When chemical reactions take place they are often accompanied by energy changes.
Chemical reactions most frequently occur in open vessels. That is, they take place at constant pressure. Enthalpy refers to energy at constant pressure
(volume may vary).
The term 'enthalpy' comes from the Greek word enthalpo , meaning warming up.
You are given a sample of methane, CH
4
. How much energy does its molecules contain? The first thing you want to know is the amount of methane present.
Chemists say that the answer is 1 mole (16 g). Energy is measured in joules, J, so you begin by thinking where to start measuring from. There seems to be no starting point; can methane molecules ever have no energy contained within them? Indeed, it is impossible to know the total amount of energy stored in these molecules. What ever its value, the total amount of energy in a given amount of a substance (sometimes called the Heat energy content ) is known as the enthalpy , denoted H .
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Methane is a fuel, so how do we get energy from it? The answer is to react it with oxygen.
CH
4
(g) + 2O
2
(g)

CO
2
(g) + 2H
2
O(l)
The above chemical equation shows that 2 moles (64 g) of oxygen molecules are required to burn 1 mole of methane. Again, it is impossible to know the total enthalpy (heat energy content) of the oxygen. Likewise, we can't know the total heat energy content of 1 mole of CO
2
and 2 moles of H
2
O (the products).
Now imagine that we could find 'molar enthalpy values' for elements and compounds in a chemical data book. This would allow us to work out the amount of energy given out when methane reacts with oxygen to form carbon dioxide and water, that is, an overall change in enthalpy ,

H , when the above reaction takes place. The following equations represent such a calculation.

H = (H
CO2
+ 2H
H2O
) - (H
CH4
+ 2H
O2
)
In general,


H =

H products
-

H reactants
But remember, this is theoretical; it is not possible to determine the absolute value of the enthalpy of a chemical element or compound. However,

H values for chemical reactions can be obtained. They can be measured experimentally, or calculated using Hess's Law (see later), or worked out in other ways.
When chemical reactions take place they are often accompanied by heat changes . The system (the reactants which form products) may give out heat to the surroundings , causing them to warm up. In this case the reactants have more stored energy (greater total enthalpy) than the products. Such chemical reactions are said to be exothermic . The system may take heat from the surroundings, causing them to cool down. In this case the reactants have less stored energy (less total enthalpy) than the products. Such chemical reactions are said to be endothermic .
Exothermic reactions give out energy to the surroundings.
These are the ones we like.
Endothermic reactions take energy from the surroundings.
It is possible to measure changes in heat energy that accompany chemical reactions. Most reactions take place in vessels that are open to the atmosphere, that is, they take place at constant pressure (volume may vary). The special name given to a change in heat energy content measured at constant pressure is enthalpy change . This change in enthalpy is denoted by

H . The value of

H
(often expressed in kJ, or kJ mol
-1
when appropriate) is given a negative sign for exothermic reactions and a positive sign for endothermic reactions , indicating whether the system loses or gains energy as a result of the reaction.
The value of

H is given a negative sign for an exothermic reaction.
The value of

H is given a positive sign for an endothermic reaction.
Precise thermochemical measurements are made in a closed vessel of fixed volume, such as a calorimeter. For a reaction involving a change in volume of gases there is a small but real difference in the measured heat change. You can read more about this here .

Enthalpy level diagrams can be used to illustrate overall exothermic and endothermic changes. They show the difference in total enthalpy of the reactants and products for a reaction:
For an exothermic reaction the total enthalpy of the products is less than that of reactants. For an endothermic reaction the total enthalpy of the reactants is less than that of the products. For each, the difference in these total enthalpies is equal to the overall enthalpy of the reaction,

H.
As well as the amounts of substances reacting (molar amounts are taken), the precise value of

H depends on both the temperature and pressure at which it is measured. For this reason

H values are expressed at standard conditions
(normally 298 K and 1 atm.). A standard enthalpy change is written as
 H°
298
.
You can read more about temperature and pressure here .
Thermochemical data for chemical reactions can be found in chemical data books.
CH
4(g)
+ 2O
½H
2(g)
2(g)

CO
2(g)
+ 2 H
2
O
(l)
 H°
+ ½I
2(s)

HI(g)
 H°
298
298
= - 890.3 kJ mol
= + 26.5 kJ mol -1
-1
Note the following changes:
CO
2(g)
+ 2 H
2
O
(l)

CH
4(g)
+ 2O
2(g)
 H°
298
= + 890.3 kJ
H
2(g)
+ I
2(s)

2HI(g)
 H°
298
= + 53.0 kJ
Units are important...
Also note how the units are expressed in these thermochemical equations. kJ

mol
-1
is used when 1 mole of a substance is burned and when 1 mole of compound is formed. Refer to the Enthalpy Definitions section below for more explanation.
Methane and oxygen do not react spontaneously when mixed. An input of energy, such as a flame, is required to get the reaction started, after which its exothermic nature will sustain it. With regard to the collision theory of reaction rates , molecules react only if in a collision they possess between them energy equal to or greater than a certain critical value. This is called the activation enthalpy , Ea . An enthalpy profile diagram illustrates this:
See also Activation Enthalpy .
There are several enthalpy definitions. The combustion of elements and compounds, and the formation of compounds from their elements, are important here.
Standard Molar Enthalpy of Combustion,
 H° c,298
. The heat evolved when
1 mole of an element or compound completely burns in oxygen, measured at
298 K and 1 atm. pressure.
Standard Molar Enthalpy of Formation,
 H° f,298
. The heat change when 1 mole of a compound forms from its elements in their standard states, measured at 298 K and 1 atm. pressure. From the definition, the enthalpy of formation of an element at 298 K and 1 atm. is zero.
The definitions refer to measurements under standard conditions, but these are sometimes omitted.

Here is a very useful equation that follows from the definition for enthalpy of formation. It can be used to calculate overall enthalpy changes when the enthalpies of formation of the reactants and products are given. It is

H =

H f (products)
-

H f (reactants)
Thermochemical equations can be written to satisfy these enthalpy definitions. Always include state symbols, for example,
C
(s)
+ O
2(g)

CO
2(g)
 H° c,298
= -393.5 kJ mol
-1
(or
 H° f,298
= -393.5 kJ mol -1 )
2C
(s)
+ 3H
2(g)
+ ½O
2(g)

C
2
H
5
OH
(l)
 H° f,298
= -277.1 kJ mol
-1
A calorimeter is used to measure heat changes directly (with adjustments made for standard conditions at constant pressure). However, this is not possible for some reactions. Their enthalpy changes can be calculated with the application of Hess's Law .
Hess's Law says that the overall enthalpy change accompanying a chemical reaction is independent of the route taken in going from reactants to products
(provided that in each case the same initial and final states of temperature and pressure apply to the reactants and products).
Hess's Law (G.H. Hess, 1840) of 'constant heat summation' follows from the
First Law of Thermodynamics, which can be stated very simply: Energy can neither be created nor destroyed .
Thermodynamics in the study of transforming energy from one form to another.
Specific heat capacity is used in some thermochemical calculations. It is the amount of energy required to raise the mass of 1 gram of a substance by 1 °C. q = specific heat capacity x mass x

T.
The specific heat capacity of water is 4.184 J g
-1
K
-1
. This means that 4.184 J of energy are required to raise each gram of water by each °C.
Heat capacity can also be used in calculations. It is the amount of energy required to raise the whole mass of a body by 1 °C. q = heat capacity x

T.

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"Kiloton" redirects here. For the similarly named weight measurements, see
Tonne .
Diagram of explosive yield vs mushroom cloud height, illustrating the difference between 22 kiloton Fat Man and 15 megaton Castle Bravo explosions
TNT equivalent is a method of quantifying the energy released in explosions.
The ton (or tonne , i.e. metric ton) of TNT is a unit of energy equal to 4.184 gigajoules , which is approximately the amount of energy released in the detonation of one ton of TNT . The megaton of TNT is a unit of energy equal to 4.184 petajoules .
[1]
The kiloton and megaton of TNT have traditionally been used to rate the energy output, and hence destructive power, of nuclear weapons (see nuclear weapon yield ). This unit is written into various nuclear weapon control treaties , and gives a sense of destructiveness as compared with ordinary explosives , like
TNT. More recently, it has been used to describe the energy released in other highly destructive events, such as asteroid impacts. However, TNT is not the most energetic of conventional explosives. Dynamite , for example, has more than 60% more energy density (approximately 7.5 MJ/kg, compared to 4.184
MJ/kg for TNT).


A gram of TNT releases 4100–4602 Joules upon explosion. To define the tonne of TNT , this was arbitrarily standardized by letting 1 gram TNT =
4184 J (exactly).
[2] This conveniently defined the energy liberated by one gram of TNT as exactly one kilocalorie .
This definition is a conventional one. The explosive's energy is normally calculated using the thermodynamic work energy of detonation, which for TNT has been accurately measured at 4686 J/g from large numbers of air blast experiments and theoretically calculated to be 4853 J/g.
[3]
The measured pure heat output of a gram of TNT is only 2724 J, [4] but this is not the important value for explosive blast effect calculations.
Alternative TNT equivalency can be calculated depending upon when in the detonation the value is measured and which property is being compared.
[5][6][7][8]
A kiloton of TNT can be visualized as a cube of TNT of 8.46 metres (27.8 ft) on a side.
Grams TNT Symbol Tons TNT Symbol Energy gram of TNT g microton of TNT μt
4.184×10 3
J kilogram of TNT kg milliton of TNT mt
4.184×10 6
J megagram of TNT Mg gigagram of TNT Gg teragram of TNT Tg petagram of TNT Pg ton of TNT t kiloton of TNT kt megaton of TNT Mt gigaton of TNT Gt
4.184×10 9 J
4.184×10 12
J
4.184×10 15
J
4.184×10 18
J
A convenient energy unit, particularly for atomic and nuclear processes, is the energy given to an electron by accelerating it through 1 volt of electric potential difference. If an electron starts from rest at the negative plate, then the electric field will do work eV on it, giving it that amount of kinetic energy when it strikes the positive plate. The work done on the charge is given by the charge times the voltage difference, which in this case is:

The abbreviation for electron volt is eV.
I find it useful to think of a battery. Most give about 1 volt but a few give 7 volts. This is clearly the characteristic energy of a chemical reaction. It comes from rearranging the outer electrons in an atom. But rearranging the nucleons in a nucleus can give energies 100,000 to 1,000,000 times as large.
THIS IS THE ONE FACTOR YOU MUST REMEMBER
Room temperature thermal energy of a molecule..................................0.04 eV
Visible light photons...........................................................................1.5-3.5 eV
Energy for the dissociation of an NaCl molecule into Na + and Cl ions:.............................................................................................4.2 eV
Ionization energy of atomic hydrogen ...................................................13.6 eV
Approximate energy of an electron striking a color television screen (CRT display) ...............................................................................20,000 eV
High energy diagnostic medical x-ray photons..................200,000 eV (=0.2
MeV)
Typical energies from nuclear decay:
(1) gamma..................................................................................0-3 MeV
(2) beta.......................................................................................0-3 MeV
(3) alpha......................................................................................2-10 MeV
Cosmic ray energies ...........................................................1 MeV - 1000 TeV

1 MeV = 10 6 eV, 1 GeV = 10 9 eV, 1 TeV = 10 12 eV
Nuclei are made up of protons and neutron , but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. This binding energy can be calculated from the
Einstein relationship :
2
For the alpha particle Δm= 0.0304 u which gives a binding energy of 28.3
MeV.
The enormity of the nuclear binding energy can perhaps be better appreciated by comparing it to the binding energy of an electron in an atom. The comparison of the alpha particle binding energy with the binding energy of the electron in a hydrogen atom is shown below. The nuclear binding energies are on the order of a million times greater than the electron binding energies of atoms.

Binding energy curve.
Nuclear units.
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HyperPhysics ***** Nuclear R Nave
The binding energy curve is obtained by dividing the total nuclear binding energy by the number of nucleons. The fact that there is a peak in the binding energy curve in the region of stability near iron means that either the breakup of heavier nuclei (fission) or the combining of lighter nuclei (fusion) will yield nuclei which are more tightly bound (less mass per nucleon).
The binding energies of nucleons are in the range of millions of electron volts compared to tens of eV for atomic electrons. Whereas an atomic transition might emit a photon in the range of a few electron volts, perhaps in the visible light region, nuclear transitions can emit gamma-rays with quantum energies in the MeV range.
The buildup of heavier elements in the nuclear fusion processes in stars is limited to elements below iron, since the fusion of iron would subtract energy rather than provide it. Iron-56 is abundant in stellar processes, and with a binding energy per nucleon of 8.8 MeV, it is the third most tightly bound of the nuclides. Its average binding energy per nucleon is exceeded only by
58
Fe and
62
Ni, the nickel isotope being
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the most tightly bound of the nuclides.
Energy in Fission
Typical fission events release about two hundred million eV (200 MeV) of energy for each fission event. By contrast, most chemical oxidation reactions (such as burning coal or TNT ) release at most a few eV per event. So, nuclear fuel contains at least ten million times more usable energy per unit mass than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic radiation in the form of gamma rays ; in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid , usually water or occasionally heavy water .
When a uranium nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus
[5]
appears as the fission energy of ~200 MeV. For uranium-235
(total mean fission energy 202.5 MeV), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion .
Also, an average of 2.5 neutrons are emitted, with a mean kinetic energy per neutron of ~2 MeV
(total of 4.8 MeV).
[6] The fission reaction also releases ~7 MeV in prompt gamma ray photons .
The latter figure means that a nuclear fission explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its energy as fast neutrons (total ~ 6%), and the rest as kinetic energy of fission fragments (this appears almost immediately when the fragments impact surrounding matter, as simple heat ). In an atomic bomb, this heat may serve to raise the temperature of the bomb core to 100 million kelvin and cause secondary emission of soft X-rays, which convert some of this energy to ionizing radiation. However, in nuclear reactors, the fission fragment kinetic energy remains as low-temperature heat, which itself causes little or no ionization.