Name: using balanced equations to predict amounts
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 57 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.4.2

Reaction Stoichiometry

Consider a balanced chemical reaction, such as the combustion of hydrogen gas. Here, the quantitative relationship between the reactants and products &#8212; H2, O2, and H2O &#8212; is that two molecules of H2 react with one molecule of O2 to produce two molecules of H2O. 

This quantitative relationship is known as stoichiometry, and it is similar to any recipe.

Suppose two slices of salami, one slice of cheese, and two pieces of Italian bread are used to make one sandwich. In order to make three sandwiches, the quantities of the ingredients are tripled. 

How many sandwiches could be made with 10 slices of salami? Since the ratio of salami to sandwiches is two to one, five sandwiches could be made.

This same process is applied to chemical reactions. For example, consider the synthesis of ammonia. The stoichiometric coefficients directly indicate the relative number of molecules, which is equivalent to the relative quantities in moles. 

One mole of nitrogen gas and three moles of hydrogen gas react to form two moles of ammonia. To make four moles of ammonia, the reactant quantities are doubled. 

The mole ratio of nitrogen gas to ammonia is one to two, while that of hydrogen gas to ammonia is three

reaction stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and products involved in the chemical change, allowing classification of the reaction. Coefficients provide the relative numbers of these chemical species, allowing a quantitative assessment of the relationships between the amounts of substances consumed and produced by the reaction. These quantitative relationships are known as the reaction&#8217;s stoichiometry, a term derived from the Greek words stoicheion (meaning &#8220;element&#8221;) and metron (meaning &#8220;measure&#8221;). 

A reaction&#8217;s stoichiometry helps predict how much of the reactant is needed to produce the desired amount of product, or in some cases, how much product will be formed from a specific amount of the reactant. 

<h4>Stoichiometric Coefficients</h4>

The coefficients of a chemical equation represent the number of moles of each substance. For example, consider the reaction of nitrogen gas and hydrogen gas to produce ammonia. Stoichiometry indicates that one mole of nitrogen and three moles of hydrogen react to produce two moles of ammonia.

<img alt="Chemical equation for ammonia synthesis: N2(g) + 3H2(g) &#8594; 2NH3(g), synthesis process, formula." src="/files/core/CDNSource/lessons/988/988_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Since one mole contains Avogadro's number of molecules, the relative numbers of molecules are the same as the relative numbers of moles. An alternative way of reading the equation is &#8220;one molecule of nitrogen and three molecules of hydrogen react to produce two molecules of ammonia.&#8221;

<h4>Stoichiometric Factors </h4>

Balanced chemical equations are used to determine the amount of one reactant required to react with a given amount of another reactant, or to yield a given amount of product, and so forth. The coefficients in the balanced equation are used to derive stoichiometric factors that permit computation of the desired quantity. In the reaction of hydrogen and nitrogen, ammonia molecules are produced from hydrogen molecules in a 2:3 ratio. This means three moles of hydrogen are stoichiometrically equivalent to two moles of ammonia.

Based on this, the following stoichiometric factors are derived:

<img alt="Stoichiometry ratios diagram; formulas for moles of H&#8322; to NH&#8323; and NH&#8323; to H&#8322; conversion." src="/files/core/CDNSource/lessons/988/988_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

These stoichiometric factors can be used to compute the number of ammonia molecules produced from a given number of hydrogen molecules, or the number of hydrogen molecules required to produce a given number of ammonia molecules. Similar factors may be derived for any pair of substances in any chemical equation.

<h4>Mole-to-Mole Conversions</h4>

For a balanced chemical reaction for the formation of ammonia from nitrogen and hydrogen, the following stoichiometric mole ratio between N2 and NH3 is 1:2. Then, the molar amount of ammonia is derived by multiplying the molar amount of nitrogen by the stoichiometric conversion factor relating the two substances of interest.

<img alt="Stoichiometry calculation, equation 2.4 moles N2 to 4.8 moles NH3, chemical formula conversion." src="/files/core/CDNSource/lessons/988/988_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

<h4>Mass-to-Mass Conversions</h4>

Converting between masses of substances based on stoichiometry requires the knowledge of mole ratios and molar masses. For example, to find the mass of hydrogen required to produce 0.170 kg of ammonia, first, the molar mass of ammonia is used to convert the mass of ammonia to the amount of ammonia (in moles). Then, the appropriate stoichiometric factor from the balanced equation converts the amount of ammonia (in moles) to the amount of hydrogen (in moles). Lastly, the molar mass of hydrogen converts the amount of hydrogen (in moles) to the mass of hydrogen.

<img alt="Stoichiometry calculation, NH&#8323; to H&#8322; mass conversion, formula chain, chemical stoichiometry method." src="/files/core/CDNSource/lessons/988/988_Equations-04.png" style="height: 120px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

Numerous variations on the beginning and ending computational steps are possible depending upon what particular quantities are provided and sought (volumes, solution concentrations, and so forth). Regardless of the details, all these calculations share a common essential component: the use of stoichiometric factors derived from balanced chemical equations.

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry">OpenStax Chemistry 2e, Section 4.3: Reaction Stoichiometry</a>.

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

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Chemical Quantities and Aqueous Reactions

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KEY TERMS AND CONCEPTS

using balanced equations to predict amounts

Balanced chemical equations show how much of each substance reacts and forms in a chemical change. The formulas identify the reactants and products, which helps classify the reaction. The coefficients show the relative numbers of each substance, which lets chemists measure the relationships between amounts consumed and produced.

These quantitative relationships are called stoichiometry. The word comes from the Greek words stoicheion, meaning element, and metron, meaning measure. Stoichiometry helps predict how much reactant is needed to make a desired amount of product. In some cases, it also helps predict how much product will form from a given amount of reactant.

The coefficients in a chemical equation represent the number of moles of each substance. For example, nitrogen gas and hydrogen gas react to form ammonia in a 1:3:2 ratio. That means 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. Because one mole contains Avogadro’s number of molecules, the same ratio also works for molecules.

Balanced equations also provide stoichiometric factors. These factors connect the amount of one substance to the amount of another substance in the same reaction. In the ammonia example, 3 moles of hydrogen are stoichiometrically equivalent to 2 moles of ammonia. The same idea can be used to find how much ammonia forms from a given amount of hydrogen, or how much hydrogen is needed to make a given amount of ammonia.

Stoichiometry often begins with mole-to-mole conversions. For the reaction that forms ammonia from nitrogen and hydrogen, the mole ratio between N2 and NH3 is 1:2. A chemist can multiply the amount of nitrogen by the proper conversion factor to find the amount of ammonia. This step uses only the balanced equation and the mole ratio it gives.

Many problems also require mass-to-mass conversions. To find the mass of hydrogen needed to produce 0.170 kg of ammonia, the mass of ammonia is first converted to moles using its molar mass. Then the stoichiometric factor from the balanced equation changes moles of ammonia to moles of hydrogen. Finally, the molar mass of hydrogen converts those moles to mass.

Other stoichiometry problems may start or end with volumes or solution concentrations. The exact steps can change with the quantities given and the quantities being found. Even so, every version depends on stoichiometric factors taken from balanced chemical equations.

Using Balanced Equations to Predict Amounts

using-balanced-equations-to-predict-amounts

Balanced chemical equations show how much of each substance reacts and forms in a chemical change. The formulas identify the reactants and products, which helps classify the reaction. The coefficients show the relative numbers of each substance, which lets chemists measure the relationships between amounts consumed and produced. These quantitative relationships are called stoichiometry. The word comes from the Greek words stoicheion, meaning element, and metron, meaning measure. Stoichiometry...

limiting reactant

Limiting Reactant in Water Formation

Limiting reactants control how much product forms in a chemical reaction. In a balanced equation, the listed amounts of reactants and products are called stoichiometric amounts. In real reactions, the reactants are not always present in those exact proportions. The limiting reactant is the reactant that gets used up first. Once it is gone, the reaction stops making more product. The other reactant is called the excess reactant. Chemists often use extra amounts of one reactant to make sure the...

reaction yield

Calculating Percent Yield in Reactions

Percent yield shows how much product a chemical reaction actually makes compared with how much it should make. The expected amount is called the theoretical yield. It comes from the balanced chemical equation and the stoichiometry, or mole ratios, in that equation. The theoretical yield assumes that the limiting reactant is completely converted into the desired product. The product collected after a reaction is the actual yield. In many cases, the actual yield is lower than the theoretical...

general properties of solutions

Solvents, Solutes, and Solution Types

Solutions are common mixtures found in everyday life. Ocean water, air, and gasoline are all examples of solutions. These mixtures contain two or more types of atoms or molecules in varying amounts. A solution is a homogeneous mixture, which means its composition is uniform throughout. This is different from a heterogeneous mixture, which has a non-uniform composition. In a homogeneous mixture, the components are evenly spread out and thoroughly mixed. Every solution has a solvent and a...

solution concentration and dilution

Molarity and Dilution in Solutions

Molarity and dilution are key ideas for describing solution concentration. Concentration means the relative amount of one component in a solution. In many solutions, one component is present in much larger amount. That component is the solvent, which acts as the medium that holds the other parts dissolved or dispersed. Water is a very common solvent, so aqueous solutions are common on Earth. A solute is the component present at a much lower concentration than the solvent. Chemists often...

electrolyte and nonelectrolyte solutions

Conductivity in Electrolyte Solutions

Electrolyte solutions conduct electricity because they contain mobile ions. The amount of current depends on how many ions are present in the aqueous solution. A solution with more dissolved ions conducts better, while a solution with few or no ions conducts poorly. Strong electrolytes dissociate completely in water. This complete dissociation is shown with a single forward arrow. Water-soluble ionic compounds such as sodium chloride split into sodium cations and chloride anions in solution.

solubility of ionic compounds

Solubility Rules for Ionic Salts

Solubility rules help predict whether ionic salts will dissolve in water. Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a set temperature and pressure. It is often measured in molarity, or moles per liter (mol/L). A compound is soluble if it dissolves in water. When a soluble salt dissolves, its ions separate and spread evenly through the solution. This physical change is called dissociation. Potassium chloride, KCl, is a simple example. Water...

chemical reactions in aqueous solutions

Aqueous Reactions and Ionic Equations

Aqueous reactions are chemical reactions that take place in water. Water is a good solvent, so it can dissolve many substances. Because of that, many chemical reactions happen in aqueous solution. In high school chemistry, these reactions are often grouped into three common patterns: precipitation, acid-base, and oxidation-reduction. A precipitation reaction happens when ions in two aqueous ionic compounds exchange partners and form an insoluble salt, called a precipitate. In an acid-base...

precipitation reactions

Predicting Precipitation in Aqueous Reactions

Precipitation reactions happen when two aqueous solutions of soluble salts are mixed and an insoluble ionic compound forms. That solid is called a precipitate. The ions in solution exchange partners, so these reactions are also called double displacement, double replacement, exchange, or metathesis reactions. These reactions are useful in chemistry for identifying metal ions in a compound. They also support gravimetric methods, which measure composition by the mass of a substance. Whether a...

oxidation-reduction reactions

Redox Reactions and Oxidation Numbers

Redox reactions are chemical changes in which electrons move between reactants, and oxidation numbers help track that change. The term oxidation once referred only to reactions with molecular oxygen, but it now covers a wider reaction class called oxidation-reduction, or redox, reactions. Some redox reactions form ionic products. A simple example is the reaction between sodium and chlorine that produces sodium chloride. To follow what happens to each reactant, chemists often write...

oxidation numbers

Assigning Oxidation States in Redox Reactions

Oxidation states help track electron transfer in redox reactions. An oxidation number, also called an oxidation state, is a hypothetical number that shows the effective charge of an atom or element. Chemists assign it by using a set of rules. In an ionic compound, oxidation numbers are based on how many electrons move between reacting species. For example, when calcium chloride forms, calcium loses two valence electrons and the two chlorine atoms each gain one electron. In CaCl2, calcium has...

acids, bases and neutralization reactions

How Acids and Bases Form Ions

Acids and bases form ions in water through acid-base reactions. These reactions move a hydrogen ion, H+, from one chemical species to another. They matter in many natural and technological processes, from chemistry inside cells and in lakes and oceans to the large-scale production of fertilizers, pharmaceuticals, and other useful substances. In water, an acid is a substance that produces hydrogen ions. This is the Arrhenius definition of an acid. The H+ ion is also called a proton. In...

synthesis and decomposition reactions

Building and Breaking in Chemical Reactions

Synthesis and decomposition are two redox reactions that show how substances can be built up or broken down. Synthesis means to make something, while decomposition means to break something apart. These reactions also involve chemical changes and energy changes. Synthesis reactions are also called combination reactions. In a synthesis reaction, two or more substances join to form one more complex substance. They are often written as A + B → AB or A + B → C. One example is the formation of...

key terms and concepts

Learn how balanced chemical equations provide mole ratios, stoichiometric factors, and conversions for predicting reactant and product amounts in chemistry.

Using Balanced Equations to Predict Amounts | JoVE K12

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Ohio’s Learning Standards and Model Curriculum

Physical Science

Study Of Matter

Reactions of matter

Interactions Of Matter

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Stoichiometry

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High School

Matter and Its Interactions

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CH.3

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HS+C.P1U1.7

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9-12.C.1.4.c

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Florida Department of Education

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South Carolina College and Career Ready Science Standards

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New York State Science Learning Standards

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Georgia Standards of Excellence

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Stoichiometry Problem-Solving

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Using Balanced Equations to Predict Amounts

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