111 Stoichiometry. Represents


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1 111 Stoichiometry What is stoichiometry? Calculations that relate the quantities of substances. It is the study of quantitative (measurable amounts) relationships in chemical reactions and equations. You will again be dealing with Senior Mole. Interpreting Balanced Chemical Equations. Remember in chapter 9 you learned how to use coefficients to balance a chemical equation. Those coefficients represent the number of particles taking place in a chemical reaction, be they atoms or molecules, they did not represent a measure of mass. They can represent moles or liters. Represents Therefore a balanced chemical equation can be used to indicate the relative (in relationship to another in that equation) number of moles of a substance be it atoms, molecules or formula units. There are five basic type of conversion problems you will be dealing with, they are mole:mole, mass:mass, mass:volume, volume:volume and limiting reactants. In each case guess who you need, yes Mr. Mole! MoleMole Problems Converting from moles of one substance to moles of another substance. The coefficients in a balanced equation can represent relative moles of a substance. There is a mole ratio relationship between all substances in an equation. Lets take a look What is the mole relationship between each substance? What if instead of 1 mole of NH 4 NO 3 you had 2.25 moles, can you calculate how many moles of each other substance you would produce? Yes! Lets try it.
2 This type of conversion is important in many other types of calculations. Lets try a few more. How many moles of HCl are needed to react with 2.3 moles of Zn? 2HCl + Zn > ZnCl 2 + H 2 How many moles of O 2 are needed to react with 0.52 mol Mg in the combustion reaction that creates magnesium oxide? How many mols of aluminum nitrate will be produced when 0.75 mols of silver nitrate react with aluminum metal in a single replacement reaction? Verifying the Law of Conservation of Matter remember that a balanced chemical equation verifies the law not only in the number and types of atoms, but the mass of the reactants will equal the mass of the products. Check it out with this equation 111 Section Review 1. Define stoichiometry and describe its use 2. What information is given by the coefficients in a chemical equation? 3. What is a ratio? How are ratios used in chemical equations? 4. How can you show mass is conserved in a chemical equation? 5. Describe all the information you can obtain form this chemical equation. 2C 2 H 2 + 5O 2 > 4CO 2 + 2H 2 O 112 Solving Stoichiometry Problems The most common stoic problems are massmass, massvol or volvol type. Let's look at each type and what you need to know to solve them. No matter what type of problem you are given the first step is to convert into moles of the given. 1. Write a balanced equation if your not given one. 2.Decide what you are given and what you want to solve for.
3 MassMass Problems given the mass of one substance, you must solve for another substance. When you see the word mass, what do you think you will need to know?? Lets check it out. Given the equation for the burning of glucose in your body. What mass of water would be produced from 1.5g of glucose. Step 1. Use (M) to convert to moles Step 2. Look at the mole ratio between what you are given and what you need to solve for Step 3 Use the (M) of what you want to solve for to get grams Instead of using three separate steps and increasing the potential for calculator error DA lets you put it all together in one neat package. Just be sure your labels cancel! Lets try a few calculations What is the mass of aluminum oxide produced when 2.3g of aluminum react with iron(iii) oxide. Hint its a sr reaction. First write the balanced equation Then use M to get out of grams of given to moles of given, then mole ratio to get to moles of what you want, then M to get to grams of what you want. Sounds easy, yes? Lets try another. What is the mass of sodium hydroxide produced when 0.25g of sodium react with water to produce sodium hydroxide and hydrogen gas (big 7?)?
4 Again. What mass of bromine (big 7) is produced when fluorine(big 7) reacts with 1.72g of potassium bromide? Again we have a single replacement reaction. MassVolume Problemsgiven the mass of one substance and asked to find the volume of another substance. You'll still need (M) and mole ratios, but now you'll need the molar volume of any gas at STP, which is L. The air bags in your car operate by a spark igniting sodium azide producing nitrogen gas. Here's the equation What volume of gas would you produce if your air bag contains 125g of sodium azide? To solve you begin with the same steps as a massmass problem because your unit of given is in grams. You must use (M) to get to moles, find your mole ratio of the substance you want to get to, then assuming your at STP use the molar volume of any gas So, in a fraction of a second the airbag would fill with 64.6L of gas. What would happen to the volume of that same gas on a hot day? Did the first airbags invented have problems? Lets try another one. Sodium bicarbonate is used to extinguish fires because when heated it decomposes into carbon dioxide which smothers fire. So here's the decomposition reaction. Sodium bicarbonate + heat yields sodium carbonate plus water plus carbon dioxide. What volume of gas would you produce with 4.0g of NaHCO 3? Again. Find the mass of aluminum required to produce 1.32L of hydrogen gas at STP. Here's the equation. 2AL + 3H 2 SO 4 > Al 2 (SO4) 3 + 3H 2 Your just working from the opposite angle now, go from liters, to molar volume, to mole ratio of what you want to (M) of what you want. Again, Again. (I'm just like a kid, eh?) How many liters of oxygen are necessary for the combustion of 340.g of ethanol(c 2 H 5 OH) with produces carbon dioxide and water. C 2 H 5 OH + 3O 2 > 2CO 2 + 3H 2 O
5 VolumeVolume Problemsgiven the volume of a gas you find the volume of another gas. These are my favorite. Guess why. Let's look at a problem How many liters of hydrogen gas will react with 15.5L of nitrogen? First decide what's given and what you want to solve for. Find the mole ratio between the two gases. The ratio in moles would be equal to the volume ratio so you can assume moles to liters. It's this easy Very Cool Lets try a few. If 0.38 L of hydrogen reacts with chlorine gas, what volume of hydrogen chloride will be produced? Again. What volume of carbon dioxide is produced when 3.2L of oxygen is consumed with methane to produce carbon dioxide and water. Again, again. What volumes of hydrogen and nitrogen gases are necessary to produce 16.0L of ammonia gas? 112 Section Review 1. What are the three main types of stoichiometry problems? Describe each in terms of the information given, what you need to use to solve, what is the unknown unit label? 2. How can a balanced equation be used to determine the mass or volume of an unknown quantity g of Carbon is burned (combusted), what mass of carbon dioxide is created? 4. Outline the steps to solve this problem. How many milliliters of oxygen gas at STP are released from the decomposition of 3.2g of calcium chlorate? Ca(ClO 3 ) 2 > CaCl 2 + 3O Describe how you could determine the amount of any product from a given reactant no matter how large or small the amount Limiting Reactants and % Yield Stoichiometric proportionwhen reactants are available in exact ratio as described by a balanced equation. When this is true all the reactants will be used up. Unfortunately, most often this is not the case, usually you will run out of one reactant before you run out of another. The one you run out of first is the limiting reactant. Identifying Limiting Reactants it is the reactant that you run out of first, the one that limits the amount of product you can form. The amount of product formed is always determined by the limiting reactant.
6 For example, I want to bake cakes for a cakewalk. I have a dozen eggs and 4 cups of oil. Each cake takes three eggs and 1/2 cup oil. Which reactant will limit the number of cakes I make? Chemistry is just like cooking, only you can't eat in lab! Determining the limiting reagent is usually easy if you have a balance equation and they give you each reactant amount in moles, but you know they won't, they'll give you grams of each or something else. The rules are still the same, if you re in grams, what do you need to get! Lets look at an example. Say you have 3.5g copper and 6.0g silver nitrate for the equation The next step is to calculate the amount of product each reactant could form In this case the limiting reactant is silver nitrate. Lets try a problem. Identify the limiting reactant when 1.7g of sodium reacts with 2.6L of chlorine gas at STP to produce sodium chloride (salt) 2Na + Cl 2 > 2 NaCl Again. Who's the limiting reagent when 10.0g of water reacts with 4.5 g sodium to produce sodium hydroxide and hydrogen(big seven!) gas. Again, again. I love it when you can't get enough of this fun! Who limits 12.5L of H 2 S at STP or 24.0g KOH to produce potassium sulfide and water
7 Limiting Reactant Problems 2.0g of sodium bicarbonate and 0.5g citric acid You can also determine the amount of sodium bicarbonate left over by calculating how much you would use The difference between the amount you started with and the amount you use is 2.0g  0.7g = 1.3g left over Lets try another. What mass of lead(ii) iodide will be produced when 16.4g of lead(ii)nitrate is added to 28.5g of potassium iodide. 1. It's a double replacement, write a balance equation 2. Determine the limiting reactant and the amount of lead iodide formed with the limited amount. Again. If 3.5g Zn and 3.5g S are mixed together and heated, what mass of ZnS will be produced? Again. What mass of barium nitride Ba 3 N 2 is produced from 22.6g barium reacting with 4.2g nitrogen gas(big7) Percent Yieldthe amount of product you actually get form a reaction. You can calculate an expected yield, that's what we've been doing, but sometimes (usually) you don't get what you expected. Get over it and calculate your percent yield! for example if you burned 4.9g of Mg in plenty of oxygen you would calculateexpect 8.1g of magnesium oxide. What if you only got 6.5g. your %yield =6.5g/8.1g x 100 = 80% 5.00g copper is placed in silver nitrate (excess) The silver metal produce has a mass of 15.2g. What is the %yield 1. Calculate your expected yield 2. Divide your actual yield by the expected (calculated) yield and x 100 Again. What is the % yield for 2.80g aluminum nitrate reacted with excess sodium hydroxide if g of aluminum hydroxide was recovered. Again. What is the % yield for the reaction between 15.0g nitrogen gas and 15.0g hydrogen gas if 10.5g ammonia is produced
8 113 Section Review 1. What is a limiting reagent. Why does it limit the product? 2. How are quantities of products determined when there is an excess of one or more reactants? 3. What is % yield and how is it determined? Special Notes
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