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 Changes in colour 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 do changes in mass 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 volume of gases 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 measuring conc changes measure rate of reaction? Limitation?
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
Continuous monitoring method: iodination of propanone
What is the iodination of propanone an example of and how is it carried out?
Shows how rate of reaction can be measured throughout reaction by using a COLORIMETER
CH3COCH3 + I2 → CH3COCH2I + HI
Iodine decolorises during reaction as it turns to IODOPROPANONE and HYDROGEN IODIDE
- uses dilute sulfuric acid calatlyst
- colorimeter measures colour absorbance proportional to the conc of the coloured species
Before the investigation begins measure the absorbance of a set of I2 standard solutions and obtain a calibration curve
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
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
