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3/18/2018
1
THERMODYNAMICSThermochemistry
Definitions2
Thermochemistry deals with changes in heat during
chemical reactions.
A main goal of the thermochemistry study is to determine
the quantity of heat exchanged between a system and its
surroundings.
The system is the part of the universe being studied,
while the surroundings are the rest of the universe that
interacts with the system.
System and surroundings3
Open system
An open system is a system that freely exchanges energy and matter with its surroundings.
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WM_ THERMODYNAMICS 5
Open systems
Closed system
A closed system exchanges energy but not matter with an outside system.Although it is typically portion of larger system, it is not in complete contact with it.
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Isolated systemWM_ THERMODYNAMICS 8
An isolated system can exchange neither energy
nor matter with its surroundings (an outside
system). While it may be portion of larger system,
it does not communicate with the outside in any
way.
Examples of such system type are:
physical universe and a closed thermos bottle
(though its isolation is not perfect).
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9Comparison of systems
10
Quiz11
•Q,
•A closed system contains 2g of ice. Another 2g
of ice are added to the system. What is the final
mass of the system?
•Q
•An isolated system has an initial temperature of 30oC.
It is then placed on top of a bunsen burner for an hour.
What is the final temperature?
WM_ THERMODYNAMICS 12
System description System type
Coffee in perfectly closed Thermos® flask
Combustion of gasoline in car engine
Mercury in thermometer
Living plant
Electric battery
Q. Identify system type (open, closed or isolated) from
descrition below and fill the enpty space in the table
below
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13
Q. Which type of thermodynamic system is:
1. an ocean?
2. an aquarium?
3. a pizza delivery bag?
4. a greenhouse?
5. a man ?
ENERGYMatter and energy
14WM_ THERMODYNAMICS
MATTER AND ENERGY
Matter is anything that has a mass and occupies some space. All bodies consist of a matter.
Mass is a measure of the quantity of a matter in a sample of any material.
The more massive an object is, the more force is required to put it in a motion.
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Energy
Energy is measure of the ability of a body or system to do work or produce anychange. No activity is possible without energy.
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Law of Conservation of Energy
Many experiments have demonstrated that all of the energy involved in any chemical or physical change appears in some form after the change.
These observations are summarized in the Law of Conservation of Energy:
Energy cannot be created or destroyed in a chemical reaction or in a physical change.
It can only be converted from one form of energy to another.
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Chemical changes always involve energy changes. However, some energytransformations do not involve chemical changes at all.
For example, heat energy may beconverted into electrical energy or into mechanical energy without any simultaneous chemical changes.
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The Law of Conservation of Matter and Energy
With the dawn of the nuclear age in the 1940s, scientists, and then the world, became aware that matter can be converted into energy.
In nuclear reactions, matter is transformed into energy.
The relationship between matter and energy is given by Albert Einstein’s now famousequation:
E = m c2
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The Law of Conservation of Matter and Energy
At the present time, we have not (knowingly) observed the transformation of matter into energy on a large scale.
It does, however, happen on an extremely small scale in “atom smashers,” or particle accelerators, used to induce nuclear reactions (like in Large Hadron Collider in CERN).
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The Law of Conservation of Matter and Energy
Now that the equivalence of matter and energy is recognized, the Law of Conservation of Matter and Energy can be stated in a single sentence:
The combined amount of matter and energy in the universe is fixed.
21WM_ THERMODYNAMICS
MEANING OF THE ENERGY IN THE LIFE
Energy is very important in every aspect of our daily lives.
The food which we eat supplies the energy to sustain life with all of its activities and concerns.
The availability of relatively inexpensive energy is an important factor in our technological society.
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„ACCOUNTING OF ENERGY’
• The concept of energy is at every heart of science.
• All physical and chemical processes are accompanied by the transfer of energy.
• Energy cannot be created or destroyed.
• We must understand how to do the “accounting” of energy transfers from one body or one substance to another or from one form of energy to another.
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OBJECT OF THE THERMODYNAMICS
In thermodynamics we study the energy changes that accompany physical and chemical processes.
Usually these energy changes involve heat—hence the “thermo-” part of the term.
There are the two main aspects of thermodynamics.
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THERMOCHEMISTRY
The first aspect is thermochemistry.
This practical subject is concerned with how we observe, measure, and predict energy changes for both physical changes and chemical reactions.
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FUNDAMMENTAL ASPECT OF THERMODYNAMICS
• The second aspect is addressed to a more fundamental aspect of thermodynamics.
• How to use energy changes to tell us
whether or not a given process can occurunder specified conditions
• to give predominantly products (or reactants)
• how to make a process more (or less) favourable.
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ENERGY - DEFINITION
We can define energy as follows:
Energy is the capacity to do work or to transfer heat.
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Heat, energy, work units
The amount of heat transferred in a process is usually expressed in joules or in calories.
The SI unit of energy and work is the joule (J), which is defined as 1 kg m2/s2.
Not SI unit of energy e.g. 1 cal = 4.184 J
WM_ THERMODYNAMICS 28
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ENERGY FORMS
Energy can take many forms:
electrical energy,
radiant energy (light),
nuclear energy,
chemical energy.
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KINETIC ENERGY
Commonly we classify energy into two general types: kinetic and potential.
Kinetic energy is the energy of motion. The kinetic energy of an object is equal to one half its mass, m, times the square of its velocity, v.
The heavier a hammer is and the more rapidly it moves, the greater its kinetic energy and the more work it can accomplish.
2
2
1vmEk
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Kinetics energy in questions
Q.
The kinetics energy of solid body with the mass of 5 kg which moved with speed 8 m s-1 is equal:
a) 40 kg m s-1; b) 320 J; c) 160 J
d) 160 kg m2s-2
WM_ THERMODYNAMICS 31
POTENTIAL ENERGY
Potential energy (EP) is the energy thata system possesses by virtue of its position or composition.
The work that we do to lift an object is stored in the object as energy.
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Ep= m g h
Where: m – mass (kg); h – body movement (change of
height, m); g – gravitational acceleration, (10 m s-2 )
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Potential energy in question
Q. When a bucket with 10 kg of water is picked up at the height of 1 m the potential energy is as follows:
a) 10 kg m; b) 100 J; c) 100 kg m2s-2;
d) 100 kg
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EXAMPLE: EpEk
If we drop a hammer, its potential energy is converted into kinetic energy as it falls, and it could do work on something it hits—for example, drive a nail or break a piece of glass.
34WM_ THERMODYNAMICS
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Q. What type of energy does a stationary
pencil contain? falling pencil?
Heat as a form of energyWM_ THERMODYNAMICS 36
Heat is a form of energy that always
flows spontaneously from a hotter
body to a colder body—never in the
reverse direction
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Heat transfer
Heat transfer concerns the generation, use, conversion, and exchange of heat(thermal energy) between physical systems.
Heat transfer is classified into various mechanisms, such as thermal conduction(diffusion), thermal convection, thermalradiation and by phase changes.
WM_ THERMODYNAMICS 37 38
Conduction is the transfer of thermal energy through direct contact between particles of a substance, without moving the particlesto a new location. •Convection is the transfer of thermal energy through movement of particles from one location to another
•Radiation is the emission of energy as waves or particles or rays.
TEMPERATURE vs. HEAT
Temperature measures the intensity of a heat, the “hotness” or “coldness” of a body.
A piece of metal at 100°C feels hot to the touch, whereas an ice cube at 0°C feels cold.
Why? Because the temperature of the metal is higher, and that of the ice cube lower, than body temperature.
39WM_ THERMODYNAMICS
CALORIMETRY, thermochemistry
We can determine the energy change associated with a chemical or physical process by using an experimental technique called calorimetry.
This technique is based on observing the temperature change when a system absorbs or releases energy in the form of heat. This is in turn the effect of chemical or physical process under study.
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The experiment is carried out in a device called a calorimeter, in which we measure the temperature change of a known amount of substance (often water) which specific heat is known.
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Specific heat
The specific heat is the amount of heatper unit of mass required to raise the temperature by one degree Celsius orKelvin with no change in phase.
WM_ THERMODYNAMICS 43
Changes in phase
Changes in phase (physical state) absorb or liberate relatively large amounts ofenergy (see Figure –NEXT SLIDE).
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SPECIFIC HEAT
The specific heat of each substance, a physical property, is different for the solid, liquid, and gaseous phases of the substance.
For example, the specific heat of:
ice is 2.09 J/g °C near 0°C;
liquid water is 4.18 J/g °C;
steam is 2.03 J/g °C near 100°C.
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SPECIFIC HEAT [c]
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WM_ THERMODYNAMICS 49SPECIFIC HEAT
Substance c in J/g KMolar CJ/mol K
Aluminum 0.900 24.3
Copper 0.386 24.5
Gold 0.126 25.6
Lead 0.128 26.4
Silver 0.233 24.9
Zinc 0.387 25.2
Mercury 0.140 28.3
Alcohol(ethyl) 2.400 111.0
Water 4.186 75.2
Ice (-10 C) 2.050 36.9
Specific heats and molar heat capacities for
various substances at 293 K (20oC)
WM_ THERMODYNAMICS 50
Q. How many heat is needed to heat up 10 g of liquid
water from 10oC to 40oC.
[c of liquid water is 4.18 J g-1 oC-1].
a) 418 J; b) 1254 J; c) 1672 J
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WM_ THERMODYNAMICS 51
Answer:
Q = m c ΔT
Q = (10 g) . (4.18 J g-1 oC-1) (40 – 10)oC
Q = 10 . 4.18 . 30 = 1254 J
WM_ THERMODYNAMICS 52
THERMAL EQUILIBRIUM
A hot object, such as a
heated piece of metal (a), is
placed into cooler water.
Heat is transferred from the hotter
metal bar to the cooler water until
the two reach the same
temperature (b).
We say that they are then at thermal equilibrium
WM_ THERMODYNAMICS 53
A 34 gram piece of an unknown metal
absorbs 351.56 Joules of energy when
the temperature increased from 10oC to
32oC. What is the specific heat of the
substance?
Hint: You are solving for Specific Heat
(Cp) not heat absorbed.
Quiz
WM_ THERMODYNAMICS 56
FUNDAMMENTAL ASPECT OF THERMODYNAMICS
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Potential energy of atom
An electron in an atom has potential energy because of the electrostatic force on it that is due to the positively charged nucleus and the other electrons in that atom and surrounding atoms.
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ATOMIC LEVEL of energy
The atomic or molecular level, we can think of each of these as either kinetic or potential energy.
The chemical energy in a fuel or food comes from potential energy stored in atoms due to their arrangements in the molecules.
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INTENSITY OF HEAT
Many forms of energy can be interconverted
and that in chemical processes, chemical
energy is converted to heat energy or vice
versa.
The amount of a heat a process uses
(endothermic) or gives off (exothermic)
can tell us a great deal about that process.
For this reason it is important for us to
be able to measure the intensity of the
heat.
59 61
https://www.quora.com/What-is-difference-between-endothermic-and-
exothermic-reaction-if-both-require-activation-energy
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ACTIVATION ENERGY
In such reactions, the total energy of the products is lower (for exothermic) orgreater (endothermic) than that of the reactants by the amount of energy as a heat released or absorbed.
Some initial activation (e.g., by heat)is needed to get these reactions started. This amount of energy is called activation energy.
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THERMOTERMIC REACTION
The amount of heat shown in such anequation always refers to the reaction
for the number of moles of reactants and products specified by the coefficients.
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Thermochemical equation
𝑁2(𝑔) + 3 𝐻2(𝑔) → 2𝑁𝐻3(𝑔) Δ𝐻0 = -92.38 kJ
2𝑁2(𝑔) + 6 𝐻2(𝑔) → 4𝑁𝐻3(𝑔)Δ𝐻0 = 2 x(-92.38 kJ ) = -184.8 kJ
1/2𝑁2(𝑔) +3
2𝐻2 𝑔 → 𝑁𝐻3 𝑔
Δ𝐻0 = (0.5) -92.38 kJ = -46.19 kJ
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2H2(g) + O2(g)
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2H2O(l)
ΔH0 = -571.8 kJ
H2 g + ½ O2(g)→H2O(l)
ΔH0 = ? kJ
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EXOTHERMIC REACTIONS
Reactions that release energy in the form of heat are called exothermic reactions.
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CH4(g) + 2O2(g)→ CO2(g) + 2H2O(l) + 890 kJ/mol
reagents products
exothermic reaction at constant pressure.
67
Exothermic reaction
Endothermic reaction
Reactions that absorb energy in the form of heat are called endothermicreactions.
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ENDOTHERMIC REACTION
NH4NO3(s) + 26 kJ NH4NO3(aq)
69WM_ THERMODYNAMICS
Reagents product
H2O
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Exothermic Reaction
Endothermic Reaction
Energy absorbed orreleased
Energy is released.It is a productof the reaction.Reaction vessel becomes warmer.
Temperature inside reaction vessel increases.
Energy is absorbed.It is a reactant of the reaction.Reaction vessel becomes cooler.
Temperature inside reaction vessel decreases.
Relative Energy of reactants & products
Energy of the reactants is greater than the energy ofthe productsH(reactants) > H(products)
Energy of the reactants is less than the energy of the productsH(reactants) < H(products)
Sign of HH = H(products) -H(reactants)
= negative (-ve)
H = H(products) -H(reactants)
= positive (+ve) Reversible reactionWM_ THERMODYNAMICS 71
N2 + 3 H2 ↔ 2NH3
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Exothermic Endothermic
Energy of the reactants
(N2 & H2) is greater
than the energy of the
products (NH3). Energy
is released.
Energy of the reactants
(NH3) is less than the
energy of the products
(N2 and H2). Energy is
absorbed.
En
erg
y p
rofile
s
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Exothermic processes Endothermic processes
making ice cubes melting ice cubes
formation of snow in clouds conversion of frost to water vapour
condensation of rain from water
vapourevaporation of water
mixing sodium sulfite and bleach baking bread
rusting iron cooking an egg
burning sugar producing sugar by photosynthesis
mixing water and strong acids mixing water and ammonium
nitrate
mixing water with an anhydrous salt making an anhydrous salt from a
hydrate
crystallizing liquid salts (as in
sodium acetate in chemical
handwarmers)
melting solid salts
Author: Fred Senese [email protected];
http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml
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WM_ THERMODYNAMICS 74
Q. Chemical reactions that absorb heat energy are called __________ . a. exothermicb. eltothermicc. endothermic
Q. Electrolysis requires energy to make it work. This means it is...a) an endothermic reaction b) an exothermic reaction c) an eltothermic reaction d) a chemical reaction
WM_ THERMODYNAMICS 75
Q. Which of the following is an endothermic
reaction?
a) Burning propane in a gas grill
b) Photosynthesis
c) Baking bread
d) Cooking an egg
e) Electrolysis of water
FIRST LAW OF THERMODYNAMICS
Some important ideas about energy are summarized in the First Law of Thermodynamics.
Energy is neither created nor destroyed in ordinary chemical reactions and physicalchanges.
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THE UNIVERSE, SYSTEM, SURROUNDINGS
The substances involved in the chemical and physical changes that we are studying are called the system.
Everything in the system’s environment constitutes its surroundings.
The universe is the system plus its surroundings.
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FIRST LAW OF THERMODYNAMICS
The system may be thought of as the part of the universe under investigation.
The First Law of Thermodynamics tells us that energy is neither created nor destroyed.
Energy is only transferred between the system and its surroundings.
78WM_ THERMODYNAMICS WM_ THERMODYNAMICS 79
Q. The first law of thermodynamics
states that energy is
a. increased during any process
b. decreased during any process
c. conserved during any process
THERMODYNAMIC STATE
The thermodynamic state of a system is defined by a set of conditions that completely specifies all the properties of the system.
80WM_ THERMODYNAMICS
THERMODYNAMIC STATE
This set commonly includes:
temperature, T,
pressure, V,
composition (identity and number of
moles of each component), n,
physical state (gas, liquid, or solid) of
each part of the system.
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WM_ THERMODYNAMICS 82
Ice liquid water
Steam
Changes in physical
state of water due
to temperature
changes
STATE FUNCTIONS
The properties of a system—such as P, V, T—are called state functions.
The value of a state function depends only on the state of the system and not on the way in which the system came to be in that state.
A change in a state function describes a difference between the two states.
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1
2
3
Direct
Indirect
Indirect
STATE FUNCTIONS
For instance, consider a sample of one mole of pure liquid water at 30°C and 1 atmpressure.
If at some later time the temperature of the sample is 22°C at the same pressure, then it is in a different thermodynamic state.
We can tell that the net temperature change is 8°C.
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STATE FUNCTION
It does not matter whether:
(1) the cooling took place directly (either slowly or rapidly) from 30°C to 22°C,
or (2) the sample was first heated to 36°C, thencooled to 10°C, and finally warmed to 30°C, then cooled to 22oC
or (3) any other conceivable path was followedfrom the initial state to the final state.
86WM_ THERMODYNAMICS
STATE FUNCTION
The change in other properties (e.g., the pressure) of the sample is likewise independent of path.
87WM_ THERMODYNAMICS
STATE FUNCTIONS
The most important use of state functions in thermodynamics is to describe changes.
We describe the difference in any quantity, X, as
When X increases, the final value is greater than the initial value, so ΔX is positive;
a decrease in X makes ΔX a negative value.
INITIALFINAL XXX
88WM_ THERMODYNAMICS
ZERO LEVEL
We can describe differences between levels of a state function, regardless of where the zero level is located.
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ZERO LEVEL
The zero levels on most temperature scales are set arbitrarily.
When we say that the temperature of an ice–water mixture is “zero degrees Celsius,” we are not saying that the mixture contains no temperature!
94WM_ THERMODYNAMICS
ARBITRARY SCALES
We have simply chosen to describe this
point on the temperature scale by the
number zero; conditions of higher
temperature are described by positive
temperature values, and those of lower
temperature have negative values, “below
zero.”
95WM_ THERMODYNAMICS
ARBITRARY SCALES
The phrase “15 degrees cooler” has the same meaning anywhere on the scale.
Many of the scales that we use inthermodynamics are arbitrarily defined in this way.
Arbitrary scales are useful when we are interested only in changes in the quantity being described.
96WM_ THERMODYNAMICS
Q. A. What is the change energy of the
sausage after heating, if original energy is
4 kJ and 20 kJ is added to it?
B. What is the total energy content of
sauage after heating?
a) 16 kJ; b) 4 kJ; c) 20 kJ; d) 24 kJ
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STATE FUNCTION
Any property of a system that depends only on the values of its state functions is also a state function.
For instance, the volume of a given sample of water depends only ontemperature, pressure, and physical state; volume is a state function.
We shall encounter other thermodynamic state functions.
98WM_ THERMODYNAMICS
ENTHALPY
Most chemical reactions and physical
changes occur at constant (usually
atmospheric) pressure.
The quantity of heat transferred into or out
of a system as it undergoes a chemical or
physical change at constant pressure, qp,
is defined as the enthalpy change, H, of
the process.
99WM_ THERMODYNAMICS
ENTHALPY - HEAT
An enthalpy change is sometimes loosely referred to a heat change or a heat of reaction.
The enthalpy change is equal to the enthalpy or “heat content,” H, of the substances produced minus the enthalpy of the substances consumed.
100WM_ THERMODYNAMICS
EHTHALPY AS STATE FUNCTION
It is impossible to know the absolute enthalpy (heat content) of a system.
Enthalpy is a state function, however, and it is the change in enthalpy in which we are interested. This can be measured for many processes.
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WM_ THERMODYNAMICS 104
Calculate the enthalpy of the ammonium nitrate
decomposition. The reaction is
and the enthalpies of the three compounds are given in
Table 1.
ΔH = [ 82+ 2(-242)] – [-366] = -36 kJ
N2O H2O NH4NO3
products reactant
105
If you reverse the previous reaction,
the sign of the enthalpy of the reaction is reversed:
Δ H = +36 kJ
WM_ THERMODYNAMICS 106
http://www.cliffsnotes.com/sciences/chemistry/chemistry/thermodynamics/e
nthalpy
WM_ THERMODYNAMICS 107
Po
ten
tiale
ne
rgy
reactants
products
ΔH > 0
products
Po
ten
tiale
ne
rgy
reactants
ΔH < 0
Reaction progress Reaction progress
EndothermicExothermic
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108
There are two ways of looking at what happens
to the enthalpy:
If the reaction is exothermic the products
have minimum enthalpy and the formation of
products (move toward the right) is favourable
If the reaction is endothermic the reactants
have minimum enthalpy and the formation of
products (move toward the right) is
unfavourable.
In this case the formation of reactants (move
toward the left) is favourable.
WM_ THERMODYNAMICS 109
•Calculate the enthalpy change for the following
reaction and classify it as exothermic or
endothermic.
WM_ THERMODYNAMICS 110
Compound ΔH0
MgCl2 (S) -642 kJ/mol
H2O (l) -286 kJ/mol
MgO (S) -602 kJ/mol
HCl (g) -92 kJ/mol
Standard enthalpies of formation
ΔH = [ΔH0 (MgO) + 2 ΔH0 (HCl)] – [ ΔH0 (MgCl2) + ΔH0(H2O)]
End of part 1
WM_ THERMODYNAMICS 111