H2SO4 + K2Cr2O7 + NaI = H2O + K2SO4 + I2 + Na2SO4 + Cr2(SO4)3

H_2SO_4 (sulfuric acid) + K_2Cr_2O_7 (potassium dichromate) + NaI (sodium iodide) ⟶ H_2O (water) + K_2SO_4 (potassium sulfate) + I_2 (iodine) + Na_2SO_4 (sodium sulfate) + Cr_2(SO_4)_3 (chromium sulfate)

Balance the chemical equation algebraically: H_2SO_4 + K_2Cr_2O_7 + NaI ⟶ H_2O + K_2SO_4 + I_2 + Na_2SO_4 + Cr_2(SO_4)_3 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2SO_4 + c_2 K_2Cr_2O_7 + c_3 NaI ⟶ c_4 H_2O + c_5 K_2SO_4 + c_6 I_2 + c_7 Na_2SO_4 + c_8 Cr_2(SO_4)_3 Set the number of atoms in the reactants equal to the number of atoms in the products for H, O, S, Cr, K, I and Na: H: | 2 c_1 = 2 c_4 O: | 4 c_1 + 7 c_2 = c_4 + 4 c_5 + 4 c_7 + 12 c_8 S: | c_1 = c_5 + c_7 + 3 c_8 Cr: | 2 c_2 = 2 c_8 K: | 2 c_2 = 2 c_5 I: | c_3 = 2 c_6 Na: | c_3 = 2 c_7 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 = 7 c_2 = 1 c_3 = 6 c_4 = 7 c_5 = 1 c_6 = 3 c_7 = 3 c_8 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 7 H_2SO_4 + K_2Cr_2O_7 + 6 NaI ⟶ 7 H_2O + K_2SO_4 + 3 I_2 + 3 Na_2SO_4 + Cr_2(SO_4)_3

+ + ⟶ + + + +

sulfuric acid + potassium dichromate + sodium iodide ⟶ water + potassium sulfate + iodine + sodium sulfate + chromium sulfate

Construct the equilibrium constant, K, expression for: H_2SO_4 + K_2Cr_2O_7 + NaI ⟶ H_2O + K_2SO_4 + I_2 + Na_2SO_4 + Cr_2(SO_4)_3 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: 7 H_2SO_4 + K_2Cr_2O_7 + 6 NaI ⟶ 7 H_2O + K_2SO_4 + 3 I_2 + 3 Na_2SO_4 + Cr_2(SO_4)_3 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 H_2SO_4 | 7 | -7 K_2Cr_2O_7 | 1 | -1 NaI | 6 | -6 H_2O | 7 | 7 K_2SO_4 | 1 | 1 I_2 | 3 | 3 Na_2SO_4 | 3 | 3 Cr_2(SO_4)_3 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_2SO_4 | 7 | -7 | ([H2SO4])^(-7) K_2Cr_2O_7 | 1 | -1 | ([K2Cr2O7])^(-1) NaI | 6 | -6 | ([NaI])^(-6) H_2O | 7 | 7 | ([H2O])^7 K_2SO_4 | 1 | 1 | [K2SO4] I_2 | 3 | 3 | ([I2])^3 Na_2SO_4 | 3 | 3 | ([Na2SO4])^3 Cr_2(SO_4)_3 | 1 | 1 | [Cr2(SO4)3] 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 = ([H2SO4])^(-7) ([K2Cr2O7])^(-1) ([NaI])^(-6) ([H2O])^7 [K2SO4] ([I2])^3 ([Na2SO4])^3 [Cr2(SO4)3] = (([H2O])^7 [K2SO4] ([I2])^3 ([Na2SO4])^3 [Cr2(SO4)3])/(([H2SO4])^7 [K2Cr2O7] ([NaI])^6)

Construct the rate of reaction expression for: H_2SO_4 + K_2Cr_2O_7 + NaI ⟶ H_2O + K_2SO_4 + I_2 + Na_2SO_4 + Cr_2(SO_4)_3 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: 7 H_2SO_4 + K_2Cr_2O_7 + 6 NaI ⟶ 7 H_2O + K_2SO_4 + 3 I_2 + 3 Na_2SO_4 + Cr_2(SO_4)_3 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 H_2SO_4 | 7 | -7 K_2Cr_2O_7 | 1 | -1 NaI | 6 | -6 H_2O | 7 | 7 K_2SO_4 | 1 | 1 I_2 | 3 | 3 Na_2SO_4 | 3 | 3 Cr_2(SO_4)_3 | 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 H_2SO_4 | 7 | -7 | -1/7 (Δ[H2SO4])/(Δt) K_2Cr_2O_7 | 1 | -1 | -(Δ[K2Cr2O7])/(Δt) NaI | 6 | -6 | -1/6 (Δ[NaI])/(Δt) H_2O | 7 | 7 | 1/7 (Δ[H2O])/(Δt) K_2SO_4 | 1 | 1 | (Δ[K2SO4])/(Δt) I_2 | 3 | 3 | 1/3 (Δ[I2])/(Δt) Na_2SO_4 | 3 | 3 | 1/3 (Δ[Na2SO4])/(Δt) Cr_2(SO_4)_3 | 1 | 1 | (Δ[Cr2(SO4)3])/(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 = -1/7 (Δ[H2SO4])/(Δt) = -(Δ[K2Cr2O7])/(Δt) = -1/6 (Δ[NaI])/(Δt) = 1/7 (Δ[H2O])/(Δt) = (Δ[K2SO4])/(Δt) = 1/3 (Δ[I2])/(Δt) = 1/3 (Δ[Na2SO4])/(Δt) = (Δ[Cr2(SO4)3])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| sulfuric acid | potassium dichromate | sodium iodide | water | potassium sulfate | iodine | sodium sulfate | chromium sulfate formula | H_2SO_4 | K_2Cr_2O_7 | NaI | H_2O | K_2SO_4 | I_2 | Na_2SO_4 | Cr_2(SO_4)_3 Hill formula | H_2O_4S | Cr_2K_2O_7 | INa | H_2O | K_2O_4S | I_2 | Na_2O_4S | Cr_2O_12S_3 name | sulfuric acid | potassium dichromate | sodium iodide | water | potassium sulfate | iodine | sodium sulfate | chromium sulfate IUPAC name | sulfuric acid | dipotassium oxido-(oxido-dioxochromio)oxy-dioxochromium | sodium iodide | water | dipotassium sulfate | molecular iodine | disodium sulfate | chromium(+3) cation trisulfate