Are chemical equations like a cooking recipe?

Sometimes, balanced chemical equations are compared to cooking recipes. Here is [one example](https://socratic.org/questions/how-does-stoichiometry-relate-to-cooking):



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> Imagine if you were baking chocolate chip cookies and measured out your ingredients incorrectly. What if you added 2 tablespoons of flour instead of two cups of flour. Or if you added one cup of salt instead of one teaspoon of salt. Your cookies would definitely not be the hit of the next bake sale.
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> This is also true of chemistry and why stoichiometry is an important aspect of the chemical process. Balancing measures of the reactants involved to maintain the expected outcomes of the products desired.
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Here are some more examples of sites using this analogy:


1. <https://saylordotorg.github.io/text_introductory-chemistry/s09-01-stoichiometry.html>
2. <https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:chemical-reactions/x2eef969c74e0d802:stoichiometry/a/stoichiometry>
3. <https://chem.libretexts.org/Courses/Bellarmine_University/BU%3A_Chem_103_(Christianson)/Phase_2%3A_Chemical_Problem-Solving/5%3A_Reaction_Stoichiometry/5.1%3A_Chemical_Recipes>


Which part of this analogy is correct, and where does the analogy fall apart?




Cooking and baking each involve a lot of chemistry themselves, and this analogy largely holds in the case of stoichiometry.


One might take liberty with a cooking or baking recipe and change it, adapt it, mess with some of the ingredients, etc. and this is not a scenario mapped to stoichiometry. The rigidity of stoichiometry comes logically from the premise that the smallest "unit" of a chemical reaction is the rearrangement of bonds between individual atoms. The law of the conservation of matter demands that all atoms at the beginning of a reaction must be present at the end of a reaction, and so these ratios of atoms are not arbitrary.


The primary way this breaks down, though, is in the end result from *messing up* the recipe. In baking, if your cake recipe calls for 2 cups of flour, 2 eggs, and 1 cup of sugar, using only 1 egg will get you a single, *terrible* cake. In chemistry, however, this will result in—to stretch the analogy—half of a *great* cake, 1 cup of flour, and half a cup of sugar. If you don't have enough of one thing in a chemical reaction, then the reaction still happens to *some* of the materials, but then it runs out and you have some "ingredients" left over.


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The difference between cooking and chemistry is in the precision. To make a cake, let's admit that you need $200$ g flour, $100$ g eggs and $100$ gram sugar. But if you take $90$ g or $110$ g sugar, the result will be the same. This is not true in chemistry. To make water from hydrogen and oxygen, you need $\ce{2.02 g H2}$ and $\ce{16.00 g O2}$. If you use $\ce{2.03 g H2}$ with the same amont of oxygen, you will obtain the same amount of water, and the $\pu{0.01 g}$ excess of $\ce{H2}$ will remain at the end of the reaction.


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Reversibility
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All chemical reactions are reversible in the presence of sufficient energy, but you cannot unbake a cake or unfry an egg. Indeed, many reactions proceed both forwards and backwards simultaneously. But mixing flour, sugar and salt in a bowl generally does not entail simultaneous mixing and unmixing of the ingredients except perhaps at the very smallest scales.


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Are chemical equations like a cooking recipe?


No. A chemical equation displays the strict theoretical relations of amount of substance of the educts and resulting products.


A cooking recipe would be closer to the description of a chemical synthesis procedure. The goal of the chemical synthesis would be to achieve a chemical reaction according to a chemical equation, but would a) be very specific about how to do things (temperature, how long for the reaction to take, how to apply certain procedures, like destillation, purificiation of products etc) and b) might use different quantities from what the actual chemical equation would theoretically suggest. Some reactions can only be made work properly if a certain excess amount of educt is used, etc.
Working out unknown chemical synthesis procedures requires a lot of creativity and playing around with the amounts of substance.
Just putting together the amounts as required by the exact chemical equation will in most cases yield disappointing results.


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