Give the Half reaction of sodium thiosulphate and calculate its equivalent weight.

Sodium thiosulfate, often represented as Na2S2O3, is a compound that plays a significant role in redox reactions, particularly in titrations involving iodine. To understand its half-reaction and calculate its equivalent weight, we first need to look at the oxidation states of the elements involved.

Half-Reaction of Sodium Thiosulfate

The half-reaction for sodium thiosulfate can be expressed in the context of its reduction. In a typical redox reaction, thiosulfate ions (S2O3^2-) are oxidized to sulfate ions (SO4^2-). The half-reaction can be written as follows:

S2O3^2- → SO4^2- + 2 e^-

In this reaction, each thiosulfate ion loses two electrons as it is oxidized to sulfate. This is crucial for understanding how thiosulfate behaves in redox chemistry.

Calculating Equivalent Weight

The equivalent weight of a substance is defined as the mass of that substance that will combine with or displace one mole of hydrogen ions (H+) or one mole of electrons in a redox reaction. To calculate the equivalent weight of sodium thiosulfate, we can follow these steps:

• Determine the molar mass: The molar mass of Na2S2O3 can be calculated as follows:
• Na: 22.99 g/mol × 2 = 45.98 g/mol
• S: 32.07 g/mol × 2 = 64.14 g/mol
• O: 16.00 g/mol × 3 = 48.00 g/mol
• Add these values together:

The total molar mass of Na2S2O3 is:

    45.98 g/mol + 64.14 g/mol + 48.00 g/mol = 158.12 g/mol
    

• Identify the number of electrons transferred: In the half-reaction, each mole of thiosulfate transfers 2 moles of electrons.
• Calculate the equivalent weight: The equivalent weight can be calculated using the formula:

    Equivalent Weight = Molar Mass / n
    

Where n is the number of electrons transferred. For sodium thiosulfate, n = 2.

    Equivalent Weight = 158.12 g/mol / 2 = 79.06 g/equiv
    

Summary

In summary, the half-reaction of sodium thiosulfate involves its oxidation to sulfate, and the equivalent weight of sodium thiosulfate is calculated to be approximately 79.06 g/equiv. This understanding is essential for performing titrations and other redox reactions involving this compound.