A) (3c) From mixing H2SO4 and KI crystals and starch reagent. A black-blue color

Question

A) (3c) From mixing H2SO4 and KI crystals and starch reagent. A black-blue color indicates that free I2 is present. Write the balanced half-reaction for I- as a reducting agent. Say what is oxidized to for what and what is reduced to form what.

B) Using the information above. Now Na2S2O3 is added. Thiosufate ion, S2O3 (2-) is oxidized by iodine to the tetrathionate ion, S4O6(2-) and iodine is reduced to iodide ion. Write the two balanced half-reation equations for tis reations. Write the balanced net ionic equation.

Explanation / Answer

A) complete equation :   SO42- + 2 H+ + 2I-   -------------------> SO3^2- + H2O + I2

    half reaction :    2I- -----------------> I2 + 2e-

I- is the reducing agent . I- is oxidised   I2 is reduced form

B)

oxidation half reaction : 2 S2O3-2   ------------------> S4O6 -2 + 2e-

reduction half reaction :   I2 + 2e- ----------------> 2I-

balanced net ionic equation :2 S2O3-2 + I2 ----> S4O62- + 2 I-

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