H2SO4 + H2S + K2Cr2O7 = H2 + K2SO4 + S + Cr(SO4)3

H_2SO_4 sulfuric acid + H_2S hydrogen sulfide + K_2Cr_2O_7 potassium dichromate ⟶ H_2 hydrogen + K_2SO_4 potassium sulfate + S mixed sulfur + Cr(SO4)3

Balance the chemical equation algebraically: H_2SO_4 + H_2S + K_2Cr_2O_7 ⟶ H_2 + K_2SO_4 + S + Cr(SO4)3 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2SO_4 + c_2 H_2S + c_3 K_2Cr_2O_7 ⟶ c_4 H_2 + c_5 K_2SO_4 + c_6 S + c_7 Cr(SO4)3 Set the number of atoms in the reactants equal to the number of atoms in the products for H, O, S, Cr and K: H: | 2 c_1 + 2 c_2 = 2 c_4 O: | 4 c_1 + 7 c_3 = 4 c_5 + 12 c_7 S: | c_1 + c_2 = c_5 + c_6 + 3 c_7 Cr: | 2 c_3 = c_7 K: | 2 c_3 = 2 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_6 = 1 and solve the system of equations for the remaining coefficients: c_2 = c_1/3 + 1 c_3 = (4 c_1)/21 c_4 = (4 c_1)/3 + 1 c_5 = (4 c_1)/21 c_6 = 1 c_7 = (8 c_1)/21 The resulting system of equations is still underdetermined, so an additional coefficient must be set arbitrarily. Set c_1 = 21 and solve for the remaining coefficients: c_1 = 21 c_2 = 8 c_3 = 4 c_4 = 29 c_5 = 4 c_6 = 1 c_7 = 8 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 21 H_2SO_4 + 8 H_2S + 4 K_2Cr_2O_7 ⟶ 29 H_2 + 4 K_2SO_4 + S + 8 Cr(SO4)3

+ + ⟶ + + + Cr(SO4)3

sulfuric acid + hydrogen sulfide + potassium dichromate ⟶ hydrogen + potassium sulfate + mixed sulfur + Cr(SO4)3

Construct the equilibrium constant, K, expression for: H_2SO_4 + H_2S + K_2Cr_2O_7 ⟶ H_2 + K_2SO_4 + S + Cr(SO4)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: 21 H_2SO_4 + 8 H_2S + 4 K_2Cr_2O_7 ⟶ 29 H_2 + 4 K_2SO_4 + S + 8 Cr(SO4)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 | 21 | -21 H_2S | 8 | -8 K_2Cr_2O_7 | 4 | -4 H_2 | 29 | 29 K_2SO_4 | 4 | 4 S | 1 | 1 Cr(SO4)3 | 8 | 8 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_2SO_4 | 21 | -21 | ([H2SO4])^(-21) H_2S | 8 | -8 | ([H2S])^(-8) K_2Cr_2O_7 | 4 | -4 | ([K2Cr2O7])^(-4) H_2 | 29 | 29 | ([H2])^29 K_2SO_4 | 4 | 4 | ([K2SO4])^4 S | 1 | 1 | [S] Cr(SO4)3 | 8 | 8 | ([Cr(SO4)3])^8 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])^(-21) ([H2S])^(-8) ([K2Cr2O7])^(-4) ([H2])^29 ([K2SO4])^4 [S] ([Cr(SO4)3])^8 = (([H2])^29 ([K2SO4])^4 [S] ([Cr(SO4)3])^8)/(([H2SO4])^21 ([H2S])^8 ([K2Cr2O7])^4)

Construct the rate of reaction expression for: H_2SO_4 + H_2S + K_2Cr_2O_7 ⟶ H_2 + K_2SO_4 + S + Cr(SO4)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: 21 H_2SO_4 + 8 H_2S + 4 K_2Cr_2O_7 ⟶ 29 H_2 + 4 K_2SO_4 + S + 8 Cr(SO4)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 | 21 | -21 H_2S | 8 | -8 K_2Cr_2O_7 | 4 | -4 H_2 | 29 | 29 K_2SO_4 | 4 | 4 S | 1 | 1 Cr(SO4)3 | 8 | 8 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 | 21 | -21 | -1/21 (Δ[H2SO4])/(Δt) H_2S | 8 | -8 | -1/8 (Δ[H2S])/(Δt) K_2Cr_2O_7 | 4 | -4 | -1/4 (Δ[K2Cr2O7])/(Δt) H_2 | 29 | 29 | 1/29 (Δ[H2])/(Δt) K_2SO_4 | 4 | 4 | 1/4 (Δ[K2SO4])/(Δt) S | 1 | 1 | (Δ[S])/(Δt) Cr(SO4)3 | 8 | 8 | 1/8 (Δ[Cr(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 = -1/21 (Δ[H2SO4])/(Δt) = -1/8 (Δ[H2S])/(Δt) = -1/4 (Δ[K2Cr2O7])/(Δt) = 1/29 (Δ[H2])/(Δt) = 1/4 (Δ[K2SO4])/(Δt) = (Δ[S])/(Δt) = 1/8 (Δ[Cr(SO4)3])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| sulfuric acid | hydrogen sulfide | potassium dichromate | hydrogen | potassium sulfate | mixed sulfur | Cr(SO4)3 formula | H_2SO_4 | H_2S | K_2Cr_2O_7 | H_2 | K_2SO_4 | S | Cr(SO4)3 Hill formula | H_2O_4S | H_2S | Cr_2K_2O_7 | H_2 | K_2O_4S | S | CrO12S3 name | sulfuric acid | hydrogen sulfide | potassium dichromate | hydrogen | potassium sulfate | mixed sulfur | IUPAC name | sulfuric acid | hydrogen sulfide | dipotassium oxido-(oxido-dioxochromio)oxy-dioxochromium | molecular hydrogen | dipotassium sulfate | sulfur |

| sulfuric acid | hydrogen sulfide | potassium dichromate | hydrogen | potassium sulfate | mixed sulfur | Cr(SO4)3 molar mass | 98.07 g/mol | 34.08 g/mol | 294.18 g/mol | 2.016 g/mol | 174.25 g/mol | 32.06 g/mol | 340.2 g/mol phase | liquid (at STP) | gas (at STP) | solid (at STP) | gas (at STP) | | solid (at STP) | melting point | 10.371 °C | -85 °C | 398 °C | -259.2 °C | | 112.8 °C | boiling point | 279.6 °C | -60 °C | | -252.8 °C | | 444.7 °C | density | 1.8305 g/cm^3 | 0.001393 g/cm^3 (at 25 °C) | 2.67 g/cm^3 | 8.99×10^-5 g/cm^3 (at 0 °C) | | 2.07 g/cm^3 | solubility in water | very soluble | | | | soluble | | surface tension | 0.0735 N/m | | | | | | dynamic viscosity | 0.021 Pa s (at 25 °C) | 1.239×10^-5 Pa s (at 25 °C) | | 8.9×10^-6 Pa s (at 25 °C) | | | odor | odorless | | odorless | odorless | | |