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WHAT DO THE COEFFICIENTS IN A REACTION TELL US??!?! Consider the equation: – 2 H 2 (g) + O 2 (g)  2 H 2 O (l) The coefficients indicate: “Two molecules.

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Presentation on theme: "WHAT DO THE COEFFICIENTS IN A REACTION TELL US??!?! Consider the equation: – 2 H 2 (g) + O 2 (g)  2 H 2 O (l) The coefficients indicate: “Two molecules."— Presentation transcript:

1 WHAT DO THE COEFFICIENTS IN A REACTION TELL US??!?! Consider the equation: – 2 H 2 (g) + O 2 (g)  2 H 2 O (l) The coefficients indicate: “Two molecules of diatomic hydrogen react with one molecule of diatomic oxygen to produce two molecules of water.” So, the coefficients also indicate: “2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water.” We can use this interpretation to help us make relationships to help us in reaction stoichiometry!!! Look at those AMU values! Sure look like molar masses to me!

2 USING THE LAW OF CONSERVATION OF MASS - Always assume a reaction proceeds 100% to the product (unless stated otherwise) Example: What does the law of conservation of mass state? Example: Think about the following reaction: 2 Na 3 N  6 Na + N 2. If 500 grams of Na 3 N completely decompose to form grams of N 2, how much Na is produced? Mass can neither be created nor destroyed, but it can be rearranged. The mass (amount of atoms) of the reactants must equal the mass (amount of atoms) of the products

3 STOICHIOMETRY PROBLEMS KEY NOTES: Stoichiometry calculations can go between any substances in a reaction… reactants or products! Stoichiometry helps us to find a theoretical yield – a maximum amount of product that SHOULD be made. (Not the actual yield!) MOLE Ratios Example: Use the balanced equation for the synthesis of water and determine the following mole ratios: 2H 2 + O 2  2H 2 0 Hydrogen to Oxygen Hydrogen to Water Oxygen to Water Water to Oxygen Water to Hydrogen

4 MOLE-MOLE problems Example: Use the balanced equation for the synthesis of water and determine how many moles of water would form if you began with moles of oxygen gas. 2H 2 + O 2  2H 2 0

5 Volume (1mole = 22.4L) MASS-MASS and MASS-PARTICLE and VOLUME CONVERSION problems Volume (1mole = 22.4L)

6 SAMPLE SETUP: Here’s what the calculation should look like for the above problem…

7 Practice: How many grams of sodium chloride could be formed in a synthesis reaction in which grams of chlorine gas are reacted with excess solid sodium?

8 Practice: How many grams of bromine are required to react completely with grams of lithium iodide forming lithium bromide and iodine?

9 Practice: How many molecules of hydrogen gas are required to react with grams of nitrogen gas in the synthesis of ammonia (NH 3 )?

10 Practice: How many moles of water could potentially be produced by reacting 5.60 grams of oxygen with hydrogen gas?

11 Practice: You are planning an experiment that requires mol of nitrogen monoxide (NO) gas. What volume of gas would you need at STP?

12 Practice: How many atoms of O 2 are in 45.6 L?

13 Practice: How many grams of CO 2 are in 28.3 L?

14 PERCENT YIELD Percent Yield allows us to express a ratio of how much product we should have gotten in comparison to how much we actually obtained. Percent Yield can be above or below 100% - very rarely will you get a 100% yield… unless you are very lucky! There is no way we’re getting 100% yield on this lab…

15 Example: The reaction of combustion of octane in an engine is known to be only 74.3% efficient. How many grams of water would you actually expect to form if grams of liquid octane are burned? 2C 8 H O 2  18H 2 O + 16CO 2

16 Practice: If 35.6 grams of zinc sulfate are actually produced when excess zinc chloride solution reacts with 47.3 grams of aluminum sulfate, what is the percent yield of this reaction? 3ZnCl 2 + Al 2 (SO 4 ) 3  3ZnSO 4 + 2AlCl 3

17 ***How do I know I have a limiting reactant problem?  You will be given the starting amounts of BOTH reactants, and asked to find the maximum amount of product you can produce. Steps to solve limiting reactant problems? TO FIND THE THEORETICAL YIELD: Take both of your reactants and find how much product they can produce. Whichever product amount is smallest is the maximum amount of product you can make. TO FIGURE OUT WHICH IS THE LIMITING REACTANT: Look to see which reactant made the least amount of product. This is your limiting reactant. The other reactant is in excess. TO FIGURE OUT THE AMOUNT OF LIMITING REACTANT LEFT OVER: Easy. It will always be NONE. You will use up all of the limiting reactant! TO FIGURE OUT THE AMOUNT OF EXCESS REACTANT LEFT OVER: Take the amount of limiting reactant you used, and use stoichiometry to calculate the amount of excess actually used. Then, subtract this value from how much you originally had. (HAD – USED = LEFT OVER) LIMITING REACTANTS (aka Limiting Reagent Problems) Limiting Reactants LIMIT HOW MUCH OF THE PRODUCT CAN BE MADE

18 Example: grams of hydrogen are mixed with grams of oxygen to form water. Write the balanced equation. What is the theoretical yield (maximum yield) of water in grams? What is the limiting reactant? How do you know? How many grams of the limiting reactant are left over? How many grams of the excess reactant are left over? This reaction was done in a laboratory environment and g of water was actually collected. What was the percent yield of this experiment?

19 Practice: grams of zinc are reacted with grams of copper (II) sulfate forming zinc sulfate and copper. Write the balanced equation What is the theoretical yield (maximum yield) of pure copper in grams? What is the limiting reactant? What is the excess reactant? How many grams of the zinc will be left over? How many grams of the copper (II) sulfate will be left over?


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