I2 + Na2S2O3 = NaI + Na2S4O6

I_2 (iodine) + Na_2S_2O_3 (sodium hyposulfite) ⟶ NaI (sodium iodide) + Na2S4O6

Balance the chemical equation algebraically: I_2 + Na_2S_2O_3 ⟶ NaI + Na2S4O6 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 I_2 + c_2 Na_2S_2O_3 ⟶ c_3 NaI + c_4 Na2S4O6 Set the number of atoms in the reactants equal to the number of atoms in the products for I, Na, O and S: I: | 2 c_1 = c_3 Na: | 2 c_2 = c_3 + 2 c_4 O: | 3 c_2 = 6 c_4 S: | 2 c_2 = 4 c_4 Since the coefficients are relative quantities and underdetermined, choose a coefficient to set arbitrarily. To keep the coefficients small, the arbitrary value is ordinarily one. For instance, set c_1 = 1 and solve the system of equations for the remaining coefficients: c_1 = 1 c_2 = 2 c_3 = 2 c_4 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | I_2 + 2 Na_2S_2O_3 ⟶ 2 NaI + Na2S4O6

+ ⟶ + Na2S4O6

iodine + sodium hyposulfite ⟶ sodium iodide + Na2S4O6

Construct the equilibrium constant, K, expression for: I_2 + Na_2S_2O_3 ⟶ NaI + Na2S4O6 Plan: • Balance the chemical equation. • Determine the stoichiometric numbers. • Assemble the activity expression for each chemical species. • Use the activity expressions to build the equilibrium constant expression. Write the balanced chemical equation: I_2 + 2 Na_2S_2O_3 ⟶ 2 NaI + Na2S4O6 Assign stoichiometric numbers, ν_i, using the stoichiometric coefficients, c_i, from the balanced chemical equation in the following manner: ν_i = -c_i for reactants and ν_i = c_i for products: chemical species | c_i | ν_i I_2 | 1 | -1 Na_2S_2O_3 | 2 | -2 NaI | 2 | 2 Na2S4O6 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression I_2 | 1 | -1 | ([I2])^(-1) Na_2S_2O_3 | 2 | -2 | ([Na2S2O3])^(-2) NaI | 2 | 2 | ([NaI])^2 Na2S4O6 | 1 | 1 | [Na2S4O6] The equilibrium constant symbol in the concentration basis is: K_c Mulitply the activity expressions to arrive at the K_c expression: Answer: | | K_c = ([I2])^(-1) ([Na2S2O3])^(-2) ([NaI])^2 [Na2S4O6] = (([NaI])^2 [Na2S4O6])/([I2] ([Na2S2O3])^2)

Construct the rate of reaction expression for: I_2 + Na_2S_2O_3 ⟶ NaI + Na2S4O6 Plan: • Balance the chemical equation. • Determine the stoichiometric numbers. • Assemble the rate term for each chemical species. • Write the rate of reaction expression. Write the balanced chemical equation: I_2 + 2 Na_2S_2O_3 ⟶ 2 NaI + Na2S4O6 Assign stoichiometric numbers, ν_i, using the stoichiometric coefficients, c_i, from the balanced chemical equation in the following manner: ν_i = -c_i for reactants and ν_i = c_i for products: chemical species | c_i | ν_i I_2 | 1 | -1 Na_2S_2O_3 | 2 | -2 NaI | 2 | 2 Na2S4O6 | 1 | 1 The rate term for each chemical species, B_i, is 1/ν_i(Δ[B_i])/(Δt) where [B_i] is the amount concentration and t is time: chemical species | c_i | ν_i | rate term I_2 | 1 | -1 | -(Δ[I2])/(Δt) Na_2S_2O_3 | 2 | -2 | -1/2 (Δ[Na2S2O3])/(Δt) NaI | 2 | 2 | 1/2 (Δ[NaI])/(Δt) Na2S4O6 | 1 | 1 | (Δ[Na2S4O6])/(Δt) (for infinitesimal rate of change, replace Δ with d) Set the rate terms equal to each other to arrive at the rate expression: Answer: | | rate = -(Δ[I2])/(Δt) = -1/2 (Δ[Na2S2O3])/(Δt) = 1/2 (Δ[NaI])/(Δt) = (Δ[Na2S4O6])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| iodine | sodium hyposulfite | sodium iodide | Na2S4O6 formula | I_2 | Na_2S_2O_3 | NaI | Na2S4O6 Hill formula | I_2 | Na_2O_3S_2 | INa | Na2O6S4 name | iodine | sodium hyposulfite | sodium iodide | IUPAC name | molecular iodine | | sodium iodide |

| iodine | sodium hyposulfite | sodium iodide | Na2S4O6 molar mass | 253.80894 g/mol | 158.1 g/mol | 149.89424 g/mol | 270.2 g/mol phase | solid (at STP) | solid (at STP) | solid (at STP) | melting point | 113 °C | 48 °C | 661 °C | boiling point | 184 °C | 100 °C | 1300 °C | density | 4.94 g/cm^3 | 1.67 g/cm^3 | 3.67 g/cm^3 | dynamic viscosity | 0.00227 Pa s (at 116 °C) | | 0.0010446 Pa s (at 691 °C) | odor | | odorless | |