Reaction Stoichiometry: Mole Ratio
Be familiar with the concept first!
Before we move on, let's recap what is this for:
Reaction Stoichiometry is useful when a scientist wants to predict how much of something will be used or comes out, or reacted and produced, during a chemical reaction. Why does this work? This works because of the Law of Conservation of Mass.
This allows us to balance equations and hence predict how much units of compound will be reacted or produced. Stoichiometric Ratios
Remember the ratios in our cooking recipe? In Chemistry, it is just the same, but Chemists call it the mole ratio or stoichiometric ratio. It's really simple.
Check this out:
2H2 + O2 → 2H2O This is a hydrogen combustion reaction, where we basically burn (remember, combustion is a reaction with oxygen) the hydrogen to get water.
Here are two practice questions:
1) If I have 2 molecules of hydrogen and an excess of oxygen, how many molecules of water will I get?
Molecule is just a word that indicates quantity, like "bits" or "pieces" or "blocks". If you get stuck because of all the big words like molecule and hydrogen, think back to the cooking recipe, and try and interpret the question this way: "If I have 2 patties, how many burgers can I make?"
Putting it in context can make it easier, so never be afraid to understand the concepts through the use of analogies. To work out how much water we can produce, let's look at the ratio:
2:1:2
which means 2 bits of hydrogen and 1 bit of oxygen can make 2 bits of water, where "bits" is a meta-syntactic placeholder for more complex words of quantity like "moles" or "molecules". So now that we have 2 molecules of hydrogen, we could just follow the ratio and lo and behold, we could produce 2 molecules of water.
By the way, excess means you have enough of the something to fully complete the reaction. In our case, we have enough oxygen to burn with hydrogen to get our water.
2) If I have 1 mole of hydrogen, how many moles of oxygen do I need to produce a corresponding amount of water?
"Mole", if you don't remember, is another noun that denotes a certain quantity - 6.023*10^23 of something to be exact (or approximate, if you like nerdy jokes). Don't worry about the exact quantity as it is irrelevant unless the question asks for the amount of molecules needed (rare) or how much mass is needed in grams (more common in quizzes, but we'll cover that later).
The working out is exactly the same as above:
Find the ratio:
2:1:2
Then multiply by a scalar quantity which corresponds with the value we're given (easier done than said!):
2:1:2
|
| *0.5
\/
1 : 1/2 : 1
So 1 mole of hydrogen needs 0.5 moles of oxygen to successfully react and form 1 mole of hydrogen.
Pretty easy, right?
Now you can make water by burning hydrogen like Matt Damon did in The Martian:
This reaction is also useful in saving the environment: right now, we are burning more and more petroleum that releases harmful greenhouse gases just to power our cars, but companies like Shell are now trying to figure out a way to power cars by burning hydrogen.
Why? Because if you look back at the reaction, burning hydrogen only releases water, which doesn't harm our environment at all. The only setback is the cost of such machines and fuel. But that's the subject of another lesson!
