Download Thermochem

Energy and Chemical Change

Energy - capacity to do work or supply heat.
1. potential energy - stored energy
a. positional - where its at
b. compositional - what it is
▪ chemical potential energy - energy
stored in chemicals
2.
kinetic energy - energy of motion
a. heat (q) - energy transferred between two
objects of differing temperature.
▪ Enthalpy (H) - heat content of a system
at constant pressure.
b. light, electrical energy, mechanical, ...........

Heat (q) – type of energy transferred because
of a difference in temperature.
 Can’t be measured directly
 Flow of energy from a warmer object to a cooler
object

Temperature – measure of the average
kinetic energy of the particles in a sample of
matter.
 Determines the direction of heat transfer
What contains more heat?
a glass of boiling water or an iceberg
What does your body sense?
temperature or heat
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Law of conservation of energy - in ordinary
reactions energy is not created or destroyed
but converted from one form to another.
Thermochemistry - study of heat changes
during chemical and physical reactions.
1.
2.
3.
4.
5.
6.
Mechanical energy of moving water
Potential energy of dammed water
Mechanical of falling water
Mechanical of moving turbine
Mechanical energy of spinning in generator
Electrical energy of moving electrons
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During exercise your body generate a certain
amount of heat (measured in Calories)
Occurs from the breakdown of organic
molecules
calorie (cal) - heat needed to raise 1 g of
water 1 C.
 Calorie (Cal) - food Calorie - 1000 cal

 1 can of pop contains 140 Calories- This means
that 1 can of pop releases 140 kilocalories when it
burns completely in the presence of Oxygen.

Joule (J) - SI unit of energy
 1Cal = 1kcal = 1000 cal = 4186 J = 4.186 kJ

Heat Capacity – heat it takes to change a
substances temperature by 1 C.
 Dependant on mass
▪ A cup of water has a higher heat capacity than a drop of
water
 Dependant on chemical composition
▪ Different substances of the same mass may have
different heat capacities
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Specific heat (C) – amount of
heat required to raise the
temperature of 1 g of a
substance 1ºC.
Measured in J/(g x ºC)
Heat affects the temp of objects
with a high specific heat much
less than the temperature of
those with a low specific heat.

Which has the highest on the list?
 Water!!
▪ This allows bodies of water to store
large quantities of energy.
▪ Cities by water have cooler
summers, warmer winters, and
wind!
▪ Land has a much lower specific heat
capacity
10.3 Specific Heat
How do you calculate the specific heat?
q = (m)(c)(∆t)
q = energy
m = mass
c = specific heat capacity
∆t = temperature change = Tfinal - Tinitial
q = (m)(c)(∆t)
mass = 68.0 g
c = 0.895 J/gºC
∆t = 34.5ºC – 2.5ºC = 32.0ºC
q = (68.0 g)(.895 J/gºC)(32.0ºC)
q = 1950 J
Solving the energy equation for the
specific heat, c, yields
q
c
m  t
Specific heat will have units of J/gºC
Thermochemistry is concerned with the flow of
heat between a chemical system and its
surroundings.
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System - what you are looking at
Surroundings - everything else
Universe = system + surroundings
Endothermic - energy in, system absorbs heat
from the surroundings(+ enthalpy change).
Exothermic - energy out, system loses heat to
surroundings(- enthalpy change).
Calorimeter – insulated device used to
measure heat changes.
If the specific heat capacity is a characteristic property
of a substance, it can be used to determine the
identity of an unknown metal.
Calorimetry- The accurate & precise measurement
of the heat change of a chemical or physical process.
Energy
cooling
copper
=
Energy
warming
water

Enthalpy (H) – The amount of heat that a
substance has at a given temperature and
pressure
 Heat changes are the same as enthalpy changes
 Change in enthalpy is symbolized as H
 Therefore H = q
 H = Endothermic Reaction
 - H = Exothermic Reaction
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The energy needed to heat up the
water can be calculated from the
mass of the water, its specific heat,
and its temperature change. (or q =
(m)(c)(∆t))
This is the same energy that the
metal releases. If you know the
mass of the metal and its
temperature change, you can
determine its specific heat.
In the lab, a 25 g sample of a metal that
has been sitting in boiling water at 100.0ºC, is
placed in a calorimeter containing 65 g of
water at 27.5 ºC. The water warms up and
reaches a high temperature of 30.0 ºC. What
is the specific heat of the metal?
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First,
qmetal = qwater
To find the energy of the cooling metal, find
the qwater.
qwater = (mwater)(cwater)(∆twater)
qwater = (65 g)(4.184J/g ºC)(30.0 ºC – 27.5 ºC)
qwater = 680 J
qmetal = -qwater = -680 J
To find the specific heat of
the metal use the equation
solved for c.
c metal
c metal
q metal

m metal  t metal
- 680 J
= .39 J/g ºC


25 g  (70.0 C)
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All reactions involve energy.
Thermochemical equation - chemical
equation with energy included.
2C8H18 + 25O2  16CO2 + 18H2O + 10900kJ
Endothermic - energy a reactant
Exothermic - energy is a product

Heat of reaction - energy shown as
enthalpy change.
2C8H18 + 25O2  16CO2 + 18H2O H= -10900kJ
H = positive(+) is endothermic
H = negative(-) is exothermic

Using the following equation, calculate the
kilojoules of heat required to decompose 2.24
mol of NaHCO3 (s)
2NaHCO3  Na2CO3 (s) + H2O (g) + CO2 (g) H = 129 kJ
129 kJ
2.24 mol NaHCO3 x __________
=
2 mol NaHCO3
145 kJ

Heat of combustion – the heat of reaction for
the complete burning of one mole of a
substance. Reported as enthalpy changes.
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (l) H= -890.4kJ
H = the heat of combustion
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Explain what happens when you drop an ice
cube on the floor using the following terms:
System, surroundings, heat, temperature.
 The ice cube is the system. The floor and air
surrounding it are the surroundings. The ice
absorbs heat from the surroundings and begins to
melt. The mixture of ice and water will remain at
0o C until all of the ice is melted.
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Molar Heat of Fusion (Hfus) – the heat
absorbed by one mole of a substance in
melting from a solid to a liquid at a constant
temperature.
Molar Heat of Solidification (Hsolid) – the
heat lost when one mole of a liquid changes
to a solid at a constant temperature.
This means that Hfus = Hsolid
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How many grams of ice at 0o C could be
melted by the addition of 2.25 kJ of heat? The
molar heat of fusion for water is 6.01 kJ/mol
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How much heat is absorbed when 24.8 grams
of H2O(l) at 100oC is converted to steam at
1000C?
P.276 in book
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So according to what you now know, what is
the molar heat of vaporization?
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Molar heat of condensation?
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How are these two related?
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Heat changes can also occur when a
substance is dissolved in a solvent.
This heat change is called the molar heat of
solution.
Ex:
 NaOH (s)  Na (aq) + OH (aq) Hsoln= -445.1kJ/mol
Is this exothermic or endothermic? Would it feel hot or cold?
Exothermic/ Hot
If you add two or more thermochemical equations to
give a final equation, then you can also add the heat
changes to give the final heat change.
 Ex: Conversion of diamond to graphite

C (s, diamond) + O2  CO2 (g) H= -395.4 kJ
CO2 (g)  C (s, graphite) + O2 (g) H=393.5 kJ
C (s, diamond)  C(s, graphite) H= -1.9 kJ
When you add these two equations together the CO2
and O2 cancel out. You can add the heat changes to
give the final heat change
Calculate the enthalpy change H (in kJ) for the
reaction
2Al (s) + Fe2O3(s)  2Fe (s) + Al2O3 (s) H= ? kJ
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Use the enthalpy changes for the combustion of
aluminum and iron:
2Al (s) + 3/2O2 (g)  Al2O3 (s) H= -1669.8 kJ
2Fe (s) + 3/2O2 (g)  Fe2O3 (s) H= -824.2 kJ
Calculate the enthalpy change H (in kJ) for
the deomposition of hydrogen peroxide
2H2O2(l)  2H2O (l) + O2 (g) H= ? kJ
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Use the enthalpy changes for the combination
of hydrogen with oxygen to make 2 different
compounds
H2 (g) + O2 (g)  H2O2(l)
H= -187.9 kJ
H2 (g) + 1/2O2 (g)  H2O (l)
H= -285.8 kJ
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Sometimes it is hard to measure the change
for a reaction. If this is the case, you can
calculate the enthalpy change of the reaction
from standard heats of formation.
Standard heat of formation (Hof) is the
change in enthalpy that accompanies the
formation of one mole of the compound from
its elements.
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Will be using table A.6 to calculate p. 787
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Use the following equation…
Ho = Hof (products) - Hof(reactants)
What is the standard heat of reaction for the
reaction of carbon monoxide gas with oxygen to
form carbon dioxide?
1. Write a balanced equation

1.
2.
2Co(g) + O2 (g)  2 CO2 (g)
Use Table A.6 in Appendix A to find the
standard heats of formation
1. Hof O2 (g) = 0.0 kJ/mol
2. Hof CO (g) =
-110.5 kJ/mol
3. Hof CO2 (g) = -393.5 kJ/mol
3. Sum the Hof values of the reactants and
then the products, taking into account the
number of moles of each
Reactants:
-110.5 kJ
2 mole CO (g) ________
= -221.0 kJ
1 mol CO
0.0 kJ
1 mol O2 (g) _________
=
1 mol O2
Products
-393.5 kJ
2 mol CO2 (g) _________
=
1 mol CO2
0.0 kJ
Total Reactants = -221.0 kJ
-787.0 kJ
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Ho = Hof (products) - Hof(reactants)
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Ho = -787.0 kJ – (-221.0 kJ) = -566.0kJ