Learning objectives. Chemical Bond. Intro to Chemical Bonding. Molecular Orbitals on the Web. Shape of Molecular Orbitals

Transcription

1 Learning objectives Why do atoms form chemical bonds? Why are some reactions fast, and others slow? How do energy and entropy play a role What is equilibrium The Elements Tom Lehrer Rates of cooking, or growth of muscles, or tightening of muscles,, or using the brain everything involves the speed of some reaction. Understanding these reactions really means getting acquainted with the molecules as if they were your friends and knowing what their nature is and what they will do... PS110A Hatch Ch ~ Henry Eyring The Elements Tom Lehrer PS110A Hatch Ch Intro to Chemical Bonding Molecular Orbitals standing probability waves for molecules these are different than the orbitals of individual atoms because of multiple nuclei more complex than those of the individual atom Why are they formed? If the electrons can achieve a lower energy arrangement the atoms form a molecule, they form a bond. Extra energy is released (usually as photons) PS110A Hatch Ch Chemical Bond A chemical bond between two atoms occurs when the electrons in the two atoms have less energy when the atoms are together than when they are apart. Atoms agree that they are happier if 1) they transfer electrons (producing ions) or 2) share electrons (producing PS110A Hatch Ch molecules). Shape of Molecular Orbitals Orbitals are electron probability standing waves Shape of orbital depends on placement of the different nuclei. New standing wave patterns (new shapes) when there are multiple nuclei Molecular orbitals Follow same rules as atomic orbitals: 2 e - / orbital (opposite spins) Fill lowest energy orbitals first s Atomic orbitals p d f Molecular Orbitals on the Web See for a three dimensional view Two positive nuclei mean new electron probability standing wave patterns (different from atomic orbitals) Bonding molecular orbitals have high electron density between nuclei Molecular orbitals PS110A Hatch Ch PS110A Hatch Ch. 20-9

2 H Molecular Orbitals & Bonding When atoms get close, MO s can form If the electrons are lower in energy in the resulting MO s than they were in the atomic orbitals, bonds form Bonding and antibonding orbitals are possible + H Bonding is lowest in Energy H H or H 2 A Thermodynamic Description of Bonding Bond formation is favorable if it leads to decreased energy for the system, and/or increased entropy for system 3 ways to accomplish this Lots of atoms share lots of electrons: metals Nonmetals take electrons from metals: ionic materials Nonmetals share electrons with each other: covalent materials energy so molecules form PS110A Hatch Ch PS110A Hatch Ch Reactivity of Elements Don t react at all He, Ne Why? React with great difficulty Ar, Kr, Xe Correlate Form diatomic gases with position H 2, N 2, O 2, F 2, Cl 2 on periodic Very reactive table? Li, Na, K, Rb, Cs Most metals Many of the diatomic gases above PS110A Hatch Ch What is a Chemical Reaction? A chemical reaction occurs when two (or more) atoms, molecules, or ions come close enough together that chemical bonds are made or broken. May need to add energy to first break some bonds and let new ones form. PS110A Hatch Ch Chemical Reaction Examples 1. C + O 2 CO 2 Oxidation of Carbon 2. HCl + NaOH NaCl + H 2 O Hydrochloric acid and sodium hydroxide to get table salt and water. 3. 2H 2 O 2 Electrolysis of water A Chemical Reaction (demo) needs energy -- heat it H + H + O + O energy released as electrons find a new, deeper energy well H 2 O Demo: Torch He balloon, & H 2 balloon Is the reaction favored energetically? Note: 1) and 2) are exothermic (release energy) P2: What happens to the energy and the disorder of the universe as water molecules are formed? 3) endothermic (requires energy) PS110A Hatch Ch PS110A Hatch Ch

3 Conservation of Matter: Balancing Chemical Reactions balanced A Chemical Equation Matter is not created or destroyed in chemical reactions This is reflected in the way we write them The equation is balanced when the number of each kind of atom is the same on both sides of the arrow Count the atoms! PS110A Hatch Ch P3: Which way of writing the reaction of hydrogen with oxygen to make water, best complies with conservation of matter and describes what happens? a) H 2 O b) H 2 + ½ O 2 H 2 O c) 2 2 H 2 O PS110A Hatch Ch Electrolysis of water: same reaction backwards Balancing Chemical Equations When an electric current passes through water, chemical bonds are broken and remade. Electrolysis - a chemical reaction The net effect is: H 2 O (balanced?) Conservation of Mass: same number of atoms on each side (like a recipe for a cake with no flour left over) H 2 O H 2 +O 2 not balanced, e.g. O: 1 on left, 2 on right; show 2 water molecules 2H 2 O balanced? No, now H s out of balance; show 2 H 2 s 2H 2 O 2 balanced? Yes, 4 H and 2 O atoms on both sides! NO! PS110A Hatch Ch PS110A Hatch Ch Electrolysis Water Hydrogen + Oxygen When Balanced 2H 2 O 2 P4: In which side of apparatus does the hydrogen collect? PS110A Hatch Ch A slightly more complicated reaction - burning of natural gas (demo) Combustion- net effect is CH 4 + O 2 C + 4H + 2O CO 2 + H 2 O this step yields energy What form does this energy take? methane + oxygen carbon dioxide + water CH 4 + O 2 CO 2 + H 2 O Balanced?? No! PS110A Hatch Ch

4 Balancing the natural gas reaction P5: Which reactions are balanced? CH 4 + O 2 CO 2 + H 2 O -- Only C balanced; work on H first, then O CH 4 + O 2 CO 2 + 2H 2 O -- now C & H balanced, but O? -- Have 2 O from CO 2 & 2 from H 2 O, need four on left side CH 4 + 2O 2 CO 2 + 2H 2 O -- now all balanced PS110A Hatch Ch (a) H 2 O (b) C + O 2 CO 2 (c) C + O 2 2CO (d) 4Na + O 2 2Na 2 O (e) 4Al + 3O 2 2Al 2 O 3 (f) C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O (g) 2K + S 2K 2 S PS110A Hatch Ch Iron and Oxygen combine at different speeds depending on conditions P6: Write a balanced reaction for combining Iron (Fe) and Oxygen (O 2 ) into rust (Fe 2 O 3 ) Fe(s) + O 2 (g) Fe 2 O 3 (s) Get Rich Your grandmother has been asked to help finance a silver extraction process based on the equation 2Ag 2 O 6Ag + O 2 Would you consider the developer as competent? PS110A Hatch Ch PS110A Hatch Ch Predicting Chemical Bonding Both energy and entropy determine chemical reactivity. Bonding resulting in lower energy is likely to happen ( downhill in energy ) Overall entropy increase is likely to happen May have a local decrease but overall, entropy increases PS110A Hatch Ch Example A lump of coal burns in air and gives off CO 2 and heat. CO 2 is a lower energy state and heat energy is given off. We go from highly order coal to highly disordered CO 2 molecules moving about randomly. This is favored energetically and by entropy PS110A Hatch Ch

5 Example: How some ice packs work Ammonium nitrate (fertilizer) is dissolved in water. Goes from a highly ordered crystal to a disordered solution of ammonium and nitrate ions. Energy is drawn from the environment to make the process go forward-uphill in energy This is favored by entropy but not by energy. PS110A Hatch Ch Reactions Take Time Going in the direction H 2 O, reaction is fast Or is it? What if you don t have a match? Electrolysis clearly takes time Simple Clock Reaction takes time Demo: simple clock reaction PS110A Hatch Ch Reactions Go at Different Speeds What Governs Speed of a Reaction? Cellulose + O 2 CO 2 + H 2 O Dead tree in forest Wood in fire Sawdust in explosion Clock reaction (oscillating clock demo) Really a complex set of racing reactions When one set leads, yellow When the other leads, purple PS110A Hatch Ch Speed of reaction: rate that reactants are consumed, or products are produced Factors: Physical state of reactants Collision rate Energetic requirements Entropic (organization) requirements Demo: steel wool, magnesium Downhill, but slow! PS110A Hatch Ch A famous LDS connection Professor Henry Eyring, Collision Rate Limits Reaction Reactants must get close for electron clouds to interact What controls collision rate? Temperature (because it is a reflection of molecular speed) Pressure (for gases) or concentration (for liquids) Physical state PS110A Hatch Ch PS110A Hatch Ch

6 Activation Energy The net energy released (or consumed) does not affect rates The energy required for system 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 Potential Energy Surface Activation Energy Net Energy Transition State PS110A Hatch Ch Methyl isonitrile PS110A Hatch Ch Acetonitrile The effect of entropy Reactants frequently must have a certain orientation or reaction will not occur, a subset of possibilities. This corresponds to more order in the transition state: the difference gives entropy of activation Iodide and methyl bromide PS110A Hatch Ch Decrease energy or increase entropy of the transition state without themselves being consumed; speed up rate H 2 O 2 decomposes spontaneously Much faster in presence of Br -. Catalyst makes new, lower energy route possible Catalysts Demo: H 2 O 2 with MnO 2, also KI 2 PS110A Hatch Ch Catalysts in biology Let s s Blow Some Stuff Up H 2 w/ Pd/C catalyst Adding O 2 allows faster mixing Hexokinase ensyme helps transfer phosphate from ATP to glucose. PS110A Hatch Ch Pd surface stretches the H-H H H bond, helping it break and lowering transition state energy Demo: bottle of H 2 O 2 PS110A Hatch Ch

7 Why do we need to use a match to explode the balloon? Products of combustion of the match catalyze reaction of Heat from the match provides the required energy of activation We need the match to pop the balloon and expose the H 2 to the O 2 Matches contain small amounts of palladium (Pd) PS110A Hatch Ch Reactants Equilibrium Products Reactions go in both directions: You ve already seen this in the reaction This dynamic balance is called chemical equilibrium PS110A Hatch Ch Meaning of Equilibrium When forward and reverse rates are equal, amount of reactant & product no longer changes Equilibrium is the state of lowest energy, maximum entropy; everything moves toward it Another Example Fe + O 2 Fe 3 O 4 P7: Write down the balanced equation. PS110A Hatch Ch PS110A Hatch Ch