Kinetics II Flashcards
What is the rate of reaction? Equation?
Change in the amount/conc of a reactant or product per unit time
Mol dm-3 s-1
Change in mount of reactant/products / time
What does the order of a reactant show?
How the conc of a chemical (reactant) affect the rate of reaction
What happens when the order of a reaction with respect to a chemical is 0, 1 and 2? What is the overall order of a reaction?
Order 0: conc of chemical has no effect on Rae of reaction
Order 1: conc of chemical is directly proptional to rate
Order 2: rate is directly proportional to the square of the conc of that chemcial
Order overall - sum of powers of reactants in rate equation
How can colorimeter measure rates of reaction?
Colorimeter measures amount of light that passes through a solution
Intensity of light reaching detector measuredevery few seconds /data plotted to show how CONC of reactants/products chnages over time
- cant be used to monitor formation of coloured precipitates as light will be scattered/blocked by precipitate
How does mass loss measure rate of reaction? Limitation to this method?
Gas produced in reaction escapes so mass decreases :
CaCO3 + HCl —> CO2
Mass measured every few seconds /change in mass over time is plotted
Mass loss = amount of reactantso graph is same as graph of reactant against time
Limitation: gas must be sufficiently dense or change in mass is too small to measure on 2/3dp balance - HYDROGEN WOULD BE SUITABLE
How does gas production measure rate of reactions?
Gas produced in reaction , trapped and its volume measured over time
E.g Mg + HCl —> H2 (g) - collect gas using gas syringe/inverted cylinder in water (if gas is not water soluble)
Volume measured every few seconds + plotted
Volume = amount of products so graph is amount of product against time
How does titration measure rate of reaction? Limitation?
Small samples removed from reaction mixture
/ quenched (stop reactant conc changing) and then titrated with suitable reagent
(NaOH can be titrated with acid , I2 titrated with sodium thiosulfate)
Measuring conc changes by Titration can affect rate of reaction (unless reaction is deliberately stopped - quenching)
- better to ‘stop the clock’ when specific visible point in reaction is reached
E.g Mg dissolves in HCl
Sodium thiosulfate + HCl produces yellow precipitate of sulfur that covers the cross (disappearing cross)
Limitation: generates one piece of data for analysis
What are clock reaction for ? What assumptions are made in this reaction?
more convenient way to measure initial rate of reaction using a single measurement
- time taken for a specific visual change in the reaction to occur is measured- a colour change or formation of a precipitate
- Initial rate proportional to 1/t
Assume: no significant change in rate of reaction between start and time when measurement taken
In clock reactions, what happens to initial rate value when time for reaction to occur is short?
As reaction progresses, conc of reactants decreases
Higher chance that the initial rate value will be closer to true value if time for reaction to occur is shorter
If time taken for reactions to occur is longer , value of initial rate will become less accurate
What is continuous montitoring method? Common ways to collect this data?
Coleicing experimetal data throughout the course of a reaction to plot a conc-time graph
Common ways: measure vol/gas evolved over time
Measuring mass of reactants lost over time
Colorimetry (iodination of propanone)
How to prepare different conc for the iodination of propanone? And how to do the method?
Vary volumes of solutions (iodine, propanone, sulfuric acid) while maintaining constant total vols using distilled water - get different concs
- solutions are measured into a small beaker, leaving the iodine in a separate beaker
- iodine is added to the other liquids, the contents mixed/ quickly transferred into the cuvette - colorimeter / data logger started
0 order conc- time graph?
CONC-TIME: conc is INVERSELY PROPORTIONAL to time
- conc of reactant decreases as time increases - straight line downwards
Gradient = RATE = k
Change in conc of reactant has no effect on rate - rate stays constant as reactant conc decreases
1st order conc-time graph?
Conc of reactant decreases with time
- curves downwards/plateaus
2nd order conc-time graphs?
Conc of reactant decreases MORE STEEPLY with time
- steeper curve downwards
How is order of reaction deduced from half life? What is half life?
Half life - time taken for the amount/conc of limiting reactant in a reaction to decrease to HALF its initial value
0 order: successive half-lives decrease with time
- less time for conc of reactant to HALF as reaction progresses
1st order : half life remains constant throughout reaction
- amount of time needed for conc of reactant to halve will be same throughout
2nd order: half life increases with time
- takes more time for conc off reactant to halve as reaction progresses
0 order of rate-conc graph?
Rate doesn’t depend on conc of reactant
- rate of reaction REMAINS CONSTANT - horizontal line
Rate = k
1st order rate-conc graph?
Rate directly PROPORTIONAL to conc of reactant
- rate increases as conc increases - straight line
Rate = k[A]
2nd order rate-conc graph?
Rate is directly proportional to the SQUARE of conc of reactant
rate of the reaction decreases more as the concentration of the reactant decreases OR VICE VERSA (increases) - curved line
rate = k[A]²
What is the rate determining step?
Slowest step in reaction
If reactant appears in the rate-determining step, then concentration of that reactant will also appear in the rate equation
Which reactants appear in the Rate equation?
The reactants that are part of the RDS
Which reactants appear in the Rate equation?
The reactants that are part of the RDS
What is the Arrhenius Equation?
K= Ae^-Ea/RT
- for reactants involving gases/in solution/on surface catalyst
- relationship between rate constant , temp and activation energy
- k only remains constant when conc of reactants is only factor that is changed- if temp changes/catalyst used , K changes
Effect of temperature/activation energy on rate constant, K and rate of reaction?
Increase Temp - greater value of K
Rate of reaction directly proportional to K , so increase in K = increase in rate of reaction
Increase Ea - proportion of molecules that have the Ea is less
So rate of reaction and K DECREASE
Y=mx + c form of the Arrhenius equation?
Ln K = (-Ea/R)(1/T) + ln A
X= 1/T
M = -Ea/R
C= ln A
For iodine clock reaction why should total volume be kept same for each experiment?
So the vol of iodate ions is PROPORTIONAL to the conc
How can rates of reaction be calculated?
Graph of conc of reactant/prodcuts VS time
GRADIENT = rate
Initial rate = rate at start of reaction - draw tangent
2 other ways to measure rate of reaction?
Measure change in electrical conductivity- works if change in no. Ions in reaction mixture
Measure change in optical actitvty - works if change in optical acidity throughout reaction - use polarimeter
What is the reaction equation for propanone and iodine in acidic solution? And titration equation?
CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + H⁺(aq) + I⁻(aq)
2S2O3 2- (aq) + I2 (aq) —> 2I- + S4O6 2- (aq)
Yellow-brown sol —> colourless sol
What is the order of the reaction with respect to iodine, propanone, and acid catalyst? What is the rate equation?
The reaction is zero order with respect to iodine, but first order with respect to propanone and acid catalyst.
Rate = K[CH₃COCH₃(aq)][H⁺(aq)]
What must the rate determining step of IODINE-PROPANONE reaction contain?
The rate determining step must contain one propanone molecule and one H⁺ ion forming an intermediate.
What is the detailed method for conducting the iodine propanone reaction?
- Add 25 cm³ of 1 mol dm⁻³ aqueous propanone and 25 cm³ of 1 mol dm⁻³ sulfuric acid to a beaker.
- Add 50 cm³ of 0.02 mol dm⁻³ iodine solution and start the clock.
- Withdraw a sample and transfer to a conical flask.
- Add sodium hydrogencarbonate to stop the reaction and record the time.
- Titrate with 0.01 mol dm⁻³ sodium thiosulfate, adding starch indicator when pale yellow.
- Repeat every 5 minutes.
Why can the order with respect to iodine be calculated?
The order with respect to iodine can be calculated because the propanone and acid are in large excess, so their concentrations do not change during the reaction.
What is a common clock reaction involving hydrogen peroxide?
Hydrogen peroxide reacts with iodide ions to form iodine, which then immediately reacts with thiosulfate ions.
What is the chemical equation for the reaction of hydrogen peroxide and iodide ions?
H₂O₂(aq) + 2H⁺(aq) + 2I⁻(aq) → I₂(aq) + 2H₂O(l)
What happens when iodine produced reacts with thiosulfate ions in clock reaction?
When the iodine produced has reacted with all of the limited amount of thiosulfate ions present, excess I₂ remains in solution, which then reacts with starch to form a dark blue-black color.
2S2O3 2- (aq) + I2 (aq) —> 2I- (aq) + S4O6 2- (aq)
How can the order of reaction with respect to a reactant in CLOCK REACTION be determined?
By varying the concentration of a reactant while keeping the concentrations of all other reagents the same and measuring the time taken for the reaction mixture to turn blue - can use this to find rate
Compare how much rate increase/decreases when conc is increased/decreased
(If conc x2 , rate x2 - 1st order)
What is the detailed method for conducting the clock reaction experiment?
- Put each of the chemicals in separate burettes.
- Measure out required volumes of potassium iodide, sodium thiosulfate, starch, and water into a small conical flask.
- Measure hydrogen peroxide into a test tube, then pour it into the conical flask and immediately start the timer while stirring the mixture.
- Time how long it takes for solution to turn blue/black
CP14: finding Ea of a reaction : METHOD?
- Pipette 10cm3 phenol solution and 10cm3 bromide/bromate solution into boiling tube
- Add a few drops methyl red indicator to mixture
- Into second boiling tube, pipette 5cm sulfuric acid
- Place both boiling tubes in water bath at 20 degrees
- Once substances have reached water temp, mix contents into one of the boiling tubes and swirl . START STOP CLOCK
- Place boiling tube containing reaction mixture in water bath
- Stop clock when methyl red indicator disappears
- Repeat the experiment at 30,40,50 and 60 degrees
Initial rate - 1/t (time taken for indicator to change colour)
How can activation energy practical results be used to calculate Ea?
Arrhenius equation rearranged :
Ln K = constant - Ea/RT
K is propertaionl to rate so can be Ln K replaced by ln(rate)
Plot graph of ln(rate) or lnK against 1/T -
GRADIENT = -Ea/R
Ea = - Grad x R (grad is always negative)
Convert to Kj mol-1
Equations for CP14 - finding Ea for reaction between bromide ions and bromate ions?
BrO3 - + 5Br- + 6H+ —> 3Br2 + 3 H2O
C6H5OH + 3Br2 —> C6H2Br3OH + 3HBr
