I2 + Na2S = S + NaI

I_2 iodine + Na_2S sodium sulfide ⟶ S mixed sulfur + NaI sodium iodide

Balance the chemical equation algebraically: I_2 + Na_2S ⟶ S + NaI Add stoichiometric coefficients, c_i, to the reactants and products: c_1 I_2 + c_2 Na_2S ⟶ c_3 S + c_4 NaI Set the number of atoms in the reactants equal to the number of atoms in the products for I, Na and S: I: | 2 c_1 = c_4 Na: | 2 c_2 = c_4 S: | c_2 = c_3 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 = 1 c_3 = 1 c_4 = 2 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | I_2 + Na_2S ⟶ S + 2 NaI

+ ⟶ +

iodine + sodium sulfide ⟶ mixed sulfur + sodium iodide

Construct the equilibrium constant, K, expression for: I_2 + Na_2S ⟶ S + NaI 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 + Na_2S ⟶ S + 2 NaI 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 | 1 | -1 S | 1 | 1 NaI | 2 | 2 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 | 1 | -1 | ([Na2S])^(-1) S | 1 | 1 | [S] NaI | 2 | 2 | ([NaI])^2 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) ([Na2S])^(-1) [S] ([NaI])^2 = ([S] ([NaI])^2)/([I2] [Na2S])

Construct the rate of reaction expression for: I_2 + Na_2S ⟶ S + NaI 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 + Na_2S ⟶ S + 2 NaI 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 | 1 | -1 S | 1 | 1 NaI | 2 | 2 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 | 1 | -1 | -(Δ[Na2S])/(Δt) S | 1 | 1 | (Δ[S])/(Δt) NaI | 2 | 2 | 1/2 (Δ[NaI])/(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) = -(Δ[Na2S])/(Δt) = (Δ[S])/(Δt) = 1/2 (Δ[NaI])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| iodine | sodium sulfide | mixed sulfur | sodium iodide formula | I_2 | Na_2S | S | NaI Hill formula | I_2 | Na_2S_1 | S | INa name | iodine | sodium sulfide | mixed sulfur | sodium iodide IUPAC name | molecular iodine | | sulfur | sodium iodide

| iodine | sodium sulfide | mixed sulfur | sodium iodide molar mass | 253.80894 g/mol | 78.04 g/mol | 32.06 g/mol | 149.89424 g/mol phase | solid (at STP) | solid (at STP) | solid (at STP) | solid (at STP) melting point | 113 °C | 1172 °C | 112.8 °C | 661 °C boiling point | 184 °C | | 444.7 °C | 1300 °C density | 4.94 g/cm^3 | 1.856 g/cm^3 | 2.07 g/cm^3 | 3.67 g/cm^3 dynamic viscosity | 0.00227 Pa s (at 116 °C) | | | 0.0010446 Pa s (at 691 °C)