Presentation on theme: "A guide for A level students KNOCKHARDY PUBLISHING"— Presentation transcript:
1A guide for A level students KNOCKHARDY PUBLISHING RATES OF REACTION - 1A guide for A level studentsKNOCKHARDY PUBLISHING
2KNOCKHARDY PUBLISHING RATES OF REACTIONINTRODUCTIONThis Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...Navigation is achieved by...either clicking on the grey arrows at the foot of each pageor using the left and right arrow keys on the keyboard
3RATES OF REACTION CONTENTS Prior knowledge Collision Theory Methods for increasing rateSurface areaTemperatureLightCatalystsPressureConcentrationCheck list
4Before you start it would be helpful to… RATES OF REACTIONBefore you start it would be helpful to…know how the energy changes during a chemical reactionknow the basic ideas of Kinetic Theoryknow the importance of catalysts in industrial chemistryCONTENTS
5CHEMICAL KINETICS Introduction Chemical kinetics is concerned with the dynamics of chemical reactions such as the way reactions take place and the rate (speed) of the process.In the following pages you will look at the QUALITATIVE and the QUANTITATIVE aspects of how the rate (speed) of a reaction can be changed.Chemical kinetics plays an important part in industrial chemistry because the time taken for a reaction to take place and the energy required are of great economic importance. The kinetic aspect of chemistry is often at odds with the thermodynamic side when considering the best conditions for industrial production.The concepts met in this topic can be applied throughout the rest of the course when studying both the theoretical and the practical aspects of chemistry.The basis of the study is COLLISION THEORY...
6COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take placenot all collisions lead to a reactionreactants must possess at least a minimum amount of energy - ACTIVATION ENERGYplusparticles must approach each other in a certain relative way - the STERIC EFFECT
7COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take placenot all collisions lead to a reactionreactants must possess at least a minimum amount of energy - ACTIVATION ENERGYplusparticles must approach each other in a certain relative way - the STERIC EFFECTAccording to collision theory, to increase the rate of reaction you therefore need...more frequent collisions increase particle speed orhave more particles presentmore successful collisions give particles more energy orlower the activation energy
8INCREASING THE RATEThe following methods may be used to increase the rate of a chemical reactionINCREASE THE SURFACE AREA OF SOLIDSINCREASE TEMPERATURESHINE LIGHTADD A CATALYSTINCREASE THE PRESSURE OF ANY GASESINCREASE THE CONCENTRATION OF REACTANTS
9INCREASING SURFACE AREA CUT THE SHAPE INTO SMALLER PIECES Increasing surface area increases chances of a collision - more particles are exposedPowdered solids react quicker than larger lumpsCatalysts (e.g. in catalytic converters) are in a finely divided form for this reason+In many organic reactions there are two liquid layers, one aqueous, the other non- aqueous. Shaking the mixture improves the reaction rate as an emulsion is often formed and the area of the boundary layers is increased giving more collisions.11CUT THE SHAPE INTO SMALLER PIECES1133SURFACE AREA= 30 sq unitsSURFACE AREA9 x ( ) = 54 sq units
10INCREASING TEMPERATURE Effect increasing the temperature increases the rate of a reactionparticles get more energy so they can overcome the energy barrierparticle speeds also increase so collisions are more frequentENERGY CHANGESDURING A REACTIONAs a reaction takes place the enthalpy of the system rises to a maximum, then fallsA minimum amount of energy is required to overcome the ACTIVATION ENERGY (Ea).Only those reactants with energy equal to, or greater than, this value will react.If more energy is given to the reactants then they are more likely to react.Typical energy profile diagram for an exothermic reaction
11INCREASING TEMPERATURE ZARTMANN’S EXPERIMENT According to KINETIC THEORY, all particles must have energy; the greater their temperature, the more energy they possess. The greater their KINETIC ENERGY the faster they travel.ZARTMANN heated tin in an oven and directed the gaseous atoms at a rotating disc with a slit in it. Any atoms which went through the slit hit the second disc and solidified on it. Zartmann found that the deposit was spread out and was not the same thickness throughout.This proved that there was a spread of velocities and the distribution was uneven.ZARTMANN’S EXPERIMENT
12INCREASING TEMPERATURE MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYNUMBER OF MOLECUES WITHA PARTICULAR ENERGYMOLECULAR ENERGYBecause of the many collisions taking place between molecules, there is a spread of molecular energies and velocities. This has been demonstrated by experiment.It indicated that ... no particles have zero energy/velocitysome have very low and some have very high energies/velocitiesmost have intermediate velocities.
13INCREASING TEMPERATURE MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYT1NUMBER OF MOLECUES WITHA PARTICULAR ENERGYT2TEMPERATURET2 > T1MOLECULAR ENERGYIncreasing the temperature alters the distributionget a shift to higher energies/velocitiescurve gets broader and flatter due to the greater spread of valuesarea under the curve stays constant - it corresponds to the total number of particles
14INCREASING TEMPERATURE MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYT1NUMBER OF MOLECUES WITHA PARTICULAR ENERGYTEMPERATURET1 > T3MOLECULAR ENERGYDecreasing the temperature alters the distributionget a shift to lower energies/velocitiescurve gets narrower and more pointed due to the smaller spread of valuesarea under the curve stays constant - it corresponds to the total number of particles
15INCREASING TEMPERATURE MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYT1NUMBER OF MOLECUES WITHA PARTICULAR ENERGYT2TEMPERATURET2 > T1 > T3MOLECULAR ENERGYREVIEWno particles have zero energy/velocitysome particles have very low and some have very high energies/velocitiesmost have intermediate velocitiesas the temperature increases the curves flatten, broaden and shift to higher energies
16INCREASING TEMPERATURE MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYNUMBER OF MOLECUES WITHA PARTICULAR ENERGYNUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIEREaMOLECULAR ENERGYACTIVATION ENERGY - EaThe Activation Energy is the minimum energy required for a reaction to take placeThe area under the curve beyond Ea corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.
17INCREASING TEMPERATURE T2 > T1MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYT1NUMBER OF MOLECUES WITHA PARTICULAR ENERGYT2EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIEREaMOLECULAR ENERGYExplanationincreasing the temperature gives more particles an energy greater than Eamore reactants are able to overcome the energy barrier and form productsa small rise in temperature can lead to a large increase in rate
18certain reactions only SHINING LIGHTcertain reactions onlyshining a suitable light source onto some reactants increases the rate of reactionthe light - often U.V. - provides energy to break bonds and initiate a reactionthe greater the intensity of the light, the greater the effectExamples a) the reaction between methane and chlorine - see alkanesb) the darkening of silver salts - as used in photographyc) the reaction between hydrogen and chlorineEquation H2(g) Cl2(g) ———> 2HCl(g)Bond enthalpies H-H kJ mol-1 Cl-Cl kJ mol-1Mechanism Cl2 ——> 2Cl• INITIATIONH Cl• ——> HCl H• PROPAGATION H• Cl2 ——> HCl Cl•2Cl• ——> Cl TERMINATION2H• ——> H2H• + Cl• ——> HCl
19ADDING A CATALYSTCatalysts provide an alternative reaction pathway with a lower Activation Energy (Ea)Decreasing the Activation Energy means that more particles will have sufficientenergy to overcome the energy barrier and reactCatalysts remain chemically unchanged at the end of the reaction.WITHOUT A CATALYSTWITH A CATALYST
20ADDING A CATALYSTMAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYNUMBER OF MOLECUES WITHA PARTICULAR ENERGYNUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIERMOLECULAR ENERGYEaThe area under the curve beyond Ea corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.If a catalyst is added, the Activation Energy is lowered - Ea will move to the left.
21ADDING A CATALYSTMAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGYNUMBER OF MOLECUES WITHA PARTICULAR ENERGYEXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIERMOLECULAR ENERGYEaThe area under the curve beyond Ea corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react.Lowering the Activation Energy, Ea, results in a greater area under the curve after Ea showing that more molecules have energies in excess of the Activation Energy
22CATALYSTS - A REVIEWwork by providing an alternative reaction pathway with a lower Activation Energyusing catalysts avoids the need to supply extra heat - safer and cheapercatalysts remain chemically unchanged at the end of the reaction.Types Homogeneous Catalysts Heterogeneous Catalystssame phase as reactants different phase to reactantse.g. CFC’s and ozone e.g. Fe in Haber processUses used in industry especially where an increase in temperature results ina lower yield due to a shift in equilibrium (Haber and Contact Processes)
23CATALYSTS DO NOT AFFECT THE POSITION OF ANY EQUILIBRIUM CATALYSTS - A REVIEWwork by providing an alternative reaction pathway with a lower Activation Energyusing catalysts avoids the need to supply extra heat - safer and cheapercatalysts remain chemically unchanged at the end of the reaction.Types Homogeneous Catalysts Heterogeneous Catalystssame phase as reactants different phase to reactantse.g. CFC’s and ozone e.g. Fe in Haber processUses used in industry especially where an increase in temperature results ina lower yield due to a shift in equilibrium (Haber and Contact Processes)CATALYSTS DO NOT AFFECT THE POSITION OF ANY EQUILIBRIUMbut they do affect the rate at which equilibrium is attaineda lot is spent on research into more effective catalysts - the savings can be dramaticcatalysts need to be changed regularly as they get ‘poisoned’ by other chemicalscatalysts are used in a finely divided state to increase the surface area
24INCREASING THE PRESSURE increasing the pressure forces gas particles closer togetherthis increases the frequency of collisions so the reaction rate increasesmany industrial processes occur at high pressure to increase the rate... butit can adversely affect the position of equilibrium and yieldThe more particles there are in a given volume, the greater the pressureThe greater the pressure, the more frequent the collisionsThe more frequent the collisions, the greater the chance of a reaction
25INCREASING CONCENTRATION Increasing concentration = more frequent collisions = increased rate of reactionLow concentration = fewer collisionsHigher concentration = more collisionsHowever, increasing the concentration of some reactantscan have a greater effect than increasing others
26RATE CHANGE DURING A REACTION Reactions are fastest at the start and get slower as the reactants concentration drops.In a reaction such as A + 2B ——> C the concentrations might change as shownReactants (A and B)Concentration decreases with timeProduct (C)Concentration increases with timethe steeper the curve the faster therate of the reactionreactions start off quickly becauseof the greater likelihood of collisionsreactions slow down with time asthere are fewer reactants to collideTIMECONCENTRATIONBAC
27Experimental Investigation MEASURING THE RATEExperimental Investigationthe variation in concentration of a reactant or product is followed with timethe method depends on the reaction type and the properties of reactants/productse.g. Extracting a sample from the reaction mixture and analysing it by titration.- this is often used if an acid is one of the reactants or productsUsing a colorimeter or UV / visible spectrophotometer.Measuring the volume of gas evolved.Measuring the change in conductivity.More details of these and other methods can be found in suitable text-books.
28MEASURING THE RATERATE How much concentration changes with time. It is the equivalent of velocity.THE SLOPE OF THE GRADIENT OF THECURVE GETS LESS AS THEREACTION SLOWS DOWNWITH TIMECONCENTRATIONyxgradient = yxTIMEthe rate of change of concentration is found from the slope (gradient) of the curvethe slope at the start of the reaction will give the INITIAL RATEthe slope gets less (showing the rate is slowing down) as the reaction proceeds
29What should you be able to do? REVISION CHECKWhat should you be able to do?Recall and understand the statements in Collision TheoryKnow six ways to increase the rate of reactionExplain qualitatively how each way increases the rate of reactionUnderstand how the Distribution of Molecular Energies is used to explain rate increaseUnderstand how the importance of Activation EnergyRecall and understand how a catalyst works by altering the Activation EnergyExplain how the rate changes during a chemical reactionCAN YOU DO ALL OF THESE? YES NO
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