Since reactions want to minimize energy you would think that the reaction would be spontaneous like a ball rolling down a hill

Transcription

1 Notes 1.1 Exothermic reactions give off heat Heat Content Since reactions want to minimize energy you would think that the reaction would be spontaneous like a ball rolling down a hill C 6 H 12 O 6 + 6O 2 6H 2 O + 6CO kJ/mol is exothermic We don t worry about sugar cubes exploding on contact with air why not? Demo sugar cub and air, and flame then powdered sugar and flame. Cornstarch and ABS. Something prevents or slows the reaction from occurring Heat Content Reaction kinetics The study of the rates of reactions and the factors that affect the rates

2 o amount can be products or reactants, Ex if 16g of HCl are used up in 12 minutes of a reaction the average rate is Try o If a reaction between CaCO 3 and HCl produces 245ml of CO 2 in 17s then what is the reaction rate? o Assign Exercises 1-5 page 2 Read 1.2 together and answer 6 as a class The slope of the graph is the rate (rise over run = amount over time) How can we determine reaction rate? There are four ways to measure the rate of the following reaction Cu (s) + 4HNO 3(aq) Cu(NO 3 ) 2(aq) + 2H 2 O (l) + 2NO 2(g) + heat Red-brown (none) blue brown 1. Colour change a. Use a spectrophotometer to measure the intensity of the blue colour (amount of Cu(NO 3 ) 2(aq) )

3 i. 2. Temperature change (heat is a product) a. 3. Pressure change (a gas is produced and could be held in a sealed container) a. 4. Mass change (run the reaction for different periods of time and weigh the copper after each trial) a. Assign 7-9 and read Activity 18A 1.3 Factors Affecting Reaction Rates Your experiments should have shown that HCl and Mg : Reacted faster at higher temperatures Reacted faster in more concentrated solution higher rate or less time Reacted faster with more surface area reactions-and-rates.jnlp Other experiments have found that: When gaseous reactant pressure increases the rate increases o (Pressure is just concentration of a gas) The nature of some reactants (chemical properties) makes them react very slowly or quickly regardless of the conditions. o We can t easily control these rates (can t slow down the explosion of nitroglycerine much by cooling it) o More or stronger bonds being broken or formed tends to slow down reactions o Look at molecules on page 7 and notice many bonds formed and broken in the first slow reaction and only an electron transferred in the second fast reaction o Assign 10-11

4 The ability of reactants to meet (remind of sugar demo) o Increasing the surface area increases the sites that the reaction can occur on Basically raises the concentration of reaction sites o Homogeneous reaction The reactants are in the same phase and perfectly mixed The reaction can happen everywhere at once 2 gases 2 substances dissolved in water two liquids completely dissolved in each other o Heterogeneous reaction Reactants are in different phases and not completely mixed Reaction can only happen on the surfaces where the reactants meet Solid and liquid Liquid and gas Solid and gas Two immiscible liquids Phase o Reactants can t move easily in solids o Reactants are closest in ionic solution o Aqueous ions > (gases or liquids)> solids Assign Catalysts Chemical that can be added to a reaction to increase the rate of reaction o After the reaction is complete there is the same amount of the catalyst present

5 Demo 2H 2 O 2 2H 2 O + O 2 with MnO 4 Most enzymes are catalysts Inhibitors Reduces reaction rate by combining with a catalyst or reactant Demo same reaction with dishsoap Examples are antibiotics and poisons Assign Notes 1.4 Experimental reaction rates A plot of [reactant] vs time doesn t give a strait line The reaction goes fast at the beginning because there is lots of reactant The reaction slows at the end as the reactant is used up Average rate looks at start and finish Exact rate at a given time is from the slope of the tangent at that time Try Collision theory (aka Kinetic Molecular Theory) Molecules act like small hard spheres that bounce off one another and transfer energy during collision

6 Molecules must collide before they react o Effective collisions produce the products Correct orientation and energies ectfolder/animations/no+o3singlerxn.html reactions-and-rates.jnlp This explains why two conditions changes can change the rate o Increasing the concentration of reactants means collisions are more frequent rate increases as collisions/sec increases o Increasing temperature makes the molecules move faster collide more often and with more energy rate increases as collisions/sec increases. Rule of thumb is as temp increases 10 o the reaction rate doubles Assign and Worksheet up to 10 Quiz tomorrow on factors controlling reaction rate Enthalpy demo endothermic and exothermic with ammonium nitrate and water and sulphuric acid and sugar Notes 1-06 Enthalpy changes As two atoms approach each other the nuclei are attracted to the electrons but the electrons repel each other H + H PE H 2 Reaction

7 They settle at a distance that minimizes the repulsive forces and maximizes the attractive forces The potential energy of the system is minimized and we say that a bond has formed. The atoms want to stay together and it will take energy to separate them Potential energy Stored energy that results from position in space Kinetic energy Energy from the movement of molecules or atoms within a molecule Bond energy The energy required to break the bond The same amount of energy is released when that bond forms Cl kJ --> 2Cl 2Cl --> Cl kJ assign 23 Most reactions are much more complicated many bonds break and form heat can move in or out work can be done by or on the system Enthalpy the sum of all the kinetic and potential energies that exist in a system when at constant pressure

8 H = H prod -H react is the change in enthalpy during the course of a reaction Draw diagram on page 14 It is H not the values of the reactants or products that is important Endothermic reactions H prod > H react and H >0 2N 2 + O KJ --> N 2 O or 2N 2 + O 2 --> N 2 O; H =+164 KJ 2N 2 O Enthalpy 2N 2 + O 2 H=+164kJ H points up to the products and is positive Energy enters the system and it is endothermic demo ammonium nitrate and water Exothermic reactions H prod < H react and H <0 H 2 + Cl 2 --> 2HCl kj H 2 + Cl 2 --> 2HCl; H = -184 kj Enthalpy (H) H 2 +Cl 2 H= -184kJ 2HCl

9 Assign H points down to the products and is negative Energy exits the system and is exothermic demo sulphuric acid and sugar Notes 1.7 Kinetic energy distributions For a reaction to occur you need collisions and sufficiently energetic molecules. At room temp the average gas molecule has collisions a second o Frequency isn t the problem, energy is. o Even at low temperatures some molecules react Kinetic energies vary from molecule to molecule # of molecules Average energy Kinetic energy At different temperatures the average kinetic changes: Number of molecules with a particular energy 25 o 200 o 400 o

10 10 o increase equals an approximate doubling of rate for slow rxns Both collision frequency and number with the minimum energy increase Assign Notes 1.8 Activation energies Minimum energy to successfully react The existence of a minimum energy means there is an energy barrier even for exothermic reactions. Molecules without enough energy just roll back down Activated complex PE R P

11 Think of rolling a ball up a hill. You increase PE as you climb but your kinetic enrgy decreases as you slow down. After you crest the hill you speed up as your potential energy is converted into kinetic energy Activated complex The arrangement of atoms which occurs when the reactants are in the process of rearranging to form products An intermediate molecule Activation energy E a Minimum energy required to change reactants into products If KE is less than E a then the atoms electron repel before the activated complex forms Ineffective collision If KE = E a then the activated complex forms but the molecules stop and could fall down either side of the hill Reaction is possible but not guaranteed If KE > E a then the activated complex forms and the molecule continues to move and can go to the products Draw diagram from hebden p 21 to relate the size of the energy hill to the number of molecules with E a at a given temperature. When the hill is high or the temp is low very few molecules have sufficient energy to form the activated complex and vice versa Assign 34-37

12 Energy is conserved during a reaction so the total E (KE and PE) stays constant through the reaction as one rises the other falls Draw page 22 Hebden A successful collision requires both sufficient KE to make contact and correct alignment of the molecules. If the alignment is off the energy hill is higher because they still need KE to wiggle into position Draw and discuss the diagram at the bottom of 22. Assign Looking at the Potential Energy diagrams there is no visible reason why molecules can t go over the hill from left to right and right to left Reactants can form products, but at the same time Products can form reactants! REACTANTS PRODUCTS PE E a (for) E a (rev) Reactants H Reaction proceeds Products

13 E a (for) is the activation energy for the forward reaction E a (rev) is the activation energy for the reverse reaction The activation energy is always endothermic It takes energy to start the forward and reverse reactions Look at and draw the other 3 diagrams on pages 24 and 25 and convince yourself that H = E a (for) E a (rev) Note If you are confused about the formula or you forget it just sketch a quick PE diagram to determine the sign of H. Assign Notes 1.9 Reaction mechanisms The sequence of steps in an overall reaction The odds of 4 or more molecules coming together and colliding at exactly the same time with exactly the right orientation are virtually nil o Complex reactions must happen in simpler steps to be possible 4HBr + O 2 2H 2 O + 2 Br 2

14 has 3 experimentally determined steps (you won t be asked to predict these on your own!) Step 1: (slow) The dots are bonds that are in the process of breaking or forming. Step 2 (fast) Step 3 (fast) There are 3 elementary processes (steps) in the reaction mechanism

15 Information you get from this mechanism: 1. the slowest step is the rate determining step (step one in this mechanism) 2. if HOOBr and HOBr are used up quickly but form slowly their concentrations can t build up 3. Since HOOBr and HOBr aren t used in the rate determining step adding more of these will not affect the overall reaction rate 4. to determine the overall reaction add all the steps together and cross off whatever appears on both sides of the arrow. a. HBr + O 2 HOOBr b. HBr + HOOBr 2 HOBr c. 2HBr + 2 HOBr 2 H 2 O + 2Br 2 d. 4HBr + O 2 2H 2 O +2Br 2 5. HOOBr and HOBr are intermediates that can exist on their own but react quickly in this mechanism 6. The formula of the activated complex is found by adding the atoms of the reacting molecules (order isn t important) Assign Break Notes 1.10 Using: a. HBr + O 2 HOOBr (slow) b. HBr + HOOBr 2 HOBr (fast) c. 2HBr + 2 HOBr 2 H 2 O + 2Br 2 (fast) Each step has it s own activated complex and therefore its own activation energy. There are 3 humps in the overall PE diagram for

16 4HBr + O 2 2H 2 O +2Br 2 PE HOOBr HOBr H 2 O HBr + O 2 Br 2 Step 1 Step 2 Step 3 The activation energy for any one step = PE (activated complex) PE (reactants for the step) draw on the graph each E act starts from a new level! Assign 54, 55 Notes 1.11 The effects of catalysts Catalyst a substance that provides an overall reaction with an alternative mechanism having a lower activation energy H is not changed but the energy hill is uncatalyzed PE catalyzed reaction proceedes

17 A catalyst increases both the forward and reverse reaction rates because more molecules have enough kinetic energy to form the activated complex uncatalyzed catalyzed KE PE # of molecules reaction proceeds Keep in mind that both pathways are still available and some may still use the uncatalyzed pathway but most take the easy route. Draw Pt catalyst diagram from page 32 Notes 1.12 The effect of a catalyst on the reaction mechanism

18 The reaction OCl - + I - lo - + Cl - has huge activation energy because it involves the approach and collision of two negatively charged species. [I--O--Cl] 2- PE I - + OCl - IO - +Cl - Reaction proceeds The reaction is therefore very unlikely to occur. A catalyst (water) provides an alternative mechanism with a lower overall activation energy Notes

19 The catalyst does take part in the reaction It is simply regenerated in a later step of the mechanism so at the end there is the same amount as was started with H for the overall reaction is the same in catalyzed and uncatalyzed reactions both intermediate species and catalysts cancel out in the overall reaction intermediate species are produced and then used up catalysts are first reactants that are later produced again Assign and assign the worksheet Notes 1.13 Some uses of catalysts Catalysts are vital to industry and one of the main reasons life is possible Enzymes biological catalysts that control virtually all the chemistry in an organism act on a substrate made of protein the complex shapes bind and then orient reactants

20 C 12 H 22 O 11 + H 2 O 2 C 6 H 12 O 6 is catalyzed by maltase Producing sulfuric acid S + O 2 SO 2 The sulfur dioxide is passed over a vanadium pentoxide or platinum catalyst 2SO 2 + O 2 2 SO 3 The sulfur trioxide in passed through water to make sulfuric acid

21 SO 3 + H 2 O H 2 SO 4 Catalytic converters Automobile engines are hot enough to combine N 2 and O 2 to make NO 2 2NO + O 2 NO 2 (smog) They also release unburned hydrocarbons The catalytic converter contains platinum, palladium and rhodium which catalyze the oxidation of the hydrocarbons to less harmful CO 2 and H 2 O Use transition metals to catalyze the reduction of NO back to N 2 and O 2 Assign Test next double block