Required practical 7-initial rate method

Question 1

write the 2 equations for the 2 successive reactions in iodine clock reaction

Answer 1

H2O2(aq)+ 2H+ (aq)+ 2I- (aq)—> I2(aq) +2H2O(l)

2S2O3^2- (aq)+ I2(aq)—> 2I-(aq) + S4O62-(aq)

Question 2

briefly explain how iodine clock experiment works

Answer 2

-When I2 reacts with limiting reagents(sodium thiosulfate ions) excess I2 reacts with starch to forming a blue-black colour

Question 3

Outline the method for iodine clock experiment.

Answer 3

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

Question 4

Issues with iodine clock?

Answer 4

some iodine concentrations may take too long to react.

-delayed stopwatch reactions

concentration may not be exact due to errors in the equipment

Question 5

what is the oxidizing agent and what does it oxidise in iodine clock experiment

Answer 5

H2O2

oxidises I oxidation state from -1 to 0

Question 6

How can the volume of H2O2 be varied

Answer 6

get a stock solution and dilute it w water(serial dilution)

Question 7

what variables need to be controlled to find out order of reaction in respect to H2O2

Answer 7

• temperature
• conc of potassium iodide
• conc. of H+

Question 8

describe the equipment used for continuous monitoring method

Answer 8

conical flask rubber bung, gas syringe

Question 9

Outline the method for continuous monitoring method using Mg and Hcl

Answer 9

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.

Question 10

How do u calc. initial rates from graph

Answer 10

concentrations at start of reaction.Draw a tangent at beginning of curve and work out gradient= rate of reaction