Effect of Energy. Which of the graphs below represents the reaction that releases the most energy?

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1 Effect of Energy The net energy released (or consumed) does not affect rates The energy required to reach the transition state ( activation energy ) has large effect Transition state The critical arrangement of atoms where the reacting system decides whether or not to make products Typically involves breaking and/or formation of bonds On Friday we blew up balloons containing methane, propane, butane, and hydrogen. Why did we need a spark? A. Because the products have more chemical potential energy than the reactants. B. Because the transition state has more chemical potential energy than the reactants. C. Both A & B Which of the graphs below represents the reaction that releases the most energy? Which of the graphs below represents a reaction that gets cold rather than hot? A B C A B C Which of the graphs below represents the reaction that requires the most energy to start the reaction? What effect does temperature have on a reaction? A. Speeds it up B. Slows it down C. Doesn t affect the rate. A B C More atoms have enough energy to make the transition state. 1

2 Effect of Entropy Br + CH 3 Cl CH 3 Br + Cl Reactants frequently must have a certain orientation at the transition state, or reaction will not occur This corresponds to order in the transition state: entropy of activation Our Handwarmers What has more entropy, a solid or a liquid? 2 things to think about. 1. Structure a. Physical State N/A b. Number of molecules 2. Energy Decrease energy or increase entropy of activation without themselves being consumed; speed up rate H 2 O 2 decomposes spontaneously Much faster in presence of Br - (or MnO 2 ). Catalyst makes new, lower energy route possible Catalysts Objectives Have you read chapter 21? Bonding in metals and semiconductors. Why metals and alloys have the properties they do. Why semiconductors behave the way that they do. a) Yes b) No, I m relying on class to fill my brain with knowledge. 2

3 This model of magnesium metal is typical of all metals. What kind of materials are metals? a) gaseous b) molecular c) noble d) network solid e) edible What makes a metal a metal? Chapter 17 Metals have low ionization energies. Metals want to give away their electrons. How do you identify metals using the periodic table? a) You find them on the right b) You find them on the left c) You find them on the top d) You find them on the bottom Energy well for each atom by itself Empty Valence shell What is a bond? Full shells A single + nucleus is pulling on the electrons. When 2 atoms are close together, there are 2 nuclei pulling on the valence electrons, and new with different energies are created. These are called Molecular Orbitals (MOs). What is a bond? What happens with many atoms? Anti-bonding Original Valence shell Bonding Full shells Anti-bonding are higher in energy than the single atom electron energies. An electron in one of these can just drop down to the single atoms state and the atoms can go their separate ways. Bonding are lower in energy. Since this low energy orbital only exists when the two atoms are close together, the atoms are stuck together until enough energy is added to raise the electron back up to the single atom energy level. The more atoms, the more MOs By the time there are a few hundred atoms, there will be a few hundred lines stacked on top of each other. Rather than individual lines, we now have bands. 3

4 So, What about Metals? Shiny and opaque Malleable and ductile Conduct both heat and electricity High melting points What should you do when you see bulleted lists in the book? a) Skip them as irrelevant b) Treat them the same as the rest of the chapter c) Label them choices A, B, C, etc and see how many ways you can think of asking a multiple choice question using them. Empty Valence Full shells Not What..Why? Remember metals don t have a lot of valence electrons, and want to get rid of the few they do have. There are lots of empty at virtually the same energies as the valence electrons. Think of the atoms and valence electrons like the adults and kids at a big family reunion in the park. FYI (it ll be important later) Empty Band Gap Full shells Non-metals have more valence electrons and combine with things so that they have full shells. There are normally NO empty within the occupied band. Think of the atoms and valence electrons like a family with small children on a trip to Disney Land. Why shiny and opaque If a photon hits the metal, there is going to be an electron that can absorb it by jumping to somewhere else in the partially full band. This means that no photons get through. The electron won t stay at the high energy for long. It ll drop back down kicking out the photon. This means that they are shiny Why Malleable and Ductile? Does it matter where the positive ions are located? Since there are plenty of available bonding, electrons can move where ever they are needed. This means you can move the ions around without breaking the bonds. 4

5 Why conductors? We just talked about how easy it is for electrons to move If you apply an electric force, the electrons will travel. Because the electrons can move anywhere, they can carry KE to any part of the metal. Why High Melting Points? Melting (and dissolving for that matter) depends on how the units are held to one another. In a metal, the units are still individual atoms. The + ions are held together by a sea of electrons. Is this force going to be small or large? An alloy is a) a mixture of a metal with a non-metal b) a mixture of a metal with another metal c) a mixture of 2 non-metals d) a mixture of a metal with a noble gas e) a friend in battle Alloys Vrs Pure Metals Impurities tend to weaken bonds. In general alloys will Not conduct as well. Not bend as easily. Melt at lower temperatures. Our Friend the Semi-conductor Have some of the characteristics of metals, and some of the characteristics of nonmetals. Can be convinced to behave more like a metal by applying electric forces. The duck-bill platypi of the periodic table All of this is within a single band of allowed. Completely full Completely empty 5

6 Our Friend the Semi-conductor Other ways to change resistance As you change temperature, how do metals behave? How do semicondutors behave? Dope the semi-conductor. If you add a few atoms that have one more electron, those electrons will be stuck up in the nearly empty band. This makes an N-type semi-conductor. More metal-like If you add a few atoms with one less electron, you get a few more holes. This is a P-type semi-conductor. More nonmetal-like. Ban doping An application: LEDs Light Emitting Diode A diode will only allow current to flow in one direction. If you put a N-type semi-conductor against a P-type, the current can only flow from N- type to P-type. This is a diode. When the current does flow, the electrons jump from the empty to the full shell. LEDs What happens when an electron jumps from one energy to another? If I compare the band gap in the RF (radio frequency) clickers to the band gap in the laser pointer, what is true? a) The RF LED has a larger band gap. b) The red LED has a larger band gap. c) I d know the answer if I could remember whether radio or visible light had a higher frequency. d) Huh? Maybe we should go over this one more time. 6