Study Guide

Stoichiometry - What is Stoichiometry?

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What is Stoichiometry?

Reactions + Equations + Chemistry + You = Unstoppable

All of this stoichiometry stuff is just fine and dandy, but who really cares how much an atom or molecule weighs? Sure, it's nice to know that each gulp of water you drink contains a bazillion molecules of H2O, but is this life-changing info? Probably not.

The truth is, being able to calculate atomic masses, molar masses, molecular weights, and all of the other concepts we discussed is just another piece of the puzzle we need to help us understand chemical reactions. A chemical reaction is the process in which a substance (or substances) is changed into one or more new substances. These reactions occur both inside and outside of our body and are in charge of everything from making our hearts beat to baking our favorite chocolate chip cookies.

Far, far, away in the land of Chemistry, there lives an ancient and often misunderstood group of people. The inhabitants of this strange land are called scientists. They often communicate in illegible lab-book scribbles using things called chemical equations. Chemical equations are used to visually show what happens during a chemical reaction. The truth is, chemical equations can seem like a foreign language at first but we're going to do make it as easy to learn as Pig Latin.

The first step in mastering this new language is learning how to write a chemical equation. We've all seen chemical equations before: a symbol here, an arrow there, and some more symbols over there. There is a method to the madness, we promise. Let's go over a simple reaction to make sure we're all on the same page. Consider what happens when hydrogen gas (H2) burns in air (which contains oxygen, O2) to form water (H2O). This reaction can be represented by the chemical equation:

H2 + O2 → H2O

The plus sign means "reacts with" and the arrow means "to yield." If we translate our chemical equation into everyday English we get, "Hydrogen reacts with oxygen to yield water." Just in case you were curious, the reaction is assumed to proceed from left to right as the arrow indicates, just like reading a book.

Element Before After
H 2 2
O 2 1

According to the law of conservation of mass, mass is neither created nor destroyed in any ordinary chemical reaction. We have to have the same number of each type of atom on both sides of the arrow. In other words, we must have a balancedchemical equation. We can achieve this by placing the appropriate coefficient (2 in this case) in front of the H2 and H2O. Don't worry; this isn't totally random. We'll teach you throughout the rest of this section how to simply and systematically figure them out.

2H2 + O2 → 2H2O

This balanced chemical equation shows that two hydrogen molecules can react with one oxygen molecule to yield two water molecules. But wait—there's more. Remember moles? Not these moles. The ratio of molecules to moles is the same. Meaning 1 mole of hydrogen contains the same number of molecules (6.023 x 1023) as 1 mole of oxygen (again 6.023 x 1023). Since this is the case, the equation also shows that 2 moles of hydrogen molecules react with 1 mole of oxygen molecules to produce 2 moles of water molecules. Thrilling? Maybe not. But knowing this will be important on chemistry exam day.

There is a bit more scientist lingo we need to address, but we promise it's as easy as pie. Take a quick peek at our equation again. We refer to H2 and O2 as reactants, which are the starting materials in a chemical reaction. Water is the product, which is the substance formed as a result of a chemical reaction. In a chemical equation the reactants are written on the left-hand side of the arrow and the products on the right-hand side of the arrow.

reactants → products

There is one more final piece of information that's usually included in chemical equations. As if these equations couldn't get any more exciting, right? Chemists often indicate the physical state of the reactants and products using the letters g, l, and s to denote gas, liquid, and solid, respectively. Sometimes the letters aq are used to represent when something is in an aqueous (that is, water) environment such as NaCl dissolved in water. Putting it all together our favorite chemical equation becomes:

2H2(g) + O2(g) → 2H2O(l)

The real difficulty in learning the language of chemical equations is becoming a master of the balancing act. Don't worry—no tightropes here.

Balancing chemical equations isn't that bad. Follow these simple steps and before you know it you'll be a balancing aficionado.

1. Identify each and every reactant and product and write their correct formulas in the form of an unbalanced equation, complete with an arrow. Here's an example:

NaOH + H2SO4 → Na2SO4 + H2O

Double-check all of your chemical formulas. Also please remember we cannot change the subscripts of any reactants or products. Changing a subscript would change the chemical identity of the reactant or products, and that's a no-no. Consider H2O (water) versus H2O2 (hydrogen peroxide). These two compounds may only differ in a single oxygen atom, but we wouldn't want to go for a dip in a pool of hydrogen peroxide.

2. Complete an element inventory in the form of a table. This is nothing more than counting how many times an element appears in the reaction. Counting is king.

Element Before After
Na 1 2
O 5 5
H 3 2
S 1 1

This is a great visual to see that our equation is, in fact, unbalanced. We need all of the numbers before and after to be the same.

3. Start playing with the coefficients. These are the numbers that come before the reactants and products. Don't worry—there is method to this madness. It's best to start with an element that only appears in one reactant and one product. In our current example, Na appears once on the left hand side (in the form of NaOH) and once on the right hand side (in the form of Na2SO4). We know we need the same number of Na atoms on each side, which means we must have 2NaOHs for every 1Na2SO4. Let's write out a new equation and a new table to see if it's balanced.

2 NaOH + H2SO4 → Na2SO4 + H2O

Element Before After
Na 2 2
O 6 5
H 4 2
S 1 1

Our new inventory shows we've balanced our Na atoms but now the O atoms have gotten all out of whack. Are we making a mistake? Are we headed in the wrong direction? Nope. Oxygen appears in both reactants and both products, so we save the balancing until the end. Let's double check that there aren't any elements (like Na) that only appear in one part of the reactant and one part of the product. Sulfur fits the bill except, but it's already balanced. Lucky us. If it weren't, we would work through the same steps we completed for Na and write out a new table.

4. Now we have two more elements to balance: H and O. Remember, we already determined that NaOH and Na2SO4 must always be in a 2;1 ratio to balance the Na atoms. For this reason, it's best to try and change the coefficients of the other reactants and products. From our latest inventory we can see that we only need one extra oxygen atom on the right hand side. The easiest thing to try would be to increase the H2O coefficient to two.

2 NaOH + H2SO4→ Na2SO4 + 2H2O

Element Before After
Na 2 2
O 6 6
H 4 4
S 1 1

Notice that our inventory shows the reaction is now balanced. Huzzah. Sometimes more challenging reactions will take more elbow grease to work though, but if you always keep a good inventory, you'll have a balanced equation in no time.

Brain Snack

One bucket full of water contains more atoms than there are buckets of water in the Atlantic Ocean.

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