H2SO4 + K2Cr2O7 + H2Se = H2O + K2SO4 + Cr2(SO4)3 + Se

H_2SO_4 sulfuric acid + K_2Cr_2O_7 potassium dichromate + SeH_2 hydrogen selenide ⟶ H_2O water + K_2SO_4 potassium sulfate + Cr_2(SO_4)_3 chromium sulfate + Se gray selenium

Balance the chemical equation algebraically: H_2SO_4 + K_2Cr_2O_7 + SeH_2 ⟶ H_2O + K_2SO_4 + Cr_2(SO_4)_3 + Se Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2SO_4 + c_2 K_2Cr_2O_7 + c_3 SeH_2 ⟶ c_4 H_2O + c_5 K_2SO_4 + c_6 Cr_2(SO_4)_3 + c_7 Se Set the number of atoms in the reactants equal to the number of atoms in the products for H, O, S, Cr, K and Se: H: | 2 c_1 + 2 c_3 = 2 c_4 O: | 4 c_1 + 7 c_2 = c_4 + 4 c_5 + 12 c_6 S: | c_1 = c_5 + 3 c_6 Cr: | 2 c_2 = 2 c_6 K: | 2 c_2 = 2 c_5 Se: | c_3 = 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 = 4 c_2 = 1 c_3 = 3 c_4 = 7 c_5 = 1 c_6 = 1 c_7 = 3 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 4 H_2SO_4 + K_2Cr_2O_7 + 3 SeH_2 ⟶ 7 H_2O + K_2SO_4 + Cr_2(SO_4)_3 + 3 Se

+ + ⟶ + + +

sulfuric acid + potassium dichromate + hydrogen selenide ⟶ water + potassium sulfate + chromium sulfate + gray selenium

Construct the equilibrium constant, K, expression for: H_2SO_4 + K_2Cr_2O_7 + SeH_2 ⟶ H_2O + K_2SO_4 + Cr_2(SO_4)_3 + Se 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: 4 H_2SO_4 + K_2Cr_2O_7 + 3 SeH_2 ⟶ 7 H_2O + K_2SO_4 + Cr_2(SO_4)_3 + 3 Se 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 | 4 | -4 K_2Cr_2O_7 | 1 | -1 SeH_2 | 3 | -3 H_2O | 7 | 7 K_2SO_4 | 1 | 1 Cr_2(SO_4)_3 | 1 | 1 Se | 3 | 3 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_2SO_4 | 4 | -4 | ([H2SO4])^(-4) K_2Cr_2O_7 | 1 | -1 | ([K2Cr2O7])^(-1) SeH_2 | 3 | -3 | ([SeH2])^(-3) H_2O | 7 | 7 | ([H2O])^7 K_2SO_4 | 1 | 1 | [K2SO4] Cr_2(SO_4)_3 | 1 | 1 | [Cr2(SO4)3] Se | 3 | 3 | ([Se])^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])^(-4) ([K2Cr2O7])^(-1) ([SeH2])^(-3) ([H2O])^7 [K2SO4] [Cr2(SO4)3] ([Se])^3 = (([H2O])^7 [K2SO4] [Cr2(SO4)3] ([Se])^3)/(([H2SO4])^4 [K2Cr2O7] ([SeH2])^3)

Construct the rate of reaction expression for: H_2SO_4 + K_2Cr_2O_7 + SeH_2 ⟶ H_2O + K_2SO_4 + Cr_2(SO_4)_3 + Se 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: 4 H_2SO_4 + K_2Cr_2O_7 + 3 SeH_2 ⟶ 7 H_2O + K_2SO_4 + Cr_2(SO_4)_3 + 3 Se 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 | 4 | -4 K_2Cr_2O_7 | 1 | -1 SeH_2 | 3 | -3 H_2O | 7 | 7 K_2SO_4 | 1 | 1 Cr_2(SO_4)_3 | 1 | 1 Se | 3 | 3 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 | 4 | -4 | -1/4 (Δ[H2SO4])/(Δt) K_2Cr_2O_7 | 1 | -1 | -(Δ[K2Cr2O7])/(Δt) SeH_2 | 3 | -3 | -1/3 (Δ[SeH2])/(Δt) H_2O | 7 | 7 | 1/7 (Δ[H2O])/(Δt) K_2SO_4 | 1 | 1 | (Δ[K2SO4])/(Δt) Cr_2(SO_4)_3 | 1 | 1 | (Δ[Cr2(SO4)3])/(Δt) Se | 3 | 3 | 1/3 (Δ[Se])/(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 = -1/4 (Δ[H2SO4])/(Δt) = -(Δ[K2Cr2O7])/(Δt) = -1/3 (Δ[SeH2])/(Δt) = 1/7 (Δ[H2O])/(Δt) = (Δ[K2SO4])/(Δt) = (Δ[Cr2(SO4)3])/(Δt) = 1/3 (Δ[Se])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| sulfuric acid | potassium dichromate | hydrogen selenide | water | potassium sulfate | chromium sulfate | gray selenium formula | H_2SO_4 | K_2Cr_2O_7 | SeH_2 | H_2O | K_2SO_4 | Cr_2(SO_4)_3 | Se Hill formula | H_2O_4S | Cr_2K_2O_7 | H_2Se | H_2O | K_2O_4S | Cr_2O_12S_3 | Se name | sulfuric acid | potassium dichromate | hydrogen selenide | water | potassium sulfate | chromium sulfate | gray selenium IUPAC name | sulfuric acid | dipotassium oxido-(oxido-dioxochromio)oxy-dioxochromium | hydrogen selenide | water | dipotassium sulfate | chromium(+3) cation trisulfate | selenium

| sulfuric acid | potassium dichromate | hydrogen selenide | water | potassium sulfate | chromium sulfate | gray selenium molar mass | 98.07 g/mol | 294.18 g/mol | 80.987 g/mol | 18.015 g/mol | 174.25 g/mol | 392.2 g/mol | 78.971 g/mol phase | liquid (at STP) | solid (at STP) | | liquid (at STP) | | liquid (at STP) | solid (at STP) melting point | 10.371 °C | 398 °C | | 0 °C | | | 217 °C boiling point | 279.6 °C | | | 99.9839 °C | | 330 °C | 684.9 °C density | 1.8305 g/cm^3 | 2.67 g/cm^3 | | 1 g/cm^3 | | 1.84 g/cm^3 | 4.81 g/cm^3 solubility in water | very soluble | | | | soluble | | insoluble surface tension | 0.0735 N/m | | | 0.0728 N/m | | | 0.1055 N/m dynamic viscosity | 0.021 Pa s (at 25 °C) | | | 8.9×10^-4 Pa s (at 25 °C) | | | 0.221 Pa s (at 220 °C) odor | odorless | odorless | | odorless | | odorless |