H3PO4 = H2O + H4P2O7

H_3PO_4 phosphoric acid ⟶ H_2O water + (HO)_2P(O)OP(O)(OH)_2 pyrophosphoric acid

Balance the chemical equation algebraically: H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_2 Add stoichiometric coefficients, c_i, to the reactants and products: c_1 H_3PO_4 ⟶ c_2 H_2O + c_3 (HO)_2P(O)OP(O)(OH)_2 Set the number of atoms in the reactants equal to the number of atoms in the products for H, O and P: H: | 3 c_1 = 2 c_2 + 4 c_3 O: | 4 c_1 = c_2 + 7 c_3 P: | c_1 = 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 H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_2

⟶ +

phosphoric acid ⟶ water + pyrophosphoric acid

Construct the equilibrium constant, K, expression for: H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_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 H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_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 H_3PO_4 | 2 | -2 H_2O | 1 | 1 (HO)_2P(O)OP(O)(OH)_2 | 1 | 1 Assemble the activity expressions accounting for the state of matter and ν_i: chemical species | c_i | ν_i | activity expression H_3PO_4 | 2 | -2 | ([H3PO4])^(-2) H_2O | 1 | 1 | [H2O] (HO)_2P(O)OP(O)(OH)_2 | 1 | 1 | [(HO)2P(O)OP(O)(OH)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 = ([H3PO4])^(-2) [H2O] [(HO)2P(O)OP(O)(OH)2] = ([H2O] [(HO)2P(O)OP(O)(OH)2])/([H3PO4])^2

Construct the rate of reaction expression for: H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_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 H_3PO_4 ⟶ H_2O + (HO)_2P(O)OP(O)(OH)_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 H_3PO_4 | 2 | -2 H_2O | 1 | 1 (HO)_2P(O)OP(O)(OH)_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 H_3PO_4 | 2 | -2 | -1/2 (Δ[H3PO4])/(Δt) H_2O | 1 | 1 | (Δ[H2O])/(Δt) (HO)_2P(O)OP(O)(OH)_2 | 1 | 1 | (Δ[(HO)2P(O)OP(O)(OH)2])/(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[H3PO4])/(Δt) = (Δ[H2O])/(Δt) = (Δ[(HO)2P(O)OP(O)(OH)2])/(Δt) (assuming constant volume and no accumulation of intermediates or side products)

| phosphoric acid | water | pyrophosphoric acid formula | H_3PO_4 | H_2O | (HO)_2P(O)OP(O)(OH)_2 Hill formula | H_3O_4P | H_2O | H_4O_7P_2 name | phosphoric acid | water | pyrophosphoric acid IUPAC name | phosphoric acid | water | phosphono dihydrogen phosphate

| phosphoric acid | water | pyrophosphoric acid molar mass | 97.994 g/mol | 18.015 g/mol | 177.97 g/mol phase | liquid (at STP) | liquid (at STP) | solid (at STP) melting point | 42.4 °C | 0 °C | 61 °C boiling point | 158 °C | 99.9839 °C | density | 1.685 g/cm^3 | 1 g/cm^3 | 1.75 g/cm^3 solubility in water | very soluble | | very soluble surface tension | | 0.0728 N/m | dynamic viscosity | | 8.9×10^-4 Pa s (at 25 °C) | 0.62 Pa s (at 25 °C) odor | odorless | odorless |