Stoichiometry “stoichio” = greek for “element” “stoichiometry” = to “measure the elements”

Chemistry is a lot like cooking (and vice versa) NOT coincidence Cooking is the process of doing chemical reactions on food! Recipes tell you the ingredients and also the proper proportions of ingredients If you get these wrong –YUCK! (try adding 3x too much sugar or 14 eggs next time you try to make cookies!)

Chemical “Recipes”: Cooking up an explosion Combustion reaction --> chemical (fuel) reacting with oxygen --> light and heat. DEMO: 38mL of butane in the can BUT, I want a bigger explosion! Let’s try it with twice as much butane. What happens? Same as cooking with too much sugar! In the demo --> too much fuel but not enough oxygen – so no boom! The “recipe” for this reaction –is this: – 2C 4 H O 2 →8CO H 2 O

Bad Chemist…. Like a bad chef, a bad chemist fails to use the correct proportions of ingredients. Can result in: – No reaction at all – Very inefficient reactions – little product produced and a waste of reactants (why old cars get worse gas mileage!) – The creation of unwanted products, i.e. the deadly carbon monoxide (instead of carbon dioxide) released from old/poorly maintained heaters.

How to be a good chemist: Use a science to figure out the right recipe so that our reactions work optimally. Also tells us how much of each product will be produced (also handy!) The name of the science: Stoichiometry! I love Stoichiometry!

Balanced Chemical Equation = Recipe! The coefficients in a balanced equation tell us the ratio of moles of the chemicals involved in the reaction. – So the equation: 2Mg + O 2 → 2MgO can be read as “2 moles of Mg atoms would react with 1 mole of O 2 molecules to produce 2 moles of MgO” (note that the 1 is not written in the formula) – Do you see how this is like saying “two cups of flour plus one cup of sugar makes 12 cookies” in a recipe?

We need to weigh the ingredients: Since we can’t actually count out the moles of atoms/molecules in the lab, we instead weigh out the proper amount of each substance by using its molar mass (we learned this last unit, right?) 2Mg + O 2 → 2MgO 2 moles of Mg = 2moles x 24.3g/mol = 48.6 grams of Mg go into reaction 1 mole of O 2 = 1 mole x 16g/mole = 32 grams of oxygen go into reaction 2 moles of MgO = 2 moles x 40.3g/mole = 80.6 g grams of MgO are produced in reaction! Compare how many grams go in to how many come out 48.6g + 32g = 80.6g in = 80.6 grams out. Why does this make sense? Now notice that 3 moles went in but only 2 came out – how did this happen? You combined 2 separate entities into one – therefore the total number of things present (moles) doesn’t have to be conserved. (imagine sticking 12 cherries on top of 12 sundaes – you still have only 12 sundaes!)

Your Turn – do the “Intro to Stoichiometry Worksheet”

To go back to our cooking comparison: Let’s say you have this recipe for pretzels: – 2 cups flour – 1 cup water – 1 package yeast – Makes 12 pretzels. Does that mean that a chef can only ever make exactly 12 pretzels? No! With some basic math a chef can make any number of pretzels she wants. The important thing is that she keep the correct ratios of ingredients from the recipe! Let’s say she wants 24 pretzels. What does she do with the recipe? What if she wants 6 pretzels? What if she wants 3 pretzels? What if she wants pretzels? The math gets a little tougher when we go to unusual numbers but the important point is this – it IS possible to calculate the right amount of ingredients to make any number of pretzels!

In addition: Given our pretzel recipe: – 2 cups flour – 1 cup water – 1 package yeast – Makes 12 pretzels. We can also answer other types of questions: – If we only have 1 cup of flour, how many pretzels can we make? How many cups of water would I need? – If I use 10 cups of flour and plenty of water and yeast, how many pretzels can I make? – If I use cups of water, how many packages of yeast do I need?

Now for the Chemistry: A chemist doesn’t always have to do the chemical reaction as stated in the equation – they can double, halve, etc. the reaction to get the amounts of products that they want! There is a cool mathematical system for doing this easily in chemistry (after all, it is tough to do 5.467x a recipe in one’s head!). The system is called the “GMMG” method (say “g – mmmmm - gah”)

Time for the GMMG Packet! G-mmmmmm- gah!

Molar Volume We keep talking about weighing out moles of atoms and molecules in the lab. This works great for chemicals that are solids and liquids. What is the problem with the idea of weighing oxygen gas?

Molar Volume It is very tough to find the mass of a gas by weighing it. So how do we collect one mole of gas (or any number of moles of gas for an experiment? First, to briefly review how we get a mole of a solid or liquid element: – We look at/calculate its molar mass in grams. – This number is different for different elements/compounds!

Molar Volume: It is actually far EASIER to measure out one mole of a gas. We measure the volume of the gas rather than weighing it. It turns out that one mole of any gas at STP has a volume of 22.4 Liters. STP = “standard temperature and pressure” = 0°C and 1 atm pressure. (We will deal with this concept more in a later unit).

How much is that? liters is actually a lot of volume. A mole of oxygen has a mass of 32 grams – only about the weight of 10 pennies. Although this is little mass, this gas occupies a space equivalent to 11 2-liter Coke bottles! Why? The molecules in a gas are very spread out – they have low density – so a little mass occupies a very large volume!

Molar Volume Stoichiometry So when working with gases, we use volume (liters) instead of mass (grams). This means that we can replace any “G” in GMMG with an “L” – LMML, LMMG, GMML, etc. The conversion factor for moles to liters of a gas is 1 mole/22.4 liters. Other than that, nothing changes – we do the stoichiometry problems the same, just paying attention to when we are using a gas and using the correct conversion factor.

Practice Problems Do “Molar Volume” worksheet P. 348 #1 (write balanced reaction first!) P. 329 # 1, 2 (write balanced reaction first!)