Study Guide

# Stoichiometry Themes

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### Combustion

Combustion reactions are an important class of chemical reactions. These reactions are vital to our everyday lives. How often do you drive around in a gas-fueled car? Or cooked a grilled cheese on a gas stove? How does your family stay warm in the winter? Perhaps a gas furnace does the trick? The combustion of fuels (wood, fossil fuels, peat, etc.) have heated and lit our homes and cooked our food for thousands of years. Our lives would be completely different if combustion reactions did not exist. Can you guess another added bonus of these gassy reactions? They provide tons of stoichiometry and balancing equations practice. Score.

A combustion reaction takes place when fuel and oxygen react, producing heat or heat and light. The most recognizable form of a combustion reaction is a flame such as a burning candle or a nice toasty campfire. Who brought the s'mores?

Combustion usually occurs when a hydrocarbon (meaning a compound composed of carbon and hydrogen) reacts with oxygen to produce carbon dioxide and water. These reactions are also highly exothermic which means they release energy often in the form of heat. That's why combustion is a great way to heat a house or run a car engine.

hydrocarbon + oxygen → carbon dioxide + water

CH4 + O2 → CO2 + H2O

But wait—the combustion reaction is not balanced. We have more oxygen atoms and fewer hydrogen atoms on the right hand side of the equation. How could we commit such a stoichiometric crime? Let's solve our conundrum by using the atom inventory technique.

 Element Before After C 1 1 H 4 2 O 2 3

It looks like we need one additional water molecule on the right hand side to even out our hydrogens before and after.

CH4 + O2 → CO2 + 2H2O

 Element Before After C 1 1 H 4 4 O 2 4

To balance the last element (oxygen) we just need one additional O2 molecule on the left hand side.

CH4 + 2O2 → CO2 + 2H2O

 Element Before After C 1 1 H 4 4 O 4 4

That wasn't so hard, was it? Try balancing these two additional combustion equations on your own:

C10H8 + O2→ CO2 + H2O

C2H6 + O2 → CO2 + H2O

### Answer

C10H8 + 12O2→ 10CO2 + 4H2O

2C2H6 + 7O2 → 4CO2 + 6H2O

### Water from a Camel: Storing Water as Tristearin

Who would have thought we could get a lesson in stoichiometry from camels? True story. What's the first thing you think of when you imaging a camel? It's probably not their dashing good looks. Instead we typically recognize camels by their humps (their humps, their lovely camel humps).

Did you ever really think why a camel has humps? There actually is a biological importance to those humps. Camels live in a harsh environment. Think about it. The desert: no food, no water, no shelter. Sometimes camels can go for days without eating or drinking. Can you guess how they survive? Their (lovely camel) humps.

Camels live in a harsh environment.

To combat the lack of resources in the desert camels actually store energy and water in their humps in the form of tristearin. It's that fancy long chain molecule shown below. Oxidation of tristearin produces water and energy. Who knew camels were so smart.

Tristearin Molecule.

Here's the reaction going on inside those humps. We balanced the equation for you.

2 C57H110O6 + 163 O2 → 114 CO2 + 110 H2O

Let's try out a problem based on our humped friend. What mass of water in grams is produced from 2.5 kg of fat (tristearin)?

First off all, let's convert 2.5 kg of fat into moles of fat. Remember the fat is the tristearin:

Now let's convert from moles of fat to moles of water:

Finally, we just need to convert moles of water to grams of water:

We'll never look at a camel the same way again.

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