7 (1.9) Measuring rate by initial + continuous monitoring Flashcards
Initial rate method?
Iodine clock experiment
Reaction equations?
Hydrogen peroxide reacts with iodide ions to form iodine and the thiosulfate ion immediately reacts with iodine:
H2O2( aq)+2H+( aq)+2I-( aq)→I2( aq)+2H2O (l)
2S2O3^2 -( aq)+I2( aq)→2I-( aq)+S4O6^2 -( aq)
Why do quantities of reactants need to be calculated carefully?
-typical gas syringe only measures 100 cm3 of gas so you don’t want a reaction to produce more than this volume
What happen if large excess or reactant?
In reactions where there are several reactants, if the concentration of one of the reactant is kept in a large excess then that reactant will appear not to affect rate and will be essentially zero order . This is because its concentration stays virtually constant and does not affect rate.
Analysis?
Plot a graph of volume of hydrogen produced on the y-axis against time in seconds for each hydrochloric acid concentration. Draw a line of best fit.
● Draw a tangent to each line of best fit at time, t = 0 s.
● Calculate the gradient of each tangent in order to deduce the initial rate of each reaction at each concentration.
● Compare the rate values obtained.
Why is measuring initial rate preferential?
Concentrations is known at start of reaction
How can rates of reaction be measured?
-initial rates method e.g iodine clock reaction
-continuous monitoring method e.g measuring volume gas released in reaction over time
How does iodine clock experiment work?
-I2 reacts with all thiosulfate ions present.
-excess I2 remains in solution, which then reacts with starch to form blue-black volor
-time how long it takes blue-black Color to appear, vary [I-] to determine order with respect to iodine ions
-issues with experiment?
-Some low I- concentrations may take too long to react
-Delayed stopwatch reactions
-Concentrations may not be exact due to measuring apparatus
What is continuous monitoring method?
Involves measuring the change in concentration of a reactant or product over time (or measuring volume of gas released) as the reaction progresses
Give an example of continuous monitoring method?
Add a strip of magnesium ribbon into a conical flask containing HCl, place a bung in the top of the flask and start timer
Tube attached to gas syringe
Note down volume of hydrogen gas collected every 15 seconds for a period of two. Five minutes
Repeat four different HCl concentrations
Analyse data from experiment?
-Plot a graph of volume of hydrogen gas produced (Y axis) against time (X axis) for each concentration of HCl
-Draw best of fit
-Draw a tangent at T equals zero seconds for each line
-to deduce the rate of each reaction, calculate the gradient of each tangent
Issues with experiment (continuous monitoring)
-Some gas may escape before the bug is added
-The magnesium strips maybe of different mass and surface area which will affect the rate of reaction
Disappearing cross?
Measure 10 cm3 of 0.2M hydrochloric acid and 10 cm3 of sodium thiosulfate in separate clean measuring cylinders. Put the solutions in separate boiling tubes
•Choose a temperature to investigate use water bath to get the two solutions to that temperature by placing the boiling tubes in the water bath.
•Place the flask on the centre of the large cross; first add the sodium thiosulfate to the flask. Then add the hydrochloric acid and start the stopwatch and swirl to mix the solutions.
•Stop the clock when the cross disappears and note the time.
•Repeat the experiment for four more different temperatures (maximum temperature should be 70 oC).
