ΔT
Q = (25g)( (4.184 J/g oC)( 6oC)
Q = 627.6J
Therefore, heat has transferred from the surroundings
into the system. This is an endothermic reaction
because the heat is -209.2J (negative).
Therefore, heat has transferred from the system to the
surroundings. This is an exothermic reaction because
the heat is 627.6J (positive).
d. The number of moles of solid used
NaCl
Given:
- M = 58.44 g/mol
- m = 1.00g
Given:
- M = 39.997 g/mol
- m = 1.00g
n = m/M
n = 1.00g / 58.44 g/mol
n = 0.017 mol
n = m/M
n = 1.00g / 39.997 g/mol
n = 0.025 mol
Therefore, the number of moles of solid used is
0.017 mol.
Therefore, the number of moles of solid used is
0.025 mol.
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NaOH
e. The quantity of heat involved per mole of solid dissolved (value from part c divided by value from part
d). This is called the molar enthalpy of solution ∆Hsoln. Assign a positive or negative sign to ∆Hsoln
depending in whether you think the dissolving process was exothermic or endothermic.
NaCl
NaOH
Qsurr = - Qrxn
Qsurr = - Qrxn
(-209.2J) = - Qrxn
627.6J = - Qrxn
209.2J = Qrxn
- 627.6J = Qrxn
ΔHrxn = Qrxn/n
ΔHrxn = (209.2J) / 0.017mol
ΔHrxn = 12305.88J/mol
ΔHrxn = Qrxn/n
ΔHrxn = (- 627.6J) / 0.025mol
ΔHrxn = - 25104 J/mol
12305.88 J/mol = 12.306 kJ /mol
- 25104 J/mol = - 25.104 kJ/mol
Therefore, this is an endothermic reaction because
there is an assigned positive sign. The amount of
energy used is greater than the amount of energy
released.
Therefore, this is an exothermic reaction because there
is an assigned negative sign. The amount of energy
used is less than the amount of energy released.
ΔH = Q/n
ΔH = (-209.2J) / 0.017mol
ΔH = - 12305.88J/mol
ΔH = Q/n
ΔH = (627.6J) / 0.025mol
ΔH = 25104 J/mol
- 12305.88 J/mol = - 12.306 kJ /mol
25104 J/mol = 25.104 kJ/mol
Therefore, this is an exothermic reaction because there
is an assigned negative sign. The final temperature also
is less than the initial (the surrounding is absorbing
heat).
Therefore, this is an endothermic reaction because
there is an assigned positive sign. The final
temperature is also greater than the initial (the system
is gaining heat).
2. Write a thermochemical equation for the dissolving process for each solid (include the heat term in each
equation).
NaCl
NaOH
NaCl(s)  Na+(aq) + Cl-(aq) + (12305.88 J/mol)
NaCl(s) + (12305.88 J/mol)  Na+(aq) + Cl-(aq)
NaOH(s) Na+(aq) + OH- (aq) + (- 25104 J/mol)
NaOH(s) + (- 25104 J/mol) Na+(aq) + OH- (aq)
3. For an ionic solid to melt, external energy must be applied to overcome the attractive forces holding its
ions together. For example, sodium hydroxide must be heated to temperatures exceeding 318oC for it to
melt. Despite such a high melting point, sodium hydroxide readily dissolves and dissociates into its ions in
water at room temperature.
a. What is the source of energy required to separate the ions from the solid sodium hydroxide?
The source of energy required to separate the ions from the solid sodium hydroxide is called Hydration
energy, or otherwise known as Hydration Enthalpy. The association of ions and water molecules is considered
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Hydration energy. The energy is the amount of heat which is discharged by the solution (sodium hydroxide),
due to it becoming hydrated when the ion disassociates.
b. Why does the resulting sodium hydroxide solution feel hot?
The resulting sodium hydroxide solution is considered hot because the reaction is exothermic. An
exothermic reaction releases energy through the forms heat and/or light. During this reaction, energy is
released due to the total energy of reactants being higher than the total energy of products. Therefore, heat is
continuously being emitted from the system. The reaction that occurs between the sodium ion and water
molecule feels hot due to the energy release.
4. Magnesium oxide, MgO, and magnesium chloride, MgCl2, are very similar, white ionic solids with the
following properties:
Compound
MgO
MgCl2
Melting Point (oC)
2800
1412
Solubility
Insoluble
Very soluble
a. Give the formula of the ions of each compound
MgO(s)  Mg2+(aq) + 02-(aq)
MgCl2  Mg2+(aq) + 2Cl1- (aq)
b. Account for the drastic difference in physical properties
Metals and non-metals attract each other and order themselves into a lattice formation
Ionic compounds are particles that form when an atom loses an electron or gains an electron (metals and nonmetals). Due to them to attracting to each other, they order themselves into a lattice formation. The high
melting points are equal to the high strength of bonds. MgO has the charges of both +2 and -2 which makes
their ionic bond stronger. Whereas for MgCl2 the charges are +2 and -1 for chlorine making it weaker than
MgO. This meaning that MgCl2 has two separate sets of bonds to break but MgO has only one with two pairs.
This indicates that MgO has a high melting point since it requires more energy to break the bond. The distance
between the ions play a big role determining the compound’s bond strength. MgO has a smaller distance than
MgCl2 which means it is harder to break. Due to small highly charged particles having a stronger bonding
strength differentiate from large slightly charged particles which plays a role in the size effecting the melting
point. The atomic radius of Chloride is bigger because of the increased number of electrons making it apparent
that chlorine is a large particle. This means the bond strength of Magnesium is weak. To sum it up, since the
bond between MgO is strong it takes up more energy to break making the melting point higher.
MgO and MgCl 2 have a significantly different melting point. The melting point of a substance is
the temperature at which it needs to be heated at to start melting. Even though these two
compounds are similar, their physical properties are quite different. This all relates to the
strength and bonding abilities of the compounds. The higher the strength of the bond, the higher
the melting point will be. In this case, MgO is made up of two elements, which are Oxygen and
Magnesium. Oxygen has a charge of -2 while Magnesium has a charge of +2. This results in
their ionic bond being strong since Oxygen needs to gain 2 electrons while magnesium needs to
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lose 2 electrons, making them a perfect match. However, MgCl 2 is made up of Magnesium and
Chlorine. Magnesium has a charge of +2 while Chlorine has a charge of -1, making their bond
weaker than MgO. Since MgO has a stronger bond, it requires more energy to break the bond,
meaning it has a higher melting point. Since MgCl 2 is weaker, it requires less energy to break and
has a lower melting point. In terms of solubility, when a compound is mixed in water, if they
have a weak bond, the water molecules will have a greater attraction for the ions and the
substance will be soluble. However, if the ions in the compound have a greater attraction with
each other, the water molecules will not attract with those ions and the solution will be insoluble.
Since MgO has a very strong bond, it is not soluble. However, for MgCl 2 , the water molecules
have a greater attraction for the ions than the ions have for each other, therefore, it is very
soluble.
5. The value of ∆Hsoln for the formation of an acetone – water solution is negative. Explain this in general
terms that discuss intermolecular forces of attraction.
An exothermic reaction occurred since the solution is negative. This concludes that the system
lost heat and the surroundings gained heat. While this reaction is taking place, the H 2 O
molecules break apart the intermolecular forces that correspond with the acetone compound.
Crystal lattice dissociation is the energy required to break apart intermolecular forces. The
energy which is released when a gaseous element attaches to form a bond is known as lattice
energy. However, Hydration energy decides whether the reaction is exothermic or endothermic.
In this case, the solute (acetone) has weaker intermolecular forces making it exothermic. If the
intermolecular bonds require larger energy the reaction would be endothermic since the system
would need more energy. Acetone evaporates quickly, meaning it has weak intermolecular
forces. However, water evaporates slowly because its molecules are attached using hydrogen
bonding, creating stronger intermolecular forces. Acetone does not have hydrogen bonding,
making its intermolecular forces comparatively weaker.
1. Make a valid conclusion, based on the correct interpretation of the results, with an explanation to the
process that will determine whether a reaction will be endothermic or exothermic.
2. Write a mathematical equation that shows the relationship between ∆Hsoln, Q (heat) and n (moles).
Lab Report Submission Details:
o Pre-Lab
o Lab Report –Doodle or flow-chart of the Procedure, Observations Table(s), Analysis Questions, and
Conclusion
o Lab report is due on: __Sunday November 29th @ 11:50pm (dropbox)____________.
RUBRIC WILL BE POSTED LATER
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