Al + CrO3 = Al2O3 + Cr

Al aluminum + CrO_3 chromium trioxide ⟶ Al_2O_3 aluminum oxide + Cr chromium

Balance the chemical equation algebraically: Al + CrO_3 ⟶ Al_2O_3 + Cr Add stoichiometric coefficients, c_i, to the reactants and products: c_1 Al + c_2 CrO_3 ⟶ c_3 Al_2O_3 + c_4 Cr Set the number of atoms in the reactants equal to the number of atoms in the products for Al, Cr and O: Al: | c_1 = 2 c_3 Cr: | c_2 = c_4 O: | 3 c_2 = 3 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 c_4 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | 2 Al + CrO_3 ⟶ Al_2O_3 + Cr

+ ⟶ +

aluminum + chromium trioxide ⟶ aluminum oxide + chromium

| aluminum | chromium trioxide | aluminum oxide | chromium molecular entropy | 28.3 J/(mol K) | 72 J/(mol K) | 51 J/(mol K) | 24 J/(mol K) total entropy | 56.6 J/(mol K) | 72 J/(mol K) | 51 J/(mol K) | 24 J/(mol K) | S_initial = 128.6 J/(mol K) | | S_final = 75 J/(mol K) | ΔS_rxn^0 | 75 J/(mol K) - 128.6 J/(mol K) = -53.6 J/(mol K) (exoentropic) | | |

Construct the equilibrium constant, K, expression for: Al + CrO_3 ⟶ Al_2O_3 + Cr 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 Al + CrO_3 ⟶ Al_2O_3 + Cr 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 Al | 2 | -2 CrO_3 | 1 | -1 Al_2O_3 | 1 | 1 Cr | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression Al | 2 | -2 | ([Al])^(-2) CrO_3 | 1 | -1 | ([CrO3])^(-1) Al_2O_3 | 1 | 1 | [Al2O3] Cr | 1 | 1 | [Cr] 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 = ([Al])^(-2) ([CrO3])^(-1) [Al2O3] [Cr] = ([Al2O3] [Cr])/(([Al])^2 [CrO3])

Construct the rate of reaction expression for: Al + CrO_3 ⟶ Al_2O_3 + Cr 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 Al + CrO_3 ⟶ Al_2O_3 + Cr 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 Al | 2 | -2 CrO_3 | 1 | -1 Al_2O_3 | 1 | 1 Cr | 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 Al | 2 | -2 | -1/2 (Δ[Al])/(Δt) CrO_3 | 1 | -1 | -(Δ[CrO3])/(Δt) Al_2O_3 | 1 | 1 | (Δ[Al2O3])/(Δt) Cr | 1 | 1 | (Δ[Cr])/(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[Al])/(Δt) = -(Δ[CrO3])/(Δt) = (Δ[Al2O3])/(Δt) = (Δ[Cr])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| aluminum | chromium trioxide | aluminum oxide | chromium formula | Al | CrO_3 | Al_2O_3 | Cr name | aluminum | chromium trioxide | aluminum oxide | chromium IUPAC name | aluminum | trioxochromium | dialuminum;oxygen(2-) | chromium

| aluminum | chromium trioxide | aluminum oxide | chromium molar mass | 26.9815385 g/mol | 99.993 g/mol | 101.96 g/mol | 51.9961 g/mol phase | solid (at STP) | solid (at STP) | solid (at STP) | solid (at STP) melting point | 660.4 °C | 196 °C | 2040 °C | 1857 °C boiling point | 2460 °C | | | 2672 °C density | 2.7 g/cm^3 | | | 7.14 g/cm^3 solubility in water | insoluble | very soluble | | insoluble surface tension | 0.817 N/m | | | dynamic viscosity | 1.5×10^-4 Pa s (at 760 °C) | | | odor | odorless | odorless | odorless | odorless