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Created by Dr. Dan Branan Stoichiometry Chemical Formulas Balancing Equations Limiting Reactant Problems Chemical Formulas Balancing Equations Limiting Reactant Problems

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Slide 2 of 17 Chemical formulas are just ratios of one atom to another in an ionic or molecular compound o “Empirical Formulas” are just the lowest possible whole number ratios For example, all sugars (such as C 6 H 12 O 6 or C 18 H 36 O 18 ) have the same empirical formula: CH 2 O (hence the name “carbo-hydrates”) Some notes on Chemical Formulas and Reactions NEXT: More on Chemical Formulas

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Slide 3 of 17 Chemical reactions are nothing more than electrons moving from one atom to another or being shared between one atom and another. o For example: this reaction (below) is just what happens when two hydrogens get close to an oxygen molecule and each hydrogen shares it’s only electron with an oxygen, forming two water molecules: OO HH HH + O O HH HH NEXT: More on Chemical Formulas Some notes on Chemical Formulas and Reactions

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Slide 4 of 17 Some notes on Chemical Formulas and Reactions When you have a balanced chemical reaction (see next slide), what you really have is a way to use dimensional analysis to turn reactants into products, or vice-versa. o For example: O 2 + 2H 2 2H 2 O o Now, if I said you have 28 molecules of hydrogen, how many molecules of water can you make? You could solve it like this: OK, that was easy, right? But it can get a lot more complicated. The critical thing is to realize that a balanced equation gives you the ratios you need to use dimensional analysis to solve these kinds of problems. o What if I gave you 28 oxygens instead? o The fractions in both cases come from the balanced equation above. Return to top of file

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Slide 5 of 17 Some notes on Chemical Formulas and Reactions When you have a balanced chemical reaction (see next slide), what you really have is a way to use dimensional analysis to turn reactants into products, or vice-versa. o For example: O 2 + 2H 2 2H 2 O o Now, if I said you have 28 molecules of hydrogen, how many molecules of water can you make? You could solve it like this: OK, that was easy, right? But it can get a lot more complicated. The critical thing is to realize that a balanced equation gives you the ratios you need to use dimensional analysis to solve these kinds of problems. o What if I gave you 28 oxygens instead? o The fractions in both cases come from the balanced equation above. Return to top of file

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Slide 6 of 17 Some notes on Chemical Formulas and Reactions When you have a balanced chemical reaction (see next slide), what you really have is a way to use dimensional analysis to turn reactants into products, or vice-versa. o For example: O 2 + 2H 2 2H 2 O o Now, if I said you have 28 molecules of hydrogen, how many molecules of water can you make? You could solve it like this: OK, that was easy, right? But it can get a lot more complicated. The critical thing is to realize that a balanced equation gives you the ratios you need to use dimensional analysis to solve these kinds of problems. o What if I gave you 28 oxygens instead? o The fractions in both cases come from the balanced equation above. Return to top of file

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Slide 7 of 17 Some notes on Chemical Formulas and Reactions When you have a balanced chemical reaction (see next slide), what you really have is a way to use dimensional analysis to turn reactants into products, or vice-versa. o For example: O 2 + 2H 2 2H 2 O o Now, if I said you have 28 molecules of hydrogen, how many molecules of water can you make? You could solve it like this: OK, that was easy, right? But it can get a lot more complicated. The critical thing is to realize that a balanced equation gives you the ratios you need to use dimensional analysis to solve these kinds of problems. o What if I gave you 28 oxygens instead? o The fractions in both cases come from the balanced equation above. Return to top of file

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Slide 8 of 17 Balancing Chemical Reactions NEXT: A Method for Balancing Equations It is vitally important to have a properly balanced equation before you start using it to help you solve a chemistry problem. “Balancing” just means that we make sure we have the same number of each atom on either side of the reaction arrow. After all, we cannot create or destroy matter! The coefficients in a balanced equation (the big numbers to the left of the chemicals) provide you with conversion factors so you can convert products to reactants and vice-versa.conversion factors When you balance equations, make sure you o nly change the coefficients o Changing the small subscript numbers makes the chemical compounds different O 2 + 2H 2 2H 2 O Coefficients NOT coefficients

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Slide 9 of 17 The “CHON” Method Balance Carbon atoms first. Balance Hydrogen atoms next. Balance Oxygen atoms next. Balance Nitrogen atoms next. Balance the remaining atoms in whatever order you want. Work in this order without backtracking until you have adjusted all the atoms in the reaction – then start with Carbon again. Keep track of atom totals on each side of the equation Change as few things as possible. Reduce coefficients to smallest whole integers. Return to top of file This is deceptively simple, but try it out -- it works! There are many ways to successfully balance equations, and if you already know one, then don’t change your method. This one might take longer than other methods sometimes, but if you stick to it it will always result in a properly balanced reaction.

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Slide 10 of 17 Limiting Reactant (Reagent) Problems Remember that the balanced equation ONLY tells you the ratios of one reactant to another, or of a reactant to a product. The reactant that is used up first limits the total amount of product you can make. It is called the “limiting reactant” Here’s how you figure out which reactant is used up first:

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Slide 11 of 17 Limiting Reactant Problems What you need: o A balanced chemical equation o A given amount of each reactant What do you do? o Choose one of the reactants o Calculate how much product you could make if you used it all up o Repeat for each reactant o The reactant that makes the LEAST product is the “limiting reactant”

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Slide 12 of 17 Limiting Reactant Problems Here’s the balanced reaction between ammonia and oxygen: If 1.50 g of ammonia reacts with 2.75 g of oxygen, how many grams of NO can you make? Let’s follow the method from the previous slide:

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Slide 13 of 17 Limiting Reactant Problems What if we used up all the ammonia? Must convert grams to moles in order to use the conversion factor from the balanced equation!

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Slide 14 of 17 Limiting Reactant Problems Now, how much NO can we make if we use up all the oxygen? Conclusion: using all the oxygen makes the least amount of product, so: Oxygen is the “limiting reactant” and the most NO I can make is 2.06 grams Return to top of file

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