O2 + As = As2O3

O_2 oxygen + As gray arsenic ⟶ As_2O_3 arsenic trioxide

Balance the chemical equation algebraically: O_2 + As ⟶ As_2O_3 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 O_2 + c_2 As ⟶ c_3 As_2O_3 Set the number of atoms in the reactants equal to the number of atoms in the products for O and As: O: | 2 c_1 = 3 c_3 As: | c_2 = 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_3 = 1 and solve the system of equations for the remaining coefficients: c_1 = 3/2 c_2 = 2 c_3 = 1 Multiply by the least common denominator, 2, to eliminate fractional coefficients: c_1 = 3 c_2 = 4 c_3 = 2 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 3 O_2 + 4 As ⟶ 2 As_2O_3

+ ⟶

oxygen + gray arsenic ⟶ arsenic trioxide

Construct the equilibrium constant, K, expression for: O_2 + As ⟶ As_2O_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 + 4 As ⟶ 2 As_2O_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 As | 4 | -4 As_2O_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) As | 4 | -4 | ([As])^(-4) As_2O_3 | 2 | 2 | ([As2O3])^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) ([As])^(-4) ([As2O3])^2 = ([As2O3])^2/(([O2])^3 ([As])^4)

Construct the rate of reaction expression for: O_2 + As ⟶ As_2O_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 + 4 As ⟶ 2 As_2O_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 As | 4 | -4 As_2O_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) As | 4 | -4 | -1/4 (Δ[As])/(Δt) As_2O_3 | 2 | 2 | 1/2 (Δ[As2O3])/(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) = -1/4 (Δ[As])/(Δt) = 1/2 (Δ[As2O3])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| oxygen | gray arsenic | arsenic trioxide formula | O_2 | As | As_2O_3 name | oxygen | gray arsenic | arsenic trioxide IUPAC name | molecular oxygen | arsenic | 2, 4, 5-trioxa-1, 3-diarsabicyclo[1.1.1]pentane

| oxygen | gray arsenic | arsenic trioxide molar mass | 31.998 g/mol | 74.921595 g/mol | 197.84 g/mol phase | gas (at STP) | solid (at STP) | solid (at STP) melting point | -218 °C | 817 °C | 312 °C boiling point | -183 °C | 616 °C | 465 °C density | 0.001429 g/cm^3 (at 0 °C) | 5.727 g/cm^3 | 4.15 g/cm^3 solubility in water | | insoluble | surface tension | 0.01347 N/m | | dynamic viscosity | 2.055×10^-5 Pa s (at 25 °C) | | odor | odorless | odorless |