7 Stoichiometry Examples in Real Life


Stoichiometry is one of the most important concepts in chemistry. The term comes from two Greek roots, “stoicheion,” which means element, and “metron,” which means measure. Just like a chef measures the ingredients before preparing a dish, stoichiometry is the science of measuring quantities or mass ratios of chemical elements in a given chemical reaction. Food preparation is an appropriate analogy to understand stoichiometry. For instance, a  recipe for making eight pancakes requires a cup of pancake mix, some milk, and an egg. The “equation” representing the preparation of pancakes per this recipe is

1 cup mix + \frac{3}{4} cup milk + 1 egg → 8 pancakes

If two dozen pancakes are needed for a big family breakfast, the ingredient amounts must be increased proportionally according to the amounts given in the recipe. For example, the number of eggs required to make 24 pancakes is

24 Pancakes × \frac{1 egg}{ 8 pancakes} = 3 eggs

Habers process

Balanced chemical equations are used in the same fashion 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. A balanced chemical equation is a concise format to express any chemical reaction. While chemical formulas provide the identities of the reactants and products involved in the chemical change, coefficients represent 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 stoichiometry. In general, it is the study of the quantities of reactants and products in a chemical reaction. The basic law that governs the stoichiometry of a given chemical reaction is the Law of conservation of mass. Since chemical reactions can neither create nor destroy matter, neither can they transform one element into another, the amount of each element must be the same throughout the overall reaction. This physical law is what makes all stoichiometric calculations possible. The general stoichiometric equation for any chemical reaction is written as:

aA + bB + … → … + yY+ zZ

It provides the information that a moles of A reacts with b moles of B to produce y moles of Y and z moles of Z. These coefficients in the equation are used to derive stoichiometric factors that permits the computations for the desired quantity. Though the stoichiometric coefficients can be fractions, whole numbers are frequently used and often preferred. Although it may not seem important, stoichiometry is the heart of the solutions to many real-life problems. Let’s take a look at few examples of how stoichiometry is being employed in practical life.

1. Airbags Design

An airbag is a safety feature inside a vehicle that would reduce the impact of the collision on drivers and passengers in an accident.  Today, airbags are mandatory in new cars to act as a supplemental safety device in addition to a seat belt. Timing is a crucial factor in the airbag’s ability to save lives in a head-on collision. The effective operation of an airbag requires it to rapidly inflate with an appropriate amount (volume) of gas and within milliseconds from the initial collision impact. One of the simplest designs for a crash sensor includes a steel ball that slides inside a smooth bore. The ball is held in place by a permanent magnet or by a stiff spring, which inhibits the ball’s motion when the vehicle drives over bumps or potholes. However, when the car decelerates very quickly, as in a head-on collision, the ball suddenly moves forward and turns on an electrical circuit, initiating the process of inflating the airbag. During the collision, the electric circuit passes an electrical current through a carefully measured amount of sodium azide {NaN}_{3} to initiate its decomposition:

{NaN}_{3} (s) → 3 {N}_{2} (g) + 2 {Na} (s)

The reaction generates gaseous nitrogen that can deploy and fully inflate a typical airbag in a fraction of a second (0.03–0.1 s). Among many other engineering specifications, the amount of sodium azide used must be appropriate for a reaction being that rapid. This is done with the help of using stoichiometry to estimate the amount of solid sodium azide. Moreover, it is also necessary that the body or head of the driver (or passenger) should not hit the airbag while it is still inflating, as the high internal pressure of the airbag would create a surface as hard as stone. For the airbag to cushion the head and torso with air for maximum protection, the airbag must begin to deflate (i.e., decrease its internal pressure) by the time the body hits it. When gaseous nitrogen stops generating, gas molecules escape the bag through vents. The pressure inside the bag decreases and, the bag deflates slightly to create a soft cushion. By 2 seconds after the initial impact, the pressure inside the bag reaches atmospheric pressure.

2. Rocket Propulsion

Humanity’s curiosity to explore the unknown, discover new worlds, and push the limits of our scientific and technical knowledge has been universal and enduring. But, space exploration is quite a demanding challenge. When we think about sending a spacecraft to another planet or into space, we ask questions like, “How can we get it off the ground?” Choosing an efficient fuel proportion for lifting off something as heavy as a rocket may sound impossible at first thought, but it pretty doable with the help of stoichiometry. Unlike jet engines which only contain fuel and have atmospheric oxygen at their disposal for oxidation, the main engines of many rockets are primarily powered by an exothermic reaction of hydrogen and oxygen. Igniting the mixture initiates a vigorous chemical reaction that rapidly generates large amounts of gaseous products. These gases are ejected from the rocket engine through its nozzle, providing the thrust needed to propel heavy payloads into space. It is very important to carefully set the proportions of fuel in the rocket because fuel tanks make up most of the rocket’s weight, and the ratio has to be exactly right for maximum propulsion with minimum mass. If the rocket has too much of one or the other reactant on board, the other one would be left unconsumed.

3. Lithium Hydroxide Scrubber

LIOH scrubber

We have already discussed the role of stoichiometry in dealing with the most basic challenge in space exploration in the previous example. Let’s now understand how stoichiometry provides an earth-like atmosphere within a space shuttle where astronauts can move around and breathe without a space-suit. While it was easy to provide a constant stream of oxygen from fuel tanks within the rocket, the extraction of carbon dioxide exhaled by the crew was one of the many unprecedented problems that came up when humans decided to go into space. The human body can survive perfectly well even in 15-17% of oxygen concentrations in the air. Anything less than this threshold can cause serious problems in space exploration and may jeopardize the whole program. The space shuttle uses an absorption method to remove carbon dioxide (CO2). The absorption of carbon dioxide is accomplished in a chemical reaction using a sorbent known as lithium hydroxide (LiOH). This method relies on the exothermic reaction of lithium hydroxide with carbon dioxide gas to create lithium carbonate (Li2CO3) solid and water (H2O).

{LiOH} + {CO}_{2} → {Li}_{2}{O}_{3} + {H}_{2}{O}

Lithium hydroxide is an attractive choice for space flight because of its high absorption capacity for carbon dioxide and the small amount of heat produced in the reaction. Disadvantages include the irreversibility of the chemical reaction. This causes the replacement of lithium hydroxide canisters to be a daily activity during space shuttle flights. The stoichiometric calculations provide the necessary data to ensure the periodic replacement of these canisters.

4. Pharmaceutical Industry

The pharmaceutical industry encompasses a wide range of chemistry with stoichiometry at the center of it. The concept of stoichiometrical calculations is important in this industry as the quantities of compounds must be exact to create the right amount of medication. Pharmacists use various measurement quantities such as milligrams to define the mass of active ingredients in a given medicine. Depending on the medical condition of the patient, doctors prescribe the dosage of certain milligrams in their prescription form. An error in measuring the active ingredient of a particular drug can drastically change the potency of a drug, even turning a medication into poison. Such errors lead to drug recall from the market and can cost millions of dollars to the manufacturing companies. Stoichiometric calculations prevent such errors by accurately estimating the amounts of reactants required to produce desirable active ingredients of a particular drug. It is also important to take a medication dosage as per the doctor’s prescription as an intake of more milligrams of the active ingredient may also lead to acute or chronic overdose.

5. Role in Industrial Sector

Several products that we use in our daily life, such as soaps, perfumes, toothpaste, etc., are prepared by some chemical reactions performed in the industries. From the scientist who forms the basic chemical equation for the product to the chemical engineer who is synthesizing that product on a large-scale, stoichiometry plays an equally important part. The chemists, who develop the synthetic route of any product, calculate excess moles of each reagent used and moles formed of the desired product. They try several proportions of reactants to get an efficient yield. The report from the lab scientist is then shared with chemists, who will try to replicate the idea on a larger scale at the pilot plant of the industries. Even there also, some stoichiometric changes are made to meet the consumer’s requirements and give the product its unique brand identity. In any industry, the cost is a factor that affects the market and for that, the process is required to be economical. Stochiometric calculations provide the most economically efficient method of designing and developing a certain product.

6. Green Chemistry

green chem

Green chemistry is another name given to sustainable chemistry. It is an area that primarily focuses on the purposeful design of chemical products and processes that minimize the use of environmentally hazardous substances and the generation of waste. One of the twelve principles of green chemistry is aimed specifically at maximizing the efficiency of processes for synthesizing chemical products. The concept of stoichiometry plays a great role in synthesizing the final product that contains the maximum proportion of the starting materials. In general terms, this process is known as maximizing the atom economy. It is a measure of efficiency and defined as the percentage by mass of the final product of a synthesis relative to the masses of all the reactants used:

atom economy = \frac{mass of product}{mass of reactants} × 100%

Though the definition of atom economy at first glance appears very similar to that for percent yield, be aware that this property represents a difference in the theoretical efficiencies of different chemical processes. The percent yield of a given chemical process, on the other hand, evaluates the efficiency of a process by comparing the yield of product actually obtained to the maximum yield predicted by stoichiometry. The synthesis of the common nonprescription pain medication, ibuprofen, appropriately illustrates the success of a green chemistry approach.

7. Ecological Stoichiometry


Bags used to manipulate food elemental stoichiometry and determine the effects on zooplankton communities

Ecological stoichiometry is the study that discovers how the chemical content of an organism shapes its ecology. Similar to chemical stoichiometry, ecological stoichiometry is founded on constraints of mass balance as they apply to organisms and their interactions in ecosystems. The study primarily focuses on the interface between an organism and its resources. The community composition for any organism is often determined by the biotic factors of its ecosystem. For instance, in aquatic systems, the stoichiometry of ambient available nutrients, such as nitrogen and phosphorus, are used to predict phytoplankton species composition. If a particular organism species can maintain the community composition despite the changes in chemical composition and the availability of resources is referred to as stoichiometric homeostasis.

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