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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.
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.
An example of an exothermic reaction:
If H >0, then qp >0
The reaction is Endothermic.
Heat goes from the
surroundings into the system.
http://www.youtube.com/watch?v=rdCsbZf1_Ng
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 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 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. If we use
KOH, what salt will form?
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
Cs material (mass)material (Tfinal-Tinitial)material = Cs water (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.
Have you ever wondered about how they determine the
calories in food? They use a Bomb Calorimeter. It can be
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
Bomb Calorimeter
An example of an exothermic reaction
from the S&T mining dept:
http://www.youtube.com/watch?v=CIGJPWAynDQ
The Computer Display Setup for Today’s Experiments
50
Temperature (oC)
40
If probe
displays less
than 15 oC,
notify your TA.
30
20
10
0
100
200
300
400
500
600
700
800
900
1000
Tim e (s econds )
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
Mixture not stirred fast
enough – Resulting
line is not vertical.
Temperature (oC)
40
30
Reaction is completed,
heat released, begin
slow cooling to ambient
20
HCl (or acetic acid)
and NaOH mixed,
reaction begins
10
0
100
200
300
400
500
600
700
800
900
1000
Tim e (s econds )
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
40
Temperature (oC)
Mixture not stirred fast
enough – Resulting line
is not vertical.
Ice cube added
30
20
Melting complete,
begin slow warming
10
0
100
200
300
400
500
600
700
800
900
1000
Tim e (s econds )
IMPORTANT: Use only 1 ice cube, the entire cube must melt.
Checkout
1 - Calorimeter (Thermos)
– Return it to the stockroom after experiment.
1 - styrofoam cup
– Return it to the stockroom after experiment.
Reagents in Lab
_____M HCl (record concentration)
_____M CH3CO2H (record concentration)
_____M NaOH (record concentration)
Important:
Use distilled water from carboys*,
NOT from the tap.
(*Distilled water from the tap is normally not at room temp.)
 Next Week (March 12-15) – No Class (Spring Recess) 
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 “Heat of Neutralization”
For March 19-22
*Thermochemistry pp 11, 13, 15, & 17
and a calculations page are due.
*Read over “Radiochemistry”
pp 19-32 green book