Stoichiometry

Definitions

A stoichiometric amount or stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion:

1. All of the reagent is consumed
2. There is no deficiency of the reagent
3. There is no excess of the reagent.

Stoichiometry rests upon the very laws that help to understand it better, i.e., law of conservation of mass, the law of definite proportions (i.e., the law of constant composition), the law of multiple proportions and the law of reciprocal proportions. In general chemical reactions combine in definite ratios of chemicals. Since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction. For example, the number of atoms of a given element X on the reactant side must equal the number of atoms of that element on the product side, whether or not all of those atoms are involved in a reaction.

Chemical reactions, as macroscopic unit operations, consist of many elementary reactions, where a single molecule reacts with another molecule. As the reacting molecules (or formula units or ion pairs) consist of a definite set of atoms in an integer ratio, the ratio between reactants in a complete reaction is also in an integer ratio. A reaction may consume more than one molecule, and the stoichiometric number counts this number, defined as positive for products (added) and negative for reactants (removed). The unsigned coefficients are generally referred to as the stoichiometric coefficients.

Each element has an atomic mass (usually given as an average in the form of the standard atomic weight), and considering molecules as collections of atoms, every compound has a molecular mass (if molecular) or formula mass (if non-molecular), which when expressed in daltons is numerically equal to the molar mass in g/mol. By definition, the atomic mass of carbon-12 is exactly 12 Da, making its molar mass 12 g/mol. The number of chemical entities per mole in a substance is given by the Avogadro constant, exactly 6.02214076×10²³ mol⁻¹ since the 2019 revision of the SI. Thus, to calculate the stoichiometry by mass, the number of molecules required for each reactant is expressed in moles and multiplied by the molar mass of each to give the mass of each reactant per mole of reaction. The mass ratios can be calculated by dividing each by the total in the whole reaction.

Elements in their natural state are mixtures of isotopes of differing mass; thus, atomic masses and thus molar masses are not exactly integers. For instance, instead of an exact 14:3 proportion, 17.031 g of ammonia consists of 14.007 g of nitrogen and 3 × 1.008 g of hydrogen, because natural nitrogen includes a small amount of nitrogen-15, and natural hydrogen includes hydrogen-2 (deuterium).

A stoichiometric reactant is a reactant that is consumed in a reaction, as opposed to a catalytic reactant, which is not consumed in the overall reaction because it reacts in one step and is regenerated in another step.

Converting grams to moles

Stoichiometry is not only used to balance chemical equations but also used in "conversions" between quantities of a substance by dimensional analysis, e.g., converting from grams to moles using molar mass as the "conversion factor", or from grams to milliliters using density. For example, to express 2.00 g of NaCl (sodium chloride) as an amount (in moles), one would do the following:

${\displaystyle \frac{2.00\text{ g NaCl}}{58.44\text{ g/mol}}=0.0342\text{mol NaCl}}$

In the above example, when written out in fraction form, the units of grams form a multiplicative identity, which is equivalent to one (g/g = 1), with the resulting amount in moles (the unit that was needed), as shown in the following equation,

${\displaystyle \left(\frac{2.00\text{ g NaCl}}{1}\right)\left(\frac{1\text{ mol NaCl}}{58.44\text{ g NaCl}}\right)=0.0342\text{mol NaCl}}$

Molar proportion

Stoichiometry is often used to balance chemical equations (reaction stoichiometry). For example, the two diatomic gases, hydrogen and oxygen, can combine to form a liquid, water, in an exothermic reaction, as described by the following equation:

2 H₂ + O₂ → 2 H₂O

Reaction stoichiometry describes the 2:1:2 ratio of hydrogen, oxygen, and water molecules in the above equation.

The molar ratio allows for conversion between moles of one substance and moles of another. For example, in the reaction

2 CH₃OH + 3 O₂ → 2 CO₂ + 4 H₂O

the amount of water that will be produced by the combustion of 0.27 moles of CH
₃OH is obtained using the molar ratio between CH
₃OH and H
₂O of 2 to 4.

${\displaystyle \left(\frac{0.27\text{ mol }\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}}{1}\right)\left(\frac{4\text{ mol }{\mathrm{H}}_2\mathrm{O}}{2\text{ mol }\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}}\right)=0.54\text{mol }{\mathrm{H}}_2\mathrm{O}}$

The term stoichiometry is also often used for the molar proportions of elements in stoichiometric compounds (composition stoichiometry). For example, the stoichiometry of hydrogen and oxygen in H
₂O is 2:1. In stoichiometric compounds, the molar proportions are whole numbers.

Determining amount of product

Stoichiometry can also be used to find the quantity of a product yielded by a reaction. If a piece of solid copper (Cu) were added to an aqueous solution of silver nitrate (AgNO₃), the silver (Ag) would be replaced in a single displacement reaction forming aqueous copper(II) nitrate (Cu(NO₃)₂) and solid silver. How much silver is produced if 16.00 grams of Cu is added to the solution of excess silver nitrate?

The following steps would be used:

1. Write and balance the equation
2. Mass to moles: Convert grams of Cu to moles of Cu
3. Mole ratio: Convert moles of Cu to moles of Ag produced
4. Mole to mass: Convert moles of Ag to grams of Ag produced

The complete balanced equation would be:

Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag

For the mass to mole step, the mass of copper (16.00 g) would be converted to moles of copper by dividing the mass of copper by its molar mass: 63.55 g/mol.

${\displaystyle \left(\frac{16.00\text{ g Cu}}{1}\right)\left(\frac{1\text{ mol Cu}}{63.55\text{ g Cu}}\right)=0.2518\text{mol Cu}}$

Now that the amount of Cu in moles (0.2518) is found, we can set up the mole ratio. This is found by looking at the coefficients in the balanced equation: Cu and Ag are in a 1:2 ratio.

${\displaystyle \left(\frac{0.2518\text{ mol Cu}}{1}\right)\left(\frac{2\text{ mol Ag}}{1\text{ mol Cu}}\right)=0.5036\text{mol Ag}}$

Now that the moles of Ag produced is known to be 0.5036 mol, we convert this amount to grams of Ag produced to come to the final answer:

${\displaystyle \left(\frac{0.5036\text{ mol Ag}}{1}\right)\left(\frac{107.87\text{ g Ag}}{1\text{ mol Ag}}\right)=54.32\text{g Ag}}$

This set of calculations can be further condensed into a single step:

${\displaystyle m_{\mathrm{A}\mathrm{g}}=\left(\frac{16.00\text{ g }\mathrm{C}\mathrm{u}}{1}\right)\left(\frac{1\text{ mol }\mathrm{C}\mathrm{u}}{63.55\text{ g }\mathrm{C}\mathrm{u}}\right)\left(\frac{2\text{ mol }\mathrm{A}\mathrm{g}}{1\text{ mol }\mathrm{C}\mathrm{u}}\right)\left(\frac{107.87\text{ g }\mathrm{A}\mathrm{g}}{1\text{ mol Ag}}\right)=54.32\text{ g}}$

Further examples

For propane (C₃H₈) reacting with oxygen gas (O₂), the balanced chemical equation is:

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O

The mass of water formed if 120 g of propane (C₃H₈) is burned in excess oxygen is then

${\displaystyle m_{{\mathrm{H}}_2\mathrm{O}}=\left(\frac{120.\text{ g }{\mathrm{C}}_3{\mathrm{H}}_8}{1}\right)\left(\frac{1\text{ mol }{\mathrm{C}}_3{\mathrm{H}}_8}{44.09\text{ g }{\mathrm{C}}_3{\mathrm{H}}_8}\right)\left(\frac{4\text{ mol }{\mathrm{H}}_2\mathrm{O}}{1\text{ mol }{\mathrm{C}}_3{\mathrm{H}}_8}\right)\left(\frac{18.02\text{ g }{\mathrm{H}}_2\mathrm{O}}{1\text{ mol }{\mathrm{H}}_2\mathrm{O}}\right)=196\text{ g}}$

Stoichiometric ratio

Stoichiometry is also used to find the right amount of one reactant to "completely" react with the other reactant in a chemical reaction – that is, the stoichiometric amounts that would result in no leftover reactants when the reaction takes place. An example is shown below using the thermite reaction,

Fe₂O₃ + 2 Al → Al₂O₃ + 2 Fe

This equation shows that 1 mole of iron(III) oxide and 2 moles of aluminium will produce 1 mole of aluminium oxide and 2 moles of iron. So, to completely react with 85.0 g of iron(III) oxide (0.532 mol), 28.7 g (1.06 mol) of aluminium are needed.

${\displaystyle m_{\mathrm{A}\mathrm{l}}=\left(\frac{85.0\text{ g }\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}{1}\right)\left(\frac{1\text{ mol }\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}{159.7\text{ g }\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}\right)\left(\frac{2\text{ mol Al}}{1\text{ mol }\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3}\right)\left(\frac{26.98\text{ g Al}}{1\text{ mol Al}}\right)=28.7\text{ g}}$

Limiting reagent and percent yield

The limiting reagent is the reagent that limits the amount of product that can be formed and is completely consumed when the reaction is complete. An excess reactant is a reactant that is left over once the reaction has stopped due to the limiting reactant being exhausted.

Consider the equation of roasting lead(II) sulfide (PbS) in oxygen (O₂) to produce lead(II) oxide (PbO) and sulfur dioxide (SO₂):

2 PbS + 3 O₂ → 2 PbO + 2 SO₂

To determine the theoretical yield of lead(II) oxide if 200.0 g of lead(II) sulfide and 200.0 g of oxygen are heated in an open container:

${\displaystyle m_{\mathrm{P}\mathrm{b}\mathrm{O}}=\left(\frac{200.0\text{ g }\mathrm{P}\mathrm{b}\mathrm{S}}{1}\right)\left(\frac{1\text{ mol }\mathrm{P}\mathrm{b}\mathrm{S}}{239.27\text{ g }\mathrm{P}\mathrm{b}\mathrm{S}}\right)\left(\frac{2\text{ mol }\mathrm{P}\mathrm{b}\mathrm{O}}{2\text{ mol }\mathrm{P}\mathrm{b}\mathrm{S}}\right)\left(\frac{223.2\text{ g }\mathrm{P}\mathrm{b}\mathrm{O}}{1\text{ mol }\mathrm{P}\mathrm{b}\mathrm{O}}\right)=186.6\text{ g}}$

${\displaystyle m_{\mathrm{P}\mathrm{b}\mathrm{O}}=\left(\frac{200.0\text{ g }{\mathrm{O}}_2}{1}\right)\left(\frac{1\text{ mol }{\mathrm{O}}_2}{32.00\text{ g }{\mathrm{O}}_2}\right)\left(\frac{2\text{ mol }\mathrm{P}\mathrm{b}\mathrm{O}}{3\text{ mol }{\mathrm{O}}_2}\right)\left(\frac{223.2\text{ g }\mathrm{P}\mathrm{b}\mathrm{O}}{1\text{ mol }\mathrm{P}\mathrm{b}\mathrm{O}}\right)=930.0\text{ g}}$

Because a lesser amount of PbO is produced for the 200.0 g of PbS, it is clear that PbS is the limiting reagent.

In reality, the actual yield is not the same as the stoichiometrically-calculated theoretical yield. Percent yield, then, is expressed in the following equation:

${\displaystyle \text{percent yield}=\frac{\text{actual yield}}{\text{theoretical yield}}}$

If 170.0 g of lead(II) oxide is obtained, then the percent yield would be calculated as follows:

${\displaystyle \text{percent yield}=\frac{\text{170.0 g PbO}}{\text{186.6 g PbO}}=91.12\mathrm{\%}}$

Example

Consider the following reaction, in which iron(III) chloride reacts with hydrogen sulfide to produce iron(III) sulfide and hydrogen chloride:

2 FeCl₃ + 3 H₂S → Fe₂S₃ + 6 HCl

The stoichiometric masses for this reaction are:

324.41 g FeCl₃, 102.25 g H₂S, 207.89 g Fe₂S₃, 218.77 g HCl

Suppose 90.0 g of FeCl₃ reacts with 52.0 g of H₂S. To find the limiting reagent and the mass of HCl produced by the reaction, we change the above amounts by a factor of 90/324.41 and obtain the following amounts:

90.00 g FeCl₃, 28.37 g H₂S, 57.67 g Fe₂S₃, 60.69 g HCl

The limiting reactant (or reagent) is FeCl₃, since all 90.00 g of it is used up while only 28.37 g H₂S are consumed. Thus, 52.0 − 28.4 = 23.6 g H₂S left in excess. The mass of HCl produced is 60.7 g.

By looking at the stoichiometry of the reaction, one might have guessed FeCl₃ being the limiting reactant; three times more FeCl₃ is used compared to H₂S (324 g vs 102 g).

Different stoichiometries in competing reactions

Often, more than one reaction is possible given the same starting materials. The reactions may differ in their stoichiometry. For example, the methylation of benzene (C₆H₆), through a Friedel–Crafts reaction using AlCl₃ as a catalyst, may produce singly methylated (C₆H₅CH₃), doubly methylated (C₆H₄(CH₃)₂), or still more highly methylated (C₆H_6−n(CH₃)_n) products, as shown in the following example,

C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl

C₆H₆ + 2 CH₃Cl → C₆H₄(CH₃)₂ + 2 HCl

C₆H₆ + n CH₃Cl → C₆H_6−n(CH₃)_n + n HCl

In this example, which reaction takes place is controlled in part by the relative concentrations of the reactants.

Stoichiometric coefficient and stoichiometric number

In lay terms, the stoichiometric coefficient of any given component is the number of molecules and/or formula units that participate in the reaction as written. A related concept is the stoichiometric number (using IUPAC nomenclature), wherein the stoichiometric coefficient is multiplied by +1 for all products and by −1 for all reactants.

For example, in the reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O, the stoichiometric number of CH₄ is −1, the stoichiometric number of O₂ is −2, for CO₂ it would be +1 and for H₂O it is +2.

In more technically precise terms, the stoichiometric number in a chemical reaction system of the i-th component is defined as

${\displaystyle {\nu }_i=\frac{\mathrm{\Delta }{N}_i}{\mathrm{\Delta }\xi }}$

or

${\displaystyle \mathrm{\Delta }{N}_i={\nu }_i\mathrm{\Delta }\xi }$

where ${\displaystyle N_i}$ is the number of molecules of i, and ${\displaystyle \xi }$ is the progress variable or extent of reaction.

The stoichiometric number ${\displaystyle {\nu }_i}$ represents the degree to which a chemical species participates in a reaction. The convention is to assign negative numbers to reactants (which are consumed) and positive ones to products, consistent with the convention that increasing the extent of reaction will correspond to shifting the composition from reactants towards products. However, any reaction may be viewed as going in the reverse direction, and in that point of view, would change in the negative direction in order to lower the system's Gibbs free energy. Whether a reaction actually will go in the arbitrarily selected forward direction or not depends on the amounts of the substances present at any given time, which determines the kinetics and thermodynamics, i.e., whether equilibrium lies to the right or the left of the initial state,

In reaction mechanisms, stoichiometric coefficients for each step are always integers, since elementary reactions always involve whole molecules. If one uses a composite representation of an overall reaction, some may be rational fractions. There are often chemical species present that do not participate in a reaction; their stoichiometric coefficients are therefore zero. Any chemical species that is regenerated, such as a catalyst, also has a stoichiometric coefficient of zero.

The simplest possible case is an isomerization

A → B

in which ν_B = 1 since one molecule of B is produced each time the reaction occurs, while ν_A = −1 since one molecule of A is necessarily consumed. In any chemical reaction, not only is the total mass conserved but also the numbers of atoms of each kind are conserved, and this imposes corresponding constraints on possible values for the stoichiometric coefficients.

There are usually multiple reactions proceeding simultaneously in any natural reaction system, including those in biology. Since any chemical component can participate in several reactions simultaneously, the stoichiometric number of the i-th component in the k-th reaction is defined as

${\displaystyle {\nu }_{ik}=\frac{\mathrm{\partial }{N}_i}{\mathrm{\partial }{\xi }_k}}$

so that the total (differential) change in the amount of the i-th component is

${\displaystyle d{N}_i=\sum _k{\nu }_{ik}d{\xi }_k.}$

Extents of reaction provide the clearest and most explicit way of representing compositional change, although they are not yet widely used.

With complex reaction systems, it is often useful to consider both the representation of a reaction system in terms of the amounts of the chemicals present { N_i } (state variables), and the representation in terms of the actual compositional degrees of freedom, as expressed by the extents of reaction { ξ_k }. The transformation from a vector expressing the extents to a vector expressing the amounts uses a rectangular matrix whose elements are the stoichiometric numbers [ ν_{i k} ].

The maximum and minimum for any ξ_k occur whenever the first of the reactants is depleted for the forward reaction; or the first of the "products" is depleted if the reaction as viewed as being pushed in the reverse direction. This is a purely kinematic restriction on the reaction simplex, a hyperplane in composition space, or N‑space, whose dimensionality equals the number of linearly-independent chemical reactions. This is necessarily less than the number of chemical components, since each reaction manifests a relation between at least two chemicals. The accessible region of the hyperplane depends on the amounts of each chemical species actually present, a contingent fact. Different such amounts can even generate different hyperplanes, all sharing the same algebraic stoichiometry.

In accord with the principles of chemical kinetics and thermodynamic equilibrium, every chemical reaction is reversible, at least to some degree, so that each equilibrium point must be an interior point of the simplex. As a consequence, extrema for the ξs will not occur unless an experimental system is prepared with zero initial amounts of some products.

The number of physically-independent reactions can be even greater than the number of chemical components, and depends on the various reaction mechanisms. For example, there may be two (or more) reaction paths for the isomerism above. The reaction may occur by itself, but faster and with different intermediates, in the presence of a catalyst.

The (dimensionless) "units" may be taken to be molecules or moles. Moles are most commonly used, but it is more suggestive to picture incremental chemical reactions in terms of molecules. The Ns and ξs are reduced to molar units by dividing by the Avogadro constant. While dimensional mass units may be used, the comments about integers are then no longer applicable.

Stoichiometry matrix

In complex reactions, stoichiometries are often represented in a more compact form called the stoichiometry matrix. The stoichiometry matrix is denoted by the symbol N.

If a reaction network has n reactions and m participating molecular species, then the stoichiometry matrix will have correspondingly m rows and n columns.

For example, consider the system of reactions shown below:

S₁ → S₂

5 S₃ + S₂ → 4 S₃ + 2 S₂

S₃ → S₄

S₄ → S₅

This system comprises four reactions and five different molecular species. The stoichiometry matrix for this system can be written as:

${\displaystyle \mathbf{N}=\left[\begin{array}{cccc}-1& 0& 0& 0\\ 1& 1& 0& 0\\ 0& -1& -1& 0\\ 0& 0& 1& -1\\ 0& 0& 0& 1\end{array}\right]}$

where the rows correspond to S₁, S₂, S₃, S₄ and S₅, respectively. The process of converting a reaction scheme into a stoichiometry matrix can be a lossy transformation: for example, the stoichiometries in the second reaction simplify when included in the matrix. This means that it is not always possible to recover the original reaction scheme from a stoichiometry matrix.

Often the stoichiometry matrix is combined with the rate vector, v, and the species vector, x to form a compact equation, the biochemical systems equation, describing the rates of change of the molecular species:

${\displaystyle \frac{d\mathbf{x}}{dt}=\mathbf{N}\cdot \mathbf{v}.}$

Gas stoichiometry

Gas stoichiometry is the quantitative relationship (ratio) between reactants and products in a chemical reaction with reactions that produce gases. Gas stoichiometry applies when the gases produced are assumed to be ideal, and the temperature, pressure, and volume of the gases are all known. The ideal gas law is used for these calculations. Often, but not always, the standard temperature and pressure (STP) are taken as 0 °C and 1 bar and used as the conditions for gas stoichiometric calculations.

Gas stoichiometry calculations solve for the unknown volume or mass of a gaseous product or reactant. For example, if we wanted to calculate the volume of gaseous NO₂ produced from the combustion of 100 g of NH₃, by the reaction:

4 NH₃ (g) + 7 O₂ (g) → 4 NO₂ (g) + 6 H₂O (l)

we would carry out the following calculations:

${\displaystyle 100\mathrm{g}\mathrm{N}{\mathrm{H}}_3\cdot \frac{1\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{N}{\mathrm{H}}_3}{17.034\mathrm{g}\mathrm{N}{\mathrm{H}}_3}=5.871\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{N}{\mathrm{H}}_3}$

There is a 1:1 molar ratio of NH₃ to NO₂ in the above balanced combustion reaction, so 5.871 mol of NO₂ will be formed. We will employ the ideal gas law to solve for the volume at 0 °C (273.15 K) and 1 atmosphere using the gas law constant of R = 0.08206 L·atm·K⁻¹·mol⁻¹:

${\displaystyle \begin{array}{cc}PV& =nRT\\ V& =\frac{nRT}{P}\\ & =\frac{5.871\text{ mol}\cdot 0.08206\frac{\mathrm{L}\cdot \mathrm{a}\mathrm{t}\mathrm{m}}{\mathrm{m}\mathrm{o}\mathrm{l}\cdot \mathrm{K}}\cdot 273.15\text{ K}}{1\text{ atm}}\\ & =131.597\mathrm{L}\mathrm{N}{\mathrm{O}}_2\end{array}}$

Gas stoichiometry often involves having to know the molar mass of a gas, given the density of that gas. The ideal gas law can be re-arranged to obtain a relation between the density and the molar mass of an ideal gas:

${\displaystyle \rho =\frac{m}{V}}$ and ${\displaystyle n=\frac{m}{M}}$

and thus:

${\displaystyle \rho =\frac{MP}{RT}}$

where:

• P = absolute gas pressure
• V = gas volume
• n = amount (measured in moles)
• R = universal ideal gas law constant
• T = absolute gas temperature
• ρ = gas density at T and P
• m = mass of gas
• M = molar mass of gas

Stoichiometric air-to-fuel ratios of common fuels

In the combustion reaction, oxygen reacts with the fuel, and the point where exactly all oxygen is consumed and all fuel burned is defined as the stoichiometric point. With more oxygen (overstoichiometric combustion), some of it stays unreacted. Likewise, if the combustion is incomplete due to lack of sufficient oxygen, fuel remains unreacted. (Unreacted fuel may also remain due to physical rather than chemical factors, such as slow combustion or insufficient mixing of fuel and oxygen – this is not due to stoichiometry.) Different hydrocarbon fuels have different contents of carbon, hydrogen and other elements, thus their stoichiometry varies.

Oxygen makes up only 20.95% of the volume of air, and only 23.20% of its mass. The air-fuel ratios listed below are much higher than the equivalent oxygen-fuel ratios, due to the high proportion of inert gasses in the air. (The ratio of 14.7 as listed for gasoline only applies for combustion in combination with a catalytic converter.)

Gasoline engines can run at stoichiometric air-to-fuel ratio, because gasoline is quite volatile and is mixed (sprayed or carburetted) with the air prior to ignition. Diesel engines, in contrast, run lean, with more air available than simple stoichiometry would require. Diesel fuel is less volatile and is effectively burned as it is injected.