KI + H3AsO4 = H2O + I2 + K2HAsO3

KI potassium iodide + H_3AsO_4 arsenic acid, solid ⟶ H_2O water + I_2 iodine + K2HAsO3

Balance the chemical equation algebraically: KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 KI + c_2 H_3AsO_4 ⟶ c_3 H_2O + c_4 I_2 + c_5 K2HAsO3 Set the number of atoms in the reactants equal to the number of atoms in the products for I, K, As, H and O: I: | c_1 = 2 c_4 K: | c_1 = 2 c_5 As: | c_2 = c_5 H: | 3 c_2 = 2 c_3 + c_5 O: | 4 c_2 = c_3 + 3 c_5 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_2 = 1 and solve the system of equations for the remaining coefficients: c_1 = 2 c_2 = 1 c_3 = 1 c_4 = 1 c_5 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 2 KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3

+ ⟶ + + K2HAsO3

potassium iodide + arsenic acid, solid ⟶ water + iodine + K2HAsO3

Construct the equilibrium constant, K, expression for: KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3 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: 2 KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3 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 KI | 2 | -2 H_3AsO_4 | 1 | -1 H_2O | 1 | 1 I_2 | 1 | 1 K2HAsO3 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression KI | 2 | -2 | ([KI])^(-2) H_3AsO_4 | 1 | -1 | ([H3AsO4])^(-1) H_2O | 1 | 1 | [H2O] I_2 | 1 | 1 | [I2] K2HAsO3 | 1 | 1 | [K2HAsO3] 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 = ([KI])^(-2) ([H3AsO4])^(-1) [H2O] [I2] [K2HAsO3] = ([H2O] [I2] [K2HAsO3])/(([KI])^2 [H3AsO4])

Construct the rate of reaction expression for: KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3 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: 2 KI + H_3AsO_4 ⟶ H_2O + I_2 + K2HAsO3 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 KI | 2 | -2 H_3AsO_4 | 1 | -1 H_2O | 1 | 1 I_2 | 1 | 1 K2HAsO3 | 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 KI | 2 | -2 | -1/2 (Δ[KI])/(Δt) H_3AsO_4 | 1 | -1 | -(Δ[H3AsO4])/(Δt) H_2O | 1 | 1 | (Δ[H2O])/(Δt) I_2 | 1 | 1 | (Δ[I2])/(Δt) K2HAsO3 | 1 | 1 | (Δ[K2HAsO3])/(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 = -1/2 (Δ[KI])/(Δt) = -(Δ[H3AsO4])/(Δt) = (Δ[H2O])/(Δt) = (Δ[I2])/(Δt) = (Δ[K2HAsO3])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| potassium iodide | arsenic acid, solid | water | iodine | K2HAsO3 formula | KI | H_3AsO_4 | H_2O | I_2 | K2HAsO3 Hill formula | IK | AsH_3O_4 | H_2O | I_2 | HAsK2O3 name | potassium iodide | arsenic acid, solid | water | iodine | IUPAC name | potassium iodide | arsoric acid | water | molecular iodine |

| potassium iodide | arsenic acid, solid | water | iodine | K2HAsO3 molar mass | 166.0028 g/mol | 141.94 g/mol | 18.015 g/mol | 253.80894 g/mol | 202.12 g/mol phase | solid (at STP) | solid (at STP) | liquid (at STP) | solid (at STP) | melting point | 681 °C | 35.5 °C | 0 °C | 113 °C | boiling point | 1330 °C | 160 °C | 99.9839 °C | 184 °C | density | 3.123 g/cm^3 | 2.2 g/cm^3 | 1 g/cm^3 | 4.94 g/cm^3 | surface tension | | | 0.0728 N/m | | dynamic viscosity | 0.0010227 Pa s (at 732.9 °C) | | 8.9×10^-4 Pa s (at 25 °C) | 0.00227 Pa s (at 116 °C) | odor | | | odorless | |