Required practical 7-initial rate method Flashcards
write the 2 equations for the 2 successive reactions in iodine clock reaction
H2O2(aq)+ 2H+ (aq)+ 2I- (aq)—> I2(aq) +2H2O(l)
2S2O3^2- (aq)+ I2(aq)—> 2I-(aq) + S4O62-(aq)
briefly explain how iodine clock experiment works
-When I2 reacts with limiting reagents(sodium thiosulfate ions) excess I2 reacts with starch to forming a blue-black colour
Outline the method for iodine clock experiment.
Method:
(1a)use a cross tile and place conical flask on it
(1a) Idea of using disappearing cross or
colorimetry
(1b) Put water,sodium thiosulfate, potassium iodide, starch, into containers on/with
cross or in colorimeter/
(1c) Add second reactant(H2O2) and start timing. Stir mixture
Measurements:
(2a)Repeat at different I2 (if number of
I2 conc. stated, must be more than two)
(2b) Record time, t, for cross to disappear / defined
reading on colorimeter
(2c) Idea of ensuring other variables (cross,
volumes, concentrations) kept constant (apart
from I2)
Use of results:
(3a) 1/t (or 1000/time, etc) is a measure of rate
(3b) plot of rate (or 1/t etc) (y-axis) against log I (xaxis) (can come from labelled axes on sketch)
(3c) sketch of plot as shown (ALLOW 3c if axes
not labelled but NOT if incorrectly labelled)
MS2019Q1.4
Issues with iodine clock?
some iodine concentrations may take too long to react.
-delayed stopwatch reactions
concentration may not be exact due to errors in the equipment
what is the oxidizing agent and what does it oxidise in iodine clock experiment
H2O2
oxidises I oxidation state from -1 to 0
How can the volume of H2O2 be varied
get a stock solution and dilute it w water(serial dilution)
what variables need to be controlled to find out order of reaction in respect to H2O2
- temperature
- conc of potassium iodide
- conc. of H+
describe the equipment used for continuous monitoring method
conical flask rubber bung, gas syringe
Outline the method for continuous monitoring method using Mg and Hcl
Mg + 2HCl –> MgCl2 + H2
1-measure 50cm3 of the 1.0 mol dm-3 HCl and add to conical flask
2-set up gas syringe in stand
3-weigh 0.20g of Mg ribbon and add to conical flask,place bung firmly into top of flask & start timer.
4-Record volume of H2 gas collected every 15 seconds for minutes.
How do u calc. initial rates from graph
concentrations at start of reaction.Draw a tangent at beginning of curve and work out gradient= rate of reaction