Name: limiting reactant in water formation
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 27 s
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Limiting Reactant

In a chemical reaction, reactants interact with each other to form products. The reactant that is completely consumed is the limiting reactant, and the reactant present in a quantity greater than necessary to react completely with the limiting reactant is the excess reactant.

A recipe analogy is helpful to understand these concepts. Here, one cup of flour, 2 eggs, and 3 tablespoons of sugar make five waffles. If there are 3 cups of flour, 4 eggs, and 8 tablespoons of sugar, how many waffles can be made? 

There is enough flour to make 15 waffles and enough sugar to make 13 1/3 waffles. However, there are only enough eggs for 10 waffles. Here, egg is the limiting reactant because it makes the least amount of waffles, while the flour and sugar are in excess.

Now, consider the combustion reaction between methane and oxygen to produce carbon dioxide and water. Recall that the coefficients of a balanced equation represent the stoichiometric amounts of the reactants and products.

Therefore, the stoichiometric mole ratio of methane to carbon dioxide is one to one, and that of oxygen to carbon dioxide is two to one.

Suppose there are 80 grams of methane and 128 grams of oxygen. Which is the

limiting reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in reality, the reactants are not always present in the stoichiometric amounts indicated by the balanced equation.

In a chemical reaction, the reactant which gets consumed first, and limits the amount of product formed, is the limiting reactant, while the other substance becomes the excess reactant. An excess of one or more reactants is often used to ensure the complete conversion of the other reactant into the product.

Consider the reaction for the formation of water represented by the equation:

<img alt="Chemical reaction equation 2H&#8322;(g) + O&#8322;(g) &#8594; 2H&#8322;O(g) representing water formation." src="/files/core/CDNSource/lessons/989/989_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

The balanced equation shows the hydrogen and oxygen react in a 2:1 stoichiometric ratio. If these reactants are provided in any other amounts, one of the reactants will nearly always be entirely consumed, thus limiting the amount of product that may be generated. This substance is the limiting reactant, and the other substance is the excess reactant. Identifying the limiting and excess reactants for a given situation requires computing the molar amounts of each reactant provided and comparing them to the stoichiometric amounts represented in the balanced chemical equation. 

Stoichiometry indicates that two moles of hydrogen and one mole of oxygen react to produce two moles of water; that is, hydrogen and oxygen combine in a 2:1 ratio. Imagine if 5 moles of hydrogen and 2 moles of oxygen are present. The ratio of the reactants is now 5:2 (or 2.5:1), which is greater than the stoichiometric ratio of 2:1. Hydrogen, therefore, is present in excess, and oxygen is the limiting reactant. Reaction of all the provided oxygen (2 mol) will consume 4 mol of the 5 mol of hydrogen provided, leaving 1 mol of hydrogen unreacted. Computing the molar amounts of each reactant provided and comparing them to the stoichiometric amounts represented in the balanced chemical equation is one way of identifying the limiting and excess reactant.

An alternative approach involves calculating the amount of product formed in moles from each reactant, as per the reaction&#8217;s stoichiometry, and then comparing the amounts. The reactant that produces a lesser amount of product moles is the limiting reactant. For example, the complete reaction of five moles of hydrogen would yield:

<img alt="Mole ratio calculation, equation showing conversion of H2 to H2O, dimensional analysis method." src="/files/core/CDNSource/lessons/989/989_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

Similarly, the complete reaction of two moles of oxygen would yield:

<img alt="Chemical equation showing mole ratio conversion for water synthesis, with visual cancellation process." src="/files/core/CDNSource/lessons/989/989_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

Oxygen produces fewer moles of water, and therefore, oxygen is the limiting reactant. Oxygen will be completely consumed once 4 moles of H2O have been produced. The stoichiometry between hydrogen and oxygen being 2:1, four moles of hydrogen are needed to react with two moles of oxygen. 

<img alt="Stoichiometric calculation formula; chemical equations for mole ratio conversion in diagram." src="/files/core/CDNSource/lessons/989/989_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

Thus, (5 mol H2 &#8722; 4 mol H2 = 1 mol H2), one mole of unreacted hydrogen will remain once this reaction is complete. 

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields">OpenStax, Chemistry 2e, Section 4.4: Reaction Yield.</a>

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

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KEY TERMS AND CONCEPTS

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 other reactant fully turns into product.

Water formation is a clear example. The balanced equation shows hydrogen and oxygen react in a 2:1 stoichiometric ratio. If the reactants are given in other amounts, one will usually be completely consumed first. That reactant is the limiting reactant, and the other one is in excess.

To find the limiting reactant, first calculate the number of moles of each reactant and compare those amounts to the balanced equation. For example, 5 moles of hydrogen and 2 moles of oxygen give a ratio of 5:2, or 2.5:1. That is greater than the 2:1 ratio required by the equation. Hydrogen is therefore in excess, and oxygen is the limiting reactant. When all 2 moles of oxygen react, they use up 4 moles of hydrogen and leave 1 mole of hydrogen unreacted.

Another method is to calculate how much product each reactant could make. For the water reaction, 5 moles of hydrogen and 2 moles of oxygen do not produce the same amount of water. Oxygen makes fewer moles of water, so oxygen is the limiting reactant. It is completely consumed when 4 moles of H2O have formed. Because 4 moles of hydrogen are needed to react with 2 moles of oxygen, 1 mole of H2 remains after the reaction is complete.

Limiting Reactant in Water Formation

reaction stoichiometry

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 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...

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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...

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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.

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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 to identify the limiting reactant and excess reactant using mole ratios, with water formation as the example reaction in this high school chemistry topic.

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Limiting Reactant in Water Formation

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