H2O + CrO3 = H2Cr2O7

H_2O water + CrO_3 chromium trioxide ⟶ H_2Cr_2O_7 dichromic acid

Balance the chemical equation algebraically: H_2O + CrO_3 ⟶ H_2Cr_2O_7 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_2O + c_2 CrO_3 ⟶ c_3 H_2Cr_2O_7 Set the number of atoms in the reactants equal to the number of atoms in the products for H, O and Cr: H: | 2 c_1 = 2 c_3 O: | c_1 + 3 c_2 = 7 c_3 Cr: | 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_1 = 1 and solve the system of equations for the remaining coefficients: c_1 = 1 c_2 = 2 c_3 = 1 Substitute the coefficients into the chemical reaction to obtain the balanced equation: Answer: | | H_2O + 2 CrO_3 ⟶ H_2Cr_2O_7

+ ⟶

water + chromium trioxide ⟶ dichromic acid

Construct the equilibrium constant, K, expression for: H_2O + CrO_3 ⟶ H_2Cr_2O_7 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: H_2O + 2 CrO_3 ⟶ H_2Cr_2O_7 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_2O | 1 | -1 CrO_3 | 2 | -2 H_2Cr_2O_7 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_2O | 1 | -1 | ([H2O])^(-1) CrO_3 | 2 | -2 | ([CrO3])^(-2) H_2Cr_2O_7 | 1 | 1 | [H2Cr2O7] 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 = ([H2O])^(-1) ([CrO3])^(-2) [H2Cr2O7] = ([H2Cr2O7])/([H2O] ([CrO3])^2)

Construct the rate of reaction expression for: H_2O + CrO_3 ⟶ H_2Cr_2O_7 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: H_2O + 2 CrO_3 ⟶ H_2Cr_2O_7 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_2O | 1 | -1 CrO_3 | 2 | -2 H_2Cr_2O_7 | 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_2O | 1 | -1 | -(Δ[H2O])/(Δt) CrO_3 | 2 | -2 | -1/2 (Δ[CrO3])/(Δt) H_2Cr_2O_7 | 1 | 1 | (Δ[H2Cr2O7])/(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 = -(Δ[H2O])/(Δt) = -1/2 (Δ[CrO3])/(Δt) = (Δ[H2Cr2O7])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| water | chromium trioxide | dichromic acid formula | H_2O | CrO_3 | H_2Cr_2O_7 Hill formula | H_2O | CrO_3 | Cr_2H_2O_7 name | water | chromium trioxide | dichromic acid IUPAC name | water | trioxochromium | hydroxy-(hydroxy-dioxo-chromio)oxy-dioxo-chromium

| water | chromium trioxide | dichromic acid molar mass | 18.015 g/mol | 99.993 g/mol | 218 g/mol phase | liquid (at STP) | solid (at STP) | melting point | 0 °C | 196 °C | boiling point | 99.9839 °C | | density | 1 g/cm^3 | | 1.66 g/cm^3 solubility in water | | very soluble | surface tension | 0.0728 N/m | | dynamic viscosity | 8.9×10^-4 Pa s (at 25 °C) | | odor | odorless | odorless |