Question

Write the chemical equations involved in this experiment and how that the rate of disappearance of [S2O8^2-] is proportional to the rate of appearance of the blue-black color of the starch-iodine complex.

Answer 1

S₂O₈²⁻

(aq) + 2I⁻

(aq) → I₂(aq) + 2SO₄

²⁻(aq)

2S₂O₃²⁻

(aq) + I₂(aq) → S₄O₆²⁻

(aq) + 2I⁻

(aq)

Explanation:

S₂O₈²⁻

(aq) + 2I⁻

(aq) → I₂(aq) + 2SO₄

²⁻(aq)

To measure the rate of this reaction we must measure the rate of concentration change of one of  the reactants or products. To do this, we will include (to the reacting S₂O₈

²⁻  and I⁻

i) a small amount of sodium thiosulfate, Na₂S₂O₃,

ii) some starch indicator.

The added Na₂S₂O₃ does not interfere with the rate of above reaction, but it does consume the I₂  as soon as it is formed.

2S₂O₃²⁻

(aq) + I₂(aq) → S₄O₆²⁻

(aq) + 2I⁻

(aq)

This reaction is much faster than the previous, so the conversion of I2 back to I⁻  is  essentially instantaneous.

$rate = \frac{dI2}{dt} = \frac{1/2 [S2O3^2^-]}{t}$