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What Is Heat Of Formation In Chemistry

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Characteristic Functions And Natural State Variables

Enthalpy of Formation Reaction & Heat of Combustion, Enthalpy Change Problems Chemistry

The enthalpy, H, expresses the thermodynamics of a system in the energy representation. As a function of state, its arguments include both one intensive and several extensive state variables. The state variables S, p, and are said to be the natural state variables in this representation. They are suitable for describing processes in which they are determined by factors in the surroundings. For example, when a virtual parcel of atmospheric air moves to a different altitude, the pressure surrounding it changes, and the process is often so rapid that there is too little time for heat transfer. This is the basis of the so-called adiabatic approximation that is used in meteorology.

Conjugate with the enthalpy, with these arguments, the other characteristic function of state of a thermodynamic system is its entropy, as a function, S, of the same list of variables of state, except that the entropy, S, is replaced in the list by the enthalpy, H. It expresses the entropy representation. The state variables H, p, and are said to be the natural state variables in this representation. They are suitable for describing processes in which they are experimentally controlled. For example, H and p can be controlled by allowing heat transfer, and by varying only the external pressure on the piston that sets the volume of the system.

Problem Solving: Calculating Enthalpy Of Reaction From Standard Enthalpy Of Formation Values

Below is a worked example of finding the standard enthalpy change for a reaction using the standard enthalpy of formation data.

The Problem:

  • NH3 fHo = 46 kJ mol-1
  • NO2 fHo = +34 kJ mol-1
  • H2O fHo = 242 kJ mol-1
  • O2 fHo = 0 kJ mol-1

Solving the Problem using the StoPGoPS approach to problem solving in chemistry.


Standard Enthalpy Of Formation Is Zero For

For an element: the form in which the element is most stable under 1 bar of pressure. One exception is phosphorus, for which the most stable form at 1 bar is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation. All elements in their standard states have a standard enthalpy of formation of zero, as there is no change involved in their formation.

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Enthalpy Of Reaction Definition

Before we define the enthalpy of reaction, we should first define enthalpy.

Enthalpy is the total heat content of a system. The formula definition is the internal energy plus the product of pressure and volume: $$H=U+PV\,\,\text$$

enthalpy of reaction.

The enthalpy of reaction is the change in enthalpy due to a chemical reaction. The general formula is: $$\Delta H_=H_-H_=q\,\,\text$$

If Hrxn> 0, the reaction is endothermic

If Hrxn< 0, the reaction is exothermic

Here is a diagram showing the difference between these two reaction types:

Endothermic reactions absorb heat, while exothermic reactions release heat. StudySmarter Original.

  • In an endothermic reaction, heat is absorbed from the surroundings for the reaction to proceed. Since the system ends with more heat than it started with, the change in enthalpy is positive.
  • For exothermic reactions, the products have a lower enthalpy than the reactants. This means that the “extra” heat is released into the surroundings, and the change in enthalpy is negative.

We also will sometimes label the change in enthalpy as something other than Hrxn based on the type of reaction that is happening. These are:

An Introduction To Heat Of Formation

Energetics of Ion Formation

In the field of Physics as well as Chemistry, the influence of compounds and mole is hugely significant. The reason behind such significance is that through the study and analysis of one mole of any and every compound, a broader, more generalized idea about the compound, or even similar compounds can be deduced and inferred. Therefore, the answer to What is enthalpy of formation is- it is the heat amount that is absorbed or even evolved during the process of formation of one mole of any compound from the constituent elements of the compound.

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Summary Heat Of Formation Vs Heat Of Reaction

Enthalpy is the energy content. The change of enthalpy indicates how much energy exchanges between reactants, products and the surrounding. The difference between heat of formation and heat of reaction is that the heat of formation is the change in enthalpy during the formation of a mole of a substance at standard conditions whereas the heat of reaction is the change in enthalpy during a chemical reaction that occurs at a constant pressure.


1. Libretexts. Standard Enthalpy of Formation. Chemistry LibreTexts, Libretexts, 9 Feb. 2017. Available here2. Britannica, The Editors of Encyclopaedia. Heat of Reaction. Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 Apr. 2016. Available here

Enthalpy Definition In Chemistry And Physics

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  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College

Enthalpy is a thermodynamic property of a system. It is the sum of the internal energy added to the product of the pressure and volume of the system. It reflects the capacity to do non-mechanical work and the capacity to release heat.

Enthalpy is denoted as H specific enthalpy denoted as h. Common units used to express enthalpy are the joule, calorie, or BTU Enthalpy in a throttling process is constant.

Change in enthalpy is calculated rather than enthalpy, in part because total enthalpy of a system cannot be measured since it is impossible to know the zero point. However, it is possible to measure the difference in enthalpy between one state and another. Enthalpy change may be calculated under conditions of constant pressure.

One example is of a firefighter who is on a ladder, but the smoke has obscured his view of the ground. He cannot see how many rungs are below him to the ground but can see there are three rungs to the window where a person needs to be rescued. In the same way, total enthalpy cannot be measured, but the change in enthalpy can.

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What Is The Importance Of Enthalpy

  • Measuring the change in enthalpy allows us to determine whether a reaction was endothermic or exothermic
  • It is used to calculate the heat of reaction of a chemical process.
  • Change in enthalpy is used to measure heat flow in calorimetry.
  • It is measured to evaluate a throttling process or Joule-Thomson expansion.
  • Enthalpy is used to calculate minimum power for a compressor.
  • Enthalpy change occurs during a change in the state of matter.
  • There are many other applications of enthalpy in thermal engineering.

Heat Of Reaction Formula

Heat of formation | Thermodynamics | Chemistry | Khan Academy

The reaction takes place in a closed system with a moveable piston that keeps the pressure constant. As nitrogen dioxide gas is generated, the piston will rise. By elevating the piston against the downward force exerted by the atmosphere, the mechanism accomplishes its task . By multiplying the external pressure P by the volume change induced by piston move), we can calculate the amount of PV work done. When external pressure is constant ,

w = PV

The sum of all of a system’s components’ kinetic and potential energy is the system’s internal energy U. Heat and work are produced by a change in internal energy. Chemists commonly employ a related thermodynamic number termed enthalpy to measure the energy changes that occur in chemical reactions. The sum of a system’s internal energy U plus the product of its pressure P and volume V is known as its enthalpy:

H = U + PV

When a chemical change occurs at constant pressure , the change in enthalpy is proportional to the change in pressure.

H =

= qp + w w

= qp

This equation is only true for a process that happens at constant pressure, as indicated by the subscript p. We can see from Equation that the change in enthalpy, H of the system, is equal to the heat received or lost at constant pressure.

H = Hfinal Hinitial= qp

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Calculating A Molar Heat Of Reaction From Formation Enthalpies

Now that we know our formulas, let’s do some calculations!

$$C_3H_+5O_\rightarrow 3CO_+4H_2O_$$

What is Hrxn for the combustion of methane given the following thermodynamic data:

H2O -241.8

Like before, we need to adjust the enthalpy values to match the molar amounts given in the chemical equation. You’ll note that there are two values of H°f for both Ca2 and H2O. You always need to pay attention to the states of your molecules when looking up thermodynamic data, since H°f depends on the state.


Now we can solve for Hrxn:



Example Change In Enthalpy Calculation

You can use the heat of fusion of ice and heat of vaporization of water to calculate the enthalpy change when ice melts into a liquid and the liquid turns to a vapor.

The heat of fusion of ice is 333 J/g The heat of vaporization of liquid water at 100°C is 2257 J/g.

Part A: Calculate the change in enthalpy, H, for these two processes.

H2O H2O H = ?H2O H2O H = ?Part B: Using the values you calculated, find the number of grams of ice you can melt using 0.800 kJ of heat.

SolutionA. The heats of fusion and vaporization are in joules, so the first thing to do is convert to kilojoules. Using the periodic table, we know that 1 mole of water is 18.02 g. Therefore:fusion H = 18.02 g x 333 J / 1 gfusion H = 6.00 x 103 Jfusion H = 6.00 kJvaporization H = 18.02 g x 2257 J / 1 gvaporization H = 4.07 x 104 Jvaporization H = 40.7 kJSo the completed thermochemical reactions are:H2O H2O H = +6.00 kJH2O H2O H = +40.7 kJB. Now we know that:1 mol H2O = 18.02 g H2O ~ 6.00 kJUsing this conversion factor:0.800 kJ x 18.02 g ice / 6.00 kJ = 2.40 g ice melted

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Enthalpy Of Formation Equation

As noted earlier, the standard enthalpy of formation terms for products, Hf °, and the standard enthalpy of formation terms for reactants, Hf °, are used to calculate the standard enthalpy of reaction, H °:


Where, , is the summation symbol and, q, and, r, are the stoichiometric coefficients of the balanced equation for products and reactants, respectively.

For example, consider the following reaction between methane gas, CH4 , and chlorine gas, Cl2 , to yield carbon tetrachloride liquid, CCl4 , and hydrochloric acid gas, HCl :

Here we have the following reaction between methane gas, CH4 , and chlorine gas, Cl2 , to yield carbon tetrachloride liquid, CCl4 , and hydrochloric acid gas, HCl :

$$1CH_4\,+4Cl_2\, \rightarrow 1CCl_4\,+4HCl\,$$

Then the enthalpy of formation equation will give the enthalpy of reaction:

\begin\Delta& =\Sigma_^n,_i-\Sigma_^m\,_i\\& =-\\& =-\\& =-433\,kJ/mol\end

Notice that in the table above, those compounds that are in their elemental state have a standard enthalpy of formation that is equal to zero.

Enthalpy Change Of Reaction

Unit 2B: Chemical Reactions and Thermochemistry

There are a few “rules” to follow when looking at enthalpy change.

First: when a reaction/process is reversed, the sign of the reaction enthalpy changes. For example, when ice melts, the reaction has a positive enthalpy change . However, when water freezes, the enthalpy change is negative since it is releasing heat .

Second: enthalpy is proportional to the amount of reactants. When you look up enthalpy values for reactions, it is assumed that it is for 1 mol of reactants. So if you wanted to know the Hrxn for 3 mols of reactant, then you would multiply the given value by three.

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Why Is The Enthalpy Of Formation Of Oxygen Zero Truong-Son N.

Enthalpy of formation means the enthalpy change which occurs when 1 mole of a compound forms from the individual elements present in the compound.

Example: the formation of of water from the elements

#”H”_2 + 1/2″O”_2 -> “H”_2″O”#

The elements which will form #”H”_2″O”# in this reaction are present in their elemental states.

The enthalpy of formation for an element in its elemental state will always be #0# because it takes no energy to form a naturally-occurring compound.

So in this case,

I’ll try and focus a little more on why the value for the standard state enthalpy of formation of elements in their natural state was set to zero.

Enthalpy, which is a state function, has a very interesting property – it depends on the initial and the final states of the system, but not on how the system got from one state to the other.

An important implication of this is that enthalpy, which essentially expresses the ability to produce heat, cannot be measured, or more specifically, absolute enthalpy cannot be measured. We can only measure changes in enthalpy.

Now, the enthalpy change for a formation reaction is called enthalpy of formation. When a substance is formed from the most stable form of its elements, a change in enthalpy takes place. You can view the reactants as the initial state and the product as the final state.

Standard Enthalpy Of Formation

As has been mentioned before, the process of heat absorption or heat evolution during the formation of a compounds mole is called the enthalpy of formation. The term enthalpy is used to refer to the internal energy of a thermodynamic mechanism or system in its totality, along with the combined product of the systems volume as well as its pressure.

Therefore, as it can be understood, the standard heat of the formation of a mole of a compound is the combined heat of the sum of its internal energy and the product of volume and pressure. Since assumptions and deductions are necessary for the formation of any and every mole, it has been observed that generally, a compound initiates its formation at 77- degree Fahrenheit temperature, where the necessary pressure is of one atmosphere. Therefore, this is considered the standardised amount of heat necessary for the formation of a mole of a compound.

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Calculate The Enthalpy Of Formation

Now you may ask, “How to calculate the enthalpy of formation?”

1. Let’s consider the enthalpy of formation of the product hydrogen chloride, HCl , from the reactants, hydrogen gas, H2 , and chlorine gas, Cl2 , under standard conditions:

$$H_2\,+Cl_2\, \rightarrow HCl\,$$

Note that the standard enthalpy of formation of elemental hydrogen gas, H2, is equal to zero H2: Hf° = 0.0 kJ/mol. However, in the present case, the reaction involves breaking the molecular hydrogen bond, which yields atomic hydrogen gas, H . The standard enthalpy of formation of atomic hydrogen gas is H : Hf° = +218.0 kJ/mol. The same is true for chlorine – the enthalpy of formation of the elemental form of chlorine gas is Cl2 : Hf° = 0.0 kJ/mol. Again, in the present case, the reaction involves the breaking of the bonds within the molecular gas, forming atomic chlorine gas Cl : Hf° = +121.0 kJ/mol. Then the actual reaction process is given by:

$$H_2\,+Cl_2\, \rightarrow 2H\,+2Cl\, \rightarrow 2HCl\,$$

Notice that we must account for the stoichiometric coefficients of the balanced equation to get the standard enthalpy of formation of reactants, such that:

Standard Enthalpy of the Formation of Reactants:

\begin\Delta& =\Sigma_^m\,_i\\& =2\cdot\Delta+2\cdot\Delta]\\& =2\cdot+2\cdot\\& =678\,kJ/mol\end

Thus, the standard enthalpy of formation for the production of 2 moles of hydrogen iodide, HCl, is given by:

$$2\cdot \Delta=678\,kJ/mol$$

The enthalpy diagram for this reaction is:

Enthalpy Of Formation Of Water

Enthalpy of formation | Thermodynamics | AP Chemistry | Khan Academy

Let’s consider the standard entahlpy of formation of water, H2O , from hydrogen gas, H2, and oxygen gas, O2. The reaction is then:

$$2H_2\,+O_2\, \rightarrow 2H_2O\,:\,\Delta=-571.6\,kJ/mol$$

Now, to write this for a reaction that produces 1 mole of water, we multiply this equation by a factor of 1/2:

$$\frac\cdot 2H_2\,+\frac\cdot O_2\, \rightarrow \frac\cdot2H_2O\,:\,\Delta=\frac\cdot=-285.8kJ/mol$$

This is then the standard enthalpy of formation for 1 mole of liquid water.

Now we might ask, “What best describes the enthalpy of formation of a substance?”

  • In all cases, the enthalpy of formation of a substance is associated with the potential energy that is released, as heat, from the breaking of a chemical bond within a compound.

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Why Is The Enthalpy Of Formation Important

In the estimation of reaction enthalpies, enthalpies of formation are incredibly useful. This is because it is possible to imagine any reaction as happening along a path through which all reactant compounds are first converted to elements and then all elements are converted into compounds of the substance.

Standard Heat Of Reaction

When one mole of a compound is generated from its constituent elements at 25° C and one atmospheric pressure, the standard heat of formation is defined as the amount of heat absorbed or evolved when each material is in its usual physical state . An element’s heat of creation is arbitrarily assigned a value of zero. The quantity of heat released when one mole of a substance is burned in excess oxygen at 25° C and one atmospheric pressure is known as the standard heat of combustion. The approach known as Hess’s law of heat summation is used to calculate heats of reactions using recorded values of temperatures of production and combustion.

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What Is Heat Of Formation

As has been mentioned earlier, the standard enthalpy of formation is used to refer to the heat amount that is absorbed or even, evolved during the process of formation of one mole of any compound from the constituent elements of the compound. Thus, as is to be understood, in order to deduce the amount of heat, whether evolved or absorbed, there is a standard formula which can be utilised to infer the amount.

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