Lesson 32
THERMOCHEMISTRY
Objectives:
-The student will explain state change.
- The student will explain the difference between heat and temp.
- The student will explain what specific heat capacity is.
- The student will perform calculations with specific heat
capacity.
- The student will explain 2 common ways to measure heat and
energy changes in substances.
- The student will apply delta H values to stoichiometric
calculations
I. Heat and Temperature are different
a.
b.
c.
d.
e.
Heat is a measure of the total amount of
energy transferred from an object of high
temperature to one of low temperature.
Temperature is a measure of the average
kinetic energy of random motion of
particles in a substance.
A change in temperature is usually
accompanied by the addition or removal
of energy in the form of heat.
The more particles that are present, the
more heat must be transferred in order
to change the temperature of the
substance.
This study of the changes of heat that
accompany chemical reactions or phase
changes is called Thermochemistry.
II.
a.
b.
c.
d.
e.
Specific heat capacity relates
temperature changes to heat
changes.
Specific Heat Capacity – the amount
of heat energy required to raise the
temperature of one gram of that
substance by one degree Celsius.
The symbol is Cp.
Each pure substance has a
characteristic specific heat capacity.
It must be measured – it cannot be
determined from a formula.
Substances with low specific heat
capacities require little energy to
change their temperature.
Substances with high specific heat
capacities require a great deal of
energy to change their temperature.
f. We can calculate a change in
heat, knowing the mass of the
substance, the specific heat
capacity of that substance, and
the temperature change in the
substance that occurred.
i. H = cp x m x t
ii. t = (Change in temperature)
=Temperaturefinal Temperatureinitial
iii. H = Change in energy or heat,
energy units (or Heat units) are in
Joules.
g. See examples in class.
Thermochemistry
Virtual Lab 2 video
III. Measuring changes in heat
energy in the lab.
a.
We can use a calorimeter to
measure changes in heat
energy.
Two main types
b.
i.
“Coffee-cup calorimeter” – this is
used to measure changes in
temperature which occur during a
reaction, usually occurring in water.
ii. “Bomb calorimeter” – this is used
when a certain amount of
substance is to be oxidized (burned)
rapidly, and the heat given off
measured.
Bomb Calorimeter
IV.
a.
Stoichiometry and heat calculations
i.
We discussed endothermic and exothermic
reactions previously.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
Enthalpy (H) is the heat content of a system at a
constant pressure.
The change in enthalpy for a chemical reaction is
called the Enthalpy of Reaction (Hrxn)
Hrxn = Hproducts - Hreactants
If Hrxn is negative, the reaction is exothermic (gives
off energy)
If Hrxn is positive, the reaction is endothermic
(absorbs energy)
Example: 4Fe + 3O2  2Fe2O3
Hrxn = -1625 kJ
(exothermic reaction)
Example: NH4NO3  NH4+ + NO3- Hrxn = 27 kJ
(endothermic reaction)
These enthalpy values can also be written as a part of
the equation. If they occur in the reactant side, it is
considered endothermic since energy is being added
to the beginning of the reaction.
If it is written as one of the products, it is considered
exothermic since energy is produced.
Examples:
4Fe + 3O2  2Fe2O3 + 1625 kJ
NH4NO3 + 27 kJ  NH4+ + NO3-
b. The H value, which can be calculated
from experiments, can be used as one
of the components of the mole ratio in
stoichiometry problems.
c. Example:
i.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = 891 kJ
ii. Read: “One mole of CH4 plus 2 moles of O2
yields 1 mole of CO2 and 2 moles of H2O,
and releases 891 kJ of energy.”
iii. Ratio: 1:2:1:2:-891 kJ
iv. If you burned 2 moles of CH4, you would get
2X as much energy.
Questions:
1. Compare and contrast the terms
“heat” and “temperature”.
2. Explain the meaning of the term
“specific heat capacity”.
3. Explain the term “enthalpy”.
4. Explain two ways in which changes
in a substance’s heat can be
measured.
Thermochemistry Problems:
1. How much heat is lost when a solid aluminum
ingot with a mass of 4110 g cools from 660.0oC to
25oC? Al Cp = .9025 J/goC
2. Suppose a piece of iron with a mass of 21.5
grams at a temperature of 100.0oC is dropped into
an insulated container of water. The mass of the
water is 132 grams and its temperature before
adding the the iron is 20.0oC. What will be the
final temperature of the system. Fe Cp = .4494
J/goC
3. How much heat is required to raise the
temperature of 854 g H2O from 23.5oC to 85.0oC?
4. Phosphorus trichloride, PCL3, is a compound used
in the manufacture of pesticides and gasoline
additives. How much heat is required to raise the
temperature of 96.7 g PCl3 from 31.70oC to
69.20oC? the specific heat of PCl3is 0.874 J/goC.
5. Carbon Tetrachloride, CCl4, was a very popular
organic solvent until it was found to be toxic.
How much heat is required to raise the
temperature of 10.35 g CCl4 from 32.1oC to
56.4oC?
CCl4 Cp = .8565 J/goC
6.
7.
8.
9.
10.
11.
If a piece of aluminum with mass 3.90 g and a
temperature of 99.3oC is dropped into 10.0 cm3 of
water at 22.6oC, what will be the final temperature of
the system? (Recall the density of water is 1.00 g/cm3.)
The color for many ceramic glazes comes from
cadmium compounds. If a piece of cadmium with mass
65.6 g and a temperature of 100.0oC is dropped into
25.0 cm3 of water at 23.0oC, what will be the final
temperature of the system. Cd Cp= .2311 J/goC
A piece of an unknown metal with mass 23.8 g is
heated to 100.0oC and dropped into 50.0 cm3 of water
at 24.0oC. The final temperature of the system is
32.5oC. What is the specific heat of the metal?
How many joules are required to heat 692 g of nickel
from 22oC to 318oC? Ni Cp = .4442 J/goC
How many joules are required to heat 18.2 g of tin from
14.7oC to 47.7oC? Sn Cp = .2274 J/goC
Dysprosium was discovered in 1886. Its freezing point
is 1400o C and its boiling point is 2600oC. If its specific
heat is 0.1733 J/goC, how many joules are required to
heat 10.0 g from its freezing point to its boiling point?
Dy Cp = .1733 J/gCo
12. Copper has a specific heat of 0.384 52 J/goC. A 105-g
sample is exposed to 15.2 kJ in an insulated container.
How many degrees will the temperature of the copper
sample increase? Cu Cp = .385 J/goC
13. Glass, which is mostly SiO2, is not a good insulator.
How much energy does a 1400-g pane of glass lose as
it cools from a room temperature of 25oC to an outside
temperature of 5.0oC? Si Cp = .7121 J/gCo
14. A swimming pool, 10.0 m by 4.00 m, is filled to a depth
of 2.50 m with water at a temperature of 20.5oC. How
much energy is required to raise the temperature of the
water to a more comfortable 30.0oC?
15. Fatty tissue is 15% water and 85% fat. When fat is
completely broken down to carbon dioxide and water,
each gram releases 9.0 kilocalories of energy. How
many kilocalories are released by the loss of 2.2
kilograms of fatty tissue? (Assume that the fat is
completely broken down.)
16. What would the final temperature of the system be if
21.2 grams of iron is added is added to 50.0 ml of water
in a closed system. The water is 25oC and the metal is
140oC before they are added together. Cp of water=
4.184 Cp of Iron = .4494 J/goC.