1. Plan, Ppt10(PS4) 1.Finish EF (Tro, 3.9)  From % masses  From combustion analysis data 2.Chemical Reaction (Tro, 3.10)  Process (takes time!); original subs  new subs  Fixed mass ratio of substances react and form  Whole-number ratio of FUs react and form 3.Chemical Equation (Balanced) [later]  Coefficients represent ratio of FUs when reaction occurs 4.Various problem types (“stoichiometry”) [later] (Tro, 4.2) 1Ppt10(PS4)

2. Empirical Formula (EF) Quick Review Need to find the subscripts => lowest whole-number ratio of atoms => lowest whole-number ratio of MOLES (of atoms) 2Ppt10(PS4)

3. Determining the Empirical Formula of a Compound From mass data (of each element A, B, C, …) of a sample: masses (of A, B, C, …)  moles (of A, B, C, … atoms)  ratio of moles  reduced (whole-number) ratio of moles  formula (subscripts) From % mass data (of each element A, B, C, …) – Get mass data, and do as above! – Assume 100 g of compound for convenience 20.4 % of 100 g = 20.4 g! (etc.)  NOTE: If molar mass is also given, it may be easiest to use that mass (rather than 100 g). You will then get the molecular formula first (which can then be reduced, if needed) 3Ppt10(PS4)

4. From Green Handout Sheet (PS4 & 5 Practice) What is the empirical formula of tetraethyllead, the “lead” in gasoline? The composition is: 29.71% C, 64.06% Pb, 6.23% H (These percentages are by mass) 4Ppt10(PS4)

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6. PS Sign-Posting The concepts and skills related to problems 14, 18, and 19 on PS4 have been covered now in this PowerPoint. Give those problems a try now! 6Ppt10(PS4)

7. From Combustion Analysis Data Often cannot easily get the elements out So, react with oxygen: – All C atoms end up in CO 2 molecules! – All H atoms end up in H 2 O molecules! Figure out C in sample from mass of CO 2 Figure out H in sample from mass of H 2 O Figure out a 3 rd atom type (O, N, S, …) by mass difference (as in Exp 4—get Cl “by difference”) 7Ppt10(PS4)

8. Combustion Analysis Figure 3.10 e.g., Mg(ClO 4 ) 2 e.g., NaOH e.g., excess O 2, sulfur oxides, nitrogen oxides, halogen-oxides, etc. 8Ppt10(PS4)

9. From Green Handout Sheet (PS4 & 5 Practice) A hydrocarbon of unknown formula C x H y was submitted to combustion analysis with the following results. What is the empirical formula of the hydrocarbon? 9Ppt10(PS4)

10. From Green Handout Sheet (PS4 & 5 Practice) Coniine, a toxic substance isolated form poison hemlock, contains only carbon, hydrogen, and nitrogen. Combustion analysis of a 5.024 g sample yields 13.90 g of CO 2 and 6.048 g of H 2 O. What is the empirical formula of coniine? 10Ppt10(PS4)

11. PS Sign-Posting The concepts and skills related to problem 17 on PS4 have been covered now in this PowerPoint. Give those problems a try now! [end of PS4 material] 11Ppt10(PS4)

12. Discussion of Meaning of a Balanced Chemical Equation Reaction vs. equation (different!) – Most chemistry texts tend to discuss this initially but then use the words interchangeably later on(!) Be careful! Chemical Reaction ◦ Initial substances (reactants) turn into new substances (products) ◦ Takes time (a process) ◦ Involves actual amounts Can be expressed in a variety of ways: grams, moles, L, etc. Chemical Equation ◦ Represents (a type of) chemical reaction (but not “literally”) ◦ Symbols (not “reality”), on paper Formulas, coefficients, an arrow ◦ Does not involve actual amounts! Only RATIO of combination and formation 12Ppt10(PS4)

13. Discussion of Meaning of a Balanced Chemical Equation (Example) Chemical Reaction 4 g of methane react with 16 grams of oxygen to produce 11 grams of carbon dioxide and 9 grams of water, all as gases. Chemical Equation CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) The reaction occurs over some period of time, during which the number of molecules of methane and oxygen steadily decrease as the number of molecules of carbon dioxide and water increase. Coefficients represent a ratio only, and they represent the ratio of formula units, most fundamentally. Here, a 1 : 2 : 1 : 2 ratio of molecules of CH 4, O 2, CO 2, and H 2 O but only as reaction occurs. 13Ppt10(PS4)

14. Discussion of Meaning of a Balanced Chemical Equation Analogy (hypothetical): Working at Lawson products 25 boxes packed up → $15 in pocket How much money is in my pocket after the “reaction” occurs? The equation alone can’t answer that! 14Ppt10(PS4) – How much “reaction” occurred? (not indicated by equation!) – How much was in your pocket before the reaction?