Chemistry Concepts Explained: Atomic Theory, Bonding, Energy Conservation

1. June 2017 Chemistry Regents Exam Questions and explanations Mrs. O’Donnell

2. % of test by topic (as best as I can estimate) Each slide tells which topic we first encountered the concept-some concepts are in more than 1 topic.

4. Remember that the atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons located in probability regions called orbitals. Topic: Atomic Theory

5. Models of the atom • It is hard to come up with a perfect model of the atom. The prior picture showed the parts of the atom nicely, but does not come close to depicting the relative size of the atom. • The nucleus is so tiny compared to the space electrons can travel in. If the nucleus was the size of a pea, the electrons would be occupying the space of a huge sports stadium!

7. WAVE MECHANICS TAUGHT US THAT electrons ARE located in probability regions called orbitals, Areas electrons exist ~90% of the time. Topic: Electrons and Periodicity

9. The word “atom” tells us this is neutral (electrons = protons). Since the atomic number from the periodic table = # protons, the atomic number of scandium (21) gives us the number of electrons. Topic: Atomic Theory

11. First, atoms will always weigh more than subatomic particles. Then, know that the protons and neutrons are essentially equal in mass, but electrons are so small, their mass is negligible (1 electron = 1/2000 the mass of a proton). Topic: Atomic Theory

13. Always choose the transition from a higher energy level to a lower energy level for energy release. Note: It doesn’t always involve returning to the ground state, just a lower state. Topic: Electrons and Periodicity

15. You’ve just got to know this one! Topic: Atomic Theory

17. This one is really getting at how the atoms bond together. It is showing the structure of the compound. “Ionic” Simplest How it really is Topic: Bonding

19. 3 things are always conserved in reactions: • Matter (mass) • Charge • energy Topic: Central Idea in Chem! + electrical energy Notice in this example all elements are equal on both sides, the total charge on each side is 2+ and, although it’s harder to “see”, the energy is conserved. Chemical potential energy is being converted into electrical energy, but the total chemical energy in the reactants = the total chemical energy and electrical energy on the products.

21. Remember our “couples” swapping partners! Topic: Reactions

23. A great way to differentiate ionic compounds is to see how they dissolve in water (Remember F?). But also notice: • They both are 5.o grams, so mass would be the same. • They have the same density, so volume would be the same for these equal masses. • If they are at STP, they would have the same temperatures! Topic: Matter

25. The Easiest way to answer questions about bonding is to draw the Lewis electron dot structure. Iodine shares 2 electrons to achieve its stable octet. Remember that each bond always contains 2 electrons. Topic: Bonding

27. Remember that nonpolar bonds occur when there is little or no electronegativity difference between the bonding elements. So, this is going to primarily occur between 2 identical elements like our diatomics. Most chemists will also count the C-H bond as nonpolar. Topic: Bonding I see a new t-shirt in my future

29. First, check to see what kind of ion potassium will form on the PT  1+! Then, recall that “kitty” cations are always smaller than their neutral element. “No touchy the protons!” (except in nuclear chemistry) Topic: Electrons and Periodicity

31. This is another “just know it” kind of questions. This is a good time to remember that thermal energy increases with mass and temperature. Be careful! If temperature is a choice, remember that this is a measure of average kinetic energy, not a form of energy. Topic: Thermochemistry

33. Whenever you see average kinetic energy, substitute the word temperature. If average kinetic energy is decreasing, temperature is decreasing. Topic: Math and Measurement

35. When asked about units, remember you have Table D to access! James Prescott Joule (1818-1889) Topic: Thermochemistry

37. Remember, gases behave most ideally at high temperatures and low pressures where they can obey the kinetic molecular theory best. *Also, condensing into a liquid would require lower temps and higher pressure as would increased solubility. Helium isn’t diatomic. Topic: Gas Laws

39. When gas particles are heated, they move faster and hit each other and their container more frequently. They also hit with more force. Physics will teach you more equations that prove this. For now, think of 2 cars crashing slowly vs. quickly. More force is involved with high speed collisions! Topic: Kinetics and Equilibrium

41. Remember, gas particles are so tiny compared to the space between them that the same volume of gas at the same temperature and pressure (in this case, STP) will contain the same number of particles (and consequently the same number of moles) regardless of the identity of the gas. Always look for the same volume and a gas. Topic: Gas Laws

43. When something can exist in different proportions it must be a mixture. (Remember our example of concentrated koolaid vs. dilute koolaid). Note: crude oil is not Koolaid. Don’t drink it! Recall, distillation is used to separate liquids with different boiling points. Topic: Matter

45. Only solids form crystal structures. They are the most ordered phase that keeps its own shape. Topic: Matter

47. Concentrations are Constant!!! Be careful, reactants and products don’t have to be equal at equilibrium, the rates of the forward and reverse reactions are equal. Topic: Kinetics and Equilibrium

49. This is another one you just need to know. It’s also a good time to recall that activation energy is the energy difference between the reactants and the activated complex (at the top of the hill), Ionization energy is the energy required to remove an electron, and heat of vaporization is the energy required to change a liquid into a gas. Topic: Thermochemistry