Module 3.2 THIS IS CHEM Flashcards
Collision theory
-in order for prticles to react what must they do ?
They msut collide .
-if particles do not collide , then they cannot react .
What happens when partilxes react ?
When particles react, chemical bonds must be broken. (This required energy is the activation energy >)
-So in order, for a reaction to happen, particles must collide with enough energy to start breaking the chemical bond.
what happens if the collision does not hve enough energy >
Particles cannot react .
-Instead they simply bounce off eachother .
What is the htird way in which for a reaction take place?
parciles must colllide in the COORECT ORIENTATIOin .
-the rat eof reaction is PROPROTIONAL to thenumber of effective collisions per second - frequency of cells .
One way to increase the rate of reaction is to increase the concentration of the reactants.
-explain this using collision theory.
a higher concentration , means we have more reactant particles int he same volume .
-as the reactant partciles are closer together ,w e have an increased chanc eof collisions taking place - increased freqency of collions .
-some of thee colliosns will be effective leading to rwzction .
-this means rate of reaction will increase .
Explain the rate of reaction in gases and how to increae it .
We can increase the rate of reaction by increasing gas pressure.
-Incresing the pressure, makes particles closer together.
-Increasing the frequency of collisions leading to an increased rate of reaction.
check sheet for the equation
what can we measure when measuring the rate of reactnt ?
-how quickly he amount of reactant decreses .
-OR how quickly , the amount of produc is formed , this depens onw ht is esiser to measure accurately .
-in reaction , making co2 , measuring volum eof gas is straightforward .
two ways of measuring the volume of gas .
-one way is to catch the gas in an upturned measuring cylinder .
-the other way is a more accurtae way which is a gas syringe .
–however , in oth ways ,w e can red the volume of gas at regular intervals fromt he scar .
check sheet , explain star one
initially , as we get a lot of product formed rapidly . This is telling us that the rate of reaction is fast .
-as w ehave a high cocnenration of reactants and a high frequency of efective collisions .
check sheet explain star two
Over time, the reaction slows down ,a s the hydrochloric acid is reacting .
-so the concentration of hcl is decreses and hcl is a LIMITING RECTANT, therefore the frequency of effective colliosions is reduced .
check sheet , explain star three
evenutally , the reaction stops and more product is formed as a result , all the hcl has reacted and there is no m ore effective collsions .
rate of reaction at any point is shownn by what ?
drawing a tangent on the graph .
units could be cm3/s –> dependsss thooo .
Recap for a reaction taking place
-for a reaction to take place reactant particles must collide in the correct orientation .
what two things must particles collide with for a reaction to take place ?
The reactant particles must also collide with enough energy to start breaking the chemical bonds in the reaction . aka as activation energy .
what is activation energy
activation energy is the minimum energy particles must have in order to start a reaction by breaking chemical bonds .
during any reaction , what must happen to the chemical bonds ?
chemical bonds must be broken and new chemical bonds must be formed .
check sheet for two moelucles a-b and c-d .
check the sheet to see the bonds being broken and forming as well .
-what happens to the two molecules .
-as the two molecules collide there comes a point where the existing chemical bonds are in the process of breaking and in the process of forming .
-scientists call this the transition state .
what happens in the transition state ?
the transition state is unstable and goes on to form the products .
-check sheet to check the graph .
-On this graph you can see that the activation energy is the difference between the highest point on the curve and reactants .
On the graph , what does the highest point on the curve represent ?
the transition state.
Now ,a s we know about transition states , we can add a bit more to our activation energy definition .
the activation energy is the enthalpy difference between the reactants and the transition state .
check graph to see two exothermic reactions which i have labelled one and two .
-explain graph one
graph one has a relatively low activation energy .
-So the reaction could take place quite rapidly at room temperature .
AS , at room temperature , there is a relatively large proportion of reactant molecules will collide with enough energy to cross the activation energy barrier and react .
explain graph 2
graph 2 , has a relatively high activation energy ,.
-This means that a room temperature ,t his reaction should be , relatively slow , as now only a smaller proportion of reactant molecules will collide with enough energy to cross the activation energy barrier .
How does a catalyst increase the rate of reaction ?
A catalyst allows the reaction to take place via an alternative pathway with an alternative pathway with a lower activation energy than an uncatalyzed reaction .
what is the effect without a catalyst
higher activation energy , meaning particles collide with a large amount of energy to form the transition state and react .
-This means a small proportion of particles have enough nervy to cross the activation energy barrier , so the rate of reaction is relatively low .
-This means ,t hat a greater proportion of particles will collide with enough energy with enough energy to form he transition state and react .
-In this case , the rate of reaction will be greater than without a catalyst .
What is the key thing about catalysts ?
Catalysts are not used up in a reaction and are not permanently changed , therefore catalysts can be reused.
What is the purpose of using a catalyst ?
-give some economic benefits and environmental benefits
-By using a catalyst , we can make reactions take place more rapidly , even at relatively low low temperatures .
-This reduces the amount of energy needed by the chemicals itself .
-which in then reduces the need to burn lots of fossil fuels , to provide this energy , this saves money providing an economic benefit .
-this also reduces carbon dioxide emissions make the chemical industry more sustainable .
What is a disadvantage for catalysts ?
some catalysts are toxic , so this is a negative impact , needs to be weighed against the benefits .
We can divide catalysts into two categories , what are they ?
Hetrogeneous ctlaysts
-Homogeneous Ctalysts
What are hetereogenous catalystistd
they have a different physcal state (phase) to the reactants .
Whatgenoeus catlaysts
thery are int he same physical satae ([ahse0 as the reactants
e are homog
give an example of hetrogeneous catlaysts and check the sheet
-In the haber porcess , the gases nitrogen and hydrogen react together tof orm the gas ammoni .
-the catalyst int his reaction is iron which is a solid .
-as the catlaysgts is ain a different physical state then the reactions it is an exmpale of hetrogenous catlayst .
how a hetrogenous ctalyst woks
-there are two steps
adsorption a nd desoprtion
…
stage 1 of hetreogenous catlayst
reactant moelcules form weakbonds with the surface of the catalyst .
-scientists call the first stage adsoprtion
ADSOPRTION NOT ABSORPTION .
stage 2 of heterogenous catalyst
second sage reaction takes place to form the [porducts .
-in the final stage porduct moeleucues are released from the surafce of the catalyst .
-scienitsts call thie final stage desoprtion .
homogenous catlaysts same pahse as the reacion
check sheet for the ecample
-oe of the reactions repsoinsible for ozonde destruction int he aormpsopehre .
-ozone is converted into oxyge , this reaction is catalsyedb y a chlorine radical .
-both reactant and altalyst are in gases so it is a hoogenous atlast .
Examples of hetrogenous catlayst
-we often see reactions with the gaseous reactants and a solid catlayst .
-moreover ,w e could also see htereogenous catlaysts with liquid reactants and a solid catalyst .
key about the homogenous catalsy
in homogenous catalsyts the catalyss reacts with the reaction to form an intemrediate .
-reaction shown (zone0 are sonsits of two tages .
-catalyst forms an itnemrediate in syage 1
-stage 3 , intermediste rects sagain and the catalyst is renewed .
if we measure the energy of all the moelcules in a gas , liquid or solution we egt a curve
this curve is the maxewell boltzman distrubtion curve .
key point one of the maxwell distrubtuion curgve
the cruve starts at 0___0 aka the origin .
-wht htis means is that htere are no moelcules with zero energy
second key point about curve , check star one
some moelcules have avery high energy , in fc , one does does not ouch the x axis at highe negries .
-telling us ther eis no maximum energy that the moeclules could have .
key point threee about the curve
-the area under the curve tells us the total number of moelcules in the suytem .
key point 4 baout the curve ,c heck star two
check str two - the most probabale enegry is directly under the pek of the curve .
-BUT tsra three — ? but the mean energy is slightly to the right of this ..
-Thsat is ecuae the particles at very high energy skew the mean to the right .
key point about five , star four
activarion enegry , remeber in order to collide effectively and reax .
-moelcules must have at least this energy
-thes emoeluelces are represented by the re under the curve shadedb lude to th right of the activtion energy
-what does a ctalst do , to the maxwell boltzman curve
catalysts porvidean alternative patheay with a lwoer acitviaiton enegry , the energy line is ec with a catalyst .
-all of the moelcules which now at least have this energy ec , cn collide efectively and rect .
-shown by the area under the curve being much greater tnumebr of meocluels comaped to the uncatalsyed line therefore , expalinw hy rof highw a calas .
another ay rof reaction increase ss by increasing the temperature
what is he effect of this on the curve ?
t1 is the normal temp
t2 is the icnrease int emp
increase in temperature effect on the curve )1_
-you will notivce , that at veyr high temoeratye rtere are more aprtuces wuth a very high eenrgy .
**can also see thag he MOST PORBABLE NEEGRY increases , but the number of partivcles witht he MOST PROBABLE energyf alls .
increase in temperautre effect on the curve t2 .
rememebrr , are aunder the curv = the total nmber of meolcuels do not change number of meoclues .
-we have any increased temp aso area sdoes not change .
–so at high temp the curve shifts to the right , but height of peak falls .
there are two effects of increasing the temperature .
-moelcules move faster ,s o frequency of all collisons increases .
-at higher tempratue , we can see the number of meocluels with at least ctivation energy has increased substantiall .
-therefore a mean greater rproptoion of the collisons are effect resulting in the rate of the reaiton to icnrease .
combustion f metahne
is an example of an irreverisble reaction .
example of a reverisble reaction
check sheet
haver osc process , reverisble rreactions as the products are able to convert bck into the reactants .
-ireverrisble arrow - a reerisble rectant tht can reach equillirbium .
What happens in a closed sytem ?
no atoms can enter or leave the systme .
star one - at the start if the reaction ,w e have a high cocnentration of reactants and no produtcs .
-as we have a higher cocnentration of reactants ,t hue rae of the forwrd reaction is very high .
-as there is no prodcuts the rate f reverse reaction is zero .
-star two ma s tge frorward reaction oriceeds tge coocnetraiton of reaction decreases , cocn of poducs increse ,r ate of rvers eincrese r ate of forwrd decreses .
star three of the clsoed sytem graph
eventually ,t here comes a point , where both the rte of the foreard reaction and reverse reactiona re talking place at the sme rate .
-at this point ,t he cocnentration of both reactants and products remian constnt stop chanign ) so the reaction has reached equilirbrium .
key point 1 fo the clsoed sytem
when we reach equillibrim , the forward and resvere reactions have not stopped
-both the forward and reverse reaction are sitll taking plac but at the sme rte whch is wh y the conc of both is constant .
S BOTH ARE STILL TAKING LACE - scienitnits cll this dynamic equlibrium .
key point 2 of the closed sytem
-at equlllirium , cocnentrarion of both reactants and porducts are no longer chnign .
-but does not mean cocnentration of both are the sme .
dtar four of the taph ,we cn see a greater conc of reactants and vice vera .
apslo equlibrium n e apporahced form either side , ljsut procudcts nor eactant sam conditions .
what does the position of the eequilibrium tell us ?
-it tells us where the equilibrium lies torrd the rectants or products , in some cases , both are the same so equilibrium does not change .
how can we change the po ?soiton of equlirbium
one way is to change the cpncentration
what does le chateiliers pirncipe sgtae ?
where an external change i applied to a sytem , at equillibriu , the equillibrium moves int he rdirection to reduce the effect of the change , .
-apply this to trh exmaple equation .
apply this concept to the reaction shown on sheet , in terms of concnetration
-if we increase the cocnentrtion of reactnt a , as we have more meolcules off a , we have applied an external chnage to the system .
-so the equillibrium now iwll move to the right to minimse the change , as the rate of the forwrd reaction is NOW greateer than the rate of the erverse reaction .
-evenutallyt he rates will be the same , and we will reach a new equillirbim .
le chartielleir princiipe ad the haber bosch procss example reaction ?
-nitorgen and hydroegen react tof orm ammonia , xn LO coneve ammonia backt o ammonia as it is a reverisble reaciton . by changig
what is agas pressure porotional to
gas pressure is propootional to the number of gaso moelcules present .
-equillobrium wills hift ot hte idse with fewer moelcules or mvore depending onw hat hte change is that needs to be couminimised .
-if we icnreas epressure ,e qullibrium will shit to ehe irgt had side as there are fewer moeclues ..
-pressure will only affecte quilirbum if there is a differenc ein the numbe orf moles on both sides .
Many reversible reactions are relatively slow at room temperature .
-what is the solution to this >
-so we often use a calasy to icnrwase the rate of reaciton
KEY ; catlasyts increse the rate of botht he forward and reeverse rection by the same amount ,s o catlays hae NO EFFECT ont he position of hte eeuillibrium .
-using a ctalsys means raction reches equilllibrium faster then if we ddid not use a catalsyt .
-HOw can qwe quanitfy the position of the equillibrium ?
by calcualitng the equillibirum contant kc check the sheet .
-imagine we have a reaction in which the reactans produce a to d .
-the lwoer case numbers sohow the balancing euqaitons and have reached equilibirum .
do the first quesiton what do u need to notice aoout this equaiton
-sall the reactan and porduct are gases aka the same pahse so it is a homogous equillibirum , check the sher how to do this
what are the values in the equationa nd what is the unit for them .?
concentration unit is moldm-3
-cgecj tge sgeet si see giw ti work out units .
-as all the units cancel out in this reaction so kc has no units .
what does equilibirum constanr depend on ?
it depends on temperature .
-so for the same rection , but different kc temperautre was used in different that why different k
what does the size of the euilbirum show ?
-show all the variation
gives us an idea of the relaitve proprotions of the reactants and products at equillirbirum
-kc+1 equillibrium lies mdiway between reactants and pordcts .
if kc is less than 1 + eeulliirum lies towards the reactant .
-if kc is more than one equillibirum lies toward poerducts .
what is the temperture used fed for the haber bosch process
althogh a low temperature should produce a high equliirum yield , the reaction owudl take place at a very low rte , so the n2 h2 moelcuels will not have enough energyt oc ross the acitivation egery barries .
-therefore the tmeerature imust be high enough to alow the reaction to proceed at relisitc rite will still producing an acceptabel equilibrium yield .
-a tmeperture of 200-500 degrees is used .
what should the pressure be for the haber bosch process ?
a high presure increses the cocnentration og the gses , incresing the reaction rate , so high presure should produce a igh equlirium yeild and hibghr ae , however , large quantitie sof energy iwll be needed dding to cotss , d it will cause sfety ipactuis ,
-so to make sure workforce is not put in danger and reduce cots a pressure of 200 atmpsheres re usd .
effect of usian iron catalyst for the haber bosch process
an iron catalyszciton allowing the equillibriumt o be estbalsihed agsyet nd leow temperatures to be used . Less enegry is needed to genrta eheat ,r educing ocsts .
