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Join Schrenk Society!
Who: Anyone with a love of chemistry!Where: Schrenk 139When: Mondays at noon
American Chemical Society
Want to get involved?Interested in Chemistry?
Purpose of the Experiment
Thermochemistry(Heat of Reaction)
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.
Thermodynamic Definition of Enthalpy (H):
H = E + PV
E = energy of the system
P = pressure of the system
V = volume of the system
Definition of Enthalpy
What is the Heat of Reaction?
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 -PV for w.
E = qp - PV
Then if we want to solve for the heat transferred, qp, at constant pressure, we simply rearrange the equation.
qp = E + PV
At Constant Pressure
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 + PVThen
H = qp
The change in enthalpy, H, is then equal to the heat transferred at constant pressure, qp.
Enthalpy = Heat Transferred
In a chemical reactionH = H products – H reactants
If H >0, then qp >0The reaction is Endothermic.
Heat goes from the surroundings into the system.
If H <0, then qp <0The reaction is Exothermic.Heat goes from the system
into the surroundings.An example of an exothermic reaction:
An example from the S&T mining dept: http://www.youtube.com/watch?v=rdCsbZf1_Nghttp://www.youtube.com/watch?v=CIGJPWAynDQ
Heat Capacity, C
T
q
turein tempera increase
absorbedheat 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.
Specific heat capacity: The energy (joules) required to raise the temperature of 1 gram of substance by 1C
Unit: J g-1K-1 or J g-1C-1
Molar heat capacity: The energy (joules) required to raise the temperature of 1 mol of substance by 1C
Unit: J mol-1 K-1 or J mol-1C-1
m
CCs =
n
CCm =
SubstanceSpecific Heat, Cs
(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
HClaq + NaOHaq NaClaq + H2O
The reaction between an acid and a basewhich results in a salt plus water.
Another example, cyanic acid and a hydroxide ion.
For example, hydrochloric acid and sodium hydroxide:
acid + base salt + water
Heat of Neutralization
Energy released by reaction = Energy absorbed by solution
Cs = q / [(mass) (Tfinal-Tinitial)]
Net ionic equation for neutralization:
H+(aq) + OH-(aq) H2O(l)
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.
q = Cs (mass) (Tfinal-Tinitial)
This can then be rearranged to solve for the heat transferred.
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
Melting (fusion) is an endothermic process
solid liquid
)()( solidHliquidHH mmfus −=
CalorimetryScience of measuring heat based on observing the temperature change when abody absorbs or loses energy as heat.
A calorimeter can be created by doing something as simple as inserting one
Styrofoam cup inside another.
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 waterCsmaterial(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)
Calorimetry
*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
Time (seconds)0 100 200 300 400 500 600 700 800 900 1000
Temperature (oC)
10
20
30
40
50
The Computer Display Setup for the Experiments
Parameters:Temperature: 10-50 oCTime: 0-1000 seconds
(Check: Probe should display 15-25 oC resting on lab bench and should read higher when warmed by hand.)
Time (seconds)0 100 200 300 400 500 600 700 800 900 1000
Temperature (oC)
10
20
30
40
50
Temperature change is important. Exact time is not important. Temperature will drift toward ambient before and after reactionTransition will be faster if NaOH is added rapidly and well stirred.
(That is you will have a more nearly vertical temp. rise)
HCl (or acetic acid)and NaOH mixed, reaction begins
Reaction is completed, heat released, beginslow cooling to ambient
The Heat of Neutralization Experiments
Time (seconds)0 100 200 300 400 500 600 700 800 900 1000
Temperature (oC)
10
20
30
40
50
IMPORTANT: Use only 1 ice cube, the entire cube must melt.
The Heat of Fusion Experiment
Ice cube added
Melting complete,begin slow warming
Checkout1-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.)
HazardsHCl, strong acid, corrosive CH3CO2H, weak acid, corrosive (neutralize acid spills with solid NaHCO3)
NaOH, strong base, pH>14, corrosive
WasteLiquid 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.