Energy and
Chemical Reactions
Chapter 6
Energy









- Thermodynamics
Basic Principles
Specific Heat Capacity and Heat Transfer
Energy and Changes of State
First Law of Thermodynamics
Enthalpy Changes of Chemical Reactions
Calorimetry
Hess’s Law
Standard Enthalpies of Formation
Product or Reactant Favored Reactions
Basic Principles


Energy - the capacity to do work
Kinetic Energy
Thermal energy - particles in motion
 Mechanical energy- macroscopic objects in
motion
 Electric Energy - electrons moving through a
conductor
 Sound compression and expansion of spaces
between particles

Basic Principles


Energy - the capacity to do work
Potential Energy





Chemical potential energy - attractions among electrons
and atomic nuclei. Rearranging electrons and nuclei
changes the potential energy
Gravitational energy - ball held above the floor
hypertextbook.com/physics/matter/energychemical
Electrostatic energy - positive and negative
ions a small distance apart.
Potential Energy can be converted to Kinetic
Energy
Conservation of Energy

The First Law of Thermodynamics
The law of conservation of energy
 The total energy of the universe is constant
 Energy can be transferred from one form to
another, but must be conserved.


All standard heat engines (steam, gasoline, diesel) work by supplying heat to a
gas, the gas then expands in a cylinder and pushes a piston to do its work. The
catch is that the heat and/or the gas must somehow then be dumped out of the
cylinder to get ready for the next cycle.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Temperature and Heat



Heat is not the same as temperature
The more thermal energy a substance has
the greater motion of its atoms and
molecules
The total thermal energy in an object is the
sum of the individual energies of all atoms,
molecules or ions in that object.
Systems and surroundings



A System - the object or collection of objects
being studied
The Surroundings - everything outside the
system that the exchange energy with a
system.
ApplicationEnergy efficiency




Combustion engine10-50%[4]
Electric motors 30-60% (small ones < 10W); 50-90
(middle ones between 10-200W); 70-99.99% above
200W
Household refrigerators low end systems ~ 20%; high
end systems ~ 40-50%
Incandescent bulbs5-10%
Directionality of Heat
Transfer:Thermal Equilibrium



Heat transfer always occurs from a hotter object
to cooler object. (directionality of heat transfer)
Transfer of heat continues until both objects are
at the same temperature (thermal equilibrium)
The quantity of heat lost by a hotter object and
the quantity of heat gained by a cooler object
are numerically equal (law of conservation of
energy)
Directionality of Heat
Transfer:Thermal Equilibrium

Exothermic process heat is transferred
from the system to the surroundings.


Symbol for heat q, q sys > 0
Endothermic process heat is transferred
from the surroundings to the system.

Symbol for heat q, q sys < 0
Energy Units

Joule is the SI unit of thermal energy
related to mechanical energy 1 J =
kilojoule (kJ) is 1000 J


1 kg . m2 / s2
1 calorie the amount of energy required to raise 1
gram of water 1 degree C (from 14.5 oC - 15.5 o C).
1 kilocalorie equals 1000 calories
4.184 Joule = 1 calorie
Specific Heat Capacity and
Heat Transfer

The quantity of heat transferred to or from
an object when its temperature changes
depends on three things:



the quantity of material
the size of the temperature change
the identity of the material gaining
or losing heat
Specific Heat Capacity ( C )
or ( S.H.)

Specific Heat Capacity
the quantity of heat required to raise the
temperature of 1 gram of a substance by one
kelvin.
 Units are J / g . K
 The quantity of heat transferred is described
by the equation

q=C
.
m
.
DT
Specific Heat Capacity and Heat Transfer
q=C


m
.
DT
D T = Tfinal - Tinitial
D T can be positive or negative



.
when D T > 0 then q> 0 exothermic
when D T < 0 then q < 0 endothermic
C has the units J / g . K

q will have units of Joule
Units for T and
Specific Heat Capacity


D T, Celsius = 100oC - 0oC = 100o C
D T, Kelvin = 373 K - 273 K = 100 C
Units of specific Heat Capacity J / mol . K
(Water 4.184 J/g . K) (18.02 g/mol) = 75.40 J/g
.K
Specific Heat Capacity and Heat Transfer
Specific Heat Capacity and Heat Transfer
In an experiment it was determined that 59.8 J was
required to change the temperature of 25.0 g of
ethylene glycol (a compound used as antifreeze in
automobile engines) by 1.00 K. Calculate the specific
heat capacity of ethylene glycol from these data.
Use q = C
Solve for C
.
m
.
DT
Specific Heat Capacity and
Heat Transfer
•
The specific heat capacity of a substance is
determined experimentally by accurately
measuring temperature changes that occur
when heat is transferred from the substance to a
known quantity of water (whose heat capacity is
known)
Specific Heat Capacity of a Metal
•
•
•
•
Assumptions the water and metal will end up at
the same temperature Tfinal
Assume no heat transferred to surroundings
Heat transferred from the metal to the water,
qmetal, has a negative value because the temp of
metal dropped conversely qwater has a positive
value
The values of qmetal and qwater are numerically
equal but of opposite sign so
•
•
- qmetal = qwater
qmetal + qwater = 0
Specific Heat Capacity of a Metal
qwater + qmetal = 0
Cwater X mwater X (Tfinal - Tinitial, water) +
Cmetal X mmetal X (Tfinal - Tinitial, metal) = 0
A 15.5 g piece of chromium, heated to 100.0 oC,
is dropped into 55.5 g of water at 16.5 oC
http://chemed.chem.purdue.edu/demos/main_
pages/5.3.html
Energy and Changes of State
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Energy and Changes of State



Heat is required to invigorate molecules to
break bonds and separate from each
other.
The heat required to convert a substance
from a solid at its melting point to liquid is
called the heat of fusion.
The heat required to convert liquid at its
boiling point to a gas called the heat of
vaporization.
Energy and Changes of State



Heat of Fusion = 333 J / g or 6.00 kJ /
mol
Heat of vaporization = 2256 J /g
Note the the units for the heats have only
2 values, J and g because there is no D T
Energy and Changes of State


Heat of Fusion = 333 J / g or 6.00 kJ / mol
Heat of vaporization = 2256 J /g
Energy and Changes of State

What is the minimum amount of ice at 0oC that
must be added to the contents of a can of diet
cola 340 mL to cool it from 20.5oC ? Assume
the Specific Heat Capacity and density of diet
cola are te same as for water and that no heat
is gained or lost to the surroundings.
The First Law of Thermodynamics



If a system does work on its surroundings, energy
must be expended and the energy content of the
system will decrease
If work is done by the surrounding on a system, the
energy content of the system increases.
Work done will change the system’s energy
content.
The First Law of Thermodynamics
D
D
E is a measurable quantity
E : measure the heat transferred and the
work done to or by the system
Enthalpy


Most experiment is a chemical laboratory are
carried out in vessels open to the atmosphere so
heat is measured under the condition of constant
pressure
Enthalpy D H, is the difference
between the final and initial enthalpy
content.
 Negative
D H specify that energy is transferred
from the system to the surroundings
 Positive D H specify that energy is transferred
from the surroundings to the system
State Function


A State Function is a quantity that
accompany chemical or physical changes do
not depend on which path is chosen in going
from the initial state to the final state.
Analogy of state function. You now have
$25 in your savings account. You could have
put in exactly $25 or you could have put in
$100 and removed $75.
Enthalpy Changes for Chemical
Reactions




Enthalpy changes are specific to the reactants and
products and their amounts. Both the identity of
reactants and products and their states (s,l,g,aq) are
important
DH has a negative value if heat is evolved (an
exothermic reaction). DH has a positive value if heat is
required (an endothermic reaction).
Values of DH are numerically that same, but opposite
in sign for chemical reactions that are the reverse of
each other.
The enthalpy change depends on the molar amounts
of reactants and products. If the amounts are doubled,
the enthalpy change is doubled.
Enthalpy Changes for Chemical
Reactions

A + B -> C
DH = + 25 kJ

C -> A+ B
DH= - 25 kJ

2A + 2B -> 2 C DH = 2 (+ 25 kJ)


Sucrose C12H22O11 is burned releasing 5645 kJ per mol. What
is the enthalpy change for burning 5 g of sucrose?
When ethane (C2H2) gas is burned according to the following
equation, the enthalpy change of -2857.3 kJ is measured.
Calculate the value of DH for 15 g of C2H6
2 C2H6(g) + 7 O2(g) -> 4 CO2(g) + 6 H2O(g)
Calorimetry

The heat transferred in a chemical or
physical process is determined by an
experimental techniques call calorimeter.

Constant pressure calorimeter


(open to the air)
Constant volume calorimeter
Coffee Cup Calorimetry
Coffee Cup Calorimetry

Insulated container with measure amount
of water
qrxn + qsolution = 0
Suppose you place 0.500 g of magnesium chips in a
coffee-cup calorimeter and then add 100.0 mL of 1.00
M HCl. The temperature of the solution increased
from 22.2 oC to 44.8 oC. What is the enthalpy change
for the reaction er mole of Mg? Assume specific heat
capacity of solution is 4.20 J/ g.K and the density of
HCl is 1.00 g/mL
Bomb Calorimetry
Bomb Calorimetry

Reaction occurs in a sealed metal
container inside a calorimeter with
water
qrxn + qbomb + qwater = 0
has units (Cbomb) ( DT)
note: qwater has units (Cbomb)( DT)(mwater)
note: qbomb
Bomb Calorimetry
qrxn + qbomb + qwater = 0
A 1.00 g of sucrose (C12H22O12) is burned in a bomb
calorimeter. The temperature of 150 g of water in
the calorimeter rises from 25.00oC to 27.32oC. The
heat capacity of the bomb is 837 J/K and the
specific heat capaccity of water is 4.20 J/K .
Calculate the
(a) the heat evolved per gram of sucrose
(b) the heat evolved per mole of sucrose.
Hess’s Law

If a reaction is the sum of tow or more other
reactions, DH for the overall process is the
sum of DH values of those reactions.
A + B -> C
DH = -25kJ
C + D -> E
DH = +70kJ
A + B + D -> E
DH = +50 kJ
Hess’s Law
Hess’s Law
Standard Molar Enthalpies of
Formation
DHof standard molar enthalpies of formation
the enthalpy change for the formation of
1 mol of a compound directly from its
component elements in their standard states
Standard Molar Enthalpies of
Formation
DHof standard molar enthalpies of formation




-the standard enthalpy of formation for an element in its
standard state is zero
-values of compounds in solution refer to the enthalpy
change for the formation of 1 M solution of the compound
from the elements making up the compounds plus th
enthalpy change occurring when the substance dissolves
in water.
most DHof are negative, indicating that formation of most
compounds from elements is exothermic.
DHof can be used to compare thermal stabilities of related
compounds.
Standard Molar Enthalpies of
Formation
Enthalpy Change for a
Reaction
DHorxn = S[DHof (products)]
- S [ DHof (reactants)]
note
DHorxn is calculated from DHof
Calculate the DHorxn for the combustion of
benzene (C6H6(l)) Given DHof [C6H6(l)] = +
48.95 kJ/mol
Enthalpy Change for a
Reaction
DHorxn = S[DHof (products)] - S [ DHof (reactants)]
In general
if
DHorxn < 0, the reaction is product-favored
if DHorxn > 0, the reaction is reactant-favored
Energy Resources



home.clara.net/darvill/altenerg/index.htm
www.energy.gov/energysources/index.htm
www.cc.utah.edu/~ptt25660/tran.html
Homework Questions Ch 6
17. What quantity of heat is required to raise the temperature of
50.00 mL of water from 25.52 to 28.75 oC? The density of
water at this temperature is 0.997 g/mL.
20. A 45.5-g sample of copper at 99.8 °C is dropped into a
beaker containing 152 g of water at 18.5 oC. When thermal
equilibrium is reached, what is the final temperature?
23. When 108 g of water at a temperature of 22.5 °C is mixed
with 65.1 g of water at an unknown temperature, the final
temperature of the resulting mixture is 47.9 oC. What was the
initial temperature of the second sample of water?
Homework Questions Ch 6
25. A 237-g piece of molybdenum, initially at 100.0 oC, is dropped into 244 g
of water at 10.0o C. When the system comes to thermal equilibrium, the
temperature is 15.3 oC. What is the specific heat capacity of molybdenum?
29. Chloromethane, CH3Cl, is used as a topical anesthetic. What quantity of
heat must be absorbed to convert 92.5 g of liquid to a vapor at its boiling
point, - 24.09 oC? The heat of vaporization of CH3Cl is 21.40 kJ/mol.
30. Ethanol, C2H5OH, boils at 78.29 oC. What quantity of heat energy (in
joules) is required to raise the temperature of 1.00 kg of ethanol from 20.0
°C to the boiling point and then change the liquid to vapor at that
temperature? (The specific heat capacity of liquid ethanol is 2.44J / g- K,
and its enthalpy of vaporization is 855 J/g.)
Homework Questions Ch 6
36. Isooctane (2,2,4-trimethylpentane), one of many hydrocarbons that make
up gasoline, burns in air to give water and carbon dioxide.
2 C8H 18(l) + 25 O 2(g) -> 16 CO 2 (g) + 18 H2O ( l)
DH rxn = -10,922 kJ
If you burn 1.00 L of isooctane (density = 0.69 g/mL), what quantity of heat is
evolved?
39. You mix 125 mL of 0.250 M CsOH with 50.0 mL of 0.625 M HF in a
coffee-cup calorimeter, and the temperature of both solutions rises from
21.50oC before mixing to 24.40 oC after the reaction.
CsOH(aq) + HF(aq) ----+ CsF(aq) + H20( l)
What is the enthalpy of reaction per mole of CsOH? Assume the densities of
the solutions are all 1.00 g/mL and the specific heats of the solutions are
4.2J/g.K.
Homework Questions Ch 6
42. Adding 5.44 g NH4N03(s) to 150.0 g of water in a coffee cup
calorimeter (with stirring to dissolve the salt) resulted in a
decrease in temperature from 18.6 to 16.2 °C. Calculate the
enthalpy change for dissolving NH4N0 3(s) in water, in kilo
joules per mole. Assume that the solution (whose mass is
155.4 g) has a specific heat capacity of 4.2J/g°K.
44. Sulfur (2.56 g) is burned in a bomb calorimeter with excess
02(g). The temperature increases from 21.25 to 26.72 oC. The
bomb has a heat capacity of 923 J/K, and the calorimeter
contains 815 g of water. Calculate the heat evolved, per mole
of S02 formed, for the reaction
S8(8) + 8 O2(g) -> 8 SO2(g))
Homework Questions Ch 6
45. You can find the amount of heat evolved in the combustion of carbon by
carrying out the reaction in a combustion calorimeter. Suppose you burn
0.300 g of C(graphite) in an excess of O 2(g) to give CO 2(g).
C(graphite) + 0 2(g) -> CO 2(g)
The temperature of the calorimeter, which contains 775 g of water, increases
from 25.00 to 27.38 oC. The heat capacity of the bomb is 893 J/K. What
quantity of heat is evolved per mole of carbon?
51. The enthalpies of the following reactions can be measured.
C2H 4(g) + 3 O 2(g) -> 2 CO 2(g) + 2 H20( l)
DHo = -1411.1 kJ
C2H5OH(l) + 3 O 2(g) -> 2 CO 2(g) + 3 H20( l)
DHo = -1367.5 kJ
(a) Use these values and Hess's law to determine the enthalpy change for the
reaction
(b) Draw an energy level diagram that shows the relationship between the
energy quantities involved in this problem.
Homework Questions Ch 6
52. Enthalpy changes for the following reactions can be determined
experimentally.
N 2(g) + 3 H 2(g) -> 2 NH3(g)
DHo = -91.8 kJ
4 NH 3(g) + 5 O 2(g) -> 4 NO (g) + 6 H20( g)
DHo = -906.2 kJ
H 2(g) + 1/2 O 2(g) -> H20( g)
DHo = -241.8 kJ
Use these values to determine the enthalpy change for the formation of NO(g)
from the elements (an enthalpy that cannot be measured directly because
the reaction is reactant-favored) .
1/2 N 2(g) + 1/2 O 2(g) -> NO(g)
DHo = ?
56. (a) Write a balanced chemical equation for the formation of 1 mol of Cr2O
3(S) from Cr and O2 in their standard states and find the value for DH; for
Cr2O 3(S) in Appendix L.
(b) What is the standard enthalpy change if 2.4 g of chromium is oxidized to
Cr2O 3(S) ?
Homework Questions Ch 6
61. The Romans used calcium oxide (CaO) to produce a very strong mortar in
stone structures. The CaO was mixed with water to give Ca(OH)2' which
reacted slowly with CO2 in the air to give CaC03.
(a) Calculate the standard enthalpy change for this reaction.
(b) What quantity of heat is evolved or absorbed if 1.00 kg of Ca(OH)2 reacts
with a stoichiometric amount of CO2?
62. The standard enthalpy of formation of solid barium oxide, BaO, is - 553.5
kJ/mol, and the enthalpy of formation of barium peroxide, BaO2' is -634.3
kJ/mol.
(a) Calculate the standard enthalpy change for the following reaction. Is the
reaction exothermic or endothermic?
(b) Draw an energy level diagram that shows the relationship between the
enthalpy of this reaction and the heats of formation of BaO(s) and BaO2 (S).
Homework Questions Ch 6
66. Use your "chemical sense" and decide if each of the
following reactions is product- or reactant-favored. Calculate
DHo rxn in each case, and draw an energy level diagram like
those in Figure 6.18.
(a) The reaction of aluminum and chlorine to produce AlCl 3(s)
(b) The decomposition of mercury(II) oxide to produce liquid
mercury and oxygen gas
Homework Questions Ch 6
74. Three 45-g ice cubes at 0 oC are dropped into 5.00 X 102 mL of tea to make ice
tea. The tea was initially at 20.0 DC; when thermal equilibrium was reached the
final temperature was 0 oC. How much of the ice melted and how much remained
floating in the beverage? Assume the specific heat capacity of tea is the same as that
of pure water.
77. A commercial product called "Instant Car Kooler" contains 10% by weight
ethanol, C2H5OH, and 90% by weight water. You spray the "Kooler" inside an
overheated car. It works because thermal energy of the air in the car will be used to
evaporate some of the alcohol and water. If the air inside an average size car must
lose 3.6 kJ of heat to drop the air temperature from 55 to 25 oC, what mass of the
ethanol water mixture must evaporate to absorb this heat? (The enthalpy of
vaporization for ethanol is 850J / g and for water it is 2260J / g.)
Homework Questions Ch 6
83. The standard molar enthalpy of formation of CS 2(g) cannot be determined
directly because the compound cannot be prepared by the reaction of carbon and
sulfur. It can be calculated from other enthalpy changes, however. The following
enthalpies can be measured.
C(s) + O 2(g) -> CO 2(g)
DHo = -393.5 kJ
S(s) + O 2(g) -> S02(g)
DHo = -296.8 kJ
CS2(g) + 3 O 2(g) -> CO 2(g) + 2 S02(g)
DHo = -1103.9 kJ
(a) Modify these equations to give a new set of equations, which, when added
together, give the equation for the formation of CS2(g) from C(s) and S(s) in their
standard states. Assign enthalpy changes to each reaction.
(b) Calculate the DHo;for CS2(g).
(c) Draw an energy level diagram that shows how the various enthalpies in this
problem are related.
(d) Is the formation of CS2(g) from its elements product- or reactant-favored?
Homework Questions Ch 6
84. The meals-ready-to-eat (MREs) in the military can be heated
on a flameless heater. The source of energy in the heater is
Mg(s) + 2 H2O (l) -> Mg(OH)2(s) + H2(g)
Calculate the enthalpy change under standard conditions (in
joules) for this reaction. What quantity of magnesium is needed to
supply the heat required to warm 250 mL of water (d = 1.00
g/mL) from 25 to 85 oC?
Homework Questions Ch 6
91. Prepare a graph of molar heat capacities for metals versus
their atomic weight. Use the data in Table 6.1 and the values in the
following table. Does any relation exist between specific heat
capacity and atomic weight? Use this relation to predict the
specific heat capacity of platinum. (The specific heat capacity for
platinum is given in the literature as 0.133J/g·K.) How good is the
agreement between the predicted and actual values?
Metal
chromium
lead
silver
Specific heat capacity (J/g· K)
0.450
0.127
0.236
tin
titanium
0.227
0.522