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Reminders

Reminders. Due today Prelab for Alka Seltzer and Lab 2 VSEPR For today lab please share your results with another 3 groups. Use the class’s data for comparison. Exam 2. On Tuesday November 12 th Covering chapter 4-6 A study guide will be posted over the weakened Similar format to exam 1

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Reminders

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  1. Reminders • Due today • Prelab for Alka Seltzer and Lab 2 VSEPR • For today lab please share your results with another 3 groups. • Use the class’s data for comparison

  2. Exam 2 • On Tuesday November 12th • Covering chapter 4-6 • A study guide will be posted over the weakened • Similar format to exam 1 • I will be available at 5:00-5:50pm Monday through Thursday of next week for extra help.

  3. Limiting Reactant A limiting reactant in a chemical reaction is the substance that • Is used up first. • Stops the reaction. • Limits the amount of product that can form.

  4. Example of Everyday Limiting Reactant How many peanut butter sandwiches could be made from 8 slices bread and 1 jar of peanut butter? With 8 slices of bread, only 4 sandwiches could be made. The bread is the limiting item.

  5. Example of Everyday Limiting Reactant How many peanut butter sandwiches could be made from 8 slices bread and 1 tablespoon of peanut butter? With 1 tablespoon of peanut butter, only 1 sandwich could be made. The peanut butter is the limiting item.

  6. Example of chemistry Limiting Reactant • Stoichiometric mixture • N2(g) + 3H2(g) 2NH3(g)

  7. Example of chemistry Limiting Reactant • Limiting reactant mixture • N2(g) + 3H2(g) 2NH3(g)

  8. Calculations of Limiting Reactants When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form? H2(g) + Cl(g)  2HCl (g) 4.00 mol 2.00 mol ??? mol • Calculate the moles of product from each reactant, H2 and Cl2. • The limiting reactant is the one that produces the smaller amount of product.

  9. Limiting Reactants Using Moles HCl from H2 4.00 mol H2 x 2 mol HCl = 8.00 mol HCl 1 mol H2 (not possible) HCl from Cl2 2.00 mol Cl2 x 2 mol HCl = 4.00 mol HCl 1 mol Cl2 (smaller number) The limiting reactant is Cl2 because it is used up first. Thus Cl2 produces the smaller number of moles of HCl.

  10. Your turn Calculate the mass of water produced when 8.00 g H2 and 24.0 g O2 react? 2H2(g) + O2(g) 2H2O(l)

  11. Oxidation and Reduction

  12. Oxidation and Reduction Cu2+ gains 2 e− Zn2+ + Cu Zn + Cu2+ Zn loses 2 e– • Zn loses 2 e− to form Zn2+, so Zn is oxidized. • Cu2+gains 2 e−to form Cu, so Cu2+ is reduced.

  13. Oxidation and Reduction Cu2+ gains 2 e− Zn2+ + Cu Zn + Cu2+ Zn loses 2 e– Each of these processes can be written as an individual half reaction: Oxidation half reaction: Zn Zn2+ + 2 e− loss of e− Reduction half reaction: Cu2+ + 2e− Cu gain of e−

  14. Oxidation and Reduction

  15. Application in medicine • A pacemaker generate small electrical impulse that triggers the heart to beat. • That impulse is generated by the redox reaction of a lithium battery.

  16. Chapter 6: Energy

  17. Nature of Energy • Energy is all around you! • You can hear energy as sound. • You can see energy as light. • And you can feel it as wind.

  18. 6.1 Energy • Energy is the capacity to do work. • Potential energy is stored energy. • Kinetic energy is the energy of motion. Kinetic energy Potential energy

  19. Energy Kinetic energy (EK) Potential energy (EP) Energy due to motion Energy due to position (stored energy) 6.1 Energy What is Energy?

  20. law of conservation of energy • The law of conservation of energy states that the • total energy in a system does not change. Energy cannot be created or destroyed. The water at the top of waterfall has potential energy because of its position. This potential energy becomes kinetic energy as the water falls.

  21. Kinetic-Potential Energy Conversions • As a basketball player throws the ball into the air, various energy conversions take place.

  22. Ball speeds up Ball slows down

  23. The Units of Energy • A calorie (cal) is the amount of energy needed to • raise the temperature of 1 g of water by 1 oC. • A joule (J) is another unit of energy. 1 cal = 4.184 J • Both joules and calories can be reported in the • larger units kilojoules (kJ) and kilocalories (kcal). 1,000 J = 1 kJ 1,000 cal = 1 kcal 1 kcal = 4.184 kJ

  24. Conversion between energy units Convert 421 kJ to kcal

  25. Nutrition calorie unit! A nutrition calorie = 1000 cal or 1Kcal

  26. Estimate calories in food Knowing a quarter pound burger with cheese has 29g of protein (4Cal/g) 40g of carbohydrate (4Cal/g) 26g of fat (9Cal/g) (29x4) + (40x4) + (26x9)= 510Cal (kcal)

  27. Your turn Knowing a snickers bar has Protein 4g (4g/cal) Carbohydrate 33g (4cal/g) 12g of fat (9Cal/g)

  28. Chemical energy • Gasoline is a form of chemical energy Chemical energy: Is potential energy stored in bonds! • A compound with lower potential energy is more • stable than a compound with higher potential energy. • Reactions that form products having lower potential energy than the reactants are favored.

  29. Chemical Energy • Fuel and food are forms of stored chemical energy.

  30. Bond energy Chemical energy: Is potential energy stored in bonds! • A compound with lower potential energy is more • stable than a compound with higher potential energy. • Reactions that form products having lower potential energy than the reactants are favored.

  31. 6.2 Energy Changes in Reactions • When molecules come together and react, bonds • are broken in the reactants and new bonds are • formed in the products. • Bond breaking always requires an input of energy. • Bond formation always releases energy.

  32. Bond Dissociation Energy • His the energy absorbed or released in a reaction; • it is called the heat of reaction or the enthalpy change. To cleave this bond, 58 kcal/mol must be added. Cl Cl To form this bond, 58 kcal/mol is released.

  33. Bond Dissociation Energy • When energy is absorbed, the reaction is said to • be endothermic and H is positive (+). • When energy is released, the reaction is said to • be exothermic and H is negative (−). To cleave this bond, H = +58 kcal/mol. Cl Cl To form this bond, H = −58 kcal/mol.

  34. Bond Dissociation Energy • The bond dissociation energy is the H for breaking • a covalent bond by equally dividing the e− between • the two atoms. • Bond dissociation energies are positive values, • because bond breaking is endothermic (H > 0). H H H + H H = +104 kcal/mol • Bond formation always has negative values, • because bond formation is exothermic (H < 0). H + H H H H = −104 kcal/mol

  35. Endothermic and exothermic reactions endothermic process(absorbs heat) As the ice melts, heat is absorbed from the air surrounding the ice and example of endothermic process. • exothermic reactions (gives off heat). Ordinary combustion of a log is an example of an exothermic reaction.

  36. Endothermic process Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat. Photosynthesis is an endothermic reaction (requires energy input from sun)

  37. reaction Endothermic process Measuring Heat Endothermic reaction, heat taken in & temperature of surrounding drops

  38. Calculations Involving H Values When H is positive: • More energy is needed to break bonds than is • released in the formation of new bonds. • The bonds broken in the reactants are stronger • than the bonds formed in the products. 6 CO2(g) + 6 H2O(l) C6H12O6(aq) + 6 O2(g) ΔH = +678 kcal/mol Heat is absorbed

  39. Burning fossil fuels is an exothermic reaction Exothermic process Exothermic process: a change (e.g. a chemical reaction) that releases heat.

  40. Calculations Involving H Values Hindicates the relative strength of the bonds broken and formed in a reaction. When H is negative: • More energy is released in forming bonds than is • needed to break the bonds. • The bonds formed in the products are stronger • than the bonds broken in the reactants. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) H = −213 kcal/mol Heat is released

  41. Bond Dissociation Energy • The stronger the bond, the higher its bond • dissociation energy. • In comparing bonds formed from elements in the • same group, bond dissociation energies generally • decrease going down the column.

  42. Calculations Involving H Values

  43. Question 6.9 • Answer the following question about the fermentation of glucose C6H12O6, the molar mass 180.2g/mole) to ethanol C2H6O and CO2∆H =-16Kcal/mole • How many kilocalories of energy is released from 6.0 mole of glucose • How many kilocalories of energy are released when 1.0 mole of ethanol is formed? C2H6O + CO2 C2H12O6 (l) -16.0 kcal 1 mol C2H12O6 -16.0 kcal 1 mol C2H12O6 = -96 kcal = -16.0 kcal 6.0mol C2H12O6 x 1.0mol C2H12O6 x

  44. Question 6.9 continue • How many kilocalories of energy are released from 20.0g of glucose? -16.0 kcal 1 mol C2H12O6 1 mol C2H12O6 x 180.2 C2H12O6 20.0g C2H12O6 x = -1.78 kcal

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