S + Fe = FeS2

S mixed sulfur + Fe iron ⟶ FeS_2 pyrite

Balance the chemical equation algebraically: S + Fe ⟶ FeS_2 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 S + c_2 Fe ⟶ c_3 FeS_2 Set the number of atoms in the reactants equal to the number of atoms in the products for S and Fe: S: | c_1 = 2 c_3 Fe: | 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 = 2 c_2 = 1 c_3 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 2 S + Fe ⟶ FeS_2

+ ⟶

mixed sulfur + iron ⟶ pyrite

Construct the equilibrium constant, K, expression for: S + Fe ⟶ FeS_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: 2 S + Fe ⟶ FeS_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 S | 2 | -2 Fe | 1 | -1 FeS_2 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression S | 2 | -2 | ([S])^(-2) Fe | 1 | -1 | ([Fe])^(-1) FeS_2 | 1 | 1 | [FeS2] 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 = ([S])^(-2) ([Fe])^(-1) [FeS2] = ([FeS2])/(([S])^2 [Fe])

Construct the rate of reaction expression for: S + Fe ⟶ FeS_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: 2 S + Fe ⟶ FeS_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 S | 2 | -2 Fe | 1 | -1 FeS_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 S | 2 | -2 | -1/2 (Δ[S])/(Δt) Fe | 1 | -1 | -(Δ[Fe])/(Δt) FeS_2 | 1 | 1 | (Δ[FeS2])/(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[S])/(Δt) = -(Δ[Fe])/(Δt) = (Δ[FeS2])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| mixed sulfur | iron | pyrite formula | S | Fe | FeS_2 name | mixed sulfur | iron | pyrite IUPAC name | sulfur | iron | bis(sulfanylidene)iron

| mixed sulfur | iron | pyrite molar mass | 32.06 g/mol | 55.845 g/mol | 120 g/mol phase | solid (at STP) | solid (at STP) | melting point | 112.8 °C | 1535 °C | boiling point | 444.7 °C | 2750 °C | density | 2.07 g/cm^3 | 7.874 g/cm^3 | 4.89 g/cm^3 solubility in water | | insoluble | odor | | | odorless