I2 + K2S2O3 = KI + K2S4O6

I_2 iodine + K_2S_2O_3 potassium thiosulfate ⟶ KI potassium iodide + KOSO_2SSSO_3K potassium tetrathionate

Balance the chemical equation algebraically: I_2 + K_2S_2O_3 ⟶ KI + KOSO_2SSSO_3K Add stoichiometric coefficients, c_i, to the reactants and products: c_1 I_2 + c_2 K_2S_2O_3 ⟶ c_3 KI + c_4 KOSO_2SSSO_3K Set the number of atoms in the reactants equal to the number of atoms in the products for I, K, O and S: I: | 2 c_1 = c_3 K: | 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 K_2S_2O_3 ⟶ 2 KI + KOSO_2SSSO_3K

+ ⟶ +

iodine + potassium thiosulfate ⟶ potassium iodide + potassium tetrathionate

Construct the equilibrium constant, K, expression for: I_2 + K_2S_2O_3 ⟶ KI + KOSO_2SSSO_3K 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 K_2S_2O_3 ⟶ 2 KI + KOSO_2SSSO_3K 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 K_2S_2O_3 | 2 | -2 KI | 2 | 2 KOSO_2SSSO_3K | 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) K_2S_2O_3 | 2 | -2 | ([K2S2O3])^(-2) KI | 2 | 2 | ([KI])^2 KOSO_2SSSO_3K | 1 | 1 | [KOSO2SSSO3K] 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) ([K2S2O3])^(-2) ([KI])^2 [KOSO2SSSO3K] = (([KI])^2 [KOSO2SSSO3K])/([I2] ([K2S2O3])^2)

Construct the rate of reaction expression for: I_2 + K_2S_2O_3 ⟶ KI + KOSO_2SSSO_3K 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 K_2S_2O_3 ⟶ 2 KI + KOSO_2SSSO_3K 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 K_2S_2O_3 | 2 | -2 KI | 2 | 2 KOSO_2SSSO_3K | 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) K_2S_2O_3 | 2 | -2 | -1/2 (Δ[K2S2O3])/(Δt) KI | 2 | 2 | 1/2 (Δ[KI])/(Δt) KOSO_2SSSO_3K | 1 | 1 | (Δ[KOSO2SSSO3K])/(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[K2S2O3])/(Δt) = 1/2 (Δ[KI])/(Δt) = (Δ[KOSO2SSSO3K])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| iodine | potassium thiosulfate | potassium iodide | potassium tetrathionate formula | I_2 | K_2S_2O_3 | KI | KOSO_2SSSO_3K Hill formula | I_2 | K_2O_3S_2 | IK | K_2O_6S_4 name | iodine | potassium thiosulfate | potassium iodide | potassium tetrathionate IUPAC name | molecular iodine | | potassium iodide |

| iodine | potassium thiosulfate | potassium iodide | potassium tetrathionate molar mass | 253.80894 g/mol | 190.3 g/mol | 166.0028 g/mol | 302.4 g/mol phase | solid (at STP) | | solid (at STP) | melting point | 113 °C | | 681 °C | boiling point | 184 °C | | 1330 °C | density | 4.94 g/cm^3 | 1.484 g/cm^3 | 3.123 g/cm^3 | 2.96 g/cm^3 dynamic viscosity | 0.00227 Pa s (at 116 °C) | | 0.0010227 Pa s (at 732.9 °C) |