Unit 1: Reaction Kinetics

Transcription

1 Unit 1: Reaction Kinetics Intro Read Hebden p. 1 Brick wall in text book is actually called. Reaction Kinetics the study of. Expressing Rates rate = quantity of a product formed or quantity of a reactant consumed unit time unit time in general: rate = (a reactant or product) g/mol/l s/min eg.) Zn (s) + 2HCl (aq) H 2(g) + ZnCl 2(aq) r = mass of Zn r = [ HCl ] (note: [ ] = molar concentration) r = volume H 2 Do ex. 1-5 p.2 S.W. (SW is Hebden s Student Workbook) 1

2 Calculations Involving Rxn Rates use conversion factors to cancel units you don t want and replace them with ones you do want! Eg.) mol =? mol min. s mol x 1 min = 3.3 x 10-4 mol 1 min 60 s s Exactly 60s/1min, so don t consider it using significant digits. The has 2 sig. digs., therefore the answer has 2. You also must use molar mass to go grams moles. Eg.) 0.26 mol Zn =? g of Zn min s You would use 22.4 L for conversions moles L (STP) for gases. 1 mol eg.) mol O 2 /s = L/s (STP) Practice: The rate of a reaction is g of Mg per second. Calculate the number of moles of Mg used up in 6.0 minutes. 2

3 Comparing rates using balanced equations ethane 2C 2 H 6 + 7O 2 4CO 2 + 6H 2 O consumed produced eg.) ethane consumed at a rate of mol /s, calculate rate of consumption of O 2 in mol /s if ethane consumed at rate of mol /s calculate rate of production of CO 2 2Al + 3Br 2 2AlBr 3 if 67.5 g of Al are consumed per second - calculate the rate of consumption of Br 2 in g/s. Eg) An experiment is done to determine the rate of the following reaction: 2Al (s) + 6 HCl (aq) 3 H 2(g) + 2 AlCl 3 (aq) It is found that the rate of production of H 2(g) is g/s. Calculate the mass of Aluminum reacted in 3.0 minutes. 3

4 Measuring Reaction Rates eg. CaCO 3(s) + 2HCl (aq) H 2 O (l) + CO 2(g) + CaCl 2 (aq) CO 2 gas is escaping in an open system CO 2(g) escapes HCl (aq) - as CO 2 escapes, mass of the of the system will Note r = mass of container and contents rate = slope of amount. vs. time graph (open system) Slope is negative if something is being and positive if something is being. Slope = rise (g) run (s) Mass of Container and Contents (g) rise (g) Rate = Slope (made +) run (s) Time (s) Note: In reality, rate(slope) is over time, rate is expressed over a certain interval 100. Average slope between 0 and 30s : straight line drawn from 0 to time 30 s. Mass (g) 20. To calculate rate at a certain point in time: it is the slope of the tangent at that point Time (s) 4

5 Monitoring Reaction Rates Which properties can be monitored (measured at specific time intervals) during a rxn in order to determine rxn rate? colour Temperature pressure mass Volume ph specific ions conductivity 1.) Colour changes - only where coloured reactant is consumed or new coloured product formed. eg1.) Cu (s) + 4HNO 3(aq) Cu(NO 3 ) 2(aq) + 2H 2 O (l) + 2NO 2(g) + heat copper clear blue clear brown Could measure: intensity of blue intensity of brown gas - colour intensity measured using a (see p. 4 S.W.) rate = colour intensity 2.) Temp changes - in exothermic rxn, temp of surroundings will - in endothermic rxn, temp of surroundings will - measured in insulated container (calorimeter) rate = temp 3.) Pressure changes (closed container) - If more moles of gas (coefficient) in products pressure will. Zn (s) + 2HCl (aq) H2(g) + ZnCl 2(aq) O m.o.g. < 1 m.o.g. - If more MOG in reactants - pressure will - If equal MOG, pressure will change: rate = pressure NO 2(g) + CO (g) CO 2(g) + NO (g) 2 m.o.g. 2 m.o.g. 5

6 4.) Mass changes eg1) If only one solid is used up, you could remove periodically and weigh it: Mg (s) + 2HCl (aq) H 2(g) + MgCl 2(aq) periodically remove Mg and note the in mass as H 2 gas escapes, mass of the container and contents. rate = mass of container & contents Factors affecting reaction rates (read p. 4-5 SW. Ex. 7-9 page 5) - Do Hand-In Assignment on Reaction Rates - Do expt 18-B (or A) To look at factors affecting rx 1.) Temperature - as temp increases rate 2.) Concentration of reactants - as conc. of one or more reactants increases, rate conc of a gas = pressure of a gas (the pressure exerted by that gas in a mixture of gases) 3.) Pressure - affects rxns that have gases in reactants. eg.) C (s) + O 2(g) ---> CO 2(g) - as pressure increases, rate Note: a decrease in the volume of rxn container will the ( and therefore rate) 4.) Nature of reactants WHY? 123: Increase chance of collision & arxn a) Bonds _ -rate of rxn depends on how strong & how many bonds in reactants need to be broken. _ in general covalent bonds are strong and slow to break. 6

7 Many bonds have to be broken and many new bonds have to form. this rxn is slow at room temp Eg.) H 2(g) + Cl 2(g) 2HCl (g) H - H + Cl - Cl covalent bonds slow at room temp. Remember, adding a flame means your rxn is NOT at room temp 5.) Catalysts - substances which can be added to the rate of a rxn. without being itself. 2H 2 O 2(l) 2H 2 O (l) + O 2(g) uncatalyzed - slow MnO 2H 2 O 2(l) 2 2H 2 O (l) + O 2(g) catalyzed - fast Inhibitors - substances which can be added to the rate of a rxn. (can combine with a or a & prevent it from reacting) -eg. -poisons (cyanide) - organophosphates (diazinon) -antibiotics -antidepressants (serotonin uptake inhibitors) -sunscreens The factor which affects only heterogeneous rxns (more than one phase) Homogeneous rxn - all reactants are in the phase 6. Surface area -when 2 different phases react, the rxn can only take place on the. - surface area by solid into smaller pieces (liquids in smaller droplets) These "inside" surfaces are added Chunk (small surface area) Sliced (larger surface area) Powder (huge surface area!) 7

8 Comparing rxn rates In general: > or > Some points 1.) Temperature affects rate of reactions 2.) Pressure (or volume) affect reactions with reactants 3.) Concentration only affects or reactants 4.) Surface area - affects only reactions. - do ex p SW. Pay close attention to the graphs in question 17! - Read pages 5-9 SW. - do ex SW. (page 8) Everyday situations which require control of reaction rate Cooking - improves taste - kills some bacteria - if too hot causes burning and productions of carcinogens (benzopyrenes) Food preservation - lower temperature - anti-oxidants (eg. ascorbic acid) - keep from O2 (sealing) - preservatives (nitrates, nitrites) Think of more! Body chemistry Factors affecting reaction rates 8

9 Monitori [- do Do ex.6 demo on page with 3 Cu of & SW.] HNO [Read page 11 and do 3 ng ex. 18- discuss 19 Reaction on p. colour, 11 mass, SW.] conc., Rates [ do pressure eg.) - metabolism - fever can destroy bacteria - neurotransmitters - awareness, sleep etc. -hormones - messengers (adrenaline, sex hormones) -catalysts - enzymes (digestive etc) - aging Fuels - concentration of O2 important - to increase combustion rate - increase [O2] - increase surface area -increase temperature - catalyst (wood stoves etc) - to decrease combustion rate - water on fire -smothers it (decreases O2) - cools it - fire retardant - forest fires - children's clothing - airplane fuels- when spilled Industrial Processes - produce product quickly eg.) - fiberglass uses catalyst (hardener) hardens fast but not too fast - glue - epoxy uses catalyst - contact cement fast - concrete - ceramics - paint - oxy- acetylene welding (must be very hot) - oil refining - sewage treatment uses microbes to speed up breakdown - slow down reactions. eg.) nitroglycerine - keep cool - if too warm explodes- keep cool & dry 2