To study the Rate of Reaction Between Potassium Iodate (KIO3) and Sodium Sulphite (Na2SO3)

When sulphite ions react with potassium iodate, it produces iodide ions. It is seen that in an acidic medium sulphite reduces potassium iodate to iodide.

• Now the newly formed iodide ions are oxidized to iodine by reaction with more iodate ions.
• When sulphite ion is fully consumed, the blue colour is by the leftover liberated iodine. 
• So, in the reaction, we can do this by the addition of a known but limited sodium sulphite solution and starch solution volume. 
• This reaction is an example of a clock reaction.
• It is because the rate of reaction is found out by the time taken for the appearance of the blue colour.

Key Terms: Sulphite ions, Potassium iodate, Sodium sulphite, Redox, Oxidation and Reduction, Chemical reaction, Starch Solution

---

Aim

[Click Here for Sample Questions]

To Study the Reaction Rate of the Reaction Between Potassium Iodate and Sodium Sulphite Using Starch Solution as an Indicator.

---

Theory

[Click Here for Sample Questions]

Indirectly, the reaction between potassium iodate and sodium sulphite includes the production of iodide ions that are oxidized in an acidic medium. The whole reaction occurs in two stages.

\(IO_3^-+3SO_3^{2-} \:\: \longrightarrow \:\: I^- + 3SO_4^{2-}\:\:\:\:........(i)\)

\(5I^-+6H^+IO_3^- \:\: \longrightarrow \:\: 3H_2O + 3I_2\:\:\:\:........(ii)\)

With a starch solution, the evolved iodine produces a blue colour. This is also known as a clock reaction.

Redox Reaction

Redox Reaction refers to the type of chemical reaction where both oxidation and reduction occur simultaneously.

Oxidation

Oxidation is the chemical change in a substance when oxygen is added to it.

Reduction Reaction

The chemical reaction where electrons are gained by one of the atoms among the two or more chemicals involved in the reaction is known as a reduction reaction.

Also check: 

---

Apparatus and Chemicals Needed

[Click Here for Sample Questions]

The following apparatus is required for the experiment

• 4 conical flasks of 250 ml each
• Stop-watch
• Measuring cylinder
• Burette
• Pipette (25 ml)

The chemicals required for the experiment are 

• M sodium sulphite solution, 
• starch solution, 
• M H₂SO₄

---

Steps of the Experiment

[Click Here for Sample Questions]

The following are the procedures of the experiment

• Mark a 250 mL conical flask as 'P'.
• Fill the container with 25 mL of 6% potassium iodate solution, 50 mL of purified water, and 25 mL of 2.0 M H₂SO₄.
• Shake the contents of the flask gently.
• Maintain a temperature bath in the flask.
• Take five conical flasks, each with a volume of 250 mL. Label them as Q, R, S, T, and U.
• Fill flasks Q, R, S, and T with 6% Na₂SO₃ solution, starch solution, and distilled water in the proportions listed below, and keep flask U to carry out the reaction.
• Fill the 'Q' conical flask with 20 mL of Na₂SO₃ solution, 75 mL of distilled water, and 5 mL of starch solution.
• Place the flask in the water bath after thoroughly mixing the contents.
• Fill the 'R' conical flask with 15 mL of Na₂SO₃ solution, 80 mL of distilled water, and 5 mL of starch solution.
• Place the flask in a water bath and stir the mixture.
• Add 10 mL of Na₂SO₃ solution, 85 mL of distilled water, and 5 mL of starch solution to conical flask 'S'.
• Place flask S in a water bath after mixing the contents.
• 5 mL Na₂SO₃ solution, 90 mL distilled water, and 5 mL starch solution in conical flask 'T'.
• Keep the contents of flask T stirred and in the water bath.
• Take the 'U'-shaped conical flask.
• Pour 25 mL of the solution from conical flask 'P' and 25 mL of the solution from conical flask Q into the conical flask.
• When half of the solution from flask Q has been added, start the timer.
• Maintain the flask in the water bath while completely mixing.
• Time how long it takes for the blue colour to show.
• Repeat the experiment with the solutions from flasks R, S, and T, using 25 mL of the solution as in the experiment with flasks Q, and record the time necessary to achieve a blue colour.
• Make a record of your observations in the table below.
• Examine the tabulated data and determine the relationship between the time the blue colour appears and the variation in Na₂SO₃ concentration.

---

Observation and Result

[Click Here for Sample Questions]

---

Experiment Precautions

[Click Here for Sample Questions]

The following are the precautions that should be taken while performing the experiment

• We have to only use freshly prepared starch solution.
• We have to only use a freshly prepared solution of sodium sulphide as it can easily get oxidized by air.
• The concentration of KIO₃ should be more than the concentration of sodium sulphite solution.

Also Read:

---

Things to Remember

• The sodium sulphide and starch solution have to be freshly prepared.
• If the former is kept in contact with air for too long, it can oxidize and the reaction will not happen
• The rate of reaction increases with the increase in the concentration of potassium iodate.
• This is an example of a clock reaction. It is because the rate of reaction is found out by the time taken for the appearance of the blue colour.
• In the reaction between magnesium metal and oxygen, magnesium is oxidized. 
• For example, in respiration, glucose reacts with oxygen and forms carbon dioxide and water.
• Here glucose is oxidized and oxygen gets reduced to water.
• The metal ores which are reduced to their metal is an example of a reduction reaction.