Learning Objectives

Define a formation reaction and also be maybe to recognize one.Use enthalpies of development to recognize the enthalpy the reaction.

You are watching: Write an equation for the formation one mole of hf(g) from its elements in its standard states.

Hess’s law allows us to construct brand-new chemical reactions and also predict what your enthalpies that reaction will certainly be. This is a very useful tool since now we don’t need to measure the enthalpy changes of every possible reaction. We require measure just the enthalpy transforms of certain benchmark reactions and then use these reaction to algebraically build any possible reaction and also combine the enthalpies the the benchmark reactions accordingly.

But what room the benchmark reactions? We require to have actually some agreed-on to adjust of reaction that carry out the central data for any type of thermochemical equation.

Formation reactions are chemistry reactions that form one mole the a substance from the constituent facets in their standard states. By standard states we typical as a diatomic molecule if the is just how the aspect exists and the appropriate phase at normal temperatures (typically room temperature). The product is one mole of substance, which may require the coefficients ~ above the reactant side be spring (a readjust from our regular insistence the all coefficients be entirety numbers). Because that example, the development reaction because that methane (CH4) is

C(s) + 2 H2(g) → CH4(g)

The formation reaction for carbon dioxide (CO2) is

C(s) + O2(g) → CO2(g)

In both cases, one of the aspects is a diatomic molecule due to the fact that that is the standard state because that that details element. The formation reaction for H2O—

2 H2(g) + O2(g) → 2 H2O(ℓ)

—is not in a conventional state since the coefficient on the product is 2; because that a appropriate formation reaction, only one mole of product is formed. Thus, we have to divide every coefficients by 2:

H2(g) + 1/2 O2(g) → H2O(ℓ)

On a molecule scale, we are using fifty percent of one oxygen molecule, which might be problematic to visualize. However, ~ above a molar level, it suggests that we space reacting only half of a mole that oxygen molecules, which need to be basic concept for us to understand.

Example 11

Which of the following are proper formation reactions?

H2(g) + Cl2(g) → 2 HCl(g)Si(s) + 2 F2(g) → SiF4(g)CaO(s) + CO2 → CaCO3(s)


In this reaction, two moles that product are produced, for this reason this is no a ideal formation reaction.In this reaction, one mole that a substance is produced from its facets in their standard states, therefore this is a ideal formation reaction.One mole that a problem is produced, however it is created from two various other compounds, not its elements. So this is no a ideal formation reaction.

Test Yourself

Is this a ideal formation reaction? explain why or why not.

2 Fe(s) + 3 P(s) + 12 O(g) → Fe2(PO4)3(s)


This is no a proper formation reaction because oxygen is not composed as a diatomic molecule.

Given the formula of any kind of substance, girlfriend should be able to write the suitable formation reaction for the substance.

Example 12

Write formation reactions because that each of the following.



In both cases, there is one mole the the substance together product, and the coefficients that the reaction may need to be fractional come balance the reaction.

Fe(s) + 1/2 O2(g) → FeO(s)2 C(s) + 3 H2(g) → C2H6(g)

Test Yourself

Write the equation because that the formation of CaCO3(s).


Ca(s) + C(s) + 3/2 O2(g) → CaCO3(s)

The enthalpy readjust for a formation reaction is called the enthalpy of formation The subscript f is the clue the the reaction of attention is a formation reaction. Thus, because that the formation of FeO(s),

Note that now we room using kJ/mol together the unit because it is construed that the enthalpy change is for one mole that substance. Note, too, by definition, that the enthalpy of formation of an aspect is exactly zero because making an element from an facet is no change. Because that example,

H2(g) → H2(g) ΔHf = 0

Formation reactions and their enthalpies are important due to the fact that these room the thermochemical data that room tabulated for any kind of chemical reaction. Table 7.2 “Enthalpies of formation for assorted Substances” lists some enthalpies of formation for a selection of substances; in some cases, however, phases can be necessary (e.g., for H2O).

It is easy to display that any kind of general chemistry equation can be written in regards to the formation reactions that its reactants and products, several of them reversed (which method the authorize must adjust in accordance v Hess’s law). For example, consider

2 NO2(g) → N2O4(g)

We can write the in terms of the (reverse) development reaction the NO2 and the development reaction the N2O4:

We must multiply the very first reaction by 2 to acquire the correct all at once balanced equation. Us are just using Hess’s legislation in combining the ΔHf worths of the formation reactions.

Table 7.2 Enthalpies of formation for miscellaneous Substances

CompoundΔHf (kJ/mol)CompoundΔHf (kJ/mol)CompoundΔHf (kJ/mol)CompoundΔHf (kJ/mol)
AgCl(s)−127.01CaCO3(s, arag)−1,207.1K(s)0Na2CO3(s)−1,130.77
Al(s)0CaCO3(s, calc)−1,206.9KBr(s)−393.8Na2O(s)−417.98
C(s, dia)1.897F2(g)0LiCl(s)−408.27PH3(g)22.89
C(s, gra)0Fe(s)0LiF(s)−616.0Pb(s)0

Sources: nationwide Institute of Standards and Technology’s Chemistry WebBook, http://webbook.nist.gov/chemistry; D. R. Lide, ed., CRC Handbook that Chemistry and also Physics, 89th ed. (Boca Raton, FL: CRC Press, 2008); J. A. Dean, ed., Lange’s Handbook that Chemistry, 14th ed. (New York: McGraw-Hill, 1992).

Example 13

Show that the reaction

Fe2O3(s) + 3 SO3(g) → Fe2(SO4)3(s)

can be written as a combination of formation reactions.


There will certainly be three formation reactions. The one because that the products will be composed as a development reaction, when the ones because that the reactants will be created in reverse. Furthermore, the formation reaction because that SO3 will certainly be multiply by 3 because there space three moles of SO3 in the well balanced chemical equation. The formation reactions room as follows:

2 Fe(s) + 3 S(s) + 6 O2(g) → Fe2(SO4)3(s)

When these 3 equations are an unified and simplified, the as whole reaction is

Fe2O3(s) + 3 SO3(s) → Fe2(SO4)3(s)

Test Yourself

Write the formation reactions that will certainly yield

2 SO2(g) + O2(g) → 2 SO3(g).


Now that us have established formation reactions together the major form of thermochemical reaction we will certainly be interested in, do we constantly need to create all the development reactions when we want to identify the enthalpy change of any type of random chemical reaction? No. Over there is an less complicated way. You may have actually noticed in every our examples that we change the signs on all the enthalpies of formation of the reactants, and we don’t change the indications on the enthalpies of formation of the products. We additionally multiply the enthalpies of formation of any type of substance by its coefficient—technically, even when the is simply 1. This permits us to make the following statement: the enthalpy adjust of any kind of chemical reaction is same to the amount of the enthalpies of formation of the assets minus the amount of the enthalpies of development of the reactants. In math terms,

where np and nr space the number of moles of products and also reactants, respectively (even if they are simply 1 mol), and ΔHf,p and ΔHf,r are the enthalpies of formation of the product and also reactant species, respectively. This products-minus-reactants scheme is an extremely useful in determining the enthalpy adjust of any kind of chemical reaction, if the enthalpy of formation data space available. Because the mol devices cancel when multiplying the amount by the enthalpy of formation, the enthalpy change of the chemical reaction has units of power (joules or kilojoules) only.

Example 14

Use the products-minus-reactants strategy to identify the enthalpy that reaction for


The enthalpies of development are multiplied by the variety of moles of each substance in the chemistry equation, and also the full enthalpy of formation for reactants is subtracted indigenous the full enthalpy of formation of the products:

ΔHrxn=<(2 mol)(−92.3 kJ/ mol)+(1 mol)(0 kJ/ mol)>−<(2 mol)(−36.3 kJ/ mol)+(1 mol)(0 kJ/ mol)>

All the mol devices cancel. Multiplying and also combining every the values, us get

ΔHrxn = −112.0 kJ

Test Yourself

What is the enthalpy that reaction for this chemistry equation?


+2.8 kJ

Food and Drink App: Calories and also Nutrition

Section 7.1 “Energy” mentioned the connection between the calorie unit and also nutrition: the calorie is the common unit of power used in nutrition, but we really think about the kilocalorie (spelled Calorie with a funding C). A daily diet of 2,000 Cal is in reality 2,000,000 cal, or over 8,000,000 J, the energy.

Nutritionists commonly generalize the Calorie contents of foodstuffs by separating it right into the three key food types: proteins, carbohydrates, and also fats. The general preeminence of thumb is as follows:

If the food isIt has this energy content
protein4 Cal/g
carbohydrate4 Cal/g
fat9 Cal/g

This table is an extremely useful. Presume a 2,000 Cal day-to-day diet, if ours diet is composed solely the proteins and also carbohydrates, we need only about 500 g the food for sustenance—a little an ext than a pound. If our diet is composed solely of fats, we require only around 220 g that food—less 보다 a fifty percent pound. Of course, many of us have a mixture that proteins, carbohydrates, and fats in ours diets. Water has actually no caloric value in the diet, so any kind of water in the diet is calorically useless. (However, that is necessary for hydration; also, plenty of forms the water in ours diet are very flavoured and sweetened, which carry other nutritional worries to bear.)

When your body works, it offers calories listed by the diet as its power source. If us eat more calories 보다 our human body uses, we gain weight—about 1 lb of weight for every extr 3,500 Cal us ingest. Similarly, if we want to shed weight, we must expend an extra 3,500 Cal 보다 we ingest to lose 1 lb of weight. No fancy or fad diets space needed; maintaining perfect body load is a straightforward issue of thermochemistry—pure and simple.

Key Takeaways

A formation reaction is the development of one mole of a problem from its constituent elements.Enthalpies of development are offered to recognize the enthalpy change of any kind of given reaction.

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Define formation reaction and give one example.