Visualizing the balancing of chemical equations By Daniel R. Barnes

1. Cardboard Disc Reactions Visualizing the balancing of chemical equations By Daniel R. Barnes Init: 1/9/2007 from previous work You may hit the “End” key at any time to jump to the hyperTable of Contents. NOTE: This presentation is meant to go along with Mr. Barnes’ cardboard disc reactions worksheet, which is meant to be used with a set of specially-colored cardboard discs, each disc representing an atom of an element indicated by the color of the disc. NOTE: As always, some of the images in this presentation have been taken from the world wide web without permission of their owners. Copying and distribution of this presentation may be, therefore, illegal. In fact, its mere existence may be illegal.

2. SWBAT . . . . . . balance chemical equations. . . . explain why some reactions are endothermic and other reactions are exothermic. . . . define “activation energy” and give real life examples.

3. You're probably going to make lots of mistakes on this worksheet, so do it in pencil.

4. reactants turn into products Skip to answer

5. Skip to answer

6. activation energy breaks old bonds formation of new bonds releases energy Skip to answer

7. Please note: charcoal briquette ashes are not made of carbon dioxide. CO2 is a colorless, odorless gas. Charcoal ashes are made of alkali metal compounds and other stuff. Charcoal is not pure carbon. If it were, there would be no ashes left after charcoal burned. Skip to answer

8. C = O = 1 C = O = 1 2 2 Here's what you should have on your worksheet for problem #1 when you're done with it.

9. Before you go to your lab tables and start working with the discs, we’d better make sure we know the definitions of . . . Chemicals absorb energy  Surroundings get colder. Endothermic Exothermic Chemicals release energy  Surroundings get hotter.

10. All chemical reactions, whether they are endothermic or exothermic, typically involve endothermic bond breakage AND exothermic bond formation. If forming new bonds releases more energy than the activation energy required to break the old bonds . . . . . . The reaction will be EXOTHERMIC If forming new bonds releases less energy than the activation energy required to break the old bonds . . . . . . The reaction will be ENDOTHERMIC

11. Sell $400 worth of hot dogs Sell $100 worth of hot dogs Spend $200 for materials Spend $200 for materials $200 PROFIT $100 LOSS

12. Yieldsmoney Yields energy Sell $400 worth of hot dogs Sell $100 worth of hot dogs Making new bonds Spend $200 for materials Breaking old bonds Spend $200 for materials $200 PROFIT $100 LOSS Costs money Costs energy

13. NOTE: The circled numbers are made up. They’re not real. Release 400 kJ of energy forming new bonds Relase 100 kJ of energy forming new bonds Making new bonds Absorb 200 kJ of energy breaking old bonds Absorb 200 kJ of energy breaking old bonds Breaking old bonds EXOthermic ENDOthermic

14. Did you get it? Q: What is the difference between an endothermic process and an exothermic process? A: Endothermic processes absorb heat, making their surroundings colder, whereas exothermic processes give off heat, making their surroundings get hotter. Q: What is activation energy? A: The energy input required to make a reaction happen, even if it’s an exothermic reaction. It’s the energy required to break old bonds.

15. BONUS DEMO: Potassium Permanganate and Glycerine Mr. Barnes, please • Open the windows. (2) Put goggles on the kids in the front two rows. (3) Put a few drops of glycerine on a dimpled pile of potassium permanganate. This reaction is definitely exothermic, as shown by the fire coming out of it. The reaction was spontaneous at room temperature. No matches, sparks, UV light, or other input of activation energy was needed to get it going. The apparently low activation energy may indicate weak bonds somewhere in the reactants.

16. BONUS DEMO: Potassium Permanganate and Glycerine One of the main reactions that happens when these two chemicals go crazy on each other is the following: 14KMnO4(s) + 4C3H5(OH)3(l) → 7K2CO3(s) + 7Mn2O3(s) + 5CO2(g) + 16H2O(g) I’ve left the coefficients covered so that you can try to balance this monster for extra credit when you’re all done with the worksheet. A slide near the end of this presentation has the coefficients on it if you want to find out what they are.

17. Please go to your lab tables.

18. C = O = 1 C = O = 1 2 2 Your worksheet should already look like this for #1. Mr. Barnes will now have you animate the reaction with cardboard discs. At this point, Mr. Barnes may wish to turn off the projector. If he does, the rest of this presentation can be viewed at hhscougars.org, on Mr. Barnes’ Chemistry Power Points page.

19. Skip to answer

20. Explosive chemical reactions can blow up a vehicle and destroy it, but they can also make vehicles move in the first place. The big, orange external fuel tank of the space shuttle contains hydrogen and oxygen. The two gasses are mixed together and ignited in the three engines under the tail fin of the orbiter. The flames that come out of those engines are made of water vapor – water vapor that’s so hot that it glows. Skip to answer The white rockets on the side of the fuel tank use a solid fuel. I don’t know what that fuel is.

21. formulas subscripts Skip to answer

22. 2 2 4 H = O = 2 4 2 H = O = 1 2 2 coefficient NOT BALANCED In a chemical reaction, atoms are neither created nor destroyed. They just change who they’re stuck to. If you start out with two oxygen atoms, you should end up with two oxygen atoms atom census Okay. Let’s re-run the reaction from the beginning, but this time we’re going to get it right the first time. Skip to answer

23. 2 2 4 H = O = 2 4 2 H = O = 1 2 2 Skip to answer

24. 2 2 4 4 H = O = 2 2 H = O = 1 2 2 Here's what you should have on your worksheet for problem #2 when you're done with it. Skip to answer

25. 2 2 4 4 H = O = 2 2 H = O = 1 2 2

26. Why Do We Balance Equations? Atoms change who they are bonded to during chemical reactions. BUT Atoms are not created or destroyed during chemical reactions. AND Atoms do not change element during chemical reactions.

27. 3. CH4 + O2 H2O + CO2 C = H = O = 1 1 C = H = O = NOT BALANCED 4 2 2 3 First, we need to see if the equation is balanced or not. We need to do an . . . . . . atom count. Skip to answer

28. C = H = O = 1 1 C = H = O = 4 2 2 3 2 2 3. CH4 + O2 H2O + CO2 4 4 4 Something is obviously wrong. Let’s animate the equation as it is and see if we notice anything weird. Looks good so far! Skip to answer

29. 2 2 3. CH4 + O2 H2O + CO2 Let's move the atoms around again just to be sure . . . Skip to answer

30. 2 2 3. CH4 + O2 H2O + CO2 C = H = O = 1 1 C = H = O = 4 2 4 4 2 4 3 Here's what you should have on your worksheet for problem #3 when you're done with it. Skip to answer

31. 2 2 3. CH4 + O2 H2O + CO2 C = H = O = 1 1 C = H = O = 4 2 4 4 2 4 3 atom census (reactants) atom census (products) “after” picture “before” picture Make sure the rest of your work on the worksheet follows this format. Skip to answer

32. 2 2 3. CH4 + O2 H2O + CO2 C = H = O = 1 1 C = H = O = 4 2 4 4 2 4 3

33. Hey, Barnes! Make sure the kids animate reaction #3 (combustion of methane) with the discs. After that disc use is optional during independent work time, but #3 is required.

34. Ammonia Fountain The famous “ammonia fountain” demonstration is something we won’t explain until chapter 14 (gases). However, if you want to see it happen, click the link below. If that doesn’t work, just go to YouTube and type in “ammonia fountain.” http://www.youtube.com/watch?v=4U-DgdWPKyo 4. NH3 + H2O NH4OH N = H = O = 1 1 N = H = O = BALANCED 5 5 1 1 Skip to answer

35. N N Ammonia Fountain The famous “ammonia fountain” demonstration is something we won’t explain until chapter 14 (gases). However, if you want to see it happen, click the link below. If that doesn’t work, just go to YouTube and type in “ammonia fountain.” http://www.youtube.com/watch?v=4U-DgdWPKyo 4. NH3 + H2O NH4OH N = H = O = 1 1 N = H = O = 5 5 1 1 Skip to answer

36. N N Your work for #4 should look like this 4. NH3 + H2O NH4OH N = H = O = 1 1 N = H = O = 5 5 1 1 Skip to answer

37. N N 4. NH3 + H2O NH4OH N = H = O = 1 1 N = H = O = 5 5 1 1

38. Carbonic Acid When water and carbon dioxide mix, they produce carbonic acid. Carbonic acid is a “weak” acid found in soda and any other carbonated beverage. It may be a “weak” acid, but it is acidic enough to partially dissolve certain rocks over time. Carbonic acid, therefore, is an agent of “chemical weathering”. 5. CO2 + H2O H2CO3 C = O = H = 1 1 C = O = H = BALANCED 3 3 2 2 Skip to answer

39. Carbonic Acid When water and carbon dioxide mix, they produce carbonic acid. Carbonic acid is a “weak” acid found in soda and any other carbonated beverage. It may be a “weak” acid, but it is acidic enough to partially dissolve certain rocks over time. Carbonic acid, therefore, is an agent of “chemical weathering”. 5. CO2 + H2O H2CO3 C = O = H = 1 1 C = O = H = 3 3 2 2 Skip to answer

40. Your work for #5 should look like this 5. CO2 + H2O H2CO3 C = O = H = 1 1 C = O = H = 3 3 2 2 Skip to answer

41. 5. CO2 + H2O H2CO3 C = O = H = 1 1 C = O = H = 3 3 2 2

42. The reaction below is an example of an acid-base “neutralization”. 6. HCl + Ba(OH)2 H2O + BaCl2 H = Cl = Ba = O = 2 3 H = Cl = Ba = O = 2 1 NOT BALANCED 1 1 1 2 You might be tempted to try to balance hydrogen first, because it’s the first element in the equation, but I wouldn’t if I were you. Notice that hydrogen appears in three different formulas in the equation. Trust me when I say that you should balance first those elements that appear in the least number of formulas. Let’s save hydrogen for later . . . Skip to answer

43. 6. HCl + Ba(OH)2 H2O + BaCl2 H = Cl = Ba = O = 2 3 H = Cl = Ba = O = 2 1 1 1 1 2 The second element in the equation, chlorine, appears in only two formulas (the smallest possible # of places). Let’s balance chlorine first. We balance an element by increasing whatever coefficient will increase the amount of that element. Remember: when you’re balancing an equation you can only change the coefficients. DON’T TOUCH THE SUBSCRIPTS! Skip to answer

44. 6. HCl + Ba(OH)2 H2O + BaCl2 2 H = Cl = Ba = O = 2 3 H = Cl = Ba = O = 2 1 1 1 1 2 EXAMPLE: Someone who didn’t know what they’re doing might foolishly put a “2” as a subscript after the “Cl” in “HCl”. DON’T DO THAT! You’re not supposed to change subscripts when you balance a chemical equation! There’s no such chemical as HCl2, and even if there were, it wouldn’t be hydrochloric acid anymore. When you balance a chemical equation, you’re not changing the identity of the chemicals in the reaction, just how much of each chemical there is. Skip to answer

45. 6. HCl + Ba(OH)2 H2O + BaCl2 2 H = Cl = Ba = O = 2 4 3 H = Cl = Ba = O = 2 2 1 1 1 1 2 The proper thing to do is to put a coefficient of “2” in front of HCl on the left. As soon as you change a coefficient, you need to adjust the atom count for any element in that formula. Doubling the amount of HCl increases the amount of H . . . . . . and also the amount of Cl. We’ve now balanced the # of Cl’s on the left and the right. Skip to answer

46. 6. HCl + Ba(OH)2 H2O + BaCl2 2 2 H = Cl = Ba = O = 2 4 4 3 H = Cl = Ba = O = 2 2 1 1 1 2 1 2 Now chlorine is fixed, but there still isn’t enough oxygen on the right. Putting a coefficient of “2” in front of H2O on the right will double the amount of oxygen, balancing it as well. Of course, the “2” doubles not only the O’s on the right, but , but also the H’s. Would you look at that? The H’s got balanced by accident! Skip to answer

47. Ba Ba Cl Cl Cl Cl 6. HCl + Ba(OH)2 H2O + BaCl2 2 2 H = Cl = Ba = O = 2 4 4 3 H = Cl = Ba = O = 2 2 1 1 1 2 1 2 And now, for the cartoon . . . Skip to answer

48. Ba Ba Cl Cl Cl Cl 6. HCl + Ba(OH)2 H2O + BaCl2 2 2 H = Cl = Ba = O = 2 4 4 3 H = Cl = Ba = O = 2 2 1 1 1 2 1 2

49. Here’s an even more classic acid-base neutraliztion: Na Na Cl Cl 7. HCl + NaOH H2O + NaCl 2 H = Cl = Na = O = 2 H = Cl = Na = O = 1 1 1 1 1 1 How boring. This one’s already balanced. Cartoon, please! Skip to answer

50. Na Na Cl Cl 7. HCl + NaOH H2O + NaCl 2 H = Cl = Na = O = 2 H = Cl = Na = O = 1 1 1 1 1 1