10.4 Chemical Reactions and Energy
chemical
bonds and
energy
Every chemical reaction involves the breaking and the formation of chemical bonds. In
order to break chemical bonds energy must be provided. When chemical bonds are
formed energy is released. Therefore every chemical reaction either releases or absorbs
energy. The amount of energy involved depends on the particular reaction. From the
burning of fossil fuels to the melting of ice on the street we rely on the energy released or
absorbed by chemical reactions.
hot packs
release energy
cold packs
absorb energy
Cold and hot packs have become the best tools
for athletic trainers. When an athlete is injured,
the trainer reaches in the first aid kit, grabs a
small plastic bag, punches it and applies it on
the injured muscle. Within seconds the bag is ice
cold. How does this happen? At another time the
trainer may reach in the first aid kit and get a
small bag which in an instant becomes very hot
and can be used to treat an injury. Where does
this heat come form? Well, both the hot and the
cold packs rely on chemical reactions to do their
job. Inside the hot pack a chemical reaction
releases energy and inside the cold pack another
chemical reaction absorbs energy.
exothermic:
release of
energy.
endothermic:
absorption of
energy
When a chemical reaction releases energy is called exothermic reaction. When a
chemical reaction absorbs energy is called endothermic reaction.
The amount of energy that is released or absorbed is denoted by the symbol ΔH which is
called the enthalpy of the reaction. Enthalpy, which means to put heat into, is another
word for energy and it has the units of Joule. A chemical reaction is presented completely
by writing the enthalpy information on the right of the chemical equation.
ΔH < 0
exothermic
Reactants → Products
ΔH
For exothermic reactions ΔH is a negative number indicating that energy is released by
the reaction.
ΔH > 0
endothermic
For endothermic reactions ΔH is a positive number indicating that energy is absorbed by
the reaction.
exothermic reaction - a reaction that releases energy. ΔH<0
endothermic reaction - a reaction that absorbs energy. ΔH>0
enthalpy - is related to the amount of energy that a chemical reaction releases to the
environment or absorbs from the environment.
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Section 10.4 Chemical Reactions and Energy
Thermochemical equations
When carbon is burned with oxygen it produces carbon dioxide and releases energy in
the form of heat and light. Since the reaction releases energy it is an exothermic reaction.
enthalpy is a
measured
quantity
For this reaction the enthalpy change is measured to be -393.5 kJ, where kJ (kilojoule) is
1000 Joules. The complete reaction is written as follows
C(s) + O2(g) → CO2(g)
ΔH = -393.5 kJ
This is called a thermochemical equation and it includes the chemical equation and
the information about the enthalpy change on the right. The negative sign for ΔH means
that the reaction is exothermic. As expected the enthalpy change is related to the amount
or material involved in the reaction. For example, the more carbon we burn the more CO2
is produced and the more energy is released. The enthalpy change refers to a certain
amount of a substance involved in the reaction. In our case the denoted enthalpy change
refers to the formation of 1 mole of CO2. If we burn 2 moles of C the enthalpy change
will be twice as great
2C(s) + 2O2(g) → 2CO2(g)
enthalpy of
formation is
used to
calculate
reaction
enthalpy
ΔH = -787 kJ
The enthalpy change is a measured quantity and it is obtained experimentally for the
various reactions. Since there are millions of possible reactions it is impossible to list all
of them with their enthalpy values. Chemists have measured and cataloged the standard
enthalpy of formation for many common substances. The enthalpy of formation
corresponds to the enthalpy change during the formation of one mole of a substance.
These values are given at standard conditions (25 °C and 1 atmosphere pressure). Using
these values they are then able to calculate the enthalpy of most reactions. Some enthalpy
values for the formation of some common substances are shown on Table 10.2.
TABLE 10.2. Enthalpies of Formation
Substance
ΔHf(kJ/mole)
ΔHf (kJ/mole)
Substance
CO2
-393.5
NO2
33.2
CO
-110.5
O3
142.7
CuSO4
-771.4
C (diamond)
1.9
H2O (l)
-285.5
C6H6
49.0
Fe2O3
-824.2
O2
0
thermochemical equation - the equation that gives the chemical reaction and the
energy information of the reaction.
enthalpy of formation - the enthalpy change during the formation of one mole of a
substance.
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Exothermic and endothermic reactions
Reverse
reactions
The enthalpy change of the reverse reaction is the negative of the enthalpy change of the
forward process. For our CO2 example reaction we have
CO2(g) → C(s) + O2(g)
ΔH = 393.5 kJ
The enthalpy of formation for each substance involved in a reaction (ΔHf) is related to
the reaction enthalpy (ΔH) by the equation
ΔH = ΔHf (products) - ΔHf (reactants)
our bodies are
powered by
exothermic
reactions
This is a mathematical expression of energy conservation and may be applied in order to
calculate an unknown reaction enthalpy or an unknown enthalpy of formation.
A chemical reaction that takes place continuously inside our bodies results from the
combination of glucose (C6H12O6) with oxygen. This is an exothermic reaction and it is
given by the thermochemical equation
C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H2O (g)
ΔH = -2808 kJ
The negative sign for ΔΗ indicates that the reaction releases energy and thus it is
exothermic. This reaction tells us that when one mole of glucose (C6H12O6), 180 g, is
combined with 6 moles of oxygen it releases 2808 kJ of energy. This energy is used by
our bodies to help us grow and move. It is the energy that makes our life possible.
The complete combustion of glucose (C6H12O6) releases 2,808kJ of energy per mole
of glucose. Calculate the enthalpy of formation of glucose.
Given:
The enthalpy of combustion of one mole of glucose is 2,808 kJ.
The combustion reaction
C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H2O (g) ΔH = -2,808 kJ
From Table 10.2 we read: The enthalpy of formation of O2 is 0.
The enthalpy of formation of CO2, ΔHf (CO2), is -393.5 kJ
The enthalpy of formation of H2O, ΔHf (H2O) is -285.5 kJ
Relationships: ΔH = ΔHf (products) - ΔHf (reactants)
Solve:
We write down the reaction for the formation of glucose
6CO2 (g) + 6H2O (g) → C6H12O6 (s) + 6O2 (g) ΔH = +2,808 kJ
From the enthalpy relation equation we have:
ΔH = ΔHf(glucose) + 6 ΔHf (O2)- 6 ΔHf (CO2)- 6 ΔHf (H2O)
2,808 = ΔHf(glucose) + 0 - 6 (-393.5) - 6 (-285.5) which gives
ΔHf(glucose) = -1,266 kJ
Answer:
The enthalpy of formation for glucose is -1,266 kJ per mole.
or -7.0 kJ per gram since there are 180 g/mole
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Section 10.4 Chemical Reactions and Energy
Calculating the enthalpy change of a reaction
rusting of iron
releases energy
The rusting of iron, as we saw in the experiment at the beginning of this chapter, results
from a chemical reaction that is exothermic. When iron (Fe) reacts with oxygen (O2) it
produces iron oxide (Fe2O3), rust, and it generates heat. For each mole of iron oxide the
enthalpy change is -824.2 kJ. This reaction is written as
2 Fe +3/2 O2 → Fe2O3
Coefficients
multiply
enthalpies of
formation
ΔΗ = -824.2 kJ
We have written the above thermochemical equation for the formation of one mole of
Fe2O3. This is the reason for the use of the fractional coefficient in front of O2. If we
were to write the coefficients using the smaller whole number possible the
thermochemical reaction would be
4 Fe +3 O2 → 2 Fe2O3
ΔΗ = -1648.4 kJ
The energy released per mole of Fe2O3 is still the same (-824.2 kJ) but now since the
chemical equation has a coefficient of 2 in front of iron oxide we must multiple the noted
enthalpy change by 2.
Endothermic
reactions
Have you noticed that when you chew certain types of gum your mouth feels cooler?
You actually feel the result of an endothermic reaction taking place in your mouth.
Ingredients in gum called polyols dissolve in the saliva resulting in an endothermic
reaction. The reaction absorbs energy from your mouth which as a result feels cooler. A
common gum ingredient is xylitol (C5H10O4) which when dissolved in water or saliva
absorbs about 22.3 kJ/mole or about 167 J/g. This is considerable energy resulting in a
cool and refreshing feeling in your mouth.
photosynthesis
stores energy
The glucose that our bodies burn was generated by plants during photosynthesis.
Photosynthesis reaction captures the energy of the sun and it is the fundamental energy
storage reaction. The photosynthesis reaction takes place inside plants and combines
water and carbon dioxide to make glucose and oxygen. The thermochemical reaction of
photosynthesis is
6CO2 (g) + 6H2O (g) → C6H12O6 (s) + 6O2 (g) ΔH = +2808 kJ
photosynthesis
is an
endothermic
reaction
Photosynthesis is an endothermic chemical reaction as indicated by the positive sign for
ΔH. The energy required to make this reaction happen is provided by sunlight.
Photosynthesis is thus the ultimate energy storage reaction. It stores the energy from the
sun and it produces oxygen which are in turn used for sustaining life on earth.
Photosynthesis has been capturing sunlight and storing it in compounds that contain
carbon and hydrogen, the hydrocarbons.
photosynthesis - the chemical reaction that combines CO2 and H2O to form
glucose C6H12O6 and oxygen. The reaction is endothermic and it driven by sunlight.
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Energy profile of chemical reactions
Why
exothermic
reactions
do not start
spontaneously
A spontaneous
reaction
The synthesis reaction of carbon dioxide is an exothermic reaction releasing a substantial
amount of energy.
C(s) + O2(g) → CO2(g)
ΔH = -393.5 kJ
However, carbon does not spontaneously catch fire! In order to burn a piece of carbon it
requires energy to start the process. It is not enough that we simply put carbon and
oxygen together. Some initial energy is needed to break the bonds between oxygen atoms
in O2 before they can re-form with carbon atoms to make CO2.
Another exothermic reaction that releases considerable energy is the reaction of sodium
(Na) with water (H2O).
Na(s) + H2O → NaOH(aq) + 1/2H2 (g)
ΔΗ=-140.9 kJ
This reaction IS spontaneous. When we drop a
piece of sodium metal in water the reaction starts
immediately with an impressive release of energy.
Both the carbon-oxygen and sodium-water
reactions are exothermic but one of them happens
spontaneously while the other needs to be initiated.
The reason for
the difference
The answer has to do with chemical bonds and
the energy that is required in order to break
them and to form them. This is best shown
with the energy diagram of the reaction. In an
energy diagram we show the bond energies of
the products and the reactants as well as the
energy path that they follow during the
reaction. This presentation gives us an energy
profile for the reaction. On the vertical axis of
the energy diagram we give the total bond
energy of reactants and products. The
horizontal axis is used to show the time
progression of the reaction from reactants to products. The energy profile may have a
barrier represented as a “hump” in the diagram. This energy barrier corresponds to the
energy input needed for the reaction to proceed. Once the reaction happens the net energy
of the reaction is the difference between the initial and the final energy levels. For an
exothermic reaction the energy level of the products is less than the energy level of the
reactants. The difference in the energy is the amount that is released from the reaction.
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Section 10.4 Chemical Reactions and Energy
The energy barrier
some reactions
need external
energy to
proceed
energy barrier
The presence of an energy
barrier in the carbon-oxygen
reaction means that we must
provide external energy for the
reaction to proceed.
The external energy is needed
in order to overcome the energy
barrier. Only the molecules that
have enough energy to go over
the energy barrier can
react.energy of a reaction is
stored in chemical bonds
Without getting into the fine details of molecular bonds we can say that all the energy
information about this reaction is included in the bonds of molecular oxygen and carbon
dioxide.
In order for the reaction to proceed the first thing that has to happen is to break the bonds
of molecular oxygen. This requires a certain amount of energy input, indicated by the
barrier in the energy profile of the reaction.
The “energy content” in the bonds of C and O2 is higher than the energy content of the
product CO2. The difference in the two energy levels corresponds to the enthalpy of the
reaction.
some reactions
happen
spontaneously
The sodium-water reaction has an
energy profile that has a very
small barrier and all the energy
that is needed to overcome it is
provided by the reactants. In this
case the reaction proceeds
without requiring any external
energy input. This type of
reaction is called spontaneous
which means that it can happen
spontaneously by simple bringing
the two reactants together.
energy barrier - The energy needed to initiate a chemical reaction.
spontaneous reaction - A chemical reaction that happens without the need for
external energy input.
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Hess’s law: A method for calculating enthalpy change.
enthalpy does
not depend on
the path taken
enthalpy is a
state function
Hess’s law
Enthalpy, like energy, is conserved and it
depends only on the initial and the final states
of the process that generate it. In chemistry we
say that enthalpy is a state function to denote
this property. Since the enthalpy depends only
on the initial and the final states, the value of
the enthalpy change depends only on the initial
and the final states. For a chemical reaction the
initial state is related to the reactants and the
final state is related to the products. As a
reaction proceeds from reactants to products,
the actual path that we take in calculating the
enthalpy change does not matter. If we go directly from Reactants to Products following
path 1 as indicated in the schematic the enthalpy change is ΔH. The change is also ΔH
even if we follow path 2 going from Reactants to A then to B and finally to Products.
Note that the enthalpy going from Reactants to A is negative, while the enthalpy going
from A to B is positive. This is a general rule called Hess’s law.
The overall enthalpy of a reaction is the sum of the reaction enthalpies of
the various steps into which a reaction can be divided.
Given that the enthalpy of combustion of graphite (Cgr) and diamond (Cd) are -393.5
kJ/mole and -395.4 kJ/mole respectively, calculate the formation enthalpy of diamond
from graphite.
Given:
Asked:
The entahlpies of combustion and the compounds of interest.
We are asked to calculate the enthalpy of the reaction:
Cgr (s) → Cd (s) ΔH = ?
Relationships: The basic relationship is Hess’s law and the combustion reactions
Cgr(s) + O2(g) → CO2(g)
ΔΗ1 = -393.5kJ/mole
Cd(s) + O2(g) → CO2(g)
ΔH2 = -395.4 kJ/mole
Solve:
A NATURAL APPROACH TO CHEMISTRY
We will apply Hess’s law by considering a path that will start with Cgr(s)
and give Cd(s). We can create such a path by considering the two reactions
Cgr(s) + O2(g) → CO2(g)
ΔΗ1 = -393.5kJ/mole
CO2(g) → Cd(s) + O2(g)
ΔH2 = +395.4 kJ/mole
Note the change of sign for ΔH2.
The first reaction produces CO2 and uses O2 and the second does the exact
opposite. The sum of the two reactions gives
Cgr(s) → Cd(s) ΔH = (395.4 - 393.5) kJ/mole = +1.9 kJ/mole
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