Chemistry 11. Unit 7 : Stoichiometry

Transcription

1 Chemistry 11 Unit 7 : Stoichiometry Name: Block:

2 Intro to Stoichiometry - Calculating with Chemical Change The reaction between phosphoric acid, H 3 P0 4, and potassium hydroxide, KOH, can produce three different products: H3P0 4 + KOH ---+ KH 2 P0 4 + H H3P KOH ---+ K 2 HP H 2 0 H3P KOH ---+ K3P H. Each of the products KH2P0 4, K2HP0 4 and K3P0 4, has different properties and different uses. For example, KH 2 P04 is used in, K 2 HP04 is used in some and antifreezes, and K3P04 is used in liquid. The products of this chemical reaction are based on the of H 3 P0 4 and KOH used. Stoichiometry (stoicheion meaning "element" and metron meaning "measure"): With stoichiometry, we can predict the amount of a specific product created when a given amount of reactant is used. Example: 2 H 2(g) + O 2(g) 2 H 2O(l) How does one state the chemical reaction equation above? It turns out that there are actually two ways: Two of hydrogen react with one of oxygen to produce two of water. In chemistry we will usually think in terms of rather than. The mole ratios of coefficients in the balanced reaction equation gives us the mole conversion factors: EXAMPLE: Consider the reaction equation N H NH3 How many molecules of N2 are required to react with 15 molecules of H2? IMPORTANT: Use completely-labelled units (eg. "molecule N2'' not just "molecule") so you know which coefficient goes on top and which goes on the bottom of the conversion factor.

3 We can use these conversion factors to move from moles of one species to moles of another. Example Suppose that 1.50 moles of aluminum were produced during a reaction between zinc and aluminum chloride. How many moles of zinc reacted? The Mole Bridge As we have seen earlier, you can convert between the different species in the chemical reaction equation. To do this, however, you must get your substances into : Only moles can cross the mole bridge! Using the mole bridge, you can convert between any of the following: a reactant and a product a reactant and a reactant a product and a product Example 1 If g of wood were combusted, then what mass of water would be expected to form?

4 Example 2 If ml of 1-pentanol were combusted then what volume of CO 2 would form, given that the volume would be determined under conditions of STP? (note: d 1-pentanol = g/ml) Example 3 What mass of C 3 H 8 is required to produce 100.0g of H 2 O?

5 Assignment #1: "Basic Stoichiometry Problems" & "Mole Ratio in Reactions" Complete ALL assignments on a separate piece of paper and attach to your booklet when handing in at the end of the unit. Be sure to clearly number each assignment with a heading. Basic Stoichiometry Problems 1. What mass of ammonia forms when 5.6 g of nitrogen reacts? N 2 + 3H 2-2NH 3 2. What mass of iron must be reacted to produce 32 grams of iron (III) oxide? 4Fe Fe 2 O 3 3. What mass of carbon dioxide forms when 1.00 kg of octane is burned? 2C 8 H, I 6CO 2 + I 8H 2 O 4. What mass of aluminum is needed to react with 6.4 g of iron (III) oxide? 2Al + Fe 2 O 3 - Al 2 O 3 + 2Fe 5. What mass of zinc will react with 50.0 ml of 3.00 M HCI? Zn + 2HCI - ZnCI 2 + H 2 6. How many ml of 2.00 M HNO 3 is needed to consume 5.4 g of aluminum? 2AI + 6HNO 3-2Al(NO 3 h + 3H Q_ ml of HCI is needed to consume 2.8g Fe. What is the concentration ofhcl? 2Fe + 6HC FeCl 3 + 3H 2 8. How many ml of 0.80 M AgNO 3 will exactly react with l 0.0 ml of 0.25 M AIC1 3? 3AgNO 3 + AIC AgCl + Al(NO 3 ) ml of 0.240M H 2 SO 4 react exactly withy 35.2 ml of KOH. Determine the concentration of KOH. H 2 SO 4 + 2KOH - K 2 SO 4 + 2H 2 O 10. How many ml of 0.60M H 3 PO 4 will react with 3Q_ g of Ca(OH) 2? 2H 3 PO 4 + 3Ca(OH) 2 - CaiPO 4 ) 2 + 6H 2 O l l. What mass of aluminum hydroxide would react exactly with 15.0 rnl of 2.00M HCl? 3HC1 + Al(OH) 3 - AIC H 2 O 12. What concentration HCI is needed so that 40.Q_ ml will react with 17.0 g of magnesium? Mg + 2HCJ - MgCl 2 + H What mass of copper will react with 10.0 ml of 12.0 M nitric acid? Cu + 4HNO 3 - Cu(NO 3 ) i + 2NO 2 + 2H 2 O 14. How many kilograms for oxygen are needed to react with 51 kg of ammonia? 4NH NO + 6H 2 O

6 Chap 7: Mole Ratio in Reactions I. How many moles of 0 2 are produced from 1.50 moles of KC1O 3? 2KC1O 3-2KC How many moles of H 2 are needed to react with 8.0 moles of N 2? N 2 + 3H 2-2NH 3 3. How many moles of HCI are needed to form 4.5 moles of H 2? 2Al + 6 HCI - 2AIC H 2 4. How many moles of water form when 0.50 moles of 0 2 react? 4NH.l NO 2 + 6H 2 O 5. How many moles of methane can react with 24.0 moles of 0 2? CH CO 2 + 2H 2 O 6. How many moles of calcium phosphate form when 2.0 moles of Ca(OH) 2 react? 3Ca(OH) 2 + 2H 3 PO 4 - CaiPO 4 ) i + 6H 2 O 7. How many moles of Mg can react with 0.40 mol HCl? Mg + 2HC1 - MgCl 2 + H 2 8. How many moles of NaHCO 3 must decompose to produce 0.80 mol H 2 O? 2NaHCO 3 - Na 2 O + 2CO 2 + H 2 O Answers: mol mol mo! mol mol mol mol mol

7 Stoichiometry Calculations Involving: Molar Concentration IMPORTANT: If a VOLUME is mentioned, and the problem involves a molarity, DO NOT assume that "22.4 L" should be used. The use of "22. 4 L" is justified only if the substance being referred to is a gas AND if the key phrase "at STP" is mentioned along with the volume. Example 1 Tums is an antacid composed primarily of calcium carbonate (chalk), and stomach acid is a dilute solution of hydrochloric acid. The neutralization reaction between CaC0 3 and stomach acid is represented by the equation: CaC03(s) + 2 HCl(aq) ----> CaCl2(aq) + C02(g) + H 2 (I) a) A tablet of Tums has a mass of g. What volume of stomach acid having [HCI] = M is neutralized by a g portion of CaCO3? b) What volume of CO 2(g) at STP is produced if 1.25 L of M HCI reacts with an excess of CaCO 3?

8 Example 2 Consider the following reaction: H 2 SO 4 + 2KOH --> K 2 SO 4 + 2H 2 O a) What mass (in g) of water is produced when 125mL of a 0.100M H 2 SO 4 solution is reacted with excess KOH? b) What volume of 0.050M KOH solution is needed to completely react with 78mL of 0.28M H 2 SO 4? Titration Reactions A TITRATION is a method used to determine the concentration of an unknown solution. How You Do It: Measure the volume of the solution of known concentration (the known solution) needed to completely react with a certain volume of the solution of unknown concentration (the unknown). Why It Works:

9 A is a process in which a amount of a solution is reacted with known volume of another solution (one of the solutions has an concentration) until a desired EQUIVALENCE POINT is reached. EQUIVALENCE POINT (Stoichiometric Point): The equivalence point is recognized by an. There are many different types of titrations but they all work on the same principle: (In Chemistry 11, we will only look at Acid-Base Reaction Titrations.) As you combine the solutions, the chemicals react, consuming each other to form products. 1. Until you ve added enough of reactant A, 2. Once you ve added just enough to complete the reaction, 3. Adding more of reactant A (after reaction is over) results in The is the point in the acid-base titration where all the reactants have been used up (and none are in excess); the number of moles of each reactant perfectly obeys the stoichiometry (mole ratios) of the reaction equation. Example 3 When 50.0 ml of HCl were titrated with M NaOH, it was determined that 75.0 ml were needed to reach the point. Determine the [HCl]. Example 4 If 19.8 ml of phosphoric acid reacts completely with 25.0 ml of M KOH, then what is the concentration of the phosphoric acid?

10 Assignment #2: Exercises # & Prepare Titration Lab! Complete ALL assignments on a separate piece of paper and attach to your booklet when handing in at the end of the unit. Be sure to clearly number each assignment with a heading. EXERCISES: 17. A student wants to put 50.0 l of hydrogen gas at STP into a plastic bag by reacting excess aluminum metal with M sodium hydroxide solution according to the reaction 2 Al(s) + 2 NaOH(aq) + 2 H 2 O(1) _. 2 NaAIO 2 (aq) + 3 H 2 (Q). What volume of NaOH solution is required? 18. What volume of M HCI is required to completely neutralize 25.0 ml of M NaOH? [Hint: what is the balanced equation for the reaction between HCI and NaOH?] 19. A technician analyzes a sample of water from the "tailings" pond of a mine for the presence of mercury. After treating and concentrating the water sample, the technician carries out the titration reaction Hg c1- HgCl 2 (s). A 25.0 ml sample of water containing mercury reacts with 15.4 ml of M c1-(asnaci). a) What is the molar concentration of the mercury in the water sample? b) What mass of HgCl 2 is formed in the reaction? 20. A 1 O. O ml sample of a saturated solution of Ca(OH) 2 is titrated with ml of O M HCI. a) What is the molarity of the Ca(OH) 2 in the saturated solution? b) What mass of Ca(OH) 2 is dissolved in 250. O ml of saturated Ca(OH)2? 21. A student titrates a ml sample of hydrogen peroxide solution, H 2(aq), according to the reaction 2 MnO (aq) +5 H 2 O 2 (aq) +6 H + (aq) _. 2 Mn 2+ (aq) + 5 O 2 (g) + 8 H 2 O(1). The supply bottle of H 2 O2 is labelled as "3.00% by volume" (3.00 ml of H 2 O 2 per 100 ml of solution), which the student calculates to have [H 2 O21 = M. a) What volume of o M MnO is required for the titration? b) What volume of 0 2 (9) at STP is produced during the reaction? 22. A 1.00 ml sample of pure phosphoric acid, H 3 PO4, is titrated with 43.8 ml of M NaOH according to the reaction 2 NaOH + H 3 PO4 _. Na 2 HPO4 + 2 H2O. a) What is the molar concentration of pure H 3 PO4? b) Calculate the density of pure H 3 PO4 23. The iron present in a sample of iron ore is converted to Fe 2+ and titrated with dichromate ion Cr Fe H + _. 2cr Fe H 2 O. If 17.6 ml of M dichromate ion is required to titrate a 25.0 ml sample of Fe 2+ solution, a) what is the molarity of the Fe 2+? b) what mass of iron is present in the 25. o ml sample? Prepare Acid Base Titration Lab: o flow chart o pre-lab questions o set up data tables

11 Stoichiometry of Limiting & Excess Quantities ANIMATION: Let's consider an analogy: Suppose I'm making a sandwich. In each sandwich I want: 2 pieces of bread (Br) 4 slices of tomato (To) 2 pieces of chicken (Ch) 2Br + 4To + 2Ch --> 1Br 2 To 4 Ch 2 BUT...what if I had 10 slices of bread, 26 tomatoes and 12 chicken slices? is the limiting reactant (reagent), because we can only make sandwiches, then we are all out of, and have an excess of...similarly, chemical reactions frequently are carried out in such a way that one or more of the reactants actually are present in amounts. Some reasons for having an excess amount: deliberately adding an excess of 1 reactant to make sure all of a second reactant used (maybe too expensive to waste, or harmful to the environment) unavoidably having a reactant in excess because a limited amount of another reactant is available.

12 T he reactant that gets used up first is called the limiting reactant because it limits how far the reaction can go. Al l other reactants that are left over after the reaction is finished are called excess reactants. Consider the simple reaction: 2 Na(s) + Cl 2(g) 2 NaCl(s). To use up all 2.17 moles of Na you would need moles of Cl 2 How many moles of Cl 2 would you need to use up 2 moles of sodium? Answer: 1 mole. What would happen if 3.0 moles of sodium and 2.0 moles of chlorine reacted? Answer: Since 3.0 moles of sodium is only sufficient to react with 1.5 moles of chlorine, all of the sodium will be consumed and 0.50 moles of chlorine will be left in excess. If 50.0 g Na and 80.5 g Cl 2 were reacted? Answer: You cannot directly compare numbers of atoms by comparing masses. You must convert to moles to compare: moles Na = 50.0 g / 23.0 g/mol = 2.17 mol moles Cl 2 = 80.5 g/ 71.0 g/mol = 1.13 mol You don t have enough Na to use up the Cl 2 (need 2.26 mol Na to use up 1.13 mol Cl 2 ) So is the limiting reactant, mole of reacts, and mole of remains. How to Determine the Limiting Reactant, Amount of Product(s) and Quantity of Excess Reactant(s): 1. Start by assuming that the first reactant is the limiting reactant (i.e. it gets used up before the others). Calculate how much product it could theoretically produce. 2. Repeat step 1, in turn, for each of the other reactants. 3. Identify the Limiting Reactant (LR) the one that can make the least amount of product. Amount of Product Formed = amount of product formed based on amount of LR that reacted 4. Identify the Excess Reactants (XSR) that remain unreacted. 5. Use the LR to find out how much of the other XSR(s) remain. Amount of XSR remaining = XSR initial XSR reacted 2 CO(g) + O 2 (g) 2 CO 2 (g)? reactants products a) Which of the reactant is in excess? b) Sketch any remaining reactants and products to show what will be present once the reaction is complete. c) How many CO2 particles are formed? d) How many excess reactant particles remain?

13 In limiting and excess problems, you will be given the mass of all of the reactant species and you must determine which of these are in excess...and more importantly, which is the limiting reactant (or reagent) that limits the amount of the product that can form. You may also have to determine how much of the excess species remains once the reaction is complete. Example 1 What volume of hydrogen gas, measured at STP, is produced when 8.0g of lithium metal is reacted with 10.0g of water? What mass of the excess reactant remains once the reaction is completed?

14 Example g of hydrogen react with g of oxygen. Identify the limiting reactant and determine the mass of water formed and the mass of the excess reactant.

15 Example 3 Determine the mass of the excess reactants when 56.8 g of ferrous chloride react with 14.0 g of potassium nitrate and 40.0 g of muriatic acid react as follows: 3 F e C l + KNO HCl 3 FeCl 3 + NO + 2 H 2 O + KCl 2 Assignment #3: Exercises # Complete ALL assignments on a separate piece of paper and attach to your booklet when handing in at the end of the unit. Be sure to clearly number each assignment with a heading.

16 Calculations of Yield In any chemical reaction, the amount of product you get is called the chemical yield. The yield always differs from the amount you expect to form: Theoretical yield = Actual yield =

17 Why does the actual yield differ from the theoretical yield? There are many possible reasons. Some are acceptable and some aren t. Here are a few examples: Carelessness or sloppiness. Reactants improperly measured out or lost due to spills, etc. Product left behind in the reaction container when collected or transferred. Products may be contaminated. The mass of the impurity results in greater mass than expected. Note: Water is frequently a source of error. Important to dry samples thoroughly before weighing. NOTE: we can never make more product than expected (unless we didn't make what we thought we did!) The of the PRODUCTS should be than the theoretical yield. 3 Types of % Yield Calculations: 1. Find the percent yield, given the mass of reactant used and mass of product formed. 2. Find the mass of product formed, given the mass of reactant used and the percentage yield. 3. Find the mass of the reactant used, given the mass of product formed and the percentage yield.

18 Example 2 A student carried out an experiment in which she completely reacted 45.8 g of potassium carbonate with an excess of hydrochloric acid. As a result, 46.3 g of potassium chloride were produced. What was the percentage yield of potassium chloride? + + H 2O + CO 2 Example 3 When a second student carried out the same reaction as above, 52.7 g of potassium chloride was obtained. Calculate the percentage yield. What most likely explains the result? Example 4 If the reaction has a 76.0% yield, what mass of K 2 CO 3(s) is produced when 1.50 g of KO 2(s) is reacted with an excess of CO 2(g) according to the reaction: 4 KO 2(s) + 2 CO 2(g) --> 2 K 2 CO 3(s) + 3 O 2(g)

19 Sample Purity When a sample is impure it means that only part of its mass is due to the pure substance. e.g. If sample of NaCl is 75% pure, then g of impure sample contains 75.0 g of pure NaCl. x % purity means x grams of = 100 grams of From this equivalence statement, we get the conversion factors: xgpure gimpure or gimpure xgpure Example 1 A sample of potassium carbonate is 58.5% pure. What mass of sample contains 87.3 g of potassium carbonate? What mass of potassium carbonate is in g of the sample?

20 Assignment #4: Exercises # Complete ALL assignments on a separate piece of paper and attach to your booklet when handing in at the end of the unit. Be sure to clearly number each assignment with a heading.

21 Chemistry 11 Stoichiometry Review Assignment Name: Date: Block: Answer the following practice questions on a separate page Define the following terms: 1. Stoichiometry 2. Stoichiometric ratio 3. Limiting reactant 4. Excess reactant 5. Percent yield (Mole-Mole Conversions) 6. The combustion of the organic fuel, decane, is outlined in the chemical equation below. You must balance the equation in order to answer the subsequent questions a-c. C10H22 + O2 CO2 + H2O a. How many moles of CO2 are produced if 5.0 moles of C10H22 react with an excess of O2? b. How many moles of O2 react with 0.75 moles of C10H22? c. How many moles of O2 would be required to produce 4.0 moles of H2O? 7. Use the following equation to solve the problems below: 3 SiO2 + 4 Al 3 Si + 2 Al2O3 a. If 6.0 moles of SiO2 react, how many moles of: i. Al react? ii. Si are produced? iii. Al2O3 are produced? b. If 2.5 moles of Al2O3 are produced, how many moles of: i. Al react? ii. SiO2 react?

22 (Mole-Mass / Mass-Mole Conversions) 8. N2 + 2O2 N2O4 a) If 15.0g of N2O4 was produced, how many moles of O2 were required? b) If 4.0x10-3 moles of oxygen reacted, how many grams of N2 were needed? 9. Cu + 2AgNO3 Cu(NO3)2 + 2Ag How many moles of Cu are needed to react with 3.50g of AgNO3? 10. Mercury (II) oxide decomposes into mercury and oxygen gas. a) Write and balance the equation. b) How many moles of mercury (II) oxide are needed to produce 125g of oxygen? c) How many grams of mercury are produced if 24.5 moles of mercury (II) oxide decomposes? (Mass-Mass Conversions) 11. Li3N(s) + 3H2O(l) NH3(g) + 3LiOH(aq) a. What mass of lithium hydroxide are produced when 0.38g of lithium nitride react? b. How many grams of lithium nitride would react with 4.05g of H2O? 12. In the combustion of 54.50g of butane (C4H6), how many grams of CO2 are produced? Write and balance the equation before solving. 13. In the following unbalanced equation, FeS2 + O2 Fe2O3 + SO2 a. How many grams of iron (IV) sulphide are used when 9.0g of O2 react? b. What is the mass of iron (III) oxide produced when 25.0g of iron (IV) sulphide are used? 14. Cu + 2AgNO3 2Ag + Cu(NO3)2 How many grams of silver are produced when 36.92g of copper react? 15. Al2(SO4)3 + Ca(OH)2 Al(OH)3 + CaSO4 Balance and answer the following questions. a. What mass of aluminum (III) hydroxide are produced if 165.7g of aluminum (III) sulfate react? b. How many grams of calcium hydroxide are needed to form 6.35g of calcium sulphate?

23 (Mass- Volume/ Volume-Volume Conversions) 16. Given the following equation: 3 NO2 (g) + H2O(l) 2 HNO3 (aq) + NO(g) Assume STP a. What mass of water is required to react with 15.5 L of Nitrogen dioxide? b. What volume of Nitrogen monoxide would be produced from g of water? c. If 42.0 L of NO(g) is produced, what volume of NO2 (g) reacted? 17. When Magnesium reacts with Nitric Acid, Hydrogen gas and aqueous Magnesium nitrate are formed. What volume of Hydrogen gas will be produced if 40.0 g of Magnesium is reacted with an excess of Nitric Acid? 18. The corrosion (rusting) of iron is represented as follows: (at STP) 3 O2 (g) + 4 Fe(s) 2 Fe2O3 (s) a. What volume of Oxygen gas would be required to produce 16.0 g of Fe2O3? b. What mass of Iron would be required to react with 10.0 L of O2 gas? 19. Mercury (II) oxide decomposes when heated to produce liquid Mercury and Oxygen gas. What mass of Mercury (II) oxide would be required to produce 30.5 L of Oxygen gas? (Assume STP) 20. How many ml of 2.00M HNO3 is needed to consume 5.4g of aluminum? 2Al + 6HNO3 2Al(HNO3)3 + 3H mL of HCl is needed to consume 2.8g Fe. What is the concentration of HCl? Fe + HCl FeCl3 + H2 22. What mass of copper will react with 10.0mL of 12.0M nitric acid? Cu + HNO3 Cu(NO3)2 + NO2 + H2O

24 REVIEW: Limiting Reagents and Percent Yield Answer all questions on separate paper and report all answers to the correct number of sig figs. 1. Identify the limiting reactant when 1.22g of O 2 reacts with 1.05g of H 2 to produce water. 2. Identify the limiting reactant when5.87g of Mg(OH) 2 reacts with 12.84g of HCl to form MgCl 2 and water. 3. Identify the limiting reactant when 6.33g of sulphuric acid reacts with 5.92g of sodium hydroxide to produce sodium sulphate and water. 4. Identify the reactant in excess if 6.25g of silver nitrate reacts with 4.12g of sodium chloride to form sodium nitrate and silver chloride. 5. If 4.1g of Cr is heated with 9.3g of Cl 2 what mass of CrCl 3 will be produced? 6. What mass of sulphur trioxide is produced when 12.4g of sulphur dioxide is reacted with 3.45g of oxygen gas? 7. If 21.4g of aluminum is reacted with 91.3g of iron (III) oxide, the products will be aluminum oxide and iron. What mass of iron will be produced? 8. If 41.6g of N 2 O 4 reacts with 20.8g of N 2 H 4, the products will be nitrogen gas and water. What mass of nitrogen will be produced? 9. What mass of NaCl will be produced by the reaction of 58.7g of NaI with 29.4g of Cl 2? 10. a. Write the balanced equation for the reaction of lead (II) nitrate with sodium iodide to form sodium nitrate and lead (II) iodide: b. If I start with 25.0 grams of lead (II) nitrate and 15.0 grams of sodium iodide, how many grams of sodium nitrate can be formed? c. What is the limiting reagent in the reaction? d. How much of the excess reagent will be left over from the reaction? 11. You calculate that using a certain amount of beryllium and hydrochloric acid you can produce 10.7g of beryllium chloride. You perform the experiment and only collect 4.5g. What was the percent yield for the reaction?

25 12. Determine the percent yield for the reaction between 45.9g of NaBr and excess chlorine gas to produce 12.8g of NaCl and an unknown quantity of bromine gas. 13. Determine the percent yield for the reaction between 44.5g of zinc sulphide and 13.3g of oxygen, if 18.4g of zinc oxide is recovered with an unknown amount of sulphur dioxide. 14. A reaction was carried out according to the following equation: FeBr KCl FeCl KBr a. What is the theoretical yield of iron (II) chloride if grams of iron (II) bromide was used in the reaction with excess potassium chloride? b. What is the percent yield of iron (II) chloride if the actual yield is 4.00 grams? 15. a. What mass of CS 2(s) is produced when 17.5 g of C (s) are reacted with 39.5 g of SO 2(g) according to the equation: 5 C (s) + 2 SO 2(g) CS 2(s) + 4 CO (g)? b. What mass of the excess reactant will be left over? 16. If g of Ba(OH) 2(s) is mixed with 15.0 ml of M HBr (aq), what mass of BaBr 2(aq) can be formed? Ba(OH) 2(s) + 2 HBr (aq) BaBr 2(aq) + 2 H 2 O (l) 17. The reaction SiO 2(s) + 4 HF (g) SiF 4(g) + 2 H 2 O (g) produces 2.50 g of H 2 O (g) when 12.20g of SiO 2(s) is treated with a small excess of HF (g). a. What mass of SiF 4(g) is formed? b. What mass of SiO 2(s) is left unreacted if only 2.50g of H 2 O is formed? c. What is the percent yield of the H 2 O (g)?