2.10.12 If you decomposed 4 moles of hydrogen peroxide, how many moles of oxygen gas would you produce if you used manganese (IV) oxide as a catalyst? HW page 301 1-3, and page 311 1-4 TYGAGT Use mole ratios to predict number of moles of product in a reaction and mass of products
stoichiometry Comes from the Greek element and to measure This is application of the law of conservation of mass, and to many is the heart of chemistry We are going to be studying reaction stoichiometry, which deals with the mass relationships between reactants and products in a chemical reaction
Example If you have 13.0 mol of Aluminum (III) oxide, how many moles of aluminum can be produced from the electrolysis of molten aluminum (III) oxide to produce aluminum and oxygen gas? Step 1: write this out in an equation Step 2: balance this equation Step 3: determine mole ratios
Mole ratios Once you have a balanced equation, what you have determined are the molar ratios between all reactants and products You can use these ratios to scale up or down the reaction In our example, the molar ratios are So the number of moles of Al that would be produced is 26.0 mol Al
Using mole ratios You can use the mole ratios to predict the masses produced (ideally) From our example, what would the mass be of the product? Another look: 13.0 mol Al 2 O 3 X 4 mol Al_ = 26.0 mol Al 2 mol Al 2 O 3 What cancels in this calculation?
You determine What mass of Oxygen was produced in the example reaction? What are your steps? What is the mole ratio? Can you use the mole ratio between Al 2 O 3 and O 2? Can you use the mole ratio between Al and O 2? Did you get 96.0 g?
Ideal Stoichiometric Ratios These are the stoichiometric predictions for reactions that happen under ideal circumstances In any lab, ideal real How do you compute the difference? Percent difference!
Example problem Carbon dioxide reacts with Lithium hydroxide to produce lithium carbonate and water. How many moles of lithium hydroxide would be required to react with 20 mol of carbon dioxide? What is the molar ratio between carbon dioxide and lithium hydroxide?
example How many moles of oxygen are required to burn 0.5 mole of methane? Write out and balance equation: CH 4 + 2O 2 2H 2 O + CO 2 Mole ratio is 2 moles of O 2 : 1 mole CH 4
continued How many grams of carbon dioxide are produced from this combustion? Use molar ratio of carbon dioxide to methane Convert to grams by using molar mass of carbon dioxide
example In photosynthesis under ideal conditions, what mass, in grams, of glucose is produced for every 3.00 mol of water? Plan Begin with a balanced equation Determine mole ratios to get a mole conversion factor Use periodic table to determine mass of glucose Multiply conversion factor X molar mass of glucose
together What mass of carbon dioxide (in grams) is needed to produce that same amount of glucose as in the previous example?
Practice (moles to mass) When magnesium burns in air, what mass of magnesium oxide is produced from 2.00 mol of Magnesium? What mass carbon dioxide is produced from the aerobic respiration of 10 mol of glucose (assume complete oxidation)?
Practice (mass to moles) The first step of the industrial production of nitric acid is the combustion of ammonia to produce nitrogen monoxide and water. In a reaction of 824 g of ammonia and excess oxygen, how many moles of each product are produced?
Practice (mass to moles) Oxygen was discovered by Joseph Priestley in 1774 when he heated mercury (II) oxide. How many moles of mercury (II) oxide are needed to produce 125 g of Oxygen gas? How many moles of mercury are produced?
2.13.12 Warmup: Tin (II) fluoride is used in toothpaste. It is made by reacting Hydrogen fluoride with Tin. How many grams of Tin(II) fluoride are produced from the reaction of 30.00 g hydrogen fluoride with Tin? Hint: start by writing it out, diagram it, balance the equation. What info DO you know? What are you trying to find out? T Practice stoichiometry, esp mass-mass and moles to mass problems
Any questions? From homework
Practice (mass to mass) When copper metal is added to silver nitrate (aq), silver metal and copper (II) nitrate are produced. What mass of silver can be produced this way from 100.0 g of Cu?
Today s assignments Page 108-109 in green book 1-23 odds
2.14.12 In the reaction of Zinc metal in hydrochloric acid, hydrogen gas and zinc chloride are produced. If you start with equal amounts of zinc and HCl, which do you run out of first? What is going to determine how much hydrogen you make? HW - look at problems on 319, bring questions tomorrow TYGAGT Calculate yields of reactions if one reagent is limiting
terms Ideal reactions are difficult to undertake, typically in a chemical reaction one reagent is limiting Limiting reactant in a reaction, that reactant that will run out first Excess reactant the unreacted remainder of the non-limiting reactant In our warmup which reactant was limiting? Which was excess?
problems Keep you head! Plan a strategy for each problem! Start with what you know! Use unit conversions (mole ratios) to get you from what you know to where you want to get to! ALWAYS ALWAYS ALWAYS include units in your calculations, they are you sign that you are on the right track --- or not!
Examples In the following reactions, which reactant is limiting? Which is in excess? Silicon dioxide + hydrogen fluoride yields silicon tetrafluoride and water. What if 6.0 mol HF is added to 4.5 mol SiO 2? Step 1: write out equation and balance Step 2: Solve yield for BOTH reactant quantities using mole ratio as conversion factor Step 3: the reactant that gives you the lesser yield is the limiting reactant Solution next slide
example 6.0 mol HF X 1 mol SiF 4 = 1.5 mol SiF 4 4 mol HF 4.5 mol SiO2 X 1 mol SiF 4 = 4.5 mol SiF 4 1 mol SiO 2 Because the 6.0 mol HF will only produce 1.5 mol SiF 4, HF is the limiting reactant
example N 2 H 4 (l) + 2H 2 O 2 (l) N 2 (g) + 4H 2 O(g) If we start with 0.750 mol N 2 H 4 and 0.500 mol H 2 O 2, which reactant is limiting? How much of the excess reactant remains? How much of each product (in moles) is formed?
example Methanol is synthesized from carbon monoxide and hydrogen gas. If 500 mol CO and 750 mol H 2 are present, which is the limiting reactant? How many moles of excess reactant remain unreacted? How many moles of methanol are produced?
Mass to mass 3Fe(s) + 4H 2 O(g) Fe 3 O 4 (s) + 4H 2 (g) If 36.0 g of H 2 O is mixed with 67.0 g Fe, which is the limiting reactant? What mass in grams of the iron oxide compound is produced? What mass in grams of excess reactant remains unreacted?
2.15.12 In a reaction between 1.000 mol of sodium hydroxide and 0.500 mol of hydrochloric acid, which reactant is limiting? Today practice practice practice Finish green book problems from both days and hand in
example Zinc and sulfur react to form zinc sulfide (recall, elemental sulfur is S 8!). If 2.00 mol of Zn are reacted with 1.00 mol of S, which is the limiting reactant? How many moles of excess reactant remain? How many moles of product are formed?
example Carbon reacts with steam to produce hydrogen gas and carbon monoxide If 2.40 mol of Carbon are reacted to 3.10 mol of steam, which is the limiting reactant? How many moles of each product are formed? What is the mass of each product formed?
practice Green books page 119 1-7 *also should finish up problems from the other day!*
2.21.12 Chlorobenzene (C 6 H 5 Cl) is produced by reacting benzene (C 6 H 6 ) with chlorine gas. Hydrogen chloride gas is a by-product. Write out and balance the equation representing this reaction. If 36.8 g benzene react with excess chlorine, what is the expected mass of chlorobenzene? If your actual yield was 38.8 g of chlorobenzene, what happened?
Homework questions? Tonight s homework look at questions on page 318 Quiz on Thursday on limiting reactants and stoichiometry topics Last call for penny labs
Today Topic percentage yield, the ratio of the amount of product we actually get over the amount of product we predict to get Express as a percentage Will never be >100% (or something really screwed up your reaction then it s a doover) There are important reasons why <100% is a rule that have nothing to do with error, we will explore them later (reaction equilibrium)
2.22.12 If 1.85 g of aluminum react with excess copper (II) sulfate, which reactant is limiting? How much copper (II) sulfate was actually consumed in the reaction? IN moles? In grams? If the actual yield of the sulfate product was 3.70 g, what was the percentage yield?
The scientific method How research is done Undergraduate and secondary totally under direction of supervisor, research ideas 85-100% from supervisor Option A from BS to MS thesis, ideas 50% yours, support 100% from supervising PhD Option B from BS/MS to PhD, ideas 90+% yours, support 100% from supervising PhD
And then Post-doctoral positions and research Research ideas your own, but YOU have to find a lab that will support your research ideas this is not a given. So are you really doing your own research? Funding comes externally, so you have to be researching something that has already gotten funding You are still under supervision and support of a PhD (even though you have one of your own)
And then You may become a staff scientist (or go back to academia) You direct post-docs and provide material, administrative and other research support Support post-docs in publishing You still do not really have any research autonomy Why not? If in academia, you have to secure your own funding, and then you do your own research and have your MS or PhD students carry it out (see previous slide)
Why supported? MS and PhD are supported because their supervisor GOT the funding, so they are for the most part carrying out their supervisor s research The supervisor has a team working on a project The team members learn how to do science so they can one day direct their own work
finally You may be a supervising scientist Why am I discussing this?
Lab Limiting reagents lab and formal lab report Will be done in groups of four - no more than five Group leader responsible for designating tasks among groups Group leader must turn in to me in writing the assignments for their group Groups turn in single paper
Paper format Title page: Title Authors (I am also an author, but should be listed last) Date, institution Abstract Introduction Explanation of stoichiometry, calculations, activity series, solubility rules of double replacement reactions, percent yield Fully referenced!
continued Methods written in passive voice (something nearly all of you lost points for in STEM fair!) Should include illustration/diagram of set-up Description of how error will be managed Description of how data will be collected Data Present in table format the averages, standard deviations, and percent yields all to same decimal value and show units Conclusions Error analysis and discussion of percentage yield, review hypothesis Did results support hypothesis? Why or why not based on discussion of previous bullet
finally Works cited in APA format Use MS word to create this (note: this requirement is PART of the assignment! You must use this format!) Also pages should be numbered, use headers feature to include title of paper on every page
grading No rubric As a contributing author and mentor, I will only accept quality submissions If your paper needs work, you will have to continue working on it When it is ready for submission, I will accept it However, you MUST have an acceptable paper ready for submission by March 9 or you are fired
Questions Is there a page length requirement? No it must be complete Can I do a different project? No Can I work by myself? No Is there any room for negotiation here? No