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American Chemical Society
Interested in Chemistry?
Want to get involved?
? Join Schrenk Society! ?
Who Anyone with a love of chemistry! Where
Schrenk 139 When Mondays at noon
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Thermochemistry (Heat of Reaction)
Purpose of the Experiment
Determine the heat of neutralization for the
reaction of a strong acid and base and for a
weak acid with a strong base. Determine the heat
of fusion of ice.
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What is the Heat of Reaction?
Definition of Enthalpy
Thermodynamic Definition of Enthalpy (H) H E
PV E energy of the system P pressure
of the system V volume of the system
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At Constant Pressure
Recall, by definition a change in energy equals
heat transferred (q) plus work (w) ?E q
w Consider a process carried out at constant
pressure. At constant pressure, work involves
only a change in volume. We can then substitute
-P?V for w. ?E qp - P?V Then if we want to
solve for the heat transferred, qp, at constant
pressure, we simply rearrange the equation. qp
?E P?V
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Enthalpy Heat Transferred
Recall our original definition of enthalpy H
E PV Then for a change in enthalpy ?H ? E
?(PV) If we set P constant, then
?H ? E P ? V Since qp ?E
P?V Then ?H qp The change in enthalpy, ?
H, is then equal to the heat transferred at
constant pressure, qp.
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In a chemical reaction ?H H products H
reactants
If ?H gt0, then qp gt0 The reaction is
Endothermic. Heat goes from the surroundings into
the system.
If ?H lt0, then qp lt0 The reaction is
Exothermic. Heat goes from the system into the
surroundings.
http//www.youtube.com/watch?vrdCsbZf1_Ng http//
www.youtube.com/watch?vCIGJPWAynDQ
An example of an exothermic reaction An example
from the ST mining dept
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Heat Capacity, C
C is an extensive property so a large object
has a larger heat capacity than a small object
made of the same material.
Using the Equation
Looking at the figures on the left, it can
be seen that the temperature change is constant,
but the heat absorbed by the larger object is
greater. This results in a larger heat
capacity for the larger object because more heat
is absorbed.
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• Specific heat capacity The energy (joules)
  required to raise the temperature of 1
  gram of substance by 1?C
• Unit J g-1K-1 or J g-1?C-1
• Molar heat capacity The energy (joules) required
  to raise the temperature of 1 mol of substance by
  1?C
• Unit J mol-1 K-1 or J mol-1?C-1

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Substance Specific Heat, Cs Specific Heat, Cs
Substance (cal/gramC) (J/kg C)
Pure water 1.00 4,186
Wet mud 0.60 2,512
Ice (0 C) 0.50 2,093
Sandy clay 0.33 1,381
Dry air (sea level) 0.24 1,005
Quartz sand 0.19 295
Granite 0.19 294
1 calorie 4.186 joules
The high heat capacity of water makes it ideal
for storing heat in solar heating systems.
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Neutralization
The reaction between an acid and a base which
results in a salt plus water.
For example, hydrochloric acid and sodium
hydroxide
HClaq NaOHaq ? NaClaq H2O
acid base ? salt water
Another example, cyanic acid and a hydroxide ion.
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Heat of Neutralization
Net ionic equation for neutralization
H(aq) OH-(aq) ? H2O(l)
Energy released by reaction Energy absorbed by
solution
Specific heat capacity, Cs, is defined as the
quantity of heat transferred, q, divided by the
mass of the substance times the change in
temperature. A value of Cs is specific to the
given substance.
Cs q / (mass) (Tfinal-Tinitial)
This can then be rearranged to solve for the heat
transferred.
q Cs (mass) (Tfinal-Tinitial)
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Enthalpy of Fusion (Melting)
Enthalpy of Fusion is defined as the heat that is
absorbed when the melting occurs at constant
pressure. If the substance freezes, the reaction
is reversed, and an equal amount of heat is given
off to the surroundings i.e., ?Hfreez -
?Hfus
solid
liquid
Melting (fusion) is an endothermic process
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Calorimetry
Science of measuring heat based on observing the
temperature change when a body absorbs or loses
energy as heat.
A calorimeter can be created by doing something
as simple as inserting one Styrofoam cup inside
another.
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Calorimetry
A Calorimeter may be used to determine the Heat
Capacity, Cs, of a material by measuring the
temperature change when a known mass of the
material at a higher temperature is placed in a
known mass of water, usually at room temperature,
and the system is allowed to reach a final
intermediate temperature. Heat
lost by hot object Heat gained by cold
water Csmaterial(mass)material(Tfinal-Tinitial)mat
erial Cswater(mass)water(Tfinal-Tinitial)water
Note The heat capacity is related to the atomic
mass and the intermolecular forces in the
material.
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Calorimetry
A Calorimeter may be used in a similar manner to
determine the enthalpy change associated with
other processes, such as

• Chemical reactions (bond energies)
• Phase changes (intermolecular forces)
• Mixing (intermolecular forces)
• Solvation (intermolecular forces)

These are the processes you will be learning
today.
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A Bomb Calorimeter is used to determine the
caloric value of food and of fuels, by burning
them in excess oxygen and measuring the amount
of heat evolved. A basic combustion
reaction CxHy O2(excess) --gt x CO2 y/2
H2O heat
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The Computer Display Setup for the Experiments
Parameters Temperature 10-50 oC Time 0-1000
seconds (Check Probe should display 15-25 oC
resting on lab bench and should read
higher when warmed by hand.)
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The Heat of Neutralization Experiments
HCl (or acetic acid) and NaOH mixed, reaction
begins
Reaction is completed, heat released, begin slow
cooling to ambient
Temperature change is important. Exact time is
not important. Temperature will drift toward
ambient before and after reaction Transition will
be faster if NaOH is added rapidly and well
stirred. (That is you will have a more nearly
vertical temp. rise)
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The Heat of Fusion Experiment
Melting complete, begin slow warming
Ice cube added
IMPORTANT Use only 1 ice cube, the entire cube
must melt.
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Checkout 1-Calorimeter Return to
stockroom. 1-styrofoam cup Return to
stockroom. Reagents in Lab _____M HCl
(record) _____M CH3CO2H (record) _____M NaOH
(record) Important Use distilled water from
carboys, NOT from the tap. (Distilled water
from the tap is normally not at room temp.)
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Hazards HCl, strong acid, corrosive CH3CO2H,
weak acid, corrosive (neutralize acid
spills with solid NaHCO3) NaOH, strong base,
pHgt14, corrosive Waste Liquid waste labeled
Calorimetry or Heat of Neutralization For
October 18-21 Thermochemistry pp 9, 11, 13,
17 and a calculations page are due. Read over
Radiochemistry pp 19-30 remember to bring
your student id.