CH 9 Notes 2012.pdf

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Ch. 9 Notes: Stoichiometry – Mass/Mass Calc.

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RETEST DEADLINE!!

In-Class Practice: Chapter 9 WS #1

Notes: Limiting reagents

Chapter 9 WS #2

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Ch. 9 Notes: Percent Yield

Mini-Lab: Percent Yield

Ch. 9 Quiz

Chapter 9 WS #3

HW: Ch 9 Review

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Chapter 9 Test Begin Final Exam

Review

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Final exam

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FINAL EXAMS

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FINAL EXAMS

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FINAL EXAMS

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Stoichiometry- Chapter 9

Vocabulary to Know:

1. stoichiometry 2. percent yield 3. actual yield

4. theoretical yield 5. gram formula mass

6. Law of Conservation of Mass

7. excess reagent 8. limiting reagent 9. mol-mol ratio

Stoichiometry – study of calculations of quantities in chemical reactions using balanced chemical equations.

2Mg + O2  2MgO 2 moles Mg + 1mole O2  2 moles MgO

 The mole ratios can be obtained from the coefficients in the balanced chemical equation.  Mole ratios can be used as conversion factors to predict the amount of any reactant or product

involved in a reaction if the amount of another reactant and/or product is known.

Mole - Mole Calculations – Mole ratios are used to calculate the number of moles of product from the given number of moles of reactant, or vice versa.

Ex: Calculate the number of moles of Na2O that will be produced when 5.00 moles of Na completely react with oxygen gas.

Write the equation: __________________________________________

Known/Unknown: __________________________________________ Solve for the answer:

Practice: How many moles of oxygen are required to react completely with 0.84 moles of Al to for Al2O3?

Write the equation: __________________________________________

Known/Unknown: __________________________________________ Solve for the answer:

Practice: How many moles of aluminum oxide (Al2O3) are formed when 17.2 moles of oxygen reacts with Al?

Write the equation: __________________________________________

Known/Unknown: __________________________________________ Solve for the answer:

Mole-Mass Calculations:

We use mole ratios from balanced chemical equations and the molar conversions from Ch. 7 to calculate amounts (grams) of substances needed or produced in chemical reactions.

Ex: How many grams of KClO3 must decompose to produce KCl and 1.45 moles O2?

Write the equation: __________________________________________

Known/Unknown: __________________________________________

Practice: How many grams of oxygen are required to “burn” 1.0 moles of C2H2? (combustion reaction)

Write the equation: __________________________________________

Practice: How many grams of NH3 can be made from 3 moles of N2 and excess H2?

Write the equation: __________________________________________

Known/Unknown: __________________________________________

Mass-Mass Calculations:

Same as mole-mass calculations, with an additional step of converting mass to moles.

Ex: When 24.0 g of Na are mixed with Cl2, are 52.0 g of NaCl produced? Explain.

Write the equation: __________________________________________

Known/Unknown: __________________________________________

Practice: CaCO3, limestone, is heated to produce calcium oxide, CaO, and CO2. What mass of limestone is required to produce 156.0 g of CaO?

Write the equation: __________________________________________

Known/Unknown: __________________________________________

Practice: How many grams of lithium bromide will be produced if 10 grams of lithium hydroxide react with HBr?

Write the equation: __________________________________________

Known/Unknown: __________________________________________

Combination Calculations – Practice:

1. At STP, how many grams of oxygen are needed to produce 19.8 L SO3 according to the balanced equation? 2SO2 (g) + O2 (g)  2SO3 (g)

2. Given the reaction between tin and hyrdoflouric acid to create tin(II) fluoride and hydrogen gas, answer the following questions.

Reaction: __________________________________________

(remember to identify the known and unknowns)

a. How many grams of SnF

2 can be made by reacting 7.42 x 10 24

molecules of HF with tin?

b. How many liters of hydrogen (STP) are produced by reacting 23.4 g of Sn with HF?

c. How many liters of HF are needed to produce 14.2 L of hydrogen? (STP)

Reactants in Excess and Limiting Reactants

• Limiting Reactant - the reactant that is used up in a reaction. When you run out of a reactant, the reaction stops and no more product is formed.

• Excess Reactant - the reactant that is not used up in a chemical reaction. • The limiting reactantdetermines the amount of product formed.

Steps to determining limiting reactant:

1. Find out how much could be produced from each amount of reactant.

2. Whichever produces less is limiting the amount produced. It is therefore the Limiting Reactant.

3. To determine how much excess reactant is left after a reaction, subtract how much of the excess reactant reacted from how much excess reactant you started with.

Practice: When 0.50 mole of aluminum reacts with 0.72 mole of iodine, to form aluminum iodide, AlI3(s), what mass of AlI3 is produced? How much of the excess reactant will remain?

2 Al + 3 I2 → 2 AlI3

Practice: Suppose 3.8 mol hydrogen reacts with 2.2 mol of oxygen to form water.

a.) What is the limiting reagent?

b.) How many moles of excess reagent are left over?

c.) How many moles of water are produced? How many grams of water? (use the limiting reagent!)

Percent Yield:

Theoretical Yield – The maximum amount of product that could be formed from given amounts of reactants. (What you should get).

Actual Yield - amount of product actually obtained in the reaction (What you really get).

Ex: If 12.5 g of copper are reacted with an excess of chlorine gas, then 25.4 g of copper(II) chloride, CuCl2(s), are obtained. Calculate the theoretical yield and the percent yield.

Practice: What is the % yield if 3.74 g of Cu is produced when 1.87 g Al is reacted with an excess of copper (II) sulfate?

Ch. 9 Summary

Study this sheet, your review, notes, and old homeworks to prepare for your test. This sheet will provide an outline on how to solve the three main types of problems we had in this unit.

Plain Ole’ Stoich:

 You will know it is this type of problem if there is one given and one want.

 Remember, stoichiometry connects two different compounds or elements in a chemical equation.  All or your units MUST cancel or you are doing it incorrectly.

 Make sure your GFM is correct. Go one number after the decimal.

 If the chemical name is written out instead of the formula (and you aren’t sure about how to GET the formula), you should look for the equation. Use that as the guide for the formula. It will have to be in the equation for you to use it. Be careful here!

 Find your mole ratio or mole bridge from the balanced equation’s coefficient. If the equation is not balanced, BALANCE it first!!!

 For sig figs, look at units of given.  Use this picture as a guide:

mol mol given want

given want

mol bridge

Limiting Reactant:

 It is this type of problem if you have 2 givens and both of the givens are REACTANTS and you are looking for the amount of product.

 Work the problems out and solve for the same “want”. The one that produces the least amount of product is the limiting reactant and determines that total amount produced.

 To determine how much of the excess reactant is left, use the given amount of the limiting reactant and convert it to the other reactant. Make sure the units for the excess amount match the given amount. Subtract the number you calculate from the given amount of excess. (I know this sounds confusing…it will make more sense when we work it out)

Percent Yield:

 It is this type of problem if there are 2 numbers given to you and you see the key word “percent yield” in the problem.

 You will immediately write “given, want, and actual”.

 Your actual is going to be the amount that is produced, yielded, given off, obtained, etc.  The want will have the EXACT same units as the actual…but you are solving for the amount.  The given will be the other number that is given to you.

 Once you find your “want”, it becomes your “theoretical” for the next part of the problem.  Actual X 100 = Percent Yield