O2 + Se = SeO3

O_2 oxygen + Se gray selenium ⟶ SeO_3 selenium trioxide

Balance the chemical equation algebraically: O_2 + Se ⟶ SeO_3 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 O_2 + c_2 Se ⟶ c_3 SeO_3 Set the number of atoms in the reactants equal to the number of atoms in the products for O and Se: O: | 2 c_1 = 3 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_2 = 1 and solve the system of equations for the remaining coefficients: c_1 = 3/2 c_2 = 1 c_3 = 1 Multiply by the least common denominator, 2, to eliminate fractional coefficients: c_1 = 3 c_2 = 2 c_3 = 2 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 3 O_2 + 2 Se ⟶ 2 SeO_3

+ ⟶

oxygen + gray selenium ⟶ selenium trioxide

Construct the equilibrium constant, K, expression for: O_2 + Se ⟶ SeO_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: 3 O_2 + 2 Se ⟶ 2 SeO_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 O_2 | 3 | -3 Se | 2 | -2 SeO_3 | 2 | 2 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression O_2 | 3 | -3 | ([O2])^(-3) Se | 2 | -2 | ([Se])^(-2) SeO_3 | 2 | 2 | ([SeO3])^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 = ([O2])^(-3) ([Se])^(-2) ([SeO3])^2 = ([SeO3])^2/(([O2])^3 ([Se])^2)

Construct the rate of reaction expression for: O_2 + Se ⟶ SeO_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: 3 O_2 + 2 Se ⟶ 2 SeO_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 O_2 | 3 | -3 Se | 2 | -2 SeO_3 | 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 O_2 | 3 | -3 | -1/3 (Δ[O2])/(Δt) Se | 2 | -2 | -1/2 (Δ[Se])/(Δt) SeO_3 | 2 | 2 | 1/2 (Δ[SeO3])/(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 = -1/3 (Δ[O2])/(Δt) = -1/2 (Δ[Se])/(Δt) = 1/2 (Δ[SeO3])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| oxygen | gray selenium | selenium trioxide formula | O_2 | Se | SeO_3 Hill formula | O_2 | Se | O_3Se name | oxygen | gray selenium | selenium trioxide IUPAC name | molecular oxygen | selenium |

| oxygen | gray selenium | selenium trioxide molar mass | 31.998 g/mol | 78.971 g/mol | 126.97 g/mol phase | gas (at STP) | solid (at STP) | solid (at STP) melting point | -218 °C | 217 °C | 118.35 °C boiling point | -183 °C | 684.9 °C | density | 0.001429 g/cm^3 (at 0 °C) | 4.81 g/cm^3 | 3.6 g/cm^3 solubility in water | | insoluble | very soluble surface tension | 0.01347 N/m | 0.1055 N/m | dynamic viscosity | 2.055×10^-5 Pa s (at 25 °C) | 0.221 Pa s (at 220 °C) | odor | odorless | |