H2 + Se = H2Se

H_2 hydrogen + Se gray selenium ⟶ SeH_2 hydrogen selenide

Balance the chemical equation algebraically: H_2 + Se ⟶ SeH_2 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2 + c_2 Se ⟶ c_3 SeH_2 Set the number of atoms in the reactants equal to the number of atoms in the products for H and Se: H: | 2 c_1 = 2 c_3 Se: | 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 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | H_2 + Se ⟶ SeH_2

+ ⟶

hydrogen + gray selenium ⟶ hydrogen selenide

Construct the equilibrium constant, K, expression for: H_2 + Se ⟶ SeH_2 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: H_2 + Se ⟶ SeH_2 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_2 | 1 | -1 Se | 1 | -1 SeH_2 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_2 | 1 | -1 | ([H2])^(-1) Se | 1 | -1 | ([Se])^(-1) SeH_2 | 1 | 1 | [SeH2] 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 = ([H2])^(-1) ([Se])^(-1) [SeH2] = ([SeH2])/([H2] [Se])

Construct the rate of reaction expression for: H_2 + Se ⟶ SeH_2 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: H_2 + Se ⟶ SeH_2 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_2 | 1 | -1 Se | 1 | -1 SeH_2 | 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_2 | 1 | -1 | -(Δ[H2])/(Δt) Se | 1 | -1 | -(Δ[Se])/(Δt) SeH_2 | 1 | 1 | (Δ[SeH2])/(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[H2])/(Δt) = -(Δ[Se])/(Δt) = (Δ[SeH2])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| hydrogen | gray selenium | hydrogen selenide formula | H_2 | Se | SeH_2 Hill formula | H_2 | Se | H_2Se name | hydrogen | gray selenium | hydrogen selenide IUPAC name | molecular hydrogen | selenium | hydrogen selenide

| hydrogen | gray selenium | hydrogen selenide molar mass | 2.016 g/mol | 78.971 g/mol | 80.987 g/mol phase | gas (at STP) | solid (at STP) | melting point | -259.2 °C | 217 °C | boiling point | -252.8 °C | 684.9 °C | density | 8.99×10^-5 g/cm^3 (at 0 °C) | 4.81 g/cm^3 | solubility in water | | insoluble | surface tension | | 0.1055 N/m | dynamic viscosity | 8.9×10^-6 Pa s (at 25 °C) | 0.221 Pa s (at 220 °C) | odor | odorless | |