H2 + Mg2Si = Mg + SiH4

H_2 hydrogen + Mg_2Si magnesium silicide ⟶ Mg magnesium + SiH_4 silane

Balance the chemical equation algebraically: H_2 + Mg_2Si ⟶ Mg + SiH_4 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2 + c_2 Mg_2Si ⟶ c_3 Mg + c_4 SiH_4 Set the number of atoms in the reactants equal to the number of atoms in the products for H, Mg and Si: H: | 2 c_1 = 4 c_4 Mg: | 2 c_2 = c_3 Si: | c_2 = c_4 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 = 2 c_2 = 1 c_3 = 2 c_4 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 2 H_2 + Mg_2Si ⟶ 2 Mg + SiH_4

+ ⟶ +

hydrogen + magnesium silicide ⟶ magnesium + silane

Construct the equilibrium constant, K, expression for: H_2 + Mg_2Si ⟶ Mg + SiH_4 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: 2 H_2 + Mg_2Si ⟶ 2 Mg + SiH_4 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 | 2 | -2 Mg_2Si | 1 | -1 Mg | 2 | 2 SiH_4 | 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 | 2 | -2 | ([H2])^(-2) Mg_2Si | 1 | -1 | ([Mg2Si])^(-1) Mg | 2 | 2 | ([Mg])^2 SiH_4 | 1 | 1 | [SiH4] 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])^(-2) ([Mg2Si])^(-1) ([Mg])^2 [SiH4] = (([Mg])^2 [SiH4])/(([H2])^2 [Mg2Si])

Construct the rate of reaction expression for: H_2 + Mg_2Si ⟶ Mg + SiH_4 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: 2 H_2 + Mg_2Si ⟶ 2 Mg + SiH_4 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 | 2 | -2 Mg_2Si | 1 | -1 Mg | 2 | 2 SiH_4 | 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 | 2 | -2 | -1/2 (Δ[H2])/(Δt) Mg_2Si | 1 | -1 | -(Δ[Mg2Si])/(Δt) Mg | 2 | 2 | 1/2 (Δ[Mg])/(Δt) SiH_4 | 1 | 1 | (Δ[SiH4])/(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 = -1/2 (Δ[H2])/(Δt) = -(Δ[Mg2Si])/(Δt) = 1/2 (Δ[Mg])/(Δt) = (Δ[SiH4])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| hydrogen | magnesium silicide | magnesium | silane formula | H_2 | Mg_2Si | Mg | SiH_4 Hill formula | H_2 | Mg_2Si | Mg | H_4Si name | hydrogen | magnesium silicide | magnesium | silane IUPAC name | molecular hydrogen | | magnesium | silane

| hydrogen | magnesium silicide | magnesium | silane molar mass | 2.016 g/mol | 76.695 g/mol | 24.305 g/mol | 32.117 g/mol phase | gas (at STP) | solid (at STP) | solid (at STP) | gas (at STP) melting point | -259.2 °C | 1102 °C | 648 °C | -185 °C boiling point | -252.8 °C | | 1090 °C | -112 °C density | 8.99×10^-5 g/cm^3 (at 0 °C) | 1.94 g/cm^3 | 1.738 g/cm^3 | 0.001313 g/cm^3 (at 25 °C) solubility in water | | decomposes | reacts | dynamic viscosity | 8.9×10^-6 Pa s (at 25 °C) | | | odor | odorless | | |