American Chemical Society
 Join Schrenk Society! 
Who: Anyone with a love of chemistry!
Where: Schrenk 139
When: Mondays at noon
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.
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
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
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.
In a chemical reaction
H = H products – H reactants
If H <0, then qp <0
The reaction is Exothermic.
Heat goes from the system
into the surroundings.
If H >0, then qp >0
The reaction is Endothermic.
Heat goes from the
surroundings into the system.
An example of an exothermic reaction: http://www.youtube.com/watch?v=rdCsbZf1_Ng
An example from the S&T mining dept: http://www.youtube.com/watch?v=CIGJPWAynDQ
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:
C
heat absorbed
q

increase in tempera ture T
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.
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
C
Cs 
m
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
C
Cm 
n
Specific Heat, Cs
Substance
(cal/gram°C)
(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.
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.
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)
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.,
ΔHfreez = - ΔHfus
solid
liquid
Melting (fusion) is an endothermic process
H fus  H m (liquid )  H m ( solid )
Calorimetry
A calorimeter can be created by doing
something as simple as inserting one
Styrofoam cup inside another.
Science of
measuring heat
based on
observing the
temperature
change when a
body absorbs
or loses
energy as heat.
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)material = Cswater(mass)water(Tfinal-Tinitial)water
Note: The heat capacity is
related to the atomic mass and
the intermolecular forces in the
material.
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.
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) --> x CO2 + y/2 H2O + heat
The Computer Display Setup for the Experiments
50
Temperature (oC)
40
30
20
10
0
100
200
300
400
500
600
700
800
900
1000
Time (seconds)
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.)
The Heat of Neutralization Experiments
50
HCl (or acetic acid)
and NaOH mixed,
reaction begins
Temperature (oC)
40
30
Reaction is completed,
heat released, begin
slow cooling to ambient
20
10
0
100
200
300
400
500
600
700
800
900
1000
Time (seconds)
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)
The Heat of Fusion Experiment
50
Temperature (oC)
40
30
Melting complete,
begin slow warming
Ice cube added
20
10
0
100
200
300
400
500
600
700
800
900
1000
Time (seconds)
IMPORTANT: Use only 1 ice cube, the entire cube must melt.
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.)
Hazards
HCl, strong acid, corrosive
CH3CO2H, weak acid, corrosive
(neutralize acid spills with solid NaHCO3)
NaOH, strong base, pH>14, 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.