Kinetics Flashcards
Rate of reaction
Increase in product/ time
Or
Dec on reactant /time
Units of rate
Mol dm-1 time-1
Three ways of obtaining rate from a graph
Rate at given time
Initial rate
Average rate
Rate at given time
Measure gradient of tangent drawn to the curve at that time
Initial rate
Graph is almost linear at t=0 the initial rate can be measured accureltybform this graph
Average rate
An average rate can be measured over a given time
Why does grad become less steep as time passes
Rate dec as reaction proceeds
AST time passes the number of unreacted particles remaining dec therefore the number of successful collisions per unit time dec ie rate dec
What do rate values depend on
Which concentration is measured or followed
(2R -> P
Rate = dec in R /time
Rate = 2X increase in P/ time)
Sampling
Ethyl ethanoate + sodium hydroxide -> sodium ethanoate + ethanol
1 the reactants are mixed and the clock started simultaneously
- Samples (aliquots) are withdrawn from the reaction mixture at regular time intervals and each sample is run into 10x it’s own volume of ice cold water (has the effect of quenching the reaction)
- The concentration of sodium hydroxide in each aliquot is then determined by titrating with STANDARD hydrochloric acid with phenolphthalein as indicator
- Plot a graph of conc. NaOH against time
- Draw a tangent at time t=0 and calculate its gradient to find the initial rate of removal of NaOH
Sampling
Br2 + methanol acid-> (H+ catalyst) 2 Br + 2H+ + CO2
- The reactants are mixed and the clock started simultaneously
- Samples (aliquots) are withdrawn form the reaction mixture at regular time intervals and each sample is run into an excess of silver nitrate solution
Ag+(aq) + Br- (aq) -> AgBr (s) - The precipitate is filtered, dried and weighed
- Plot a graph of Br- conc. against time
- Draw a tangent at t=0 and calculate its gradient to find the initial rate of formation of Br-
Continuous measurement
Change in colour
Br2 + methanol acid-> (H+ catalyst) 2 Br + 2H+ + CO2
- The reactants are mixed and the clock started simultaneously
- Using a colorimeter measure the absorbance of the solution at regular time intervals
- Use a calibration curve to convert absorbance into [Br2]
- Plot a graph of [Br2] against time
- Draw a tangent at t=0 and calculate its gradient to find the initial removal of Br2
Continuous measurement
Change in volume of gas evolved
Hydrogen perixude-> water + oxygen
- The reactants are mixed and the clock started simultaneously
- Using a gas syringe, measure the volume of gas formed at regular time intervals
- Convert the volume of oxygen formed to [hydrogen peroxide] remaining
- Plot a graph of [H2O2] against time
- Draw a tangent at t=0 and calculate its gradient to find the initial removal of H2O2
Change in volume of gas evolved
Why is temp important
Constant temp is important as the volume of gas inc as temp inc
Continuous measurement
Change in electrical conductivity
BrO3-(aq) + 5Br-(aq) + 6H+ (aq) -> 3Br2 (aq) + 3H2O(l)
- The reactants are mixed and the clock started simultaneously
- The electrical conductivity of the solution is measured at regular time intervals
- Covert the electrical conductivity to [reactant] or [product]
- plot a graph of [reactant] or [product] against time
- Draw a tangent at t=0 and calculate its gradient to find the initial removal/ formation of a reactant/ product
Continuous measurement
Change in pH
Ethyl ethanoate + sodium hydroxide -> sodium ethanoate + ethanol
- The reactants are mixed and the clock started simultaneously
- The pH of the solution is measured at regular time intervals
- Covert the pH to [OH-] using pH= -Log[H+] Kw= [OH-][H+]
- plot a graph of [OH-] against time
- Draw a tangent at t=0 and calculate its gradient to find the initial removal of OH-
