Clock Rate Reaction

Purpose

To determine the reaction rate law and effect of concentration for the redix reaction between peroxydisulfate and iodide ions

Procedure

1. Prepared 4 solutions as reactant 1 in a 250 mL erlenmeyer flask

2. In a separate beaker, prepared 4 solutions as reactant 2

3. Prepared 6 test tubes with 1.00 ml of thiosulfate

4. Placed a magnetic stir bar at the center in the reaction beaker

5. Poured reactant 2 into reactant 1 (100mL total) in reaction beaker then recorded time

6. Once blue-black color appeared, poured 1 test tube of thiosulfate and recorded time color change occured

7. Repeat until all 6 test tubes are used up

Repeat for next 3 solutions with 6 new test tubes of thiosulfate each

What were the 5 components in reactant 1 solution?

0.2M KNO₃, 0.2M KI, 0.4M S₂O₃^2- , 1% starch, 0.1M EDTA

What was the role of Potassium Nitrate? (KNO₃)

To have a constant number of ions

What is the role of Potassium Iodide? (KI)

KI contains I- which is reactant 1

What was the role of EDTA?

To remove metal impurities

What is the role of thiosulfate?

To reverse the reaction by destroying iodide

What was the limiting reagent?

Thiosulfate

What was the one component reactant 2?

Peroxydisulfate

How many batches of thiosulfate for each experiment?

6

What was the color of the solutions containing K+ ions?

Colorless

What was the color of the solutions containing I- ions?

Brown

What was the color of the Iodide-starch complex?

Blue-Black

Reaction Rate

Change of concentration over time - negative for reactants, positive for products

What do rates of reactions depend on?

reactants, concentration of reactants, temperature, and presence of catalysts

Rate Law

The relationship of the rate of reaction K to the concentrations of the reactants raised to some powers

Are the order of reactants the same as the stochioemtry ratios?

NO

Total reaction order

sum of the exponents for the reactants

Reaction Order 1 example

Rate = [A][B]

If [A] is doubled in Rate = [A][B], how is the rate changed?

The rate is doubled

If Rate = [A][B]² what is the overall reaction order?

A= 1

B = 2

1+2 = 3

If [B] is doubled in Rate = [A][B]² how does the rate change?

The rate is quadrupled

What is the rate for this experiment?

Rate = K [I-][S₂O₈]

What is the overall reaction order for this experiment?

Second

What is the reaction order for [I-] in this experiment?

First

What is the reaction order for peroxidisulfate in this experiment?

First

What is the clock reaction?

First thiosulfate is added and solution is clear, then more Iodide forms and the solution is blue black, and finally more thiosulfate is added and solution is clear

Describe solution 1 and 2 in the experiment

Solutions 1 and 2 have the same amount of KI

Describe solution 3 in the experiment

Solution 3 had twice as much KI as 1 and 2

Describe solution 4 in the experiment

Solution 4 had 1/2 as much KI as 1 and 2

What was the x and y axis of the reaction rates graph?

X = Time in seconds, Y = moles of peroxydisulfate reacted

How to calculate rate of reactions for solutions 1-4

Slope/0.100L = Rate (mol x L^-1 x s^-1 )

How was rate measured in the experiment?

Measured in the intervals between the color changes

Which solutions measure the effect of [I-]?

Solutions 1 and 3 measured the effect of doubling [I-]

Which solutions measured the effect of [S₂O₈²⁻]?

Solutions 1 and 2 measured the effect of doubling [S₂O₈²⁻]

what does the blue-black color represent?

The completion of the clock reaction

Which chemical species is responsible for the blue black color change?

I₂

What’s the reaction we’re studying?

S₂O₈ + 2I- —> I₂ + 2SO₄

_{peroxydisulfate + 2Iodide —> Pure Iodide + 2Sulfate}

S₂O₈ role

Oxidize Iodide from I- to I₂

Chemical Formula of Peroxidisulfate

S₂O₈^2-

Chemical Formula of Thiosulfate

S₂O₃ ^2-